What has been proved about the big bang, and what has not?

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Solution 1

Spencer's comment is right: we never "prove" anything in science. This may sound like a minor point, but it's worth being careful about.

I might rephrase the question like this: What's the smallest size of the Universe for which we have substantial observational evidence in support of the standard big-bang picture?

People can disagree about what constitutes substantial evidence, but I'll nominate the epoch of nucleosynthesis as an answer to this question. This is the time when deuterium, helium, and lithium nuclei were formed via fusion in the early Universe. The observed abundances of those elements match the predictions of the theory, which is evidence that the theory works all the way back to that time.

The epoch of nucleosynthesis corresponds to a redshift of about $z=10^9$. The redshift (actually $1+z$) is just the factor by which the Universe has expanded in linear scale since the time in question, so nucleosynthesis occurred when the Universe was about a billion times smaller than it is today. The age of the Universe (according to the standard model) at that time was about one minute.

Other people may nominate different epochs for the title of "earliest epoch we are reasonably sure about." Even a hardened skeptic shouldn't go any later than the time of formation of the microwave background ($z=1100$, $t=400,000$ years). In the other direction, even the most credulous person shouldn't go any earlier than the time of electroweak symmetry breaking ($z=10^{15}$, $t=10^{-12}$ s.)

I vote for the nucleosynthesis epoch because I think it's the earliest period for which we have reliable astrophysical evidence.

The nucleosynthesis evidence was controversial as recently as about 10 or 15 years ago, but I don't think it is anymore. One way to think about it is that the theory of big-bang nucleosynthesis depends on essentially one parameter, namely the baryon density. If you use the nucleosynthesis observations to "measure" that parameter, you get the same answer as is obtained by quite a variety of other techniques.

The argument for an earlier epoch such as electroweak symmetry breaking is that we think we have a good understanding of the fundamental physical laws up to that energy scale. That's true, but we don't have direct observational tests of the cosmological application of those laws. I'd be very surprised if our standard theory turns out to be wrong on those scales, but we haven't tested it back to those times as directly as we've tested things back to nucleosynthesis.

Solution 2

For an alternate answer, consider the inflationary universe. (Look up Inflation (Cosmolgy) in Wikipedia.)
Cosmologists say we have substantial evidence for inflation. The time of inflation is somewhat uncertain, but with high probability it is very early, if it occurred at all. Wikipedia says 10^-36 to 10^-32 seconds.

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Updated on August 01, 2022

Comments

  • HappyDeveloper
    HappyDeveloper over 1 year

    Ok so the universe is in constant expansion, that has been proven, right? And that means that it was smaller in the past.. But what's the smallest size we can be sure the universe has ever had?

    I just want to know what's the oldest thing we are sure about.

