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From Wikipedia:

The horizon problem is the problem of determining why the Universe appears statistically homogeneous and isotropic in accordance with the cosmological principle. For example, molecules in a canister of gas are distributed homogeneously and isotropically because they are in thermal equilibrium: gas throughout the canister has had enough time to interact to dissipate inhomogeneities and anisotropies. The situation is quite different in the big bang model without inflation, because gravitational expansion does not give the early universe enough time to equilibrate.

https://en.wikipedia.org/wiki/Inflation_(cosmology)

The argument is that the early universe could not have had time to evolve to thermal equilibrium without an inflationary period. But why we are assuming that the early universe needed to evolve into this state in the first place? Why couldn't it have formed already in thermal equilibrium? The quoted argument seems to make unnecessary assumptions. To suggest that the universe needed time to equilibrate is to suggest that it was once - sometime prior to recombination - not in a state of thermal equilibrium, correct? But why would we make such an assumption?

I have had a look at this Why does the homogeneity of the universe require inflation? question and the accepted answer claims:

Creating a universe where the temperatures were random in different parts of space and had an opportunity to come in thermal equilibrium before going out of causal contact (as a result of inflation) is more natural

But I don't agree that this is more 'natural'. To me this logic is totally backwards. A universe with a random temperature distribution is much harder to explain than a uniform one. If you're going to suggest that the early universe contained random energy fluctuations, surely you have to explain why that would be the case? If the entire universe somehow came into being in one great Big Bang, then surely it makes significantly more sense that it would be the same temperature and composition at every point in space? Only developing anisotropies afterwards (probably due to quantum fluctuations).

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  • $\begingroup$ There is no "horizon problem", because the entropy is minimal at the Big Bang when the universe is completely uniform and absolutely cold. However, this forum is about "conventional science" and someone has decided without a good explanation that the "horizon problem" is "conventional". $\endgroup$
    – safesphere
    Commented Oct 3, 2017 at 6:27
  • $\begingroup$ Can you elaborate? It sounds like you're sort of saying the same thing as I am? That the horizon problem isn't really a problem if you assume that the universe started in a uniform state? But the 'mainstream' seems to think this interpretation is a "fine-tuning" hypothesis (which I really don't think it is but all right...) and so they reject it out of hand. However, to my mind it seems much more far-fetched to suggest that the universe needed to exchange information and come into thermal equilibrium before expanding. Why not just form with a uniform energy density to begin with? $\endgroup$
    – JeneralJames
    Commented Oct 4, 2017 at 7:06
  • $\begingroup$ You said it. This "problem" seems to be based on unjustified assumptions. $\endgroup$
    – safesphere
    Commented Oct 4, 2017 at 7:28
  • $\begingroup$ I think the "fine-tuning" point often referenced is misrepresented by claiming that random initial data is preferred or more "natural". In my mind, the better statement is that your hypothesis of an initially uniform universe, while not impossible, invokes a specific set of initial conditions, but it's much more satisfying and robust if the model can begin with a very wide array of plausible initial conditions and still result in what we observe. We have very little information about the universe's initial conditions, so we'd like to assume as little about them as possible. $\endgroup$
    – jawheele
    Commented Apr 8, 2019 at 16:59

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The argument is that the early universe could not have had time to evolve to thermal equilibrium without an inflationary period.

This is a misunderstanding of the argument. When one looks at the most perfect black body radiation curve we have measured , the cosmic microwave background curve

cmb

it is evident that we see a thermal equilibrium, but the puzzle is not that time was needed, it was that the different regions in space could not communicate between themselves due to the light cone problem of special and general relativity. There could be no interactions between the masses in the total region observed of the CMB because no energy and momentum could be exchanged between them due to the velocity of light being c, and the light cone of each region:

Given an event E, the light cone classifies all events in spacetime into 5 distinct categories:

Events on the future light cone of E.

Events on the past light cone of E.

Events inside the future light cone of E are those affected by a material >particle emitted at E.

Events inside the past light cone of E are those that can emit a material particle and affect what is happening at E.

All other events are in the (absolute) elsewhere of E and are those that cannot affect or be affected by E.

It is the elsewhere part of the universe part of it seen in the CMB plot

cmbnight

that is the problem.

Now a model that would posit a "uniform big bang" as a hypothesis, is less attractive than the inflation model, which assumes quantization of gravity and the inflaton field doing the homogenisation .

It explains the origin of the large-scale structure of the cosmos. Quantum fluctuations in the microscopic inflationary region, magnified to cosmic size, become the seeds for the growth of structure in the Universe (see galaxy formation and evolution and structure formation). Many physicists also believe that inflation explains why the Universe appears to be the same in all directions (isotropic), why the cosmic microwave background radiation is distributed evenly, why the Universe is flat, and why no magnetic monopoles have been observed.

So it is a hypothesis that satisfies most physicists, but is still open to research , particularly as quantization of gravity is still an effective theory and no definite model has been established.

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  • $\begingroup$ Okay, good answer, but I understand light cones and the reason why this theory is invoked, my dispute is with the necessity of it. If the universe is supposed to have started in this incredibly dense state, low entropy state, I would have thought it much easier to assume that it began life already in thermal equilibrium. That is to say, the different regions of space did not need to interact with each other to come into this state, they were initialised in a common state for reasons we do not yet understand. The inflationary model assumes that the universe needed to exchange (cont...) $\endgroup$
    – JeneralJames
    Commented Sep 29, 2017 at 8:49
  • $\begingroup$ information to evolve into thermal equilibrium but I see no good reason to make this assumption when dealing with the beginning of the universe itself! To posit that these different regions must have needed to communicate with eachother to come into equilibrium is to suggest that at some earlier stage, the energy distribution of the universe was not uniform. I see no reason to suggest this. A uniform Big Bang - were the universe literally began it's existence with the same temperature at all points in space - requires fewer assumptions to my mind. $\endgroup$
    – JeneralJames
    Commented Sep 29, 2017 at 8:52
  • $\begingroup$ But perhaps this is a case of me being too much of an idealist? Thinking of the Big Bang as the beginning of existence. Is there a general consensus that the universe may have existed for a long time prior to inflation? Perhaps having existed in a hot, dense state for many billions of years, even an infinite amount of time? I suppose this is getting too speculative to answer so I will accept you answer, thanks. $\endgroup$
    – JeneralJames
    Commented Sep 29, 2017 at 8:56
  • $\begingroup$ Your statement is an alternative hypothesis which does not lead to understand the 10^-5 level inhomogeneity in the CMB data, which are coincident with clusters of galaxies. Also psychologically, Bangs are expected to be highly inhomogeneous from classical bangs. en.wikipedia.org/wiki/Inflation_(cosmology)#/media/… . It is a theory that at the moment fits the data. It could change. en.wikipedia.org/wiki/Inflation_(cosmology) $\endgroup$
    – anna v
    Commented Sep 29, 2017 at 10:16

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