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How can we see the Big Bang?

  1. Mar 30, 2014 #1
    Like many, I have just recently learned about the theory of inflation and, though learning about, noticed a problem with the Big Bang Theory. To gather information on the Big Bang, telescopes point their "cameras" into deep space so we can "see" the early universe as it was forming. However, this does not make sense to me. If all matter and energy started at approximately the same point and then spread, the light should have spread faster than the matter that would make up our planet. Therefore, all the light from the early universe should have already passed us. You can see why this is a problem. For a visual, imagine a standard model of our solar system with concentric orbits. Now, replace the sun with the epicenter of the big bang, Mercury with our planet and have Pluto be the light from the Big Bang. Can someone please explain this, or, if my explanation is unclear, tell me why my problem is confusing (I have never been good at articulation). Thanks
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  3. Mar 30, 2014 #2


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    There is no "epicenter" to the big bang. There is no center to the universe. The big bang was not an explosion that happened at a single point.

    I suggest you Google "surface of last scattering" to get a start on understanding what you are asking about, and then come back if you still have questions (as you likely will :smile:)
  4. Mar 31, 2014 #3


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    You did not notice a problem with the theory, you noticed a problem with your understanding of the theory. Did you really think thousands of physicists would make such an obvious error (if it would be an error), and you were the first to find it?
    phinds pointed out where the problem is.
  5. Apr 8, 2014 #4


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    Like the two posters above said, the solution is that there is no one point in our current universe on which the big bang occurred. In other words, it's not like you can point to a direction in the sky and say "the big bang happened that way!". That one point from which all emerged IS our current universe (ALL the infinity of it!). The big bang happened everywhere, because back then, everywhere was only 1 point, that 1 point expanded to be everything that we see! (Actually, one should probably not take the collapse back to the point of the big bang, as the singularity itself is indescribable with our current physics). Admittedly, that is a very weird notion, so it's not your fault that you made this mistake. The light that we see all around is light that came after the big bang, the farther we look (in any direction!), the farther back in time we see.

    As it turns out; however, we can only see to the "surface of last scattering" (this is the CMBR). This is the radiation from the time when the electrons and protons cooled enough to combine into atoms (this is called recombination for some reason...even though it was the first time this happened) and let all the light out (previously, the light was trapped). This surface corresponds to a time of ~400,000 years after the big bang happened. So, currently, we can't see anything that happened before ~400,000 years after the big bang because the light couldn't move around before then.

    Now, if we could get real neutrino observatories set up and watch the neutrinos produced from the big bang, we would be able to see much farther into our past (I don't know off the top of my head the exact number estimated for a neutrino's "surface of last scattering"). But neutrinos are notoriously difficult to detect, and the signal would be very very faint.
  6. Apr 8, 2014 #5


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    Stating that the universe is infinite in extent is personal speculation on your part. It might well be true or it might not. The universe could be finite but unbounded.
  7. Apr 8, 2014 #6
    Thank you, phinds. Your balloon analogy helped me to understand and then an article that came up when I googled "surface of last scattering" as you told me to helped me to understand completely. However, the article has a completely separate problem I hope you can help me understand. The essay says "this scattering kept the Universe in a state of thermal equilibrium. Eventually the Universe cooled to a temperature at which electrons could begin to recombine into atoms". The problem with this is that the cooling of a closed system (which I assume the Universe is) in thermal equilibrium goes against the first law of thermal dynamics. I must be making another mistake, so could you please point it out? Thanks in advance.
  8. Apr 8, 2014 #7
  9. Apr 8, 2014 #8


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    I'm not much on thermodynamics (MAN I hated that course in undergraduate school) but I think "closed system" doesn't apply to a system that is expanding, which the universe has been doing ever since the singularity.

    I hope someone here with a better understanding of thermodynamics can give you a more definitive answer.
  10. Apr 8, 2014 #9
    The cooling is a reduction in the energy density due to the expansion, so it isn't an issue for the first law of thermodynamics.

