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Review of Mainstream Cosmology

  1. May 28, 2005 #1

    SpaceTiger

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    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 The field is advancing very rapidly, so it's possible that even the most reliable websites will be woefully out of date, both in terms of results and the evidence for them.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, for example), so web surf if you want to know more.
     
    Last edited: May 28, 2005
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  3. May 28, 2005 #2

    Chronos

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    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 :smile:
     
    Last edited: May 28, 2005
  4. May 28, 2005 #3

    SpaceTiger

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    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.
     
  5. May 28, 2005 #4

    Chronos

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    question - could gamma bursters have ionized the early universe? I have a follow up question.
     
  6. May 28, 2005 #5

    Chronos

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    Another question [pardon my curiousity], could SMBH have formed in the early universe from collapsing gas clouds [skipping the merger thing, just formed directly]?
     
  7. May 28, 2005 #6

    SpaceTiger

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    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.
     
    Last edited: May 28, 2005
  8. May 28, 2005 #7

    SpaceTiger

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    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.
     
  9. May 28, 2005 #8

    SpaceTiger

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    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.
     
  10. May 28, 2005 #9

    wolram

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    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".
     
  11. May 28, 2005 #10

    Garth

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    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
     
    Last edited: May 28, 2005
  12. May 28, 2005 #11

    Chronos

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    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.
     
  13. May 28, 2005 #12

    SpaceTiger

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    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.
     
  14. May 28, 2005 #13

    SpaceTiger

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    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.
     
  15. May 28, 2005 #14

    wolram

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    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.
     
  16. May 28, 2005 #15

    SpaceTiger

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    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
     
  17. May 28, 2005 #16

    SpaceTiger

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    I'll address the dark matter issue in a bit. Hopefully that will clear things up.
     
  18. May 28, 2005 #17

    Garth

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    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” 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
     
    Last edited: May 28, 2005
  19. May 28, 2005 #18

    Chronos

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    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.
     
  20. May 28, 2005 #19

    SpaceTiger

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    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.


    Which part of the following did you not understand?

     
  21. May 29, 2005 #20

    Garth

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