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The question that concerns my mind is that in the time period before the cosmic microwave background, did humans have any observations or not? I mean, are all the materials mentioned in physics about this time period based on theory, or are there any observational evidences? What exactly do we know about this time period based on observational evidence?

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    $\begingroup$ Your comments on my answer lead me to ask you to define what you mean by "observational evidence". $\endgroup$
    – ProfRob
    Commented May 17 at 10:25

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In astronomy there are broadly 4 ways you can "observe" or collect information about the universe (which is what I assume you mean by "observational evidence").

  1. By receiving electromagnetic radiation.
  2. By collecting physical material.
  3. By receiving gravitational waves.
  4. By receiving/studying particles (e.g. cosmic rays and neutrinos).

When thinking about the pre-CMB universe (roughly when the universe was $<$400,000 years old, we know that any light emitted then has been thermalized and scrambled into the CMB. Nevertheless, their remain subtle traces in that radiation field (in terms of its angular distribution and polarisation) that do tell us about pre-CMB conditions. i.e. There is observational evidence of the pre-CMB conditions imprinted in the CMB. The CMB itself is evidence for a hot, dense plasma in the past.

Gravitational waves are also expected to be generated during the early phases of the big bang and would pass unhindered through the universe. Evidence for these might be found in the CMB itself, but direct detection is also possible, but will probably have to wait for spacecraft interferometers.

We do expect the universe to be flooded with a similar cosmic neutrino background, with a characteristic temperature of 2.3K. These neutrinos will be weakly interacting and abundant, but very hard to detect at these low energies (and they have not yet been detected). Similarly, any dark matter that is around now would almost certainly have arisen in the pre-CMB era, but again, none has yet been detected.

Instead, the evidence that you might argue has come directly from the big bang, does come in the form of particles in the universe and indeed materials that have been collected in our own Solar System. These are in the form of nuclei of hydrogen, helium, deuterium and lithium, that were manufactured in the first 10 minutes or so after the big bang (according to that model). The majority of the hydrogen, helium and deuterium that exists today comes directly from the big bang (lots of lithium has been both created and destroyed in stars and is more complicated). The inferred relative abundances of primordial helium and deuterium to hydrogen are in remarkable agreement with the concordance big bang cosmology model, with just one free parameter - the ratio of photons to baryons at the epoch of nucleosynthesis. Since nuclei require temperatures of $>10^8$ K to fuse effectively, this is also strong observational evidence for a dense, hot phase, and much hotter than the phase that produced the CMB.

To summarise - there is plenty of evidence for a very hot, dense universe at some point in the pre-CMB past. The primordial abundances and various features of the CMB are in accord with a (big bang) model. That model also makes other predictions that have not been observationally tested yet - for example there should be neutrino and gravitational wave backgrounds. There other "observations" that the model cannot explain (and that is why is it a model and not a theory, in my opinion) - these would include what "caused" the big bang, why there was a matter/anti-matter imbalance that led there to be $\sim 10^9$ photons for every baryon and what determines the ratios of the various components of the universe (e.g. the matter to dark matter ratio).

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  • $\begingroup$ How you can say exact time of formation atomic nucleuses in 10 minutes after BB ?and if I say they formed after CMB in dark age ,Is there any reason for evaluating? $\endgroup$
    – QQQ
    Commented May 17 at 7:22
  • $\begingroup$ The big bang model has an epoch of nucleosynthesis that happens in the first 10 minutes, because that is when it was hot enough to cause nuclear fusion. @QQQ If you have another model of how nuclei are created then presumably it makes predictions? $\endgroup$
    – ProfRob
    Commented May 17 at 7:30
  • $\begingroup$ No, I just want to know how much of what is said about the beginning of the universe up to the CMB is based on valid observational evidence and how much is based on theory and models and the like? $\endgroup$
    – QQQ
    Commented May 17 at 7:40
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    $\begingroup$ Then you need to define what you mean by "valid observational evidence." You clearly know that we cannot "see" past the CMB, so what are you looking for? I suspect you have a very frequentist rather than Bayesian view of the world. What we do is we have a model of how things work, that model makes predictions that we compare against observations. The model isn't "knowing". It is the comparison of the model predictions with observations that validates various aspects of the model. @QQQ $\endgroup$
    – ProfRob
    Commented May 17 at 8:45
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    $\begingroup$ For example, when I measure the position and velocity at some point on the trajectory of a projectile, would you say that is "valid observational evidence" of the speed and direction with which it was launched? @QQQ $\endgroup$
    – ProfRob
    Commented May 17 at 9:10

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