Review of Mainstream Cosmology [Archive]

SpaceTiger

May28-05, 03:06 AM

Chronos

May28-05, 03:19 AM

SpaceTiger

May28-05, 03:25 AM

ST, would you be shocked if I agree 100%? I would like to discuss metallicity in the early universe. Some think this is a huge issue. I think it is not. I am further annoyed by suggestions that metallicity does not evolve with redshift. Is that fair game in this thread?

Yes, of course, if it's within the context of the standard model. As I mentioned in my other recent post, I don't think it represents a particularly good cosmic clock, so it would be difficult for metallicity observations at high redshift to produce strong evidence for or against the Big Bang model.

Chronos

May28-05, 03:34 AM

question - could gamma bursters have ionized the early universe? I have a follow up question.

Chronos

May28-05, 03:40 AM

Another question [pardon my curiousity], could SMBH have formed in the early universe from collapsing gas clouds [skipping the merger thing, just formed directly]?

SpaceTiger

May28-05, 03:53 AM

question - could gamma bursters have ionized the early universe? I have a follow up question.

Although it's true that we don't understand Pop III evolution enough to know what their supernovae look like, I'm fairly certain that such gamma-ray bursts couldn't provide enough consistent flux to ionize the early universe. Remember that any one of these events is very brief and the area that they effect is very small (relative to the size of the universe). They would certainly ionize the gas in their vicinity, but in the absence of another source of radiation, it would quickly recombine. There is thought to be a steady flux from the first generation of stars, however, so they may indeed reionize the universe while being simply in a steady state.

SpaceTiger

May28-05, 04:00 AM

Another question [pardon my curiousity], could SMBH have formed in the early universe from collapsing gas clouds [skipping the merger thing, just formed directly]?

It's true that some of the processes which cause molecular clouds to fragment and limit the maximum mass of a star will not necessarily occur in a metal-free environment. I find it hard to believe, however, that a cloud as massive as an SMBH (106-109 solar masses) could collapse without fragmentation or self-destruction (by radiation pressure). I'm not sure, however, so I will do a little research on the subject.

SpaceTiger

May28-05, 04:43 AM

3) Homogeneity and Isotropy

Most models of the universe assume that it is uniform to translations in space (homogeneous) and uniform in direction (isotropic). This does not mean that every point in space is the same on all scales (it obviously isn't), but rather that the universe is smooth on the largest scales. By analogy, the surface of a spherical balloon is homogeneous and isotropic, despite having small bumps and wiggles if you look at it closely enough. Although this point is not controversial (even believers in steady-state cosmology like homogeneity and isotropy), it is actually more difficult to prove than, for example, expansion. Difficult, but not impossible.

The first and most convincing line of evidence (if you believe the big bang) is the cosmic microwave background radiation. If it really is a fingerprint of the early universe, then its extreme uniformity implies homogeneity to one part in 104. There are some indications of a possible asymmetry in the recent WMAP measurements of the CMB, but it is very small and seems to line up with the ecliptic, indicating that it may be due to contamination from the solar system.

There are many other things that we can observe to test homogeneity and isotropy, including galaxies, radio sources, the x-ray background, and lyman-alpha absorption clouds. A nice (though outdated) review can be found here:

http://arxiv.org/PS_cache/astro-ph/pdf/0001/0001061.pdf

Efforts are currently underway to test it with the Sloan Digital Sky Survey (SDSS), but they are not yet conclusive.

wolram

May28-05, 06:20 AM

Space Tiger.

Could you help in clarifying, the missing particle dilemma, axions, higgs,graviton
etc? will cosmology work without them? or at least some? if any which could
remain "undiscovered".

Garth

May28-05, 07:57 AM

ST, is there a problem with your scenario?

Would not the earliest PopIII stars have very low, primordial BB, metallicity Z = 10-13Zsolar?
They are required to re-ionise the CMB and provide early metallicity.

However, would not these stars therefore have to be very massive
M = >105Msolar to gravitationally collapse? If so then they would leave behind your SMBHs as their final stage of stellar evolution.

Should these PopIII stars be visible (m ~ 27 or brighter is quoted, estimates are given of one per arcsec)?

Are they seen?
If not why not?

Garth

Chronos

May28-05, 03:01 PM

I'm still stuck on the SMBH issue. They appear to be a necessary precursor to forming galaxies and it just seems to take too long for them to form via mergers. So I'm kind of going opposite of Hawkings tiny primordial black hole idea, I'm expecting huge black holes conspiring with DM to seed the the observed structure of the universe. Has anyone N-modeled such an idea? I've read much about the DM model producing filament structures [which are not quite right vs observation] but nothing about what happens if you inject various sized massive bodies into the formula.

SpaceTiger

May28-05, 04:11 PM

Space Tiger.

Could you help in clarifying, the missing particle dilemma, axions, higgs,graviton
etc? will cosmology work without them? or at least some? if any which could
remain "undiscovered".

Standard cosmology doesn't actually need any specific particles to exist, but it does need a WIMP. Any WIMP will do, however. The only reason we look for specific particles (like the Higgs boson) is to test the theories of particle physics.

SpaceTiger

May28-05, 04:18 PM

However, would not these stars therefore have to be very massive
M = >105Msolar to gravitationally collapse? If so then they would leave behind your SMBHs as their final stage of stellar evolution.

Nothing about Pop III stars will present a challenge to any model until we are more confident in the physics that govern them. Even local star formation is very poorly understood. To say that we expect a certain mass, lifetime, metallicity, or whatever of a Pop III star is certainly untrue. Just a casual search of the literature will reveal that.

wolram

May28-05, 04:22 PM

Space Tiger.

Standard cosmology doesn't actually need any specific particles to exist, but it does need a WIMP. Any WIMP will do, however. The only reason we look for specific particles (like the Higgs boson) is to test the theories of particle physics.

So there "has", to be a WIMP, and "not", a modification to GR.

SpaceTiger

May28-05, 04:26 PM

I'm still stuck on the SMBH issue. They appear to be a necessary precursor to forming galaxies and it just seems to take too long for them to form via mergers.

The theories are favoring accretion as the primary means of SMBH growth at the moment. Why do you say they're a necessary precursor to the formation of galaxies? I certainly agree that the tend to end up at the centers of galaxies. The dominant theory of structure formation involves the gradual collapse of larger and larger density perturbations. See this article for details:

http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1974ApJ...187..425P&db_key=AST&h igh=424800249005443

SpaceTiger

May28-05, 04:28 PM

So there "has", to be a WIMP, and "not", a modification to GR.

I'll address the dark matter issue in a bit. Hopefully that will clear things up.

Garth

May28-05, 05:08 PM

Nothing about Pop III stars will present a challenge to any model until we are more confident in the physics that govern them. Even local star formation is very poorly understood. To say that we expect a certain mass, lifetime, metallicity, or whatever of a Pop III star is certainly untrue. Just a casual search of the literature will reveal that.
Is not the problem in getting any stellar mass to collapse under self-gravitation that of removing the pressure (i.e. heat) supporting the mass against collapse?
Does not metallicity play a crucial role in radiating this heat energy away?

So if we remove the metallicity then we require super-massive Jeans' masses to condense?

These super PopIII stars would then be expected to leave behind SMBHs that should be observed today, but are not AFAIK.

However, if there were high primordial BB metallicity (Z = 10-8Zsolar) as predicted by the “Freely Coasting” (http://arxiv.org/abs/astro-ph/0306448) model, then more moderate 102 - 103 Msolar PopIII stars might form that leave behind IMBHs of the same order of mass size, which may not yet be detected.

DM identified?

Just a thought,

Garth

Chronos

May28-05, 05:49 PM

The theories are favoring accretion as the primary means of SMBH growth at the moment. Why do you say they're a necessary precursor to the formation of galaxies? I certainly agree that the tend to end up at the centers of galaxies. The dominant theory of structure formation involves the gradual collapse of larger and larger density perturbations. See this article for details:

http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1974ApJ...187..425P&db_key=AST&high=424800249005443Those are excellent links, and I mostly agree with what they say. But, I don't think they are necessary, just convenient. While I like convenient explanations. I am still trying to connect the dots between early metallicity and pop III gamma bursters.

SpaceTiger

May28-05, 09:19 PM

These super PopIII stars would then be expected to leave behind SMBHs that should be observed today, but are not AFAIK.

Limits on the masses of Pop III stars range from 100 solar masses to 105 solar masses, depending on which model you subscribe to. The black holes created by these stars are certainly not a problem for the standard model, as they would not be numerous enough to be observed. In addition, if they were in large overdensities (i.e. galaxies), they would eventually merge with the central SMBHs by processes like dynamical friction.

However, if there were high primordial BB metallicity (Z = 10-8Zsolar) as predicted by the “Freely Coasting” model, then more moderate 102 - 103 Msolar PopIII stars might form that leave behind IMBHs of the same order of mass size, which may not yet be detected.

Which part of the following did you not understand?

Note that we are discussing mainstream cosmology, so this is not the place to present your favorite non-standard model for the universe.

Garth

May29-05, 02:26 AM

OK sorry, but my point is should such large PopIII stars be observable today?
Garth

SpaceTiger

May29-05, 02:35 AM

OK sorry, but my point is should such large PopIII stars be observable today?

What I'm saying is that we simply don't know. It's possible that they're observable, but even if they are, there's no guarantee that we would have seen them already. You realize how dim 27th magnitude is, right?

Nereid

May29-05, 08:49 AM

Standard cosmology doesn't actually need any specific particles to exist, but it does need a WIMP. Any WIMP will do, however. The only reason we look for specific particles (like the Higgs boson) is to test the theories of particle physics.Amplification: SpaceTiger is, of course, talking about observational cosmology here, not astronomy or astrophysics is general ("we" the cosmologists).

Specific particles and their behaviour are important in the study of cosmic rays, neutrino astronomy, the finer details of high energy shocks (collapsars, Type 1a SN, SNR? magnetars??), and may also be important for the finer details of the quasar engine, whether or not non-BH objects more exotic than neutron stars could exist, and so on.

There was a time when at least some other properties of the WIMPS (other than the WI) did matter ... hot? warm? cold? a mixture?

Garth

May29-05, 09:48 AM

What I'm saying is that we simply don't know. It's possible that they're observable, but even if they are, there's no guarantee that we would have seen them already. You realize how dim 27th magnitude is, right?
Yes, but I quote from Schild Some Consequences of the Baryonic Dark Matter Population (http://arxiv.org/abs/astro-ph/0409549)The mystery of how did the universe become re-ionized by a Pop III that should have been seen at redshifts 6 to 8, now under scrutiny from direct spectroscopic observation, is cleanly side-stepped.......

Garth

wolram

May29-05, 11:58 AM

Nereid
Amplification: SpaceTiger is, of course, talking about observational cosmology here, not astronomy or astrophysics is general ("we" the cosmologists).

Yes, i did not want to bother ST to much, i imagine his time is at a premium.

SpaceTiger

May29-05, 01:00 PM

Amplification: SpaceTiger is, of course, talking about observational cosmology here, not astronomy or astrophysics is general ("we" the cosmologists).

That's right.

There was a time when at least some other properties of the WIMPS (other than the WI) did matter ... hot? warm? cold? a mixture?

You're right, I was being overly terse, but I intend to write longer post on the issue later, so I figured it would be better to hold off detailed discussion until then. I was simply trying to emphasize the point that the standard model allows for a variety of particles to be the WIMP and would not be falsified by the non-detection of a specific particle.

SpaceTiger

May30-05, 09:47 PM

4) Age of the Universe

One of the obvious implications of the Big Bang Theory is that the universe has a finite age. A precise determination of the age of the universe will come out of the cosmological model, including all of the parameters, but we can make several independent estimates from other arguments.

Firstly, there are globular clusters. From what we know about stellar evolution, we can model populations of stars and, under the assumption that they were all born at the same time, determine their age. When we do this with Milky Way globular clusters, we get an age of around 12 +- 3 billion years. Not technically a determination of the universe's age, but certainly a lower limit.

