# What is the scientific reasoning behind Inflation after the big bang?

## Main Question or Discussion Point

I understand what inflation is, but what are the reasons behind why scientists are near definitive that it indeed happened? Please go as in depth, complex, or over the top as you need/want.

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Chronos
Gold Member
Inflation was proposed as a solution to the flatness and horizon problem. The flatness problem addresses why the universe is Euclidean [spatially flat] within the limits of our measurement ability. The horizon problem addresses why the CMB temperature is almost perfectly homogenous in every direction. Inflation provides a simple solution to both of these issues. Scientists are suckers for simple explanations.

So why do they take the position of a singularity and its spacetime 'inflating' many times the speed of light, instead of a position of the Big Bang came from a tremendously massive object that had a volume many light years across?

Simon Bridge
Homework Helper
Because that (big bang from a very large structure) would be begging the question of where the volume for such an object came from. Again, it's simpler.

Of course, one may equally ask why not just consider the Universe to have always been much as it is now (steady state)?

Chronos
Gold Member
So why do they take the position of a singularity and its spacetime 'inflating' many times the speed of light, instead of a position of the Big Bang came from a tremendously massive object that had a volume many light years across?
Because it fails to resolve either problem [flatness / horizon]. Try to refrain from proposing a personal theory here.

Because that (big bang from a very large structure) would be begging the question of where the volume for such an object came from. Again, it's simpler.

Of course, one may equally ask why not just consider the Universe to have always been much as it is now (steady state)?
I'm aware of plenty of reasons why we shouldn't assume the universe isnt stationary (but I don't have to tell you that because you already know). What I'm not seeing as clearly though is why its written off having a singularity is simpler on the pretense of its begging the question of where the volume came from initially. Wouldn't asking where the infinitely dense singularity came from be just as much of a legitimate question as to where the matter/energy came from?

Because it fails to resolve either problem [flatness / horizon]. Try to refrain from proposing a personal theory here.
I'm not proposing personal theories. I'm trying to understand the reasoning behind a current one by knowing the fundamentals of why it was proposed. I also read somewhere that inflation explained why we do not see any monopoles in nature. Could someone elaborate on that one?

Simon Bridge
Homework Helper
What I'm not seeing as clearly though is why its written off having a singularity is simpler on the pretense of its begging the question of where the volume came from initially.
(1) That's not a pretense! - it really does beg the question in the same way as the statement "God created the Universe" begs the question "who created God?"
(2) The macroscopic initial state requires more to be accounted for than the singularity one ... that's why the "question-begging" is important. For instance:
Wouldn't asking where the infinitely dense singularity came from be just as much of a legitimate question as to where the matter/energy came from?
Yep - perfectly legit. However, a macroscopic initial state also needs to explain the matter/energy as well as where all the space-time came from. "Where the singularity came from?" is a much easier problem. (note: I didn't mean to say it's simpler because the other begs the question, I meant it's simpler, in addition to the other begging the question. But since you got me thinking about it...)

There are a lot of cosmological models - we pick the one that makes life easiest for cosmologists. If you really want to understand why a particular model is favored you need to have studied general relativity, and topology, and such things. Of course, if you have, then the discussion can get a lot more precise.

You do realize that not all inflationary (big bang) models have a singularity as an initial condition - though they do have to account for why it looks like that from here.

In the end this is what is being said: it looks an awful lot like the Universes initial condition was some kind of a singularity ... the obvious model is one that says "well is was a singularity" and see what you have to do to it to make it work for what we see ... maybe it's wrong? We'll know when the tweaks get too much and one of the other models manages to work out simpler.

Right now there is no way to give you a clear picture because I'm stuck with plain English. If you want to know why one model is simpler than another: you have to do the math.

BTW:
I kinda assumed that you meant that the state accounted for in the BB model by the rapid expansion phase was the macroscopic start point. Reasoning something like: if the really small scale universe cannot create the flatness/horizon thing, then perhaps that's where the Universe began? You don't have a "before" that stage to explain?

... otherwise you get Chronos' point that the state also fails to account for the flatness/horizon problem. (It'd have to be bigger than the scale you suggest I suspect...)

This sounds conceptually simpler and is the id of reasoning very useful for solving conjuring tricks. Unfortunately it makes life quite hard for the theorist.

