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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.

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"Proven" is a dangerous word to use here... –  kharybdis Jun 15 '11 at 18:13
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3 Answers

up vote 27 down vote accepted

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.

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thanks a lot, very enlightening –  HappyDeveloper Jun 15 '11 at 19:21
    
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 Oct 25 '12 at 23:37
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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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PS The size, as opposed to age, of the universe was much larger than 10^-36 light seconds and may have even been infinite. –  Jim Graber Jul 5 '11 at 10:05
    
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" –  Helder Velez Jul 26 '11 at 17:04
    
Here some arguments for and against inflation : physics.princeton.edu/~steinh/0411036.pdf –  jjcale Oct 25 '12 at 19:33
    
@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. –  Ron Maimon Oct 25 '12 at 23:38
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BBT is not proved and it can not be proved, and I point in this answer to a new model that will knock it down.
While a physical law was never broken, and is expected to be unbreakable; a theory come and go as time goes by.

In the begining of BBT (Big Bang Theory) it was expected that gravity will, in due time, make the universe to collapse. A few years ago, against the expectations, it was measured an acceleration of the expansion.
Because the matter content of Universe is not working to bring an end to the expansion we can deduce that Bariogenesis can not be an argument in favor of BBT (matter plays a minor role).

The cosmological constant and the Dark Energy appeared magically to explain the feature, without explaining. Its only a parameter inserted in the equations, and proceed as usual.

To explain the observed, i.e. measured, expansion of the space we have to consider what stands behind 'making a measure', what referential can we chose, etc,... and one have to search for more than the naive explanation that you know of.

ΛCDM model (BBT) has 6 parameters: H0 , w, ΩΛ , ΩM , ΩR and ΩK

You can find here a killing theory with only one parameter (H0); this paper was dated July 1, 2011 and submitted to PRX; it is easy reading and any undergrad student can understand it. It is my hope that the teachers will follow. I'm very proud to see my name in this foundational paper, and I will stand up to argue in favor of it.

A self-similar model of the Universe unveils the nature of dark energy

The abstract

This work presents a critical yet previously unnoticed property of the units of some constants, able of supporting a new, self-similar, model of the universe. This model displays a variation of scale with invariance of dimensionless parameters, a characteristic of self-similar phenomena displayed by cosmic data. The model is deducted from two observational results (expansion of space and invariance of constants) and has just one parameter, the Hubble parameter. Somewhat surprisingly, classic physical laws hold both in standard and comoving units, except for a small new term in the angular momentum law that is beyond present possibilities of direct measurement. In spite of having just one parameter, the model is as successful as the ΛCDM model in the classic cosmic tests, and a value of H0 = 64 km s−1 Mpc−1 is obtained from the fitting with supernovae Ia data from Union compilation. It is shown that in standard units the model corresponds to Big Bang cosmologies, namely to the ΛCDM model, unveiling what dark energy stands for. This scaling (dilation) model is a one-parameter model that seems able of fitting cosmic data, that does not conflict with fundamental physical laws and that is not dependent on hypotheses, being straightforwardly deducted from the two observational results above mentioned.

I will try to explain this novel theory in a few words:
All system of units we use are 'atomic units' because are based in the properties of the atoms around us. And we only knew how to apply the laws of nature if we use those units.
We are 'anthropocentric' and we can only imagine that the atom is absolute, invariant in time. Yes, we are using an 'absolute reference' and also denying that an 'absolute reference' can exist.

A measure is an adimensional quantity, a ratio of two quantities: e.g. if we are measuring a distance in the numerator is the amount of length and in the divisor is the amount corresponding to the unit of measure.
As we are measuring an expansion we have to consider : the space expands, the unit of measure is decreasing (our atom) or a mix of both.

In the paper it is shown that the laws of nature can support a scaling property of the universe, where the atom is no more invariant. It is shown that by choosing a proper referential, a comoving one, the dark energy and the cosmological constant are artifacts of BBT. The BB is no more needed; the Inflation era is no more needed, etc.

The unexpected: (quote)

Atomic measures are number counts
We will now see that the measures of bodies’ properties using atomic units are independent of the base quantities and dependent on the number of particles or atoms.

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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. –  anna v Jul 5 '11 at 10:27
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@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... –  Marek Jul 25 '11 at 11:17
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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) –  Benjamin Horowitz Jul 25 '11 at 13:00
    
@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. –  Helder Velez Jul 25 '11 at 13:27
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@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! –  Marek Jul 25 '11 at 15:17
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