What about radioactive elements? Can we somehow use them to infer the age of the universe? It turns out that we can. Recent detections of Uranium-238 and Thorium-232 in stars have allowed us to use the traditional radioactive dating method to obtain an age of 12.5 +- 3 billion years. Again, a lower limit, but completely independent from and consistent with that from stars.

Finally, there are the measured cosmological parameters. When brought together and analyzed carefully, we can very tightly constrain the age of the universe to be 13.7 +- 0.2 billion years. It is very reassuring that this is consistent with both of the above ages. In fact, the standard model predicts that the Milky Way should have formed very early in the life of the universe, so the fact that the other two ages are of the same order (and not much less) is also consistent. One way to falsify the standard model would be to find something that is significantly older than 13.7 billion years. For a while, the globular cluster measurements were thought to represent such a falsification, but with the improvement of both our globular cluster measurements and our cosmological measurements, we are now finding nice agreement.

Note: I got the numbers from this nice review paper:

http://lanl.arxiv.org/find/astro-ph/1/au:+Primack_J/0/1/0/all/0/1

SpaceTiger

Jun2-05, 09:15 PM

5) Flatness

What do we mean when we say the universe is flat? Well, in short, we mean that the space can be described by normal Euclidean geometry; for example, the angles of a triangle add up to 180 degrees. In fact, the latter is exactly what we usually use in our attempts to determine flatness. One could actually go out and perform such an experiment by constructing a giant triangle (with, say, laser beams shooting from one mountain to another) and measure the angles of this giant triangle. If, within the uncertainties, the angles added up to 180 degrees, one would conclude that the space in that region was approximately flat. Of course, we know now that the space near the earth's surface is very well approximated as flat, but there was no way for the ancients to be sure of this.

Likewise, without a direct measurement, there's no way that we can be sure whether or not the space in the observable universe is flat. This kind of thing is very difficult to do locally because we only expect the universe's curvature to be noticable on large scales (that is, at high redshift). It turns out the most effective method is to analyze the anisotropies in the cosmic microwave background (CMB), a last-scattering "surface" that was formed at around z ~ 1100 [For more information on the microwave background, see marlon's What is Cmb (http://www.physicsforums.com/showthread.php?t=72153) thread]. By looking at the length scale on which the CMB is most anisotropic, we can determine very precisely the flatness of the universe. Using WMAP, we were able to determine that the universe was flat to very high precision:

\Omega=1.02 \pm 0.02

For those not familiar with that notation, \Omega=1 is a flat universe. Buried in this notation, however, is an important assumption. What it really means is

\Omega=\frac{\bar{\rho}}{\rho_c}=\frac{8\pi G\bar{\rho}}{3H^2}

where \bar{\rho} is the average density of the universe and H is Hubble's constant. This is an elegant description of how mass curves space. That is, general relativity tells us that not only can we measure the geometry of space itself, but we can also infer its geometry by measuring how much mass and energy occupy it. This should be kept in mind when one considers that the total energy density of the universe has been measured to correspond approximately to that needed to flatten the universe. In other words, the pictures are consistent -- the geometry is flat and the contents are sufficient to flatten it.

In the following sections, I'll describe exactly what those contents are and how we measure their total contribution to the curvature of the universe.

Chronos

Jun3-05, 02:12 AM

What a waste. ST, I am so dang tired of trying to explain why all the evidence points towards a CONSISTENT model, I will let it drop in your lap for a week.

SpaceTiger

Jun3-05, 02:33 AM

If anyone wants me to explain any of the concepts further, just say so and I'll be happy to devote a sub-section to it. My goal is not to write a textbook, so I'll only dwell on those details requested.

Chronos

Jun4-05, 04:32 AM

Can we talk about spallation yet?

Garth

Jun4-05, 10:17 AM

Chronos "Can we talk about spallation yet?"
Deuterium is very fragile.
The standard model assumes that any deuterium left today has been left over from the BB as any other source, such as stellar nuclear fusion reactor cores, would not only have created deuterium but destroyed it ‘instantly’ as well.

The deuterium relative abundance (D/H ~ 2 x 10 -5) is therefore assumed to be a very accurate trace of what was happening in the BB and puts a fine constraint on cosmological constraints. (1% < Omegabaryonh2 < 1.5%) (h2 ~ 0.5) It is more or less concordant with a 3% - 4% baryon closure density.

If another significant source of deuterium exists then that would throw this standard model out.

Deuterium production by high-energy particles (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1977ApJ...212..595E&db_key=AST) The production of the cosmic abundance of deuterium by high-energy spallation reactions is examined. The large energy requirements and the concomitant production of other nuclei and gamma-rays impose severe constraints on this sort of mechanism. Violent pregalactic events, which might occur shortly after recombination or in early quasarlike objects, are found to be possible sites for deuterium production. Some constraints on the origin of the diffuse gamma-ray background also are obtained.

The question is; how significant are these other possible sites for deuterium production? If the D/H ratio is partly explained by such then the standard model has some explaining to do. If all the D/H ~ 2 x 10 -5 can be explained in this way then nucleosynthesis might continue in a more slowly evolving universe and produce all the DM as baryons, but that would not be "mainstream cosmology".

Garth

meteor

Jun4-05, 09:26 PM

Another question [pardon my curiousity], could SMBH have formed in the early universe from collapsing gas clouds [skipping the merger thing, just formed directly]?

I find myself quite skeptical with respect to the possible formation of black holes if that formation doesn't come from one of these two processes:
a)the collapse of a star
b)the formation of miniblackholes in the early stages of the Universe

though I have read some other persons proposing that it could be possible the formation of a black hole by the collapse of a cloud, these propositions have been in Internet forums, so possibly only based in wild speculation, but then I've found this paper in arxiv
http://arxiv.org/abs/astro-ph/0505136
Black Hole Formation from Collapsing Dark Matter in the Background of Dark Energy

that more or less proposes that black holes can be formed if a cloud of dark matter and dark energy collapses. Given that dark matter halos were one of the first structures to form in the Universe, then this paper open the door to the possibility that some massive black holes in the center of galaxies could have formed this way. I'm by no means subscribing to the point of view of the authors of the paper, only presenting a curious theory

Garth

Jun5-05, 02:15 AM

There is another problem with SMBH's - the formation of the BH, either from the end result of super massive stars, or directly from DM & DE, which would also drag a lot of baryonic matter with it, would be a very energetic and bright event. Should we be seeing these very early hyper-novas?

That something did go on in the pre-galactic era seems very likely as there is a lot of re-ionisation and early metallicity to explain. However if there were a few very large BH formation events then the re-ionisation and metallicity would be very localised and patchy. This does not seem to be the case, although there is variation in the metallicity.

Perhaps these events were not as large as the SMBH scenario requires, and there were many more of them. IMBHs ([102 - 104]Msolar) could explain the DM today.

Garth

Chronos

Jun5-05, 03:25 AM

One of the problems with finding very low or metal free galaxies is low metallicity results in low surface brightness. Low metallicity galaxies are progressively under represented in surveys as redshift increases. They could be very numerous at z=6+, but are simply too faint to be seen.

Garth

Jun5-05, 08:52 AM

So the "Mainstream Cosmology" model predicts:
1 Large SMBH's, but their bright formation process has not been observed - too faint?
2 An evolution in early metallicity - but that has not been observed - selection effect?
3 The vast proportion of the universe 73% in the form of Dark Energy - but nobody has any idea of what that actually is and certainly have not verified its existence in a laboratory or Earth bound observation.
4 23% of the universe in the form of non-baryonic Dark Matter - but nobody has any idea of what form that might take - ditto as with DE.
5 A process of explosive Inflation in the earliest universe because of the action of the Higgs field - but nobody has discovered the Higgs boson that causes that process.
6. A antigravity effect that causes acceleration of the expansion of the universe - DE? - this effect is massively switched on in the Inflation epoch, switched off for the nucleosynthesis epoch, switched on for the distant SN Ia epoch and finally switched off again for the recent epoch.

All to make the mainstream cosmological model fit the data.

Am I being too critical in my analysis of that model? Perhaps I am a natural cynic, but then again perhaps not.
Just a few thoughts.

Garth

Chronos

Jun5-05, 11:40 PM

Just focusing on metallicity issues for now, Garth. It has been notoriously difficult to obtain good metallicity samples. Reliable, low z sources are rare and high z sources are contaminated by selection effects [low metallicity galaxies are inherently fainter]. The data is, however, accumulating and evolutionary trends are emerging:

The Age-Metallicity Relation of the Universe in Neutral Gas: The First 100 Damped Lya Systems
http://arxiv.org/abs/astro-ph/0305314

Chemical Abundances in SFG and DLA
http://arxiv.org/abs/astro-ph/0504389

Damped Lyman Alpha Surveys and Statistics - A Review
http://arxiv.org/abs/astro-ph/0505479

Garth

Jun6-05, 03:53 AM

Chronos yes, I am not arguing that there has been no metallicity evolution as I believe red shift to be cosmological, and therefore high z systems are ancient. Since then stellar nucleosynthesis has obviously taken place after all the stars are luminous! I was exaggerating for the sake of making a point in my (2) above.

The question is; "At what high z does the standard model expect this metallicity to drop to zero and is this bottoming out observed?" Your first link above states Regarding the lower limit to the DLA metallicities, it appears possible that we will never identify a damped LyA system with [M/H] < −3, a value that significantly exceeds our detection limit. This lower bound has important implications for the presence of primordial gas (zero metallicity) within these galaxies. If primordial gas with significant surface density and cross-section exists in high redshift galaxies, then it is always surrounded by metal-enriched gas yielding a mass weighted metallicity exceeding 1/1000 solar. Note the assumption that primordial gas has to have zero metallicity. It is precisely this assumption that is cosmological model dependent and that which I question.

Garth

SpaceTiger

Jun9-05, 11:55 PM

All to make the mainstream cosmological model fit the data.

These things were invented to explain the data, not the other way around.

1 Large SMBH's, but their bright formation process has not been observed - too faint?
2 An evolution in early metallicity - but that has not been observed - selection effect?

Again, I think it's really silly that crackpots put so much emphasis on these observations as evidence for non-standard cosmology. They're extremely sketchy and littered with selection biases. The mainstream model does not claim to have a solid understanding of the process of quasar growth or metallicity evolution, so it should be of no surprise that they produce difficult observations.

3 The vast proportion of the universe 73% in the form of Dark Energy - but nobody has any idea of what that actually is and certainly have not verified its existence in a laboratory or Earth bound observation.

"Dark energy" is little more than a description of an observation at this point. We don't have a solid theory for it yet, so emphasizing its non-detection is redundant.

4 23% of the universe in the form of non-baryonic Dark Matter - but nobody has any idea of what form that might take

That's untrue. In fact, particle physicists expect a particle at exactly the energy scale needed to solve the dark matter problem. Furthermore, alternative gravity models have not successfully predicted (or even explained) all of the observed phenomena. More on this later.

5 A process of explosive Inflation in the earliest universe because of the action of the Higgs field - but nobody has discovered the Higgs boson that causes that process.

The evidence for an inflationary epoch is getting stronger, but is still not entirely convincing. Again, more on this later.

6. A antigravity effect that causes acceleration of the expansion of the universe - DE? - this effect is massively switched on in the Inflation epoch, switched off for the nucleosynthesis epoch, switched on for the distant SN Ia epoch and finally switched off again for the recent epoch.

The standard model has the current acceleration being caused by dark energy and the inflationary epoch by some other scalar field. This point is redundant with those two previous ones.

Many of the things people refer to as "holes" in the standard model are not actually inconsistencies, just things that are not completely understood (like dark energy). The simple fact is that the standard model uses known physics (i.e. GR and QFT) to explain multiple independent observations. This is what makes it so compelling. That we don't have a full understanding of everything should hardly be surprising. Most alternative models invoke arbitrary new physics, usually with serious observational inconsistencies. I doubt that we have everything figured out; in fact, I hope that we don't. I do think, however, that we have done a fairly good job of parameterizing the universe so far and it's clear that the community has been approaching consensus on the basic cosmological parameters. Currently, the only inconsistencies in the standard model are either barely significant or explainable by alterations in less fundamental theories (such as quasar growth).

