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In his book Gravitation and Cosmology, Steven Weinberg says that the Cosmic Microwave Background (CMB) makes it "difficult to doubt that the universe has evolved from a hotter denser early stage".

In my understanding, CMB is just a peculiar isotropic radiation representing a black body at ~ 2.7K.

How and why does CMB point to the early Universe being hotter and denser?

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    $\begingroup$ this needs a context, as it is in a book, at least say page number ( found a pdf but it is an image). he simple answer is that the statement is consistent with getting the CMB characteristics from the big bang model $\endgroup$ – anna v Feb 11 at 5:29
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    $\begingroup$ we can not only see the current microwave background (CMB) but also at earlier times - up to the theoretical maximum around 150,000 years after the big bang , when the nuclei of the primordial nucleo-synthesis fetched their electrons for the first time. The very isotropy speaks for itself, like G.Smith writes $\endgroup$ – eagle275 Feb 11 at 13:26
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    $\begingroup$ The Axis Of Evil invalidates CMB as coming from the surface of last scattering. The only plausible explanation of CMB is the light of our own galaxy that has made a full circle around the closed universe. $\endgroup$ – safesphere Feb 14 at 3:20
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    $\begingroup$ Our own galaxy emits a perfect blackbody spectrum does it? @safesphere $\endgroup$ – Rob Jeffries Feb 25 at 18:39
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    $\begingroup$ @RobJeffries I hope not since cosmic background is not black body: physics.stackexchange.com/questions/196366 - Also it was emitted at $t\approx 26\,My$, $z\approx 536$, and $T\approx 1460K$ before Milky Way or larger clusters formed when things were more uniform (none of this is FLRW of course). $\endgroup$ – safesphere Feb 26 at 4:57
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Beyond the fact that the cosmic microwave background (CMB) is a direct prediction of the big bang model, there is the question of how you would produce it in any other way. It is remarkably close to being isotropic and remarkably close to being a blackbody spectrum - i.e. it is almost a perfect blackbody radiation field.

A blackbody radiation field is emitted by material in complete thermodynamic equilibrium (CTE). An example would be the interior of a star. A requirement for (CTE) is that the matter and radiation field are characterised by the same temperature and that the material is "optically thick" - meaning that it is opaque to that radiation at basically all wavelengths.

Given that the universe is mainly made up of hydrogen, helium and (presently) traces of heavier elements, we can ask how is it possible to produce a perfect blackbody radiation field? Cold hydrogen and helium are transparent to microwaves. To make them opaque they need to be ionised, so that the free electrons can be a source of opacity at all wavelengths via Thomson scattering. But this requires much higher temperatures - about 3000 K.

How do we uniformly raise the temperature of a gas (adiabatically)? By squeezing it. A smaller, denser universe would be hot enough to have ionised hydrogen and would be opaque to the radiation within it. As it expanded and cooled, the electrons combined with protons to form atoms and the universe becomes transparent, but filled with a perfect blackbody radiation spectrum. The light, originally at a temperature of 3000 K and mainly in the visible and infrared, has had its wavelengths stretched by a factor of 1100 by expansion of the universe, meaning we now see it mainly as microwaves.

Additional evidence for this model is that the radiation field is not absolutely isotropic. These small ripples encode information such as the expansion rate of the universe at the time of (re)combination and the density of matter. When inferred from measurements, these parameters agree very closely with other determinations that are independent of the CMB, such as the Hubble redshift distance relationship and estimates of the primordial abundance of Deuterium and Helium.

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  • $\begingroup$ Dear Rob, Thank you for your most excellent and insightful answer! Could you please add the same answer to my question on Astronomy SE (asked a week before this one) for the benefit of the community over there as well? $\endgroup$ – Ritesh Singh Feb 25 at 12:57
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The Big Bang model of an expanding Friedmann universe, with a hot and dense early era that cooled and thinned as the universe expanded, predicted the existence, isotropy, and approximate temperature of the cosmic microwave background. So the discovery of this CMB is considered strong confirmatory evidence for that remarkably simple and elegant model. See this list of predictions of the CMB temperature.

This paper reviews alternate explanations of the CMB, and why they are not considered as persuasive as the Big Bang model.

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