The Cosmic Microwave Background provides the left over radiation of the big bang. Cosmologists have theorised what happened at the Planck epoch era of the big bang.
Where is the evidence of what happened during the Planck epoch era of the big bang?
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2$\begingroup$ Any such theories are extremely tentative, since we need a theory of quantum gravity to properly deal theoretically with that epoch, and we can't explore those energies in collider experiments, it's way beyond our technology, and may remain so indefinitely. $\endgroup$– PM 2RingCommented Jan 14, 2019 at 18:18
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$\begingroup$ The CMB gives us information about the time after the universe became transparent to light, which IIRC was about 10,000 years after the big bang. It doesn't tell us anything about earlier times. AFAIK the best information from earlier times comes from nuclear abundances. $\endgroup$– user4552Commented Jan 15, 2019 at 1:24
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2$\begingroup$ @Ben The recombination era was about 378,000 years after the BB. $\endgroup$– PM 2RingCommented Jan 15, 2019 at 9:41
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$\begingroup$ Nobody can prove that the Cosmic Microwave Background provides the left over radiation of the big bang. It's just a theory like the Big Bang. Maybe there was no Big Bang. So there is no Planck epoch. :-) $\endgroup$– Sebastian SiemensCommented Jan 15, 2019 at 15:04
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$\begingroup$ @SebastianSiemens Your comment is a null statement; are you trolling us? Nobody can prove anything in science; what we look for is the best match of theory (or models) with observation. There is extensive evidence to support the theory of recombination/decoupling 13.4b years ago, and the science is pretty robust all the way back to $10^{-12}$ seconds after $t_0$ (i.e. from the Quark Epoch onwards). $\endgroup$– Chappo Hasn't ForgottenCommented Jan 21, 2019 at 6:19
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I am quoting from this source
Before a time classified as a Planck time, $10^{-43}$ seconds, all of the four fundamental forces are presumed to have been unified into one force. All matter, energy, space and time are presumed to have exploded outward from the original singularity. Nothing is known of this period.
Italics mine.
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In the era around one Planck time, $10^{-43}$ seconds, it is projected by present modeling of the fundamental forces that the gravity force begins to differentiate from the other three forces. This is the first of the spontaneous symmetry breaks which lead to the four observed types of interactions in the present universe.
Looking backward, the general idea is that back beyond 1 Planck time we can make no meaningful observations within the framework of classical gravitation
so it is necessary to be able to quantize gravity so that a continuity exists in the models before and after the Planck time.
This give a picture
The inflationary hypothesis offers a way to deal with the horizon problem and the flatness problem of cosmological models.
These problems are directly connected with observations and are currently modeled in the Big Bang in a satisfactory way, but there is no observational check to a model for the times before $10^{-36}$ seconds and the Planck time. Only speculations.
#/media/File:History_of_the_Universe.svg){kind=link}
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$\begingroup$ The change in the rate of expansion at 10^-35 in the picture looks sharp and instantaneous. $~$ So, after inflation ended, did inflationary expansion instantly turn into non-inflationary, or did inflationary expansion gradually decelerate to non-inflationary due to gravity? $\endgroup$– ForgeCommented Jan 18, 2019 at 8:00
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$\begingroup$ @Forge Inflation stopped because of the continuous expansion and the falling of the energy density which allowed theoretically a standard model (including the gravitons) matter particles in an unbroken version ( hypothetical model at present) to appear. see also answers here physics.stackexchange.com/questions/454765/… $\endgroup$– anna vCommented Jan 18, 2019 at 11:45
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$\begingroup$ Could you explain what you mean by “Inflation stopped because of the continuous expansion” ? $~$What continuous expansion? $\endgroup$– ForgeCommented Jan 18, 2019 at 19:07
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$\begingroup$ “the falling of the energy density which allowed theoretically a standard model matter particles in an unbroken version to appear” $~$Did you mean the falling energy density of the inflaton field? $\endgroup$– ForgeCommented Jan 18, 2019 at 19:11
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$\begingroup$ @Forge of the universe, at the end of the infation era, where the expansion returns to the rate of the original Big Bang model, look at the plot., it returns because the inflaton field breaks down into what would be a string-or guts -or -whatever-with-matter particles model, Same progress with inflation as with original BB, different rates during inflation period. $\endgroup$– anna vCommented Jan 19, 2019 at 4:15