SpaceTiger

Jun10-05, 12:27 AM

6) The Matter Density

The matter density is, quite simply, the average space density of matter in the universe. It is usually parameterized relative to the critical density:

\Omega_m=\frac{\rho_m}{\rho_c}

This is the density of all non-relativistic matter, including the stuff we're made of (baryonic matter) and the dark matter that has so far eluded our detectors. It does not include photons, relativistic particles, or dark energy.

Since it includes the stuff we can't see, the estimates of \Omega_m must be dynamical; that is, they must be inferred from gravitational influence of the matter. Doing this in a variety of systems (on both small and large scales), we can directly measure the total amount of matter in the universe. These methods tend to give values in the range:

\Omega_m \sim 0.2 - 0.3

Remember that \Omega_m=1 would mean that the matter density was exactly sufficient to flatten the universe. Recently, several other independent measurements, including the peculiar velocity field of galaxies, the power spectrum, and the CMB, have given values that are in the same ballpark. In fact, measurements of the matter density have been confirmed in so many different ways that it was previously believed that we lived in an open universe with \Omega\simeq \Omega_m \simeq 0.3. With the recent CMB and supernovae measurements, however, we now believe that the remainder of the energy density required to flatten the universe is in some other form, this mysterious dark energy.

SpaceTiger

Jun10-05, 12:50 AM

There is another problem with SMBH's - the formation of the BH, either from the end result of super massive stars, or directly from DM & DE, which would also drag a lot of baryonic matter with it, would be a very energetic and bright event.

The whole point of dark matter is that it's weakly-interacting and therefore does not emit much light. The collapse of an overdensity consisting only of dark matter would not need to emit a lot of observable photons. Likewise, there's no reason that a dark energy field should have to produce photons upon collapsing to a black hole.

Should we be seeing these very early hyper-novas?

It's hard enough to detect supernovae out to z=1, I don't know why you'd expect to see them at z>6.

That something did go on in the pre-galactic era seems very likely as there is a lot of re-ionisation and early metallicity to explain. However if there were a few very large BH formation events then the re-ionisation and metallicity would be very localised and patchy. This does not seem to be the case, although there is variation in the metallicity.

I can't emphasize enough how sketchy our observations of that era are. To be honest, I don't even entirely trust the WMAP reionization results.

Perhaps these events were not as large as the SMBH scenario requires, and there were many more of them. IMBHs ([102 - 104]Msolar) could explain the DM today.

How many times do I have to emphasize that this thread is not the place for your pet theories? Stop trying to plug your model in my thread.

Garth

Jun10-05, 02:52 AM

In my posts #35 & #37 I raise questions about the "mainstram cosmology" model. Questions that are raised by others in the cosmological community. These questions might well be answered in the future within that paradigm.

But at what point did I plug my model?

Surely the test of a robust model is that it is open to cross examination?
These things were invented to explain the data, not the other way around.
Stand back and think!

Garth

SpaceTiger

Jun10-05, 03:21 AM

In my posts #35 & #37 I raise questions about the "mainstram cosmology" model. Questions that are raised by others in the cosmological community. These questions might well be answered in the future within that paradigm.

What's your point?

But at what point did I plug my model?

I quoted it. The IMBH as DM is purely speculation that you've recently introduced as part of your attempt to do away with non-baryonic matter.

Stand back and think!

It's amusing to me how crackpots try to defend their point of view by playing "open-minded", yet seldom seem to understand the observational evidence for the models they're trying to topple. I suppose it hasn't occurred to you that I actually have thought about my opinions? Do I strike you as the sort who's ignorant of the observational support for our current theoretical understanding? Have you heard me give resounding support for every mainstream model? (hint: do a search for my posts on inflation)

Garth

Jun10-05, 03:51 AM

SpaceTiger I appreciate all that you have said on these Forums and the considerable thought that you have demonstrated in your clear and informative posts.

Nevertheless, in astrophysics and cosmology we are dealing with data that has to be interpreted as "Remote Sensing". The problem with such remote sensing is that of "Ground Truth"; in our case the task of explaining the physics of the cosmos ‘out there’ by the physics of the laboratory ‘down here’.

Today there is a huge amount of precision data, which has to be interpreted, but the interpretation of that data set is theory dependent. i.e. Change the paradigm and that interpretation changes too.

Therefore a critical analysis of the subject has to be open to other possible interpretations, if only to subsequently reject them as internally inconsistent, non-concordant with the data set, incompatible with laboratory experiment and finally inelegant i.e. requiring a multiplication of “entities” [Ockham’s (Occam’s) razor ”Entia non sunt multiplicanda praeter necessitatem” (Entities should not be unnecessarily multiplied).

Let me repeat for clarification – I was not trying to be rude -
These things were invented to explain the data, not the other way around.As in ‘epicycles’?

Garth

SpaceTiger

Jun10-05, 04:31 AM

Today there is a huge amount of precision data, which has to be interpreted, but the interpretation of that data set is theory dependent. i.e. Change the paradigm and that interpretation changes too.

If you wish to entirely change a paradigm, you must re-interpret all of the observational evidence in the context of the new paradigm before you can safely say that your theory is viable. This is what Einstein did with relativity; in fact, he went a step further and made predictions. None of the alternative theories on the table have done this successfully, even MOND. I will consider any alternative theory that is prepared to do this, provided that it doesn't arbitrarily "invent" too many new forces, effects, etc.

Therefore a critical analysis of the subject has to be open to other possible interpretations, if only to subsequently reject them as internally inconsistent, non-concordant with the data set, incompatible with laboratory experiment and finally inelegant i.e. requiring a multiplication of “entities” [Ockham’s (Occam’s) razor ”Entia non sunt multiplicanda praeter necessitatem” (Entities should not be unnecessarily multiplied).

I am open to new interpretations, but that's not the point of this thread, as I very clearly stated at the beginning. The point of this thread is to review why we believe or disbelieve the standard model, not to present alternatives to it.

Let me repeat for clarification – I was not trying to be rude -
As in ‘epicycles’?

They are indeed analogous to epicycles, but really, the vast majority of new phenomena are explainable by small extensions to existing theories. There have as yet been no Keplers to come forward and reinterpret everything within a simpler and predictive framework. In the absence of a viable alternative theory, adding components to the existing ones (which have already been tested) is not necessarily an unwise thing to do. If I discover a new star that isn't described by existing theory, should my first instinct be to rewrite stellar astrophysics? That's really not good critical thinking, IMO.

It's one thing to remain skeptical, it's another to have an axe to grind. Despite the "epicycles", the standard model has, so far, been consistent. Unlike Ptolemy, we've only had to add two or three. If that turns out to be sufficient (or, even better, we detect dark matter and/or dark energy), I will not see a need to revise our physics. Till then, if you have new models, I suggest you make predictions (in a separate thread, please) and wait for them to be tested.

turbo-1

Jun10-05, 07:51 AM

The whole point of dark matter is that it's weakly-interacting and therefore does not emit much light. The collapse of an overdensity consisting only of dark matter would not need to emit a lot of observable photons.If dark matter is so weakly interacting as to be undetectable to us, how can it be persuaded to distribute itself "just so" to flatten the rotation curves of galaxies, provide gravitational binding forces for clusters, etc?

Garth

Jun10-05, 07:59 AM

SpaceTiger my question here is not the acceptance of the standard model, but the confidence placed in that acceptance. While the Higgs boson, the DM particle and the nature of DE are all undiscovered, the veracity of the concepts of inflation, DM and DE must be open to question.

In this thread are we not even allowed to question that "mainstream model"? You seemed to take exception to my doing just that in my posts above.
To be specific:
Perhaps these events were not as large as the SMBH scenario requires, and there were many more of them. IMBHs ([102 - 104]Msolar) could explain the DM today. How many times do I have to emphasize that this thread is not the place for your pet theories? Stop trying to plug your model in my thread. Actually I was reflecting on your post #30 in the 'Dark Matter!' thread.For discussion of observational constraints on black holes as dark matter, see here . Basically, the only workable regime is ~100 - 104 solar masses.
Which I found to be an extremely interesting piece of information that might explain the problem of IGM metallicity and re-ionisation.

Garth

SpaceTiger

Jun10-05, 03:51 PM

If dark matter is so weakly interacting as to be undetectable to us, how can it be persuaded to distribute itself "just so" to flatten the rotation curves of galaxies, provide gravitational binding forces for clusters, etc?

They're not weakly interacting gravitationally. In that respect, they act the same as any other form of mass/energy. A spherical "halo" is a natural configuration for a collection of bodies interacting only by the gravitational force.

SpaceTiger

Jun10-05, 03:55 PM

SpaceTiger my question here is not the acceptance of the standard model, but the confidence placed in that acceptance. While the Higgs boson, the DM particle and the nature of DE are all undiscovered, the veracity of the concepts of inflation, DM and DE must be open to question.

In this thread are we not even allowed to question that "mainstream model"? You seemed to take exception to my doing just that in my posts above.

I only take exception to your use of my thread to push obscure and untested ideas (like IMBHs as dark matter).

Actually I was reflecting on your post #30 in the 'Dark Matter!' thread.
Which I found to be an extremely interesting piece of information that might explain the problem of IGM metallicity and re-ionisation.

Not within the standard model. This idea only makes any sense in the context of your cosmology and that's why I take exception to you bringing it up. If you want to discuss it, do so somewhere else.

Garth

Jun10-05, 04:10 PM

The whole point of dark matter is that it's weakly-interacting and therefore does not emit much light. The collapse of an overdensity consisting only of dark matter would not need to emit a lot of observable photons. Likewise, there's no reason that a dark energy field should have to produce photons upon collapsing to a black hole.
However, as you have said They're not weakly interacting gravitationally So as the DM/DE collapsed it would attract also whatever baryonic matter was around. This matter would also be collapsed into a very small volume under extremely high temperatures and pressures and presumably form some kind of supernova. It would very likely be a bright event, would it not?

Garth

Garth

Jun10-05, 04:12 PM

Actually I was reflecting on your post #30 in the 'Dark Matter!' thread.
Which I found to be an extremely interesting piece of information that might explain the problem of IGM metallicity and re-ionisation.
Not within the standard model. This idea only makes any sense in the context of your cosmology and that's why I take exception to you bringing it up. If you want to discuss it, do so somewhere else.
Alright, how does the standard model explain early metallicity and re-ionisation?

Garth

SpaceTiger

Jun10-05, 04:47 PM

Alright, how does the standard model explain early metallicity and re-ionisation?

Simple, population III stars. WMAP measures reionization to occur at z~20, indicating that there were a considerable number of stars around before the high-z quasars were observed (z~6). A stellar population can build up supersolar metallicities in 300 Myr with sufficient quantities of star formation. See here:

http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002AJ....123.2151P&db_key=AST&h igh=42a9f128e013453

Do I think we understand these things? Heck no, but I certainly don't think there is strong evidence for a theoretical contradiction, particularly on the cosmological front.

SpaceTiger

Jun10-05, 04:54 PM

However, as you have said So as the DM/DE collapsed it would attract also whatever baryonic matter was around. This matter would also be collapsed into a very small volume under extremely high temperatures and pressures and presumably form some kind of supernova. It would very likely be a bright event, would it not?

Did you actually read the paper (or my post, for that matter)? They were talking about the direct collapse of dark matter and dark energy only. If baryons are included, then the process is the usual star formation process.

Garth

Jun10-05, 04:58 PM

Yes indeed I did read that paper, but how would stop ordinary matter also being dragged in?

Garth

SpaceTiger

Jun10-05, 05:13 PM

Yes indeed I did read that paper, but how would stop ordinary matter also being dragged in?

That's what makes the paper so implausible. It's hard to imagine a situation in which dark matter and energy will exist in isolation of baryonic matter. I think it was only being presented as a theoretical exercise, determining what would happen if there were only dark matter and/or dark energy. I think it's fair to say that the only plausible methods of BH formation at this point are stellar collapse or relics from the early universe.

Garth

Jun10-05, 05:47 PM

So a more realistic exercise would conclude that such DM/E BH formation would be bright?