I also assumed that you understood that the big-bang is not an explosion of anything into space.

So why do they take the position of a singularity and its spacetime 'inflating' many times the speed of light, instead of a position of the Big Bang came from a tremendously massive object that had a volume many light years across?
This makes no sense. We observe that the universe is homogeneous and isotopic, both today and through the CMB. Along with this is Hubble's Law, which demonstrates that the universe expands from every point in intergalactic space.

So, the big bang was the hot, dense, state that the early universe was in. It wasn't an explosion. Hence, it 'coming from a tremendously massive object that had a volume many light years across' simply doesn't make sense.

Also, your question about the monopoles - many GUTs predicted a large abundance of magnetic monopoles, of extreme mass. However, we don't observe any. Inflation explains this because the monopoles would have been spread out over the enormous expansion during inflation.

We have good reason to believe monopoles exist. Dirac showed that allowing the existence of one monopole forced electric charges to come in integer multiples of one charge, which we see (the electron charge).

Simon Bridge
Homework Helper
So, the big bang was the hot, dense, state that the early universe was in. It wasn't an explosion. Hence, it 'coming from a tremendously massive object that had a volume many light years across' simply doesn't make sense.
That was my initial reaction too - but I realized that the same statement could be read in terms of the initial state of the Universe having a larger volume that supposed in the regular BB models. So the "object" includes the space-time that contains the energy in this early stage. Which is to say, why not just start the model off at the point where we suppose the initial rapid expansion ended?

Mind you - I agree that we need for this to be clear now. Does OP imagine the big bang to propose an explosion into space?

Mind you - I agree that we need for this to be clear now. Does OP imagine the big bang to propose an explosion into space?
No, im in agreeance that the big bang wasn't an explosion per se. In my mind it is logical to think of the big bang as described by traditional science, except replace a singularity with a massive object and then inflation (a phenomenon that sticks out to me) would not need to be assumed to have existed. I know science has its foundations for its mainstream theories like inflation, and my hope is to understand them to the same point as others here.

phinds
Gold Member
2019 Award
... except replace a singularity with a massive object ...
It has been explained to you why this is not possible, based on our current observations of the universe, as it REQUIRES a point-origin for the big bang, which then contradicts the observations of isotropy and homogeneity.

If you do not understand that, I suggest you read up on those two terms. They are observational fact and if you get to the point of realizing that, you will drop this nonsense of the "massive object".

So why do they take the position of a singularity and its spacetime 'inflating' many times the speed of light, instead of a position of the Big Bang came from a tremendously massive object that had a volume many light years across?
1) Because you don't solve the horizon problem. Suppose you have an object that is five light years across. Then it will take information five years to get from point A to B. You would expect that if the gas is denser or less dense at point A, that point B won't know for five years. You can then take the time lag and calculate the likely difference in density between point A and B, and that's a lot larger than what we see.

The inflation picture was that during the early universe A and B were very close to each other. Because they were close, the differences in density smoothed out, and *then* the universe expanded a lot. This means that A and B are going to be very smooth, but because of "random atomic fluctuations" they aren't going to be totally smooth which gets you to......

2) Because you can make observable predictions from the first and not from the second.

If you assume that the universe "inflated" and then random changes in density that come from quantum mechanics will get magnified, and you end up with what's called a "scale free power spectrum."

At the atomic level, there are going to be random density changes. The universe inflates and then those atomic level random density changes become density changes at much large scales. You then have "new" random density changes at the atomic level.

I wish I had a blackboard.

Imagine a balloon. Draw some really tiny squiggles. Now blow up the balloon a little. Those tiny squiggles become bigger squiggles. Draw some new tiny squiggles. Now blow up the balloon a little, you now have three sets of squiggles. Keep blowing up the balloon and drawing tiny squiggles.

You end up with a fractal. We see part of that fractal.

What you end up with is a calculation for density fluctuations in the universe, and that's what we see.

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I wish I had a blackboard.

What you end up with is a calculation for density fluctuations in the universe, and that's what we see.
You did a pretty good job without a blackboard. Thanks.

So inflation gets you
• Solution of the flatness problem
• Solution of the horizon problem
• Primordial fluctuations
In fact, the observed primordial fluctuations have the sort of spectrum that one would expect of inflation.