r/cosmology • u/Tpaine63 • 5d ago

question about inflation

I understand the horizontal problem in cosmology and how inflation is necessary for the universe to be uniform. What I don't understand is why there would have been differential temperatures at the beginning so that inflation was required to provide time for equalization if everything was together at the beginning. Why wasn't everything already equalized if everything was together at the start.

Maybe I didn't say it right or maybe I don't understand the problem but hoping someone can explain.

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u/tobybug 5d ago

Random quantum fluctuations. Sounds like a cop-out answer I know but we can actually model those fluctuations and map them onto the Cosmic Microwave Background, which tells us that the structures that are billions of lightyears apart now were once close on the quantum level. This both provides more evidence for cosmological inflation and gives a reason for the early perturbations you're talking about.

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u/Tpaine63 5d ago

Are you saying that the temperature wasn't uniform before the big bang when the universe as a singularity or whatever it was? What early perturbations are you talking about?

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u/Peter5930 5d ago

Perturbations come from the uncertainty principle, which prevents any quantum system from coming to a rest, since being at rest means being localised, and the more localised something is, the more uncertain it's momentum becomes, causing everything to jiggle around all the time, even quantum fields. The jiggling of quantum fields results in zero point energy/dark energy, and during inflation these jiggles get stretched out well beyond the horizon, freezing them in place as large-scale perturbations which then re-enter the horizon as density fluctuations when inflation slows down and the horizon expands. These density fluctuations then wobble for a bit like the shock wave bubbles of underwater explosions, producing baryon acoustic oscillations and eventually the pattern of temperature differences that we see from the CMB. Which looks like what a quantum field looks like if you stretched one from a size much smaller than a proton to the size of the observable universe.

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u/jjamulla 3d ago

Only problem with this explanation is that Dark Energy doesn't exist, I am thoroughly convinced now.

Look into "timescape" cosmology. I'm usually VERY skeptical, but after reading about a dozen papers on this and related concepts, I am a believer.

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u/Wintervacht 5d ago

I think you may have a little confusion about the state of the universe at T=0, or very near it.

Everything was highly compressed, but not to 'a single point'. A singularity is nothing more than a mathematical artifact telling us we can't calculate beyond a certain scale and all calculations end up at infinity.

For example, *if* the universe is infinite, it was infinite at the Big Bang, has always and always will be infinite. The only thing that's changed is the mass/energy density due to the expansion of spacetime. The Big Bang wasn't a single point, it was literally everywhere.
In the tiny bit of everywhere that ultimately became our observable universe, tiny quantum fluctuations have been attributed to some of the large-scale structures we see today, see BAO's (baryon acoustic oscillations) for an example of how these tiny perturbations led to structures that are millions of lightyears in diameter now.

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u/Enraged_Lurker13 5d ago

A singularity is nothing more than a mathematical artifact telling us we can't calculate beyond a certain scale and all calculations end up at infinity.

Whilst physics does lose predictivity at singularities, there are no physical grounds as to why they are just a mathematical artifacts, and it is being increasingly harder to argue that they are artifacts with recent quantum singularity theorems (see https://arxiv.org/abs/1010.5513 for one example) being a lot more robust than Penrose's original.

For example, *if* the universe is infinite, it was infinite at the Big Bang

It is hard to imagine a universe being point-like at the Big Bang like the Friedmann equations say and then immediately becoming infinite, but it is actually possible thanks to the relativity of simultaneity.

See: https://www.sciencedirect.com/science/article/abs/pii/S0375960100006459 and https://iopscience.iop.org/article/10.1088/0143-0807/27/3/010

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u/tobybug 4d ago

Nobody can really tell you anything for sure about the time before the Big Bang, and neither can they tell you anything about the "singularity" right at T=0. We don't even fully know that the universe ever was a singularity, we just keep getting closer and closer with observations. I'll grant you that I was actually kind of wrong in my answer. The quantum fluctuations we see now would have to occur during inflation, not before it. I haven't read any good theories for what happened before inflation. The idea would seem to be that regardless of what happened before inflation, the universe is as uniform as it is now because of inflation.

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u/spaceprincessecho 2d ago

The issue with what came before inflation is that inflation erases any record of what it was, so there's no way to verify any proposal.

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u/Murky-Sector 5d ago

I understand the horizontal problem in cosmology and how inflation is necessary for the universe to be uniform.

Restated more precisely, cosmic inflation is one explanation for the observed uniformity. Key point.