Garth

SpaceTiger

Jun10-05, 05:50 PM

So a more realistic exercise would conclude that such DM/E BH formation would be bright?

No, a more realistic exercise would be star formation, as I said. The self-interaction of associated baryons prevents large concentrations of DM or DE from collapsing directly into black holes. Instead, black holes must form by the process of star formation.

Garth

Jun10-05, 05:52 PM

From ST's link in post #51The results are indistinguishable from those of lower redshift quasars and indicate little or no evolution in the metal abundances from z~6 to 2. The line ratios suggest supersolar metallicities, implying that the first stars around the quasars must have formed at least a few hundreds of megayears prior to the observation, i.e., at redshifts higher than 8.
Interesting! - Over to Chronos?.

Garth

Garth

Jun10-05, 06:06 PM

ST what mass range of PopIII stars are we talking about and what would they bequeath to the present epoch?

Garth

turbo-1

Jun10-05, 06:49 PM

They're not weakly interacting gravitationally. In that respect, they act the same as any other form of mass/energy. A spherical "halo" is a natural configuration for a collection of bodies interacting only by the gravitational force.Are you saying that the dark matter halos are naturally spherical, and that the spherical distribution can account for the galactic rotation curves of all galaxies? My understanding is that the DM spheres must have hollow cores with specific density gradients to explain flat galactic rotation curves.

Here's a quick example, using lensing to estimate galactic mass distributions.

http://www.control.com.au/bi2003/articles241/feat3_241.shtml

The cold dark matter theory predicts that dark matter should clump together in the centre of galaxies and dominate the galactic centre. In the galaxy we studied, however, the dark matter plays an insignificant role in its centre, accounting for less than 4% of the mass within the gravitationally lensed images. Instead the mass here is dominated by the stars in the bulge of the galaxy.

The dark matter does play a very large role in the overall galaxy, contributing about 60% of the total mass within the radius of the visible light of the galaxy, but its contribution is primarily to the outer regions. Other work has suggested a similar distribution of dark matter in other galaxies, but this is the first galaxy to be used that definitively discredits the current theory.From the same authors:

http://e-collection.ethbib.ethz.ch/ecol-pool/poster/poster_18.pdf

Garth

Jun11-05, 07:28 PM

Chronos

Jun12-05, 03:00 AM

Let's revisit the metallicity issue. What is the population distribution of low metallicity stars with respect to the core of any given galaxy?

Garth

Jun12-05, 12:16 PM

First define metallicity – i.e. the fraction of elements heavier than hydrogen and helium in astrophysical parlance. One measure is the ratio of iron to hydrogen, the [Fe/H] ratio, this is defined relative to the solar abundance of iron as:
[Fe/H] = log10(NFe/NH)star/medium - log10(NFe/NH)Solar and has an extreme range in stars of -4.5 < Fe/H < +1.

In the younger thin disc of our galaxy this range is -0.5 < Fe/H < +0.3
and in the older thick disc of our galaxy it is -0.6 < Fe/H < -0.4.

The older stars are considerable less "metallic" than our Sun.

This would lead us to believe that there was evolution (i.e. metallicity increasing with age) taking place, and indeed it would be crazy to think otherwise as stars are producing 'metals' through nucleosynthesis all the time and discharging them into the ISM through S/N explosions or enhanced stellar winds.

However, notice the great variation in stars of the same epoch, nearly an order of magnitude. It is not surprising then that there is a similar or greater variation in metallicity in the IGM, measured for example in the Lyman alpha forests of different quasars, as this has many different sources.

In Table 1 of ”THE AGE-METALLICITY RELATION OF THE UNIVERSE IN NEUTRAL GAS: THE FIRST 100 DAMPED Lyα SYSTEMS“ (http://arxiv.org/abs/astro-ph/0305314) we see variation in [Fe/H] from 0.0 at z = 0.526 to –3.13 at z = 3.684, yet close to the -3.13 system, at z = 3.727 there is a value [Fe/H] = 0 again! However, the general trend is for [Fe/H] to decrease with z, and the authors conclude they have found evolution in metallicity.

That notwithstanding, the paper “VLT Optical and Near-Infrared Observations of the z = 6.28 Quasar SDSS J1030+0524”, ST’s link in his post #51, concluded that even out to that high z the metallicity was indistinguishable from lower red shift quasars. So the onset of this metallicity was very early in the epoch prior to z = 6.

Question: When did it start?

Garth

Chronos

Jun12-05, 10:09 PM

WMAP results suggest reionization occured around z=20. I suspect metallicity trends would become very evident in that realm.

turbo-1

Jun12-05, 10:20 PM

WMAP results suggest reionization occured around z=20. I suspect metallicity trends would become very evident in that realm.Do you have a telescope capable of observing objects at that redshift? Standard cosmology predicts wonderful events at particle physics energies and observational redshifts that we may never be able to record. Doesn't that give you at least a little discomfort? My ZPE model gives at least 5 or 6 predictions that can make the model falsifiable. At present, the standard model has so many freely adjustable parameters that it cannot be falsified by any method that I can imagine.

matt.o

Jun12-05, 11:02 PM

Note that we are discussing mainstream cosmology, so this is not the place to present your favorite non-standard model for the universe. However, please do feel free to discuss observational evidence (or the lack thereof) for the standard theories.

hmmm. :grumpy:

Garth

Jun13-05, 04:11 AM

Back to the mainstream model.WMAP results suggest reionization occured around z=20. I suspect metallicity trends would become very evident in that realm.
Thank you - lets put some numbers into the time-line of that model.
The 'look-back' time tl as a function of red shift z is given by:
tl/tH = (2/3)(1 - 1/(1 + z)3/2)

Note: This is for the mater dominated era of the Friedmann model,
R(t) ~ t2/3, the radiation dominated era shortens the initial time period, while the epoch of recent acceleration lengthens 'look-back' time, but apparently does not apply to these earliest epochs.

With tH = 10.2/h Gyrs.
WMAP determines h = 0.72 so tH = 14.2 Gys.
and the age of the universe = 2/3tH = 9.44 Gyrs.

Let tz= be the age of the universe, after the BB, at red shift z.

So for "re-combination" - the surface of last scattering of the CMB,
z = 1000,
tz=1000 = 300,000 yrs.

for the onset of metallicity, i.e. Pop III stars, z = 20
tz=20 = 100 Myrs.

for quasar 'ignition' z = 8
tz=8 = 350 Myrs.

for 'modern' metallicity in Quasar SDSS J1030+0524 z = 6.28
tz=6.28 = 480 Myrs.

Food for thought…
[For a comparison with the Freely Coasting model see my new thread "Comparison of the Mainstream and the Self Creation Freely Coasting models"]
Garth

turbo-1

Jun13-05, 07:56 AM

hmmm. :grumpy:You're right Matt, and Space Tiger, please accept my apologies for contaminating your thread. I must say that it's quite frustrating to see critical falsification tests for the standard model being pushed into inaccessible regions (extreme redshifts, extreme accelerator energies, etc). Scientific models must make testable predictions in order to be confirmed. Just a few of the things that the standard model predicts are the existence of gravitons, Higgs bosons, and magnetic monopoles. At what level of non-detection can the standard model be considered falsified? Is there any such level, or are we dealing with a matter of faith?

Locrian

Jun13-05, 10:23 AM

Just a few of the things that the standard model predicts are the existence of gravitons, Higgs bosons, and magnetic monopoles.

What? That is almost as strange as

May I remind you that many of the critical tests of GR have failed to support GR? So far, no graviton, no Higgs boson (the expected energy level keeps getting pushed up, leading to more powerful accelerators), no dark matter detection, no dark energy...

but not quite. Are you sure you understand what these models are, how they work and what they predict?

turbo-1

Jun13-05, 02:06 PM

What? That is almost as strange as...but not quite. Are you sure you understand what these models are, how they work and what they predict?Lets see...the standard model of particle physics predicts the existence of the Higgs Boson, the hypothetical mediating particle of the all-pervasive Higgs field. In this model, all matter derives its mass from interaction with the Higgs field. This is analagous to Sakharov's suggestion that all objects derive their mass and inertia from interaction with the quantum vacuum fields, although he did not propose a mediating particle, to my knowledge. Accelerators have probed energies up to about 115 Gev and have not yet found the Higgs Boson.

Most (not LQG) quantum gravitational theories require gravitons - plentiful, attractive (not repulsive) and acting over long distances. Theoretically, they may be detected by their interaction with gamma rays in results from GLAST, although the results are also eagerly awaited by Fotini Markopoulou Kalamara, a LQG researcher who expects the results to define fine structure of space-time, not detect the effects of gravitons.

At the time of symmetry breaking, the standard model predicts that magnetic monopoles were very plentiful, and in a non-inflationary BB model, they should still be plentiful, yet none are detected. This is one motivation for inflation, since the inflation would allow the universe to have been much smaller at the time of their production, and monopoles could be much less plentiful today. Still, a zero detection rate is puzzling.

ohwilleke

Jun13-05, 02:42 PM

Garth

Jun13-05, 04:12 PM

Mainstream cosmology, so far as I know, also pretty much universally includes inflation as a core element. I'm not aware of mainstream cosmologists who think that magnetic monopoles are necessary for inflationary BB theory to work, but I say so modestly and am willing to be proven wrong.
You are correct, inflation is necessary, or at least useful, to explain the non-detection of magnetic monopoles, but monopoles are not necessary for inflation.

Garth

turbo-1

Jun13-05, 06:24 PM

I agree that the Standard Model predicts a Higgs.

I would call both gravitons and magnetic monopoles outside of the Standard Model. The Standard Model pretty expressly does not say anything about gravity (hence the quest for new quantum based theories which do). The Standard Model of particle physics also does not include any particles that have magnetic monopoles and does not address cosmology either.It is the intersection of GR, cosmology, particle physics and quantum physics where things get messy. GR does not supply a mechanism for gravity, only a mathematical model of its effects, so the mechanism has to be addressed by ancillary fields of physics, including particle physics and quantum physics. Some (but not all) schools of quantum gravity require a graviton, while it has been more widely accepted that the Higgs field should produce really massive mediating particles that endow mass upon matter through some form of interaction.

Mainstream cosmology is firmly rooted in classical GR rather than quantum gravity. Indeed, one speculation and hope of many quantum gravity theorists is that quantum gravity might provide alternate answers to cosmological questions as a result of distinctions between the two -- particularly in relation to black holes, the BB singularity and inflationary behavior. Those quantum effects that are considered by mainstream cosmologists are, to the best of my knowledge, non-gravitational ones.You are right about mainstream cosmology being rooted in classical GR. This is the reason that we are looking for dark matter - the gravity model in GR predicts gravitational effects far smaller (and more oddly distributed) than we observe (compared to the luminous matter that we observe). The ball is out of the cosmology court and back in the court of the ancillary fields of physics, as possible DM candidates are sought out. It is the intersections of particle physics, quantum physics, and GR cosmology where the important conflicts occur and the potential for breakthroughs exist.

Mainstream cosmology, so far as I know, also pretty much universally includes inflation as a core element. I'm not aware of mainstream cosmologists who think that magnetic monopoles are necessary for inflationary BB theory to work, but I say so modestly and am willing to be proven wrong.Garth got this right. Magnetic monopoles are not responsible for inflation. Inflation is proposed as a reason why they have never been detected. In this scenario, the universe was so tiny at the time of symmetry-breaking that the concentrations of monopoles NOW can theoretically be very thin, while at the time of symmetry-breaking they are expected to have been very numerous. This does not make the current non-dection of magnetic monopoles understandable, but perhaps a bit more palatable.

