Is the Universe Really Expanding?

[SpaceTiger]

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 The field is advancing very rapidly, so it's possible that even the most reliable websites will be woefully out of date, both in terms of results and the evidence for them.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 modeled 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, for example), so web surf if you want to know more.

 

[Chronos]

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? By the way, I have an arsenal of material on this topic

 

[SpaceTiger]

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]

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

 

[Chronos]

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]

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 anyone 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]

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]

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]

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]

ST, is there a problem with your scenario?

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

However, would not these stars therefore have to be very massive
M = >10⁵M_solar 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]

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 Hawking 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]

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]

However, would not these stars therefore have to be very massive
M = >10⁵M_solar 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]

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]

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&high=424800249005443

 

[SpaceTiger]

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]

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^-8Z_solar) as predicted by the “Freely Coasting” model, then more moderate 10² - 10³ M_solar 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]

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=424800249005443

Those 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]

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]

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

 

[SpaceTiger]

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]

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]

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

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]

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]

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]

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]

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 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]

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]

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]

Can we talk about spallation yet?