Science a GoGo's Home Page Forums General Discussion Physics Forum The Flatness Problem?

At the time of the BB the Universe was smaller than an atomic nucleus. It is sometimes described as being "infinitely small".

Unless, at t = 10^-43s, the Universe had a diameter greater than 10^-35m, every part should have been in contact.

If this is the case, why is inflation necessary in order to explain the uniformity of the CMB, which, presumably, reflects the uniformity of the temperature of the Universe at the period of last scattering?

I started out to try to figure out how to explain how the Big Bang takes care of the flatness problem, but here is a better one.

Ethan Siegel explains it all

Bill Gill

Thanks Bill.

That's a very good layman-type explanation of inflation. Just the sort of thing we non-scientific folk need.

However, as I see it (and I may be missing something crucial), this makes a basic assumption about the time before the Big Bang, which is justified only on the grounds that it solves a specific problem.

That's fair enough, but he says: "And all of this happened before the big bang ever took place. Was it for the first 10^-35 seconds of the Universe?"

So are we looking at something pre-BB, or the first 10^-35s post-BB?

Also, in a straightforward BB scenario, the Universe is supposed to start "infinitely small". Presumable this is not greater than 10^-35m. If as Siegel seems to suggest, we could be looking at the first 10^-35s, what is the problem? Light is capable of travelling 10^-35m in 10^-35s, with some to spare, so all parts of the nascent Universe could be in contact.

Keep in mind that he is using a definition that equates inflation with the Big Bang. So he doesn't know anything about what was happening before that. But QM says that nothing can be infinitely small, so the universe didn't really come from an infinitely small singularity.

As you say all parts of the universe were indeed in contact prior to the Big Bang. So that prior to the Big Bang everything was homogenous. Then the inflationary period was so short that nothing had time to become non-homogenous. After inflation/Big Bang the universe maintained its homogenous nature. If the universe had expanded at a much slower rate then there would have been time for a lot of clumping to occur and there would be a lot more diversity in the universe.

Also keep in mind that inflation was indeed invented to explain the flatness problem. And so far all observations can be explained through the mechanism of inflation. Nobody has come up with a theory that has any thing like the explanatory power of inflation. So, at least for now, inflation is accepted as the real way it happened. In science of course everything is open to correction. Any day now somebody may come up with another explanation that will be even better. I'm not going to hold my breath waiting for it to happen, but it could.

Bill Gill

Bill, your explanation places a different, and helpful, slant on the flatness problem. In most popular explanations it appears that all that is needed to provide the necessary homogeneity in the in the early Universe to result in the presently observed flatness is that all parts should have been in contact.

What you seem to be saying is that even if the necessary contact/homogeneity had been achieved in the very beginning, it could/would have been lost if the Universe had expanded relatively slowly. That seems to make more sense, but leads to the next question: Would it necessarily have been lost?

I don't know for a fact that it would have been, but think about it for a minute. When everything is pretty much in contact, or at least is close enough together that they would all influence each other everything would pretty much average out. When they slowly expanded to the point that they were not all in close communication then a deviation might occur in one place, but not be communicated to another place. So there could be greater deviation between different parts of the universe. With the quick inflation widely separated areas would still have the same basic structure they had before the inflation, and that structure would be very uniform.

Bill Gill

That makes a lot more sense than the frequently encountered explanation that there was not enough time, in the first instant, for all parts to be in contact.



Could you say a bit more about that, please?

One of the basics of QM is the uncertainty principle. That says you can't know both the momentum and the position of a particle. So we wind up with the idea that anything MUST occupy a certain finite volume. Since we can't locate it in any specific place it has to be no smaller than the volume we can locate it in. If we can't locate it then it can't be infinitely small. The smallest that anything can be is of course the Planck length 1.6 * 10^-35 meters. So nothing can be compressed beyond that point.

Of course this disagrees with General Relativity (GR), which doesn't have that restriction. That is one of the big problems facing physics today. Reconciling the 2 theories, both of which are RIGHT.

Bill Gill

So, if we can locate it then it can be infinitely small? Surely not.



Something that is infinitely small cannot have been compressed to that state; it must always have been infinitely small. Would that make a difference?

Does the concept of "infinitely small" have any meaning?

Would it not be the same as "non-existent"?

Ok, you caught me. I thought I checked that carefully and I still missed that one. I of course meant "If we can locate it"


A lot smarter people than I am have wrestled with the meaning of infinity and come away looking for aspirin, or maybe a stiff drink. I am not even going to try to figure it out.

Bill Gill