Locrian

Jun14-05, 10:01 AM

Most (not LQG) quantum gravitational theories require gravitons

The standard model is NOT a quantum gravitational model. Magnetic monopoles are not a prediction of the standard model, but a prediction of other models that extend on it. I just don't understand how you could be so brazen in your failure to know anything about these models you spend so much time talking about. Dark energy and dark matter critical tests of GR? Standard model predicting gravitons? This is just comedy.

turbo-1

Jun14-05, 09:04 PM

The standard model is NOT a quantum gravitational model. Magnetic monopoles are not a prediction of the standard model, but a prediction of other models that extend on it. I just don't understand how you could be so brazen in your failure to know anything about these models you spend so much time talking about. Dark energy and dark matter critical tests of GR? Standard model predicting gravitons? This is just comedy.Lets keep it simple. Comedy? If the gravitational model of GR is correct, then DM is absolutely necessary to explain how the rotation curves of galaxies and the binding energies and lensing abilities of clusters can be explained. The non-detection of DM is thus a falsification of Einstein's model of GR gravitation. If GR cannot be falsified by any such observations, then it perhaps it has has passed through science to faith.

Garth

Jun15-05, 02:07 AM

The non-detection of DM is thus a falsification of Einstein's model of GR gravitation.Absence of evidence is not evidence of absence.

Garth

Chronos

Jun16-05, 12:01 AM

turbo-1

Jun16-05, 07:57 AM

You reach a point where first cause principles come into play. There is no mechanism to explain any of the four fundamental forces of nature. It is no more appropriate to question the mechanism responsible for gravity than it is to question why the speed of light is what it is. Some things just are what they are, and that will always be the case, in my mind - no matter how deeply we probe the mysteries of the universe.That is a defeatist attitude. It is appropriate to question the mechanism for everything, no matter how basic it might be. If everybody was satisfied with the reality and fundamental nature of GR's mathematical model of gravitation (with no underlying mechanism), there would be no String theorists, no LQG theorists, etc. These are pretty popular fields... The question "why" is always appropriate except perhaps in matters of religious faith.

As for "first cause principles": Calculating galactic and cluster masses using GR gravity results in shortfalls of observed matter. This is a hint that gravitation may be not be a fundamental force, but instead may be emergent, and susceptible to more complex behavior than envisioned in GR. Before you ask, I mean emergent in much the same way that inertia arises from acceleration with respect to a universal rest frame in Mach's model.

Absence of evidence is not evidence of absence.You are correct. It is impossible to disprove the existence of DM and thus provide falsification for GR. The more tightly the nature of DM is constrained, the more exotic it becomes, without limit, and it will never go away unless a more predictive paradigm for gravitation becomes established.

Chronos

Jun17-05, 02:06 AM

Why does gravity get singled out? Why not question the mechanism responsible for the strong nuclear force?

turbo-1

Jun17-05, 08:01 AM

Why does gravity get singled out? Why not question the mechanism responsible for the strong nuclear force?You may feel free to examine the basis of any of the fundamental forces you wish. I concentrate on gravitation because:
1) it is extremely weak, yet seems to act over long distances, suggesting that it is mediated by a weak but easily polarizable field.
2) the GR model of "mass curves space-time" seems to be impossible to express on quantum scales, suggesting that we need a quantum mechanical process by which gravitational attraction can be explained
3) current theories posit the existence of a Higgs field and require the existence of Higgs bosons, which interact with all physical matter to endow mass upon it - these bosons have not been detected
4) many of the current theories (not all) also require the existence of gravitons to mediate the gravitational force between massive objects - ditto on the non-detection
5) if the fields from which the particles in 3) and 4) arise are not perfectly congruent to the nth degree everywhere, gravitation will not behave according to the same rules everywhere. We do not observe these inconsistencies, suggesting that mass and gravitational attraction arise from the SAME field.
6) measuring the masses of galaxies and clusters using GR gravitation routinely results in a shortfall of matter relative to the observed gravitational effects.
7) simplicity and elegance. The rules of the universe are likely to be simple, and not require the cooperation of multiple entities to produce the most fundamental force that organizes it on large scales.

There are more reasons, and you have heard them all before. Just remember, there is more mental horsepower being applied to developing a quantum mechanical model of gravitation than perhaps any other knotty problem in physics. I am not alone in "singling out" gravitation for study.

Chronos

Jun18-05, 02:48 AM

In the spirit of this thread, I would like to add a couple links here from my list of favorites addressing current issues in mainstream cosmology:

http://arxiv.org/abs/gr-qc/0503107
Understanding Our Universe: Current Status and Open Issues

The current status of observational cosmology
http://www.arxiv.org/abs/astro-ph/0409131

Nereid

Jun18-05, 09:44 AM

Can we get back to SpaceTiger's review of mainstream cosmology now, please?

His last post, other than to comment on some questions, was #39.

While many questions and comments have been helpful, there have also been many which are similar to 'attacking QCD for failing to predict details of the (economic) theory of comparative advantage'.

Similarly, there are certainly 'holes' and 'weaknesses' in mainstream cosmology; but let's discuss them in terms of the (physics) foundations and good observational results.

SpaceTiger

Jan27-06, 01:30 PM

7) Dark Energy

I've been seeing a lot of dark energy questions of late and thought it might prudent to add at least one more section to my review, since this is one of the most important and controversial subjects in modern cosmology.

Dark energy refers to the energy component that is driving the current acceleration of the universe. The key relation that describes it is the equation of state; that is, the relationship between pressure and density. The simplest form of such an equation would be:

p=w\rho

where p is the pressure and \rho is the energy density. If you include a dark energy component in a general relativistic cosmological model, you'll find that, in order to cause acceleration, the "dark energy" must have w < -\frac{1}{3}.

But what is this stuff? Pretty much everything we know of and can do experiments on has a positive pressure. Well, one possibility was pondered by Einstein (though for different reasons) back in 1917. He considered that perhaps the vacuum naturally had an energy associated with it and that, as the universe expanded, more energy would be created as space expanded. Another way of saying this is that he proposed a cosmological constant -- a constant energy density associated with space itself. With this addition, his famous equation took the form:

R^{\mu \nu}-\frac{1}{2}Rg^{\mu \nu}-\Lambda g^{\mu \nu}=8\pi T^{\mu \nu}

where \Lambda is the cosmological constant. Because of the nature of the metric (g^{\mu \nu}), it turns out that this cosmological constant corresponds to a dark energy equation of state, w=-1.

Let's now fast forward to the end of the 20th century. In 1998, a group of astronomers observing supernovae announced that their data were inconsistent with a decelerating universe. In fact, the universe seemed to be accelerating and, in order to explain it, they needed 70% of the energy density of the universe to be made up of dark energy. This was greeted with a great deal of skepticism, partially because the methods were questionable and partially because it was physically difficult to explain. It wasn't until 2003, when the WMAP satellite announced its results from an analysis of the cosmic microwave background (CMB), that dark energy became a fixture in our cosmological models. Quite simply, the satellite made an independent measurement of the dark energy density and came to the same conclusion that the supernovae people did -- 70% of the universe is composed of a dark energy component.

That leads to what we call the concordance model, or \Lambda CDM. This is a general relativistic model of the universe that includes ~30% matter (~90% of which is cold dark matter) and ~70% dark energy in the form of a cosmological constant. Does the dark energy have to be in the form of a cosmological constant? No, it can be in the form of a scalar field (much like the one that led to inflation), but it must have an equation of state near that of the cosmological constant because observations constrain w ~ -1 to about 30% (depending on which observations you believe). Also, it's possible that general relativity fails at large scales and our observations are simply parameterizing the breakdown of Einstein's theory.

Dark energy is one of the most puzzling aspects of modern cosmology and I think it would be naive of us to claim that we really understand what's going on here. Astronomers are working overtime to understand and quantify its effects, but we would still like a physical understanding of the mechanism that gives the vacuum energy. Is it the zero-point energy of QFTs? Is it a scalar field? Is it some exotic kind of particle? I'm happy to say that we still don't know and there is still much to be learned from our universe. :wink:

Garth

Jan27-06, 03:28 PM

Thank you SpaceTiger for that clear and concise post.

May I add a couple of comments?

The need for the WMAP data to have DE arises because its constraints on all matter density are no more than 30% closure density (best estimate 27%) and the peaks in the power spectrum of the WMAP anisotropies are consistent with flat space that requires 100% closure density, therefore something is required to fill the gap and DE fits nicely.

Secondly all that is actually observed in the distant Type Ia Supernovae is that at around z ~ 1 they are fainter than expected and beyond they become brighter than expected again. This is interpreted as first a period of deceleration in the scale factor R(t), then acceleration (z ~ 1) and probably now deceleration again, caused by the action of DE under a specific equation of state, which is still being determined.

However the geometry of the universe, as well as the scale factor R(t), affects the expected brightness of these distant standard candles.

These conclusions are geometry dependent. If the geometry of space is not flat then it is back to the drawing board as far as DE is concerned.

Why might the geometry of space be different? As I have pointed out before, as the WMAP data is angular in nature and conformal transformations are angle preserving the WMAP data is also consistent with conformally flat space.

Is there any indication that this might be the case?

The peaks of the WMAP power spectrum are all in the correct place for flat, or conformally flat space, but the large scale low-l mode fluctuations appear to be genuinely deficient, which is not consistent with infinite flat space. However, these two observations, first peak at ~ 10 plus a deficient quadrupole, could be explained by a finite conformally flat universe.

This conclusion is not easy to reconcile with the standard theory but it might be just what the data is telling us.

Garth

Chronos

Jan28-06, 04:00 AM

I lean towards deformed special relativity as the most promising current approach. WMAP year 2 will unveil the prophet, IMO. I'm impatient to see the results [like everyone else] but I'm sure it will be worth the wait. The fact they have spent so much time trying to get it right is very exciting to me.

Garth

Jan28-06, 04:18 AM

Chronos, what do you mean by "deformed special relativity"? Is this a new theory you are proposing or just a description of ordinary GR (i.e. SR with curvature)?

There is another conumdrum with DE. If we look at the dimensions of the Einstein field equation:

R^{\mu \nu}-\frac{1}{2}Rg^{\mu \nu}-\Lambda g^{\mu \nu}=8\pi T^{\mu \nu},
where first we note that in this form geometric units are being used, in which G and c are unity, then we can recognise a coincidence with DE.

The components of the terms of the tensor on the right hand side are density and pressure, which is energy density, therefore the \Lambda on the left hand side has the dimensions of density - in this case an energy density.

If DE is the cosmological constant, i.e. \omega = -1, then it is a constant energy density. As the universe expands the vacuum retains the same DE density and so the total DE grows with the universe; it is not conserved as the matter density is. The total proportion of the universe's mass that is DE will constantly grow.

However the present constituents of the \LambdaCDM model are 4% baryonic matter, 23% exotic non-baryonic DM and 73% DE.

These amounts are roughly equal to an OOM, is this not rather a coincidence? But why?

If \omega varies in some even more exotic theory of DE, then it is observed to be approximately unity now, but this is yet another coincidence; why should \omega ~ -1 in this present epoch?

The concepts of DM & DE grew out of a GR model that suffered Inflation in the earliest stages. Inflation is a theory unverified in the laboratory that was invoked to explain the horizon, density, smoothness and magnetic monopole problems (coincidences) of the 'raw' GR model.

Yet it seems to be able explain one set of coincidences only by replacing them with another.

Garth

SpaceTiger

Jan28-06, 06:25 PM

Thank you, Garth. The low quadrupole and "cosmic coincidence" problems are indeed recognized issues with the standard model (or perhaps, in the former case, the measurements). If I have time, I may do a more general review of such problems and give some of the more conventional explanations.

Chronos

Jan29-06, 03:09 AM

I'm more the spectator type. If I propose a new theory, take my car keys and give me a ride home [Guiness makes my clothes fall off]. Deformed special relativity emerged from the shadows about 10 years ago. It did not generate a great deal of interest until 2003. Girelli, Levine and Oriti have been the most notable current advocates. Here is a good place to start, and one you may find interesting:

Modified Relativity from the kappa-deformed Poincare Algebra
http://arxiv.org/abs/gr-qc/9602016

jhe1984

Jan30-06, 12:09 AM

Several points of confusion that can hopefully be (elementarily and concisely) cleared up:

If the universe is expanding, doesn't that mean that it has to be getting less dense?

Do we know if dark energy is a force (pushing, sucking, whatever) or do we only know that it is an observation as yet without explanation?

If the typical model of particle physics finds both particles and anti-particles (in an environment where particles outnumber anti-particles to a small extent), why can't gravity (or a graviton, per se) have an anti-graviton that happens to outnumber it in a much higher ratio?