    That isn't to say that the expansion of the universe doesn't present problems for the first law of thermodynamics though. The vacuum energy is an issue, but then general relativity doesn't get on too well with it anyway.
    Last edited: Apr 8, 2014
  11. Apr 8, 2014 #10
    The universe is treated as a close system as their is no outside influence. Just prior to last scattering their was a tremendous reheating phase due to the end of inflation. This high energy state allows thermal equilibrium. Different particle species will remain in thermal. equilibrium, only if they interact with each other often enough .Since the Universe expands, particle densities become smaller and smaller, and ultimately the various particle species decouple from each other

    First law of thermodynamics: Because energy is conserved, the internal energy of a system changes as heat flows in or out of it. Equivalently, machines that violate the first law (perpetual motion machines) are impossible. Heat is the flow of thermal energy from one object to another.

    if this is the law your referring to this law doesn't apply to cosmology as vacuum energy and quantum tunneling. Also the Heisenburg uncertainty principle is involved in quantum virtual particle production processes. Essentially the process is originally described by Allen Guth's false vacuum inflationary model. Which later included the inflaton for chaotic eternal inflation.
    In essence a higher energy potential region (true vacuum) can quantum tunnel to a lower vacuum potential (false vacuum)(hopefully I got the sequence correct lol if not I'm positive Bapowell will politely correct me :redface:)

    Through the above process and the Heisenburg uncertainty principle, its quite possible to have a universe develop from nothing. Lawrence R Krauss has written and supported this process

    edit I did get the false vacuum true vacuum sequence wrong lol. the false vacuum is the local minimum but has a higher energy potential than the ground state (lowest energy potential true vacuum.) So tunneling will go from false vacuum to true vacuum

    Last edited: Apr 8, 2014
  12. Apr 8, 2014 #11


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    Can't a guy get some artistic license in writing a response on here? lol. The Universe is spatially flat to a very good approximation, so even if it were closed and bounded, it would be unimaginably huge compared even to our currently unimaginably huge observable universe! Ok I revise my statement to "All the possibly infinite, or less likely, but still possibly closed and bounded, but without boundary, volume of the universe."

    [STRIKE]As for the first law of thermodynamics and the expansion of the universe, I don't see any violation there. Consider the adiabatic free expansion of an ideal gas. In that case T goes down even without any heat transfer or the performance of any pdV work. [/STRIKE]

    The below points are more complicated and might require some knowledge of general relativity:

    The energy lost by physical particles and light due to the expansion of the universe is more problematic as that potentially violates conservation of energy as a whole. But one finds that since the FLRW metric is NOT time independent (unlike for static solutions), there is no Killing field associated with the coordinate time, and therefore free particle energies are not conserved. Energy conservation in General Relativity is a very touchy subject since the potential energy of the gravitational field, for non perturbative solutions of GR itself is very ill defined.

    Edit: Actually for an ideal gas adiabatic free expansion does not lead to temperature decrease. I have redacted that part of my statement. I'll think about it a bit more.
    Last edited: Apr 8, 2014
  13. Apr 9, 2014 #12

    if you want to get deep into the thermodyamics I recommend this article

  14. Apr 9, 2014 #13


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    Right, but I doubt that this plasma consisting of electrons and protons in thermal equilibrium with radiation behaves like an ideal gas. At least the temperature of the radiation is inversely proportional to the expansion.
  15. Apr 9, 2014 #14


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    Nah, they hired me to be the resident nit-picker :smile:
  16. Apr 9, 2014 #15
    why not? if they are in thermal equilibrium, their reaction rates is higher than the expansion rate then it can be described as a Bose-Einstein and fermi-dirac distributions.

    see this article chapter 4


    of course you also have to account for entropy density, chemical potential and spin of each particle species however at high enough temperatures all degrees of freedom become relativistic

    section 4.1

    also doesn't the equations of state also show the relations between relativistic and non relativistic particles of an ideal gas. in cosmology?