    • spencer nelson
      spencer nelson over 12 years
      "Proven" is a dangerous word to use here...
  • Jim Graber
    Jim Graber over 12 years
    PS The size, as opposed to age, of the universe was much larger than 10^-36 light seconds and may have even been infinite.
  • anna v
    anna v over 12 years
    I have not read the paper, and I have not been down voting. It would be a service to all of us if the people who are down voting would document their objections, other than "big bang is the consensus cosmology" which , for lack of explanation, I assume. Nevertheless, it would be more useful on the down voters part to document where they believe this proposal is violating conservation laws, or some such.
  • Marek
    Marek over 12 years
    @anna: I don't think it's worthwhile to spend even a minute discussion on this obviously crackpot physics... The point about big bang is not that it is consensus but rather that it explains everything rather well (i.e. there has been no falsification of the theory yet). Looking for an alternative theory that might have fewer parameters is of course completely fine but if someone states "new model that will knock it down." there's no point in reading any further. Just imagine Planck and Einstein would say "QM will knock classical physics down". Of course, they were smart so they hadn't...
  • Benjamin Horowitz
    Benjamin Horowitz over 12 years
    Also, proposing your (/your friends) theories do not at all answer the question the poster was asking. S/He was asking what was known by the scientific community about the big bang, scientific community $\neq$ un-peer-reviewed theories. (Generally, I am a bit disapproving of how Helder Velez has used this site for a launching point for these types of "theories", answering the most tangentially related questions with references to them)
  • Helder Velez
    Helder Velez over 12 years
    @Marek I'm free to be an evangelist (using emphasis as you do, but politely) of a new theory that has no "free lunchs" ('space expansion' and 'dark energy' are always growing without known cause) and no one knows how to apply it locally. You are free to be an evangelist of the long lived BBTheory. Your comment imply that you obviously know how to unprove the formal (and quite simple) evidence that mass is NOT invariant (PSE-post). If you please.
  • Marek
    Marek over 12 years
    @Helder: I am not an evangelist. Rather, I like to consider myself a scientist: any theory that has passed a test of time (as has BB for past decades and even more so with recent observations) is a good theory and worth defending against "a theory" (for a lack of a better word) you present. What does invariance of mass has to do with anything by the way? It's a standard QFT observation that has been known at least since 1940s...
  • Helder Velez
    Helder Velez over 12 years
    @Marek When I was a little boy I needed 40 steps to cross the road in front of my home. Now I measure it to be 20 steps and we have a consensus, among all that grew with me, that the road is shrinking, backed with careful measures. My special world is made of equal kids and when I was told that our height and weight had changed I could not beleive. Mass/length atom invariance is a hidden claim of BBT (not explicit but it is unavoidable and was never proved). The unit of distance is accomplished with a local atom. If you attach the referential to the atom you can not see how they can vary.
  • Marek
    Marek over 12 years
    @Helder: there is no such assumption in BBT. BBT is just about cosmological models in general relativity. Now, to obtain reasonable models of the "beginning of the universe", one also needs to add particle physics as well (most importantly nucleogenesis that Ted talks about). If one does this, one finds out that based on just few parameters the model agrees with all observations ever made. Again, one can try to find a better model but BBT is correct in the same sense classical physics is correct: any more fundamental theory has to agree with it and explain it, not knock it down!
  • Helder Velez
    Helder Velez over 12 years
    @Marek A complicated problem, in any domain, can only be solved by parts and the BBTheory took decades to build, layer after layer. I believe that in the future, because you are young, you will have the opportunity to see the evolution of new theories. Physical laws were never disproved but theories come and go, and this is good for young scientists like you. Take your time to read the paper. (forget the 'knock it down' expression)
  • Helder Velez
    Helder Velez over 12 years
    data about the evidence, or lack of it at 90%CL, see Large-angle anomalies in the CMB by Copi et al (2010/11) "The striking feature of the two-point angular correlation function as seen in Fig. 5 is not that it disagrees with CDM (though it does at > 90% C.L.) but that at large angles it is nearly zero. This lack of large angle correlations is unexpected in infationary models"
  • Jerry Schirmer
    Jerry Schirmer about 11 years
    @HelderVelez: do you get the H/He abundance right? If you don't, your theory is ruled out as inferior to BBT.
  • jjcale
    jjcale about 11 years
    Here some arguments for and against inflation : physics.princeton.edu/~steinh/0411036.pdf
  • Ron Maimon
    Ron Maimon about 11 years
    The inflationary phase has observational evidence, and it is prior to electroweak symmetry breaking. I wouldn't go earlier than that, but it's a lot earlier in terms of energy scales, something like 10 orders of magnitude at least.
  • Ron Maimon
    Ron Maimon about 11 years
    @JimGraber: You should best measure size by distance to the cosmological horizon, not size of a "now" patch, and then the inflationary era the universe is small.
  • Helder Velez
    Helder Velez about 11 years
    @Jerry looking for dates in [WP-Baryogenesis] (en.wikipedia.org/wiki/Baryogenesis) found a doc from 1997 titled ("Baryogenesis, 30 Years After). 1967 is many years after the inception of BBT is enough answer to your remark. Baryogenesis is independent of BBT and is listed in the en.wikipedia.org/wiki/Category:Unsolved_problems_in_physics and the 'Lithium problem' is still a problem. Matter-antimatter assymetry is not solved.
  • Jerry Schirmer
    Jerry Schirmer about 11 years
    @HelderVelez: then why do diferent matter content models give different answers for the problem? The Lithium problem is real, bt just as real is the fact that the H/He ratio is a tight constraint on big bang models.
  • Helder Velez
    Helder Velez almost 11 years
    @Jerry with the BBT we have a hot beggining w/ equal atoms all the time. With this new model the origin is at 0ºK with a possible scenario: bigger long lived neutrons, progressivelly decaying into H, and giving occurrence to all the sort of ratio of isotopes (see [Webb-1998] (arxiv.org/pdf/astro-ph/9803165) when he got a varying ratio of Mg24/Mg26 as a possible way to explain the measured varying alpha constant (pg4 1st §)). When the community will address the problem of a varying alpha?. (the nuclear decay is also function of the varying (nuclear mass)/(nuclear volume)) thru time.
  • Helder Velez
    Helder Velez over 7 years
    After further analysis I dropped the notion that there was a 'drift' in the ratio of the nuclides (see previous comment).
  • ProfRob
    ProfRob about 7 years
    The abundance of lithium is still a problem; but whether with the BBM or stellar physics is uncertain.
  • freecharly
    freecharly about 7 years
    What do you mean by size of the universe here? There seem to exist views that the universe was infinite right from the beginning.