Is there any explanation for what is "space" other than "other" or "void" - I'm thinking now about "space" as in the phrase "space is expanding"?

---tries to avoid getting trampled---

Chronos

Jan30-06, 01:03 AM

In modern theory, not every particle has an anti-particle equivalent. It's a spin thing.

oldman

Dec8-06, 05:18 AM

A recent review in American Scientist by Joe Polchinsky (of Woit and Smolin's books debunking string theory) includes the following remark:

...... it may be that string theory has already made a connection with observation, one of immense significance. Positive dark energy is the greatest experimental discovery of the past 30 years regarding the basic laws of physics. Its existence came as a surprise to almost everyone in physics and astronomy, except for a small number, including, in particular, Steven Weinberg.

This "greatest discovery of the last 30 years" was of course a cosmological one, and was confirmed by the WMAP results, as was pointed out in this forum more than a year ago:

7) ......when the WMAP satellite announced its results from an analysis of the cosmic microwave background (CMB), that dark energy became a fixture in our cosmological models. Quite simply, the satellite made an independent measurement of the dark energy density and came to the same conclusion that the supernovae people did

Could someone please explain to me how exactly this independent measurement was inferred from the WMAP data?

George Jones

Dec8-06, 07:39 AM

Could someone please explain to me how exactly this independent measurement was inferred from the WMAP data?

Curvature causes distortions in the small temperature anisotropies present in the backgorund radiation. The sizes of the anisotropies seen indicate that the universe is (very close to being) spatially flat (I don't know the details of the calculation), which requires that the matter/energy density of the universe by very close to the critical value. Even when dark matter is taken into account, we can only get a density of about 25-30% the critical value.

The supernova data indicates that dark energy/cosmological constant is responsible for a density of about 70-75% of the critical density.

These two results strongly reinforce each other.

oldman

Dec9-06, 02:50 AM

Curvature causes distortions in the small temperature anisotropies present in the backgorund radiation. The sizes of the anisotropies seen indicate that the universe is (very close to being) spatially flat (I don't know the details of the calculation), which requires that the matter/energy density of the universe by very close to the critical value. Even when dark matter is taken into account, we can only get a density of about 25-30% the critical value.

The supernova data indicates that dark energy/cosmological constant is responsible for a density of about 70-75% of the critical density.

These two results strongly reinforce each other.

George, thanks for your prompt reply. Your post clarifies my understanding of this aspect of the WMAP results, which now runs along the following lines (please correct me --- I've probably got it wrong):

1. The first peak in the angular power spectrum of the CMB temperature fluctuations is caused by changes in the metric that occur while the CMB radiation is crossing overdense regions of the (almost homogeneous and isotropic) universe, en route to us.

2. The position of this peak depends on the spatial geometry of the universe. Its measured position shows that this geometry is very nearly Euclidean. In the context of a FRW model, Euclidean geometry fixes the total density of mass/energy in the universe (given the measured value of the Hubble constant) at the so-called critical density.

3. The total mass and energy density of the universe measured from luminosity ratios (visible matter) and gravitational effects (galaxy rotation curves, virialised cluster-galaxy speeds) is only about 25% of this critical density.

4. The resultant density deficit (of about 75% of the critical value) agrees with the deficit required to account for small deviations from linearity of the upper end of the Hubble plot, deduced from the use of type 1a supernovae as standard candles. The deficit is assumed to be made up of "dark energy", which, as noted in post #82 of this forum, is:

......one of the most puzzling aspects of modern cosmology and I think it would be naive of us to claim that we really understand what's going on here. Astronomers are working overtime to understand and quantify its effects, but we would still like a physical understanding of the mechanism that gives the vacuum energy. Is it the zero-point energy of QFTs? Is it a scalar field? Is it some exotic kind of particle? I'm happy to say that we still don't know and there is still much to be learned from our universe.

SpaceTiger

Dec9-06, 04:57 AM

1. The first peak in the angular power spectrum of the CMB temperature fluctuations is caused by changes in the metric that occur while the CMB radiation is crossing overdense regions of the (almost homogeneous and isotropic) universe, en route to us.

All of the peaks in the angular power spectrum are fluctuations in the gas that emitted the radiation and don't come into being en route. These are called primary anisotropies and are created by the sound waves at the surface of last scattering. Some of the weaker anisotropies are created during the voyage of the radiation between z=1100 and z=0 and are called secondary anisotropies.

The rest of your points are correct.

oldman

Dec9-06, 05:38 AM

All of the peaks in the angular power spectrum are fluctuations in the gas that emitted the radiation and don't come into being en route. These are called primary anisotropies and are created by the sound waves at the surface of last scattering. Some of the weaker anisotropies are created during the voyage of the radiation between z=1100 and z=0 and are called secondary anisotropies.

The rest of your points are correct.

Thanks for clearing up my confusion with the Sachs-Wolfe effect, which I had thought was involved.

SpaceTiger

Dec9-06, 06:17 AM

Thanks for clearing up my confusion with the Sachs-Wolfe effect, which I had thought was involved.

The Sachs-Wolfe effect is involved, in fact, but note that it is the non-integrated Sachs-Wolfe effect. The acoustic peaks are imprinted at the surface of last scattering, at which time the emitted photons must climb out of the potential wells from which they were emitted. In contrast, the Integrated Sachs-Wolfe (ISW) effect occurs as the photons pass through potential wells en route to our telescopes. The non-integrated variant is a primary anisotropy and the integrated variant is secondary.

oldman

Dec10-06, 02:26 AM

The Sachs-Wolfe effect is involved, in fact, but note that it is the non-integrated Sachs-Wolfe effect .................. The non-integrated variant is a primary anisotropy and the integrated variant is secondary.

I've been struggling somewhat to understand the WMAP results. I suspect that there is a screen of computer modelling with sophisticated codes between basic physics that I can grasp and the remarkable conclusions of the mission.

Thanks again for this further clarification. It's most helpful. I have one further confusion, though, of a general nature, which I hope you can remove for me:

It has to do with one of the purposes served by the inflationary "scenario", namely to explain why the sky and in particular the CMB is so uniform on large angular scales --- i.e to solve the horizon problem by suggesting that the observable universe is an inflated fragment of a thermalised portion of the primeval universe.

If this is so, why are the most prominent temperature fluctuations seen by WMAP (namely those of the first peak in in the power spectrum) deemed only to be of linear dimensions of order 1/(H at last scattering)?

Should inflation not have amplified many primeval fluctuations to dimensions greater than 1/(H at last scattering), just as it is supposed to have amplified "thermalised uniformity" to cover the whole sky?

So my difficulty boils down to: why are large-scale temperature fluctuations not more prominent in the WMAP results?

oldman

Dec10-06, 07:14 AM

Garth: in asking some questions later in this thread (my posts 90 and 96) I had missed these comments of yours. The first answers the query I raised in #90 (now resolved by Space Tiger). The second seems to involve a similar difficulty (my emphasis) that I describe in #96.

My apologies for not referring to your comments.

The need for the WMAP data to have DE arises because its constraints on all matter density are no more than 30% closure density (best estimate 27%) and the peaks in the power spectrum of the WMAP anisotropies are consistent with flat space that requires 100% closure density, therefore something is required to fill the gap and DE fits nicely......

The peaks of the WMAP power spectrum are all in the correct place for flat, or conformally flat space, but the large scale low-l mode fluctuations appear to be genuinely deficient, which is not consistent with infinite flat space. However, these two observations, first peak at ~ 10 plus a deficient quadrupole, could be explained by a finite conformally flat universe.

This conclusion is not easy to reconcile with the standard theory but it might be just what the data is telling us.

Garth

Chronos

Dec11-06, 01:39 AM

oldman

Dec11-06, 03:10 AM

Oldman, 'h' does not change in the inflationary scenario so far as I know. It remains a universal constant tied to 'c'. Of course that implies 'c' is more fundamental than 'h'. Is that the essence of your question?

Chronos: thanks for your reply. It's not quite the question I was asking --- perhaps I foolishly confused the issue by talking about H. Let me rephrase my difficulty.

I'm concerned that inflation seems to conflict with the WMAP results in one important respect.

It is this: if one relies on inflation to spread "uniformity" over our sky (so as to resolve the horizon problem), one should expect inflation to also spread large-scale thermal irregularities. My understanding is that both thermal uniformity and tiny quantum fluctuations on all scales are postulated to be characteristic of the pre-inflation universe. It seems to me that when inflated, both should become features of the present observable universe.

But, as Garth pointed out, the WMAP results show that "the large scale low-l mode fluctuations appear to be genuinely deficient" .

I would like to know how one can "have one's cake" (solve the horizon problem) and at the same time "eat it" (accept the deficiency in low-l modes).

hellfire

Dec11-06, 08:19 AM

My understanding is that both thermal uniformity and tiny quantum fluctuations on all scales are postulated to be characteristic of the pre-inflation universe.
Quantum fluctuations arise during inflation and their spectrum contains all wavelenghts. Modes at short wavelengths are strongly redshifted by the inflationary expansion of space so that their wavelength becomes larger than the horizon. Beyond the horizon at long wavelengths, the modes freeze out to a nonzero values of the amplitude. Later on, after inflation, frozen modes with equal wavelength reenter the horizon at the same time perturbing the distribution of the energy density.

This perturbations lead to the oscillatory behaviour of the plasma before of recombination. The plasma stops oscillating after recombination. The latest modes that reenter the horizon at the time before of recombination have a wavelenght of the size of the horizon at that time. At the time of last scattering the mode which enters the horizon at that time will lead to maximum fluctuations (first peak), and some modes that have entered before will produce standing waves with nodes at the edges of the horizon (other peaks).

Regarding thermal uniformity, the theory of inflation can be formulated in a way that a homogeneous distribution of energy density is available in any case, even if the initial conditions are not homogeneous.

Mad_Morlock

Dec11-06, 09:05 PM

I'm not sure this will be well recieved, but I had an idea regarding the topology of the universe that might do well as a model....

[edited non-mainstream theory]

Wallace

Dec11-06, 10:56 PM

But Mad Morlock where is the evidence for your idea? What does it predict that improves upon the predictions of the current model? The universe, on current evidence is going to continue to expand at an accelerated rate henceforth (though extrapolation is always risky, particularly as we really have no idea about the physics of dark energy).

You are right about one thing though, your post is unlikely to be well received :tongue:

oldman

Dec12-06, 01:17 AM

Beyond the horizon at long wavelengths, the modes freeze out to a nonzero values of the amplitude. Later on, after inflation, frozen modes with equal wavelength reenter the horizon at the same time perturbing the distribution of the energy density........

Thanks for trying to educate me, Hellfire, but I'm not as smart as you give me credit for, and I lost you about here. I still fear that there's a conflict between the WMAP results and the resolution of the horizon problem...........

Chronos

Dec12-06, 01:53 AM

No conflict if you accept the LCDM model. It handily describes the observational results. It is a very good model that explains many things. Hellfire gave the basics for formulating a theory that preserves the assumptions without tossing out the baby.

hellfire

Dec12-06, 05:53 AM

Thanks for trying to educate me, Hellfire, but I'm not as smart as you give me credit for, and I lost you about here. I still fear that there's a conflict between the WMAP results and the resolution of the horizon problem...........
Do you agree that inflation solves the homogeneity problem (or horizon problem) regardles of the initial conditions? Inflation was formulated to solve this problem. Providing homogeneity, it was clear that the theory had to provide a mechanism to account for the small inhomogeneities in the CMB that are the seeds of the matter structures. It was some time later when it was realized that this mechanism could be quantum fluctuations during inflation.

I recommed to read the following article by Alan Guth:

Inflation and Cosmological Perturbations
http://arxiv.org/abs/astro-ph/0306275

It contains a brilliant narration of the historical steps for the formulation of this mechanism and it describes also some technical aspects.

oldman

Dec12-06, 07:01 AM

the LCDM model... handily describes the observational results. It is a very good model that explains many things.

