Why is the Temperature of the Universe the Same Everywhere? | The Horizon Problem

[@jmcgarryify]

Enjoyed your 3 videos on the flatness, monopole and horizon problem.

[@ChrisPattisonCosmo]

Thanks very much!

[@mat_nova]

really cool video chris

[@ChrisPattisonCosmo]

Thanks a lot!

[@asmath1231]

Great explanation

[@ChrisPattisonCosmo]

Thanks! Glad it was clear :)

[@StevenLyall]

Great, and I completely accept it. But - every time I read about the horizon problem, I get stuck on the notion that it all started at a billionth the size of proton, so it would be uniform from the start, regardless of the rate of expansion. I know I just need help to understand it.

[@ChrisPattisonCosmo]

Hi Steven, that's a great question, and the answer might not be that satisfying to you. You're right that if the universe starts off completely uniform, then the rate of expansion doesn't matter and it would stay uniform. The issue is that there is no reason (that we know of) to assume that the universe started off uniform. Even when it was subatomic in size, there is no guarentee that the density was uniform, and there could have been fluctuations, even on incredibly tiny scales. If this happened, then any over-densities would grow over time, unless they get washed out by some incredibly fast period of inflation. Even inflation doesn't actually get rid of the variations, it just hides them on scales much larger that the observable universe, but on the scales we can see, it looks uniform to us. If we could see scales much larger than our observable universe, we would likely see variations. Of course, if the universe did start uniform, then there is no horizon problem anyway, but this is called "fine-tuning" as we have to make an assumption that doesn't seem to be justified, and we try to avoid doing that.

[@Tom_Quixote]

@@ChrisPattisonCosmo I also came to this video looking for an answer to this question. It seems to me that since we don't understand the process that generated the universe, we don't know if the initial condition would have been symmetrical or not. And if all this energy appeared from one single point, I think the most obvious assumption is that it must have appeared in a symmetrical fashion like a ripple from a stone thrown into a pond. So I still don't understand why there even is a "horizon problem".

[@ChrisPattisonCosmo]

Hi Tom, you're right that we don't know how the universe came into being, but that's why we can't assume everything was uniform to begin with. If the universe is infinite as we think it might be, then it was also infinitely large when it started existing, and this means the big bang doesn't correspond to a single point in space, and so it's not obvious that anything had to start off uniform. There might not be a horizon problem if for some reason everything was uniform to start with, but the fact we don't know whether that's the case or not leads us to propose inflation to try to give a physical explanation to the observations we make now.

[@Salsadans123]

@@ChrisPattisonCosmo I understood somewhere that the universe was the size of a bouncing ball. What does this mean? If the universe has been like a dimensionless point, isn't that the actual most compact size? What would have been the consequences if the universe were not flat? Would it have collapsed and could not have existed? isn't it true that the universe has been splattered in and out until a uniform division came after an almost infinite move in and apart?

[@Zonydeep]

Obviously not uniform. Relatively hot galaxies in some areas, empty. Space others.

[@Salsadans123]

Confusing. So inflation is actually a derivative of three problems that have no explanation otherwise. Inflation is still working and is faster than the speed of light. Can inflation not be tested by observations?

[@ChrisPattisonCosmo]

You're exactly right, inflation was proposed to solve those three problems, which otherwise require "fine-tuning" of the inital conditions of the universe to explain. In our observable universe, inflation has ended, and we think the current accelerated expansion is due to dark energy. Inflation predicted the spectrum of fluctuations we see on the CMB, which is a good check that it works, but a "smoking gun" observation to confirm inflation would be detecting gravitational waves from inflation. This should be possible in the next decade or so, but we haven't seen them yet!

[@Salsadans123]

@@ChrisPattisonCosmo How nice of you to respond! If I looked at the "bell" of the universe, you see a great expansion in a very short time and then at present a steady and faster removal in the future. I didn't know that the distinction between inflation and expansion could be made that way. Inflation is therefore triggered in a different way than the expansion that is supplied with energy by the "dark" matter. Does that have to do with the properties of the elementary forces of particles or does the energy for inflation came from somewhere else? It seems to me that the gravitational waves of the Big Bang are much stronger than those of two colliding black holes. Why is that more difficult to detect?

[@ChrisPattisonCosmo]

The energy for inflation comes from somewhere else. Our best theories say that in the early universe there was a field that drove inflation, called the "inflaton field". At some point, inflation ended and this field decayed into the particles of the standard model of particle physics through a process called "reheating". The grav waves from the big bang would have been very violent, but they were so long ago (which is the same as very far away from us) that they have lost energy and are now very weak. They are hard to detect because they are so weak now, and also because stronger gravitational waves now "wash them out" and they are very hard to find in the data.

[@Salsadans123]

@@ChrisPattisonCosmo very interesting information, I’ve been searching for more background a lot. I will keep doing so. Again thanks for responding!

[@ChrisPattisonCosmo]

No problem at all, thanks for the great questions!

[@dantodd2196]

Ecree!!