There's copious documents about how Inflation solves the problem that General Relativity predicts a lumpy CMB. That influation 'smooths' out the curvature fluctuations and, so, predicts a CMB at thermal equilibrium. The perfect Black Body radiation curve is proof that the visible sky was in causal contact with all other parts of the visible sky at some point.

But I don't understand where this problem comes from and can't find a lucid description in any of the text. At t=0, the universe was in causal contact and in thermal equilibrium. It must be a perfect Black Body at this point. At $t=\delta$, the peak temperature of space is at some finite value, and again, the radiation would give a perfect profile because there's no physics to randomly distribute the initial energy (no prime mover) of the big bang. This goes on for $t=2\delta$, $t=3\delta$, and so on.

Time passes and we eventually get a quantum fluctuations in the fields. One of the fields that can fluctuate is energy/mass. So we get a blip in the curvature because one small section has more/less mass than the average. Are the inhomogeneities that Inflation fixes caused by the fact that the scale factor is now permanently different in the more/less dense regions?

  • $\begingroup$ Related question here. $\endgroup$
    – knzhou
    Jun 21, 2019 at 15:05
  • $\begingroup$ en.wikipedia.org/wiki/Horizon_problem $\endgroup$ Aug 19, 2019 at 3:51
  • $\begingroup$ I would be helpful if you read the question before posting an answer. The Wiki page, like all the textbooks, state that what we observe is a homogeneous universe. At no point do they discuss why this is a problem. At $t = 0$, by definition. all the universe was in causal contact and would have had the same temperature. What caused the inhomogeneities that Inflation seeks to correct. $\endgroup$
    – user32023
    Aug 19, 2019 at 12:46

2 Answers 2


A black body does not have a uniform temperature. Any object above absolute zero experiences random statistical fluctuations in its energy distribution so it has hotter regions and colder regions. In a body that is in thermal equilibrium these fluctuations are continually being created and disappearing so the temperature overall has a constant and well defined value.

The problem with the universe is that in the early stages of its expansion it was expanding too fast for the hotter fluctuations to heat the colder fluctuations and disappear. As soon as a fluctuation appeared the expansion would tear apart the hotter and colder regions and preserve the temperature difference.

In fact the subsequent expansion increases the inhomogeneity because the colder denser regions expand less and the hotter less dense regions expand more. The end result is that by the time of the CMB the thermal fluctuations would have created temperature differences many orders of magnitude larger than are observed in the CMB. This is the fundamental problem that inflation solves.

  • $\begingroup$ Does a region with a higher temperature imply more energy/mass? If so, does that imply that the scale factor is different than a region with less energy/mass? If so, does that imply that regions of space expand at different (measurable) rates? $\endgroup$
    – user32023
    Jun 21, 2019 at 15:07
  • $\begingroup$ @DonaldAirey the expansion rate increases as you go back towards the Big Bang and becomes infinite at the singularity. The fall in expansion rate going forward in time is because the collective gravity of all the matter in the universe slows the expansion rate. Less dense regions slow less than more dense regions so the less dense regions end up expanding faster than more dense regions at any given time after the Big Bang. However at the time of the CMB the difference was so small it took decades to make accurate measurements of it. $\endgroup$ Jun 21, 2019 at 15:44
  • $\begingroup$ Looking at the universe today we see no detectable differences in the expansion rate, though of course any measurement is difficult because of the small scale inhomogeneities like superclusters and smaller. $\endgroup$ Jun 21, 2019 at 15:46
  • $\begingroup$ >> the expansion would tear apart the hotter and colder regions and preserve the temperature difference Yes. I understand that part, but what magnitude of difference is 'frozen' in? Is it more than the 1 part in 500,000 that we see today (not considering the effect that you mention in the following paragraph of your answer). $\endgroup$
    – user32023
    Jun 21, 2019 at 16:28
  • $\begingroup$ @DonaldAirey I don't have the numbers to hand, but the temperature differences would be orders of magnitude greater than are observed in the CMB. $\endgroup$ Jun 21, 2019 at 16:35

The problem, or claim is that inflation solves the flatness problem. Concretely, the problem is that in cosmology there’s a dynamical variable (ie, one that depends on time), called the curvature density parameter. It’s by construction dimensionless (doesn’t have units) and its value today is smaller than 0.1.

The standard model of hot big-bang cosmology requires initial conditions which are problematic in two ways:

  1. The early universe is assumed to be highly homogeneous, in spite of the fact that separated regions were causally disconnected (horizon problem); and
  2. Tthe initial value of the Hubble constant must be fine tuned to extraordinary accuracy to produce a universe as flat (i.e., near critical mass density) as the one we see today (this is the flatness problem!).

These problems would disappear if, in its early history, the universe super-cooled to temperatures of orders of magnitude below the critical temperature for some phase transition.

A huge expansion factor would then result from a period of exponential growth, and the entropy of the universe would be multiplied by a huge factor when the latent heat is released. Such a scenario is completely natural in the context of grand unified models of elementary-particle interactions.

For further reading see Lecture 1 here.

  • $\begingroup$ I read the lecture and it has the exact same problem as all the other text: it says Inflation fixes the problem, but doesn't describe how it came to be a problem. Here's an example: "This means that they should look like very different from one another". Why? The universe was in equilibrium at $t=0$, so what what was the prime mover that caused the inhomogeneity that Inflation fixes? $\endgroup$
    – user32023
    Jun 21, 2019 at 17:02

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