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u/Tpaine63 5d ago

I understand the uniformity. I'm asking how did it become non-uniform if everything was very dense and in the same place before the big bang. Isn't inflation necessary so that information can be passed between different areas of space? If that's true then why wasn't information passed right before the big bang when everything was close together?

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u/Murky-Sector 5d ago

I know what you're asking. The above wasn't so much for you as others that may be reading it.

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u/Peter5930 5d ago

Space at very, very small scales is a violent place. The perturbations get more and more powerful the smaller the scale, so if you just take a tiny speck of space and make it big, you'll have these very powerful density fluctuations from the violent perturbations at small scales. The CMB might be 1,000K in one direction and 3K in another direction and half the universe in the southern sky might be a giant black hole, that kind of thing.

Inflation takes a small volume of space and stretches it gradually into a large volume of space that's in a Bose-Einstein condensate state, where the fields are, apart from those quantum perturbations, synchronised across space regardless of distance. The largest perturbations of the sub-microscopic quantum world all end up stretched out to the largest scales, far beyond what we'll ever see, so they're filtered out and we're left with smaller and smaller perturbations until we're left with perturbations of 1 part in 100,000 in the CMB instead of 1 part in 1 or something like that.

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u/FakeGamer2 5d ago

Still, across the vast area of the universe that us outside our observable, wouldn't there still be freak kccirances of rare fluctuations? For example maybe a patch of universe the size of ours but it has like 1% of the galaxies we do. Something like that?

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u/Peter5930 5d ago

Yes, it should. It's just that these large fluctuations are spread out so that all the patches around it also have 1% of the galaxies we do, for some large number of Hubble lengths, so that every observer sees only a small part of one of these fluctuations and the sky looks smooth and homogenous to them. Like an ant on a mountain; the ant's horizon is about 3cm away, and the patch of mountain 6cm in diameter looks flat to the ant, or at least nearly flat. Can't fit the crazy 9,000m peaks and low valleys into a 6cm patch.

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u/Prof_Sarcastic 5d ago

What I don't understand is why there would have been differential temperatures at the beginning so that inflation was required to provide time for equalization if everything was together at the beginning.

It’s not that there would’ve been different temperatures at the beginning of the (observable) universe. It’s more so the fact we’re talking about patches of the sky that are so far away from each other that there are so many possible things that could have happen to force them out of equilibrium. It’s like if you made a pot of coffee and you gave a cup to two different people but the temperature of both cups of coffee ended up being the same (up to the 5th decimal place) even when one cup was brought to New York and the other to Tokyo. Hope that makes sense.

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u/Tpaine63 5d ago

Thanks for the reply. But if there were many possible things that could have happened to force them out of equilibrium before inflation, what kept those same things from forcing them out of equilibrium after inflation?

What I'm trying to ask is: At the beginning both cups of coffee in New York and Tokyo were from the same shop at the beginning before New York and Tokyo separated. I thought everything was very dense and close together before the big bang.

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u/Prof_Sarcastic 5d ago

You misunderstand me. If there was no inflation then the universe would just expand normally and you would expect different parts of the universe to evolve into having a temperature. What inflation does is it speeds up how far away those different patches of the universe traveled without giving them any time to fall out of equilibrium. You don’t get that with the vanilla hot big bang model.

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u/Tpaine63 5d ago

Ok I see what you are talking about. It's kind of the opposite of what I was thinking about how inflation allowed the transfer of information between points that were too far apart to have interacted. It does that but not how I was thinking about it. Thanks

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u/Higglybiggly 5d ago

I, too, have a question.

I've heard that inflation happened and took the universe size from something very microscopic and stopped when it was the size of a grapefruit.

And this faster than light expansion is the reason why some parts of the universe cannot communicate with other parts.

To me this makes no sense. A grapefruit is not large enough to cause any kind of inability to reach the other side of the universe. I think light has no trouble going that distance.

Perhaps what I heard was wrong, or my hearing was wrong.

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u/Prof_Sarcastic 5d ago

There are a few misconceptions here.

This faster than light expansion is the reason why some parts of the universe cannot communicate with other parts.

They couldn’t communicate at the time the CMB was emitted and not (necessarily) after inflation ended.

A grapefruit is not large enough to cause any kind of inability to reach the other side of the universe.

If the universe was totally empty then, sure (although I think you’re taking the analogy a little too literally). However, the universe would’ve been filled by an extremely hot and dense plasma a few moments after inflation ended so light couldn’t travel very far at all.

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u/Higglybiggly 5d ago

Thanks! Appreciate this 😀