I'm certainly not disputing this kind of general statement, Chronos, but the devil is always in the details! My original query was about one detail, namely:

my difficulty boils down to: why are large-scale temperature fluctuations not more prominent in the WMAP results?

and in a following post:

I would like to know how one can "have one's cake" (solve the horizon problem) and at the same time "eat it" (accept the deficiency in low-l modes)..

These quotes should also make it clear that I do agree with Hellfire

... that inflation solves the homogeneity problem (or horizon problem) (and that)..... the theory .... provide(s) a mechanism to account for the small inhomogeneities in the CMB that are the seeds of the matter structures......this mechanism could be quantum fluctuations during inflation.

But nether of you have yet mentioned the deficiency in low - l modes, which the WMAP results now seem to show are real. This conflict is what bothers me. Or am I tilting at windmills?

I hadn't seen the 2003 paper by Guth that Hellfire kindly referred me to. I've glanced at it now, and it seems to complement his 1997 book (which I have)excellently. Thanks for the reference, Hellfire.

SpaceTiger

Dec12-06, 07:11 AM

But nether of you have yet mentioned the deficiency in low - l modes, which the WMAP results now seem to show are real. This conflict is what bothers me. Or am I tilting at windmills?

There is a deficiency in the low-l modes, but it's only a relative deficiency...they still expected much less power on those scales than at the scale of the acoustic peak. The interpretation of this deficiency is still a matter of some debate. Some believe it is a sign of a non-trivial topological structure for the universe, some a signature of modified gravity, and some just a statistical fluke exaggerated by a posteriori statistics. Only time will tell.

Garth

Dec12-06, 07:20 AM

(Edit: Crossed with ST's post.)

Not many low-l mode anisotropies are to be expected in the first instance, as they are the 'large' ones. Even so there are fewer than predicted by the LCDM model, is this perhaps just a statistical fluke?

It could be, however if the 'Axis of Evil' is removed as being some kind of local contamination then the chance that the remaining increased deficiency at the low-l modes be a statistical fluke becomes vanishing small.

Perhaps the Axis of Evil is indeed real and part of the cosmological structure of the CBM? In which case its alignment with local geometry is an 'a posteri' recognised statistical chance event and not significant. And the low-l mode deficiency may be disregarded as statistically insignificant.

However, one explanation for the deficiency may be that although the universe's spatial geometry appears flat and therefore the universe is infinite or very large compared with our horizons, it is in fact smaller so there was not enough room for the deficiencies to grow in the early, pre-Surface of Last Scattering universe but also spatially conformally flat. This, however, would require a modification of the theory by which the CMB data is analysed, i.e. it would require a modification of GR.

Garth

oldman

Dec12-06, 09:52 AM

...... The interpretation of this deficiency is still a matter of some debate ........Only time will tell. That satisfies my curiosity. I hope that not too much time will pass before all becomes cut and dried. Thanks.

oldman

Dec12-06, 10:21 AM

..... one explanation for the deficiency may be that although the universe's spatial geometry appears flat and therefore the universe is infinite or very large compared with our horizons, it is in fact smaller so there was not enough room for the deficiencies to grow ....

I'm slightly puzzled by this suggestion, because the uniformity of the universe is postulated to have been grown by inflation to encompass the whole observed universe, thus solving the horizon problem. If there was room for this (call it l = 0?) mode to grow, why not room for low-l modes? But I don't trust my reasoning , and accept that others will probably resolve this anomaly in time.

If it were to "require a modification of the theory by which the CMB data is analysed, i.e. and .... a modification of GR", as you mention, the interpretation of much else might also change --- a welcome astonishment for a somewhat sceptical spectator like myself.

Thank you for your interest.

Garth[/QUOTE]

Chronos

Dec13-06, 01:34 AM

I feel the non-trivial topology hypothesis has legs. The persistent redshift anomalies certainly suggest this possibility. In a finite universe, this [IMO] is a very plausible explanation - there may be quantum fluctuations embedded in the inflationary phase that produce localized variations in H0. I would be fascinated by any study that examines this possibility. I have toyed with this idea using LEDA and VizieR data and cannot rule it out.

Garth

Dec13-06, 03:03 AM

Might the explanation of the low-l mode deficiency and apparent alignment be explained by an exotic topology? CMB Alignment in Multi-Connected Universes (http://arxiv.org/abs/astro-ph/0612308) Not yet apparently! We have investigated the question whether the strange alignment observed in low CMB multipoles can be explained by multi-connected space forms. There are several examples of such spaces which can explain the missing anisotropy power at large angular scales as measured by the temperature correlation function C(ϑ), ϑ > 60◦, or the angular power spectrum Cl for l = 2, 3. It is thus natural to ask for the alignment properties in such spaces.

................................

But in no case we have found a model, where a significant fraction of the simulations possesses the alignment observed in the CMB sky. It remains to be seen whether there are other space forms which can more easily explain the alignment than the space forms considered in this paper.

Garth

heusdens

Jan14-07, 05:05 PM

With all the crazy ideas that get thrown around in this forum, I thought it would be good to step back and review the mainstream view on cosmology in 2005. The field is advancing very rapidly, so it's possible that even the most reliable websites will be woefully out of date, both in terms of results and the evidence for them. Let's review, starting from the most secure and ending with the most puzzling/dubious aspects of the standard theories. I'll do this over the course of multiple posts, and feel free to interject and discuss at any point. Note that we are discussing mainstream cosmology, so this is not the place to present your favorite non-standard model for the universe. However, please do feel free to discuss observational evidence (or the lack thereof) for the standard theories.

1) Expansion

The universe is, without a doubt, expanding. The most striking evidence for this is the fact that nearly every object in the sky exhibits a redshift in the spectrum of light that is emitted from it. Furthermore, more distant objects are observed to have larger redshifts, exactly what you would expect for expansion. Alternative theories (such as Zwicky's "tired light hypothesis") were put forth and seriously considered in the first half of the 20th century, but have produced no correct predictions, nor are they consistent with any known physics. They have not been seriously considered by the mainstream for quite some time.

It should be noted that redshift is not the only reason we think the universe is expanding, but it was certainly the first evidence. Since the discovery of Hubble's Law in 1929, many more things have been deduced under the assumption of expansion (most notably, the Big Bang Theory) that also produce testable predictions. The success of these theories can be viewed retroactively as evidence for the expanding universe.

2) The Big Bang Theory

There is a lot of confusion amongst the general public about what the Big Bang Theory is really saying and which aspects of it are taken as gospel truth by the scientific community. In its simplest form, you can think of the argument as follows:

"If space is expanding and the universe has a finite size, then it must have been much smaller in the past".

How much smaller? Well, the standard assumption is that the universe had a creation event and expanded from a singularity to its present size. Such a distant extrapolation can't possibly be verified by the current observations, but we can safely say that the universe expanded from a much smaller size than its current one. There is good observational evidence for an epoch of nucleosynthesis approximately one minute after the creation event (z ~ 108). Physical models of the conditions in this early phase of the universe were able to predict the relative abundances of the light isotopes (including hydrogen, helium, and deuterium) to very high accuracy.

There is even stronger evidence for recombination, an event that occurred when the scales in the universe were a 1000 times smaller than today (~400,000 years after the big bang). Recombination is what gives rise to the cosmic microwave background (CMB) radiation, a nearly blackbody spectrum that can also be modelled very accurately. The models are so accurate, in fact, that they have also allowed us to precisely measure some of the parameters of our universe. More on this later.

In addition, there is increasing evidence for an epoch of inflation, thought to occur 10-35 seconds after the creation event, during which the universe may have expanded by as much as a factor of 1050! If we could observationally confirm such a hypothesis, it would be an overwhelming success for both the Big Bang Theory and the scientific method itself. I'll also discuss the evidence for this in more detail later. There are a lot of nice websites on the Big Bang Theory (see here (http://www.umich.edu/~gs265/bigbang.htm), for example), so web surf if you want to know more.

I agree on general on this outline of BB theory, except for the stuff marked in red. Although it can be theoretized that the theory of GR shows a singularity at the beginning, it is not mainstream scientific or part of the BB theory to claim that the singularity actually "happened", because it would mean that there was a moment in time at which time began (there was no time prior to it). But we don't know that at all, it is NOT a scientific fact!
What we can talk about and back up with observational evidence goes back to the origin of the CMBR, and theoretically we can go back to the Planck time. What happened before the Planck time we can not predict with current scientific theories (GR and QM), unless there is a theory of quantum gravity, and although some scientific theories have emerged here, to my knowledge there is not yet an accepted theory there.
What we can NOT talk about, scientifically, is talking about the actual singularity as a real event (happenening in space and time), since the very theory that comes up with it, is known to break down there, so it ain't scientific to say that there was a singularity at the begin, or to even suggest that there could have been a begin. (*)
So the singulairy is only a fictional point which boils up in models, but is not a scientific fact that this singularity actually exist. It would have contained infinite density for instance.
As a model for the early universe, the theory of inflation has been the most succesfull over the last couple of 20 years, and it has been shown that:

inflation can start at in fact chaotic conditions (which means, not requiring special conditions for the early universe)
once started, inflation could go on forever, although for every worldline, the period of inflation is finite. [compare that with a wildfire, every tree just burns for some specific time, but the fire itself can go on forever, at least theoretically; in a theoretically infinite universe, inflation could go on forever].
there is no need for inflation to have begun at some time, it could have been past eternal.

(*)
And as a remark, from a philosophical point of view, it is necessary to acknowledge that as such the universe can not have a begin, as the universe contains all there is, since if there was, the universe would have needed to start out from nothing. But a nothing is only nothing, not a begin of even an infinitisimal small something. A nothing is merely a pure concept of thought, and not a concept that applies to physical reality. Don't interchange that with a vacuum (the absence of ordinary matter), since a vacuum definitely contains something (energy, fields, spacetime metric, etc.)

hellfire

Jan14-07, 05:50 PM

SpaceTiger's claim is correct if you take singularity = an event or a location at which general relativity and the classical notion of space-time break down.

Chronos

Jan15-07, 02:19 AM

heusdens

Jan15-07, 07:33 AM

A universe from 'nothing' is not as far fetched as it may sound. Under the laws of thermodynamics, it is virtually required. Entropy is illogical in an infinitely old universe that includes causality.

That is not correct, if 'nothing' causally connects to any something, it is not 'nothing', but already a potential something.

You can't make up arbitrary physics law from nothing.

Qauntum mechanics for instance, is not applicable to 'nothing', because a 'nothing' is not part of any reality and has no physical description, it is just a concept of thought. If it would, then 'nothing' would not be 'nothing' but full of potential something.

And btw. this whole issue is outside the domain of physics, if there ain't a physical description of reality, there is no physician or physical law that can deal with it.

That one can not perceive of reality without contradictions somewhere, does not mean that reality does not exist, but that our way of describing it is incorrect.

The law of Thermodynamics (2nd law) is used over and over again to somehow 'proof' that the world would have needed a begin (a begin from 'nothing' !), but what one does not see is that the law of thermodynamics is not usefull in that context. It is restricted to finite systems only, which are not thermally connected to the rest of the universe.

The infinite universe is not a problem, since in any case, any timeline need to be strictly finite. The bottom line however is that one can not perceive of infinity without a contradiction.

heusdens

Jan15-07, 07:34 AM

SpaceTiger's claim is correct if you take singularity = an event or a location at which general relativity and the classical notion of space-time break down.

Therefore, it is not an 'event'.

hellfire

Jan15-07, 08:11 AM

Therefore, it is not an 'event'.
Ok, you may be right with the wording. What I was trying to point out is that the claim of SpaceTiger is correct, as long as you take the definition of singularity that is given within general relativity. This definition can be formulated in a very precise an formal way and there are singularity theorems that prove that the universe must have had such a singularity in its past. This is actually the mainstream view.

