# Why does the homogeneity of the universe require inflation?

They say inflation must have occured because the universe is very homogeneous. Otherwise, how could one part of the universe reach the same temperature as another when the distance between the parts is more than light could have traveled in the given time?

Why can't this problem be solved without inflation? If each part started with the same temperature to begin with, then they can have the same temperature irrespective of the distance between them. Am I missing something here?

The short answer is that physicists/astronomers want to avoid fine tuning wherever possible. Creating a universe where the temperature everywhere was essentially the same requires exceptional fine tuning. 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. If this was all that inflation solved, it maybe would not be considered as likely to have occurred as it is. It also solves another fine tuning problem though, in that the universe is very nearly flat (if not exactly so), and inflation naturally would produce such a universe as well.

Edit: I should also mention that inflation naturally explains the absence of observations of magnetic monopoles as well. This problem was actually the primary motivation of Alan Guth, who first developed the idea of inflation. This issue is not so much one of fine tuning though, unlike the flatness problem and the homogeneity problem.

• Pedantically, inflation only explains the rarity of magnetic monopoles, not their absence. (That is, "Magnetic monopoles are so rare we have never seen one," vs. "Magnetic monopoles do not exist. Anywhere.") Although never seen one might be a bit too strong of a statement. Tantalizing clues have been spotted from time to time, but have never been reproducible. – Andrew Mar 21 '12 at 12:12
• Thank you for the suggestion. I did not notice the ambiguity in what I wrote. It is certainly worth clarifying, as the whole point of inflation was to explain why magnetic monopoles were not seen, when they are considered so likely to exist. – jdmcbr Mar 21 '12 at 14:32
• I must say I completely disagree with this notion that a random temperature distribution is more "natural" than a uniform one! What nonsense is this?! It is far, far easier to comprehend of a universe that started off the same everywhere. Otherwise, you need to explain why it is different in different places! homogeneity is always easier to explain surely? – JeneralJames Sep 27 '17 at 12:58
• I want to amplify the issue that @JeneralJames raised. What is your prime mover? At $t=0$, you had thermal equilibrium. All locations in the universe were in causal connection with every other location. At $t=\delta$, what physics caused the temperature of one section of space to be different from another section? – Quarkly Jun 26 '19 at 12:29

What Big Banged To Produce The Universe

A commonsensible conjecture is that Universe Contraction is initiated following the Big-Bang event, as released moving gravitons (energy) start reconverting to mass (gravity) and eventually returning to black holes, steadily leading to the re-formation of The Universe Singularity, simultaneously with the inflation and expansion, i.e. that universal expansion and contraction are going on simultaneously.

Conjectured implications are that the Universe is a product of A Single Universal Black Hole with an extremely brief singularity of ALL the gravitons of the universe, which is feasible and possible and mandated because gravitation is a very weak force due to the small size of the gravitons, the primal mass-energy particles of the universe.

This implies also that when all the mass of the presently expanding universe is consumed by the present black holes, expansion will cease and be replaced with empansion back to THE Single Universal Black Hole.