    this article defines the thermodynamic behavior to Gibb's law (ideal gas form)

    Last edited: Apr 9, 2014
  17. Apr 9, 2014 #16


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    Why do you think that the perfect fluid (part of the FRW model) coincides with the plasma of the last scattering? Represents the latter part of said model too?
    Last edited: Apr 9, 2014
  18. Apr 9, 2014 #17
    Well, I have a couple of questions/observations (what's really the difference?)
    First, what would have been the diameter of the universe at recombination? Obviously some parts of the universe were already separated by this amount at this time and depending on the rate of expansion, I doubt we will EVER see those guys, could we?
    Second, if space began at one point then we wouldn't be able to look in every direction to see the beginning or as close as we can get to the beginning? Or perhaps at some point in the future, when we have extremely detailed detection capabilities, we can look in every direction and see exactly the same event. So the question is, wouldn't space then have to include at least one other dimension in order to see a single item in every direction that you look?
    And lastly, have the Hubble guys ever considered taking a Deep Field at the exact same spot and exactly the same exposure etc. several years apart to see if any "new" galaxies appeared or disappeared?
  19. Apr 9, 2014 #18


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    Your whole post is predicated on the totally erroneous assumption that the big bang singularity happened at a point. It did not. It happened everywhere and the universe at that time might have been infinite or it might have been finite but unbounded.

    Also, galaxies do not appear or disappear in a matter of years. Hundreds of thousands of years is likely closer to reality.
  20. Apr 9, 2014 #19

    look at equations 4 and 5 then read further down, he isn't specifying the reheating phase, he applies Gibb's law to the current cosmology conditions as well as covering the radiation dominant era. via an effective EoS. This is also done in the other link I provided as well as Dodelson's Modern Cosmology 2nd edition. If you want further proof look at the references in the first article.

    Interacting cosmic fluids in power–law Friedmann-Robertson-Walker cosmological


    "Usually the universe is modeled with perfect fluids and with mixtures of non-interacting perfect fluids" however in this case he has interactions. The articles I posted previously show the perfect fluid forms for fermions and baryons. So its essentially two perfect fluids however they didn't go into interaction between the two.

    perfect fluid solutions are used extensively in cosmology, even to modelling spcific regions of stars, black-hole accretion disks etc. Yes they serve at best as approximations however they are used in nearly every application of cosmology.

    further examples

    "The Fluid Nature of Quark-Gluon Plasma"

    "The Dynamical Behavior of a Star with Perfect Fluid"

    "As is well known, static spherically symmetric perfect fluid distributions in general relativity, are described by a system of three independent Einstein equations for four variables (two metric functions, the ene
    rgy density and the isotropic pressure" quote from this paper.

    All static spherically symmetric anisotropic solutions of Einstein's equations


    its even used in quantum applications

    Perfect fluid quantum Universe in the presence of negative cosmological constant

    Perfect fluid spheres with cosmological constant

    Exact and Perturbed Friedmann-Lemaitre Cosmologies

    The evolution of cosmological gravitational waves in f(R) gravity


    as you can see perfect fluid calculations are involved in a wide variety of aspects

    section 5.2 has the equation of the effective EoS for different species in regards to the CMB.

    in this article


    so effectively you can calculate the EoS for each species and derive an effective EoS then apply that to a perfect fluid solution. Or you can also choose to treat each species as a separate perfect fluid. with or without interactions with each other as per the dark matter dark energy example above. In the last article he also shows a derivative of a curvature fluid.
    Last edited: Apr 9, 2014
  21. Apr 9, 2014 #20
    Sorry but I was speaking timewise - a point in time - it makes more sense to me to think of the universe in time except in some circumstances where I would need distance as an example. And why wouldn't galaxies appear or disappear in a matter of years? If we can just barely detect a couple of photons of an early galaxy now we surely wouldn't see anymore in a few years. And, the reverse could also be true. Suddenly one year a few photons would appear, then more, etc. especially once our detection abilities become more sensitive.
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