Mike2

Jan15-07, 08:29 AM

The law of Thermodynamics (2nd law) is used over and over again to somehow 'proof' that the world would have needed a begin (a begin from 'nothing' !), but what one does not see is that the law of thermodynamics is not usefull in that context. It is restricted to finite systems only, which are not thermally connected to the rest of the universe.
The requirement of causality is that the next fact or event is materially implied by the previous condition. It is logically valid that a true conclusion can be implied by a falsehood, ie. 0->1 is logical. Thus the universe can come from nothing. Logic was developed from the most complete generalization of physical observations. One thing seemed to lead to another until we generalized the "one thing" as a premise and "another" as the consequences until now we accept that this is the way to think about anything. Now we are in the habit of assigning the property of being true to physical facts which do exist, and false to those supposed facts whcih really don't exist. We assign truth to existence and fasle to non-existence. So the very first fact would represent "True" materially implied by non-existence which is false. The first sample of existence completely distinguishes true from false. And the certainty of its existence is 100%. If anything at all is knowable with absolute certainty, it is that reality exists. What we can say about the universe after that would be less certain. And we can only decompose the absolute certaintly about the property of existence to other properites which are not as certain. So it seems probabilities would enter the calculations about subsystems of reality.

heusdens

Jan15-07, 09:08 AM

The requirement of causality is that the next fact or event is materially implied by the previous condition. It is logically valid that a true conclusion can be implied by a falsehood, ie. 0->1 is logical. Thus the universe can come from nothing. Logic was developed from the most complete generalization of physical observations. One thing seemed to lead to another until we generalized the "one thing" as a premise and "another" as the consequences until now we accept that this is the way to think about anything. Now we are in the habit of assigning the property of being true to physical facts which do exist, and false to those supposed facts whcih really don't exist. We assign truth to existence and fasle to non-existence. So the very first fact would represent "True" materially implied by non-existence which is false. The first sample of existence completely distinguishes true from false. And the certainty of its existence is 100%. If anything at all is knowable with absolute certainty, it is that reality exists. What we can say about the universe after that would be less certain. And we can only decompose the absolute certaintly about the property of existence to other properites which are not as certain. So it seems probabilities would enter the calculations about subsystems of reality.

I don't think you're on the right track, because to begin with "false" has no meaning without "true". "False" and "true" are just opposites of each other, that belong to each other, and one can not exist without the other.
There is no dark without light. There is no nonbeing without being.

In other words those opposites (like false & true, light & dark, being & nonbeing) have no independend existence, but exist only in their unity of opposites.

Further, and as seen by dialectics, these union of opposites form a higher unity, so the unity of being & nonbeing is just becoming.

See for example Hegel, Science of Logic, Doctrine of Being:
http://www.marxists.org/reference/archive/hegel/works/hl/hlbeing.htm

heusdens

Jan15-07, 09:12 AM

Ok, you may be right with the wording. What I was trying to point out is that the claim of SpaceTiger is correct, as long as you take the definition of singularity that is given within general relativity. This definition can be formulated in a very precise an formal way and there are singularity theorems that prove that the universe must have had such a singularity in its past. This is actually the mainstream view.

Yeah, but be aware that you might be deluded.
If I measure a tree that is growing, and calculate back to it's past, I might conclude that also that tree once was zero length. [let us suppose the observer here is an intelligent ant, who is able of measuring, but has no notion about how trees reproduce, etc. ]
But that ain't the case and can't be the case. Even if the model correctly predicts that!

The problem is that the model is sometimes seen as reality itself, if the model claimed that at one of it's limiting conditions something weird happens, people tend to think that such a thing was a real event.

Don't confuse the model with reality. Trees don't start their growth from zero length, neither does the universe start out from a singularity.

PS.
Even mainstream ideas can be wrong.
If you would ask any person where the world came from or came into being, considering the fact that there is a majority of people considering themselves religious, the answer would then be that 'God did it'.

logomach

Jan15-07, 11:25 PM

Yeah, but be aware that you might be deluded.
If I measure a tree that is growing, and calculate back to it's past, I might conclude that also that tree once was zero length. [let us suppose the observer here is an intelligent ant, who is able of measuring, but has no notion about how trees reproduce, etc. ]
But that ain't the case and can't be the case. Even if the model correctly predicts that!

The problem is that the model is sometimes seen as reality itself, if the model claimed that at one of it's limiting conditions something weird happens, people tend to think that such a thing was a real event.

Don't confuse the model with reality. Trees don't start their growth from zero length, neither does the universe start out from a singularity.

PS.
Even mainstream ideas can be wrong.
If you would ask any person where the world came from or came into being, considering the fact that there is a majority of people considering themselves religious, the answer would then be that 'God did it'.

Just to chime in as a completely unrelated observer, I agree with your reasoning, Heusdens, and I found reading your perspective quite interesting. But the title of this thread is "Review of Mainstream Cosmology".

"Even mainstream ideas can be wrong" is not really a position that should be described and championed so much in this thread. Maybe you should start a new thread, perhaps entitled "Critique of Mainstream Cosmology"?

I, at least, would enjoy reading it. But I think you will agree that the topic is seperate from the one that this thread was intended for.

heusdens

Jan16-07, 10:34 AM

Just to chime in as a completely unrelated observer, I agree with your reasoning, Heusdens, and I found reading your perspective quite interesting. But the title of this thread is "Review of Mainstream Cosmology".

"Even mainstream ideas can be wrong" is not really a position that should be described and championed so much in this thread. Maybe you should start a new thread, perhaps entitled "Critique of Mainstream Cosmology"?

I, at least, would enjoy reading it. But I think you will agree that the topic is seperate from the one that this thread was intended for.

Although my post relates to the physical theory and what it tells is about the world (and just to note, I am not an opponent of the BB theory as a scientific theory, I am just commenting on some commonly held beliefs and intepretations of the BB theory, which are not even part of the theory itself (there is nothing in the BB theory that says or states anything about the singularity - even if it digs up in the underlying theory - that it would have to be regarded as a 'real event'), but are misnoted common beliefs or interpretations of the BB theory, which seem to be pretty mainstream - it is not a comment that has to deal with the physics/cosmology itself, but the philosophical implications it might or might not have.

The comment is not just in regard to any specific physical theory, but in general on all physical theories. So, philosophy of science is in fact the forum it belongs to.

Allday

Jan19-07, 03:17 AM

hi everyone, i just read thru this whole thing so this post might drag on a bit but i wanted to address some of the points brought up.

1. There is another piece of evidence for dark energy that is not mentioned very often. This paper (http://arxiv.org/abs/astro-ph/0307335) describes a measurement of the correlation between the integrated Sachs-Wolfe effect and the location of LRGs from Sloan. Basically photons falling into potential wells that are shrinking due to accelerated expansion come out with extra energy because the hill they have to climb out of isnt as steep as the one they went down. Scranton et al. measure a positive correlation between hot spots on the CMB and luminous red galaxies in Sloan.

2. As has been mentioned, the study of pop III stars is at a very early stage and little is known about their properties. Here is a recent paper detailing a simulation (http://arxiv.org/abs/astro-ph/0610174) Of course this is not as good as observation, but it is a good start and I believe that simulations will be necessary to help analyze the observational data when it comes in.

3. A promising way to get at some of the properties of pop III stars is to make maps of the nuetral hydrogen in the early universe and look for the dark spots (ionized regions). These observations have to be done with large radio telescopes because the 21cm line from that era has been redshifted into the radio. LOFAR and PAST are two such telescopes that have the potential to make these kind of measurements in the next couple years.

Here are some recent papers on reionization that ive found interesting ...

self regulated reionization
http://arxiv.org/abs/astro-ph/0607517

lecture notes on observations of the high Z univese
http://arxiv.org/abs/astro-ph/0701024

short description of LOFAR
http://arxiv.org/abs/astro-ph/0610652

Also Ive read a couple people say something about the Universe decelerating in the present era? thats news to me.

heusdens

Jan21-07, 09:32 AM

The most important idea behind the Big Bang theory is that, acc. to the general relativity theory, the metrics of space are expanding, which becomes observable in the form of an appearant redshift-distance relation (Hubble relation).

But what is in fact this concept of "expansion of space". Is it some physical event, with an underlying physical process explaining it, or what?

Space itself, when we see it from a "classical" point of view, is not a substance or something. The metrics of space in the classical sense are purely defined by measuring distances between bodies in space. Measuring "space itself" is something clearly out of the question. At least in our classical conception of space. In the classical conception of space, the expansion of space could not even be stated, we could only state that bodies distantiate themself in space, which is of course the same as stating that those bodies recede away from each other, and have therefore a nonzero velocity relative to one another.
In the classical sense a change of the metrics of space would not be a physical phenomena. In the same way as changing our rulers (units of measure) would not change anything physically. All physical phenomena stay the same when we would switch our units of measurement. (the only thing that would change were our textbooks of physics, which would have to be rewritten in the new measurement units). This is even true when we would have a "flexible" measurement unit (one that changes as a function of some other physical entity, for instance time; if we were to state that the unit of lenght were to double each year, then of course all physical interactions become much more difficult to calculate, yet the physical world itself does not change because of this weird choice of measurement unit).

Now quantum mechanics and relativity come into play, in which our classical concept of space ain't correct any more. We can adress physical properties to space. Electro magnetic theories also adress properties of space.
Also we know that space can't be exactly empty, acc. to the Heisenberg Uncertainty Principle.

So, my question then is, if we apply this to the concept of expansion of space, and reason that only the changing metric does not change anything physically (which is the case in classical point of view), what physical changes take place in space that would cause space to expand.
It can't just be a change in the metric or scale factor, as they are not themselves something physical (i suppose), or are they?

In a theoretical situation of a universe as large as ours, and we have two very distantiated objects (like some billion light year), how can we ever state that the appearant increase in distance (which we somehow measure) is due to expansion of space, instead of a relative velocity (objects receding from one another)?

Sometimes this is misnoted as that in one case, the two very far apart objects are not moving (relative to surrounding space), and therefor the appearant recission speed is due to expanding of space, and in the other case, the objects do recede from each other (they are moving relative to surrounding space).

Yet, this whole explenation is somehow inconvincing, since it makes use of the concept of "moving relative to surrounding space". Acc. to relativity itself , this is not a valid concept, since we can only state something about objects moving relative to each other (and not relative to space itself!).

Am I misconceiving something, or how is it that all of a sudden we must make statements to explain what expansion of space realy is (and how it differs from normal relative motion), which uses concepts which by the very same theory that would cause this effect in the first place, is simply not a valid concept.

So either the motion of an object relative to space itself is a valid concept, or not. If it is not a valid concept, then how do we explain the difference between simple motion of objects relative to each other (very far remote objects) and the expansion of space?

If it is a valid concept, then how can we calculate our relative motion to surrounding space. Perhaps relative to the CMBR?

SpaceTiger

Jan21-07, 07:41 PM

the standard assumption is that the universe had a creation event and expanded from a singularity to its present size. Such a distant extrapolation can't possibly be verified by the current observations, but we can safely say that the universe expanded from a much smaller size than its current one.

Seems fine to me. Standard models of the universe use a simple GR model with a beginning and an end, the beginning marked by a singularity. Nobody views this as scientific fact, just an approximation that covers the essential features that are currently verifiable by experiment. Eternal inflation is possible, as is a quantum gravity removal of the singularity, but I didn't feel it was appropriate to get into these details in a basic BB summary. I think you're just picking at semantics.

Allday

Jan21-07, 08:40 PM

Heusdens,

let me address some of your statements.

"Space itself, when we see it from a "classical" point of view, is not a substance or something. The metrics of space in the classical sense are purely defined by measuring distances between bodies in space. Measuring "space itself" is something clearly out of the question."

The fact that space is expanding was deduced from measurements of the distances to galaxies and supernovae, and the fact that their recession velocity increased the further away they were.

If you want to explain this fact by postulating that the galaxies are simply moving through a static space away from us and that space is not expanding, then we are at the center of this "explosion" and occuppy a very special place in the universe. Also in this scenario the galxies that are distant enough will be moving faster than the speed of light. Expanding space solves both of these problems.

special and general relativity introduced the concept of a dynamic space-time that can curve and stretch. In some sense this is the assumption of GR and as its predictions are verified to a higher and higher degree you should think of space in this sense as opposed to the static sense.

If you are looking for the answer to the question "what exactly is spacetime and where does it come from?" then you will have to look beyond GR.