Is the Universe Really Expanding?

[Chronos]

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]

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(N_Fe/N_H)_star/medium - log₁₀(N_Fe/N_H)_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“ 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]

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

 

[turbo]

WMAP results suggest reionization occurred 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]

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.

 

[Garth]

Back to the mainstream model.

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

Thank you - let's put some numbers into the time-line of that model.
The 'look-back' time t_l as a function of red shift z is given by:
t_l/t_H = (2/3)(1 - 1/(1 + z)^3/2)

Note: This is for the mater dominated era of the Friedmann model,
R(t) ~ t^2/3, 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 t_H = 10.2/h Gyrs.
WMAP determines h = 0.72 so t_H = 14.2 Gys.
and the age of the universe = 2/3t_H = 9.44 Gyrs.

Let t_z= 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,
t_z=1000 = 300,000 yrs.

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

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

for 'modern' metallicity in Quasar SDSS J1030+0524 z = 6.28
t_z=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]

hmmm.

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]

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]

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]

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.

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.

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]

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]

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]

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]

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]

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]

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.

 

[turbo]

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]

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

 

[turbo]

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]

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]

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]

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.

 

[Garth]

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]

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]

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 $\Lambda$CDM 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]

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]

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]

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]

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

 

[oldman]

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?