Is the Universe Expanding or Are Galaxies Just Moving Apart?

[gabrielh]

Hi. I have a question about the expansion of the universe. First off, I'm only 16 and I don't have much knowledge of cosmology or astrophysics other than what I've read in a few books and on this site. That said, this question may be laughed at and it may have a basic answer, but I'll ask anyway.

I've always heard that the universe was expanding, but could it be that the universe itself isn't expanding, but the galaxies and such are just moving away from each other? I always seen galaxies moving away from each other not as the universe getting bigger, just what is in it is getting further apart.

I hope this isn't a stupid question. Thanks in advance.

 

[cristo]

Hi gabriel,

Your question is not stupid, and I think the root of the question lies in the definition of the expansion of the universe. This term is thrown around by almost everyone nowadays, especially popular science writers, but it was a term that originated as a "scientific shorthand" used by cosmologists. There are various different equivalent definitions for the expansion of the universe, but the one that I prefer to use is the one that features in your question: that is, the expansion of the universe is defined as the distances between all galaxies increasing. Thus, what you are thinking of is indeed what cosmologists talk about, and it is that which is translated into the "popular" expression "expansion of the universe."

I hope this helps!

 

[CaptainQuasar]

Yes, this is the way I've always understood it, that it's something like the coordinate system of the universe that's expanding. I've also read somewhere that this ought to exert a force on atomic and molecular bonds, but is that something that has actually been demonstrated?

Another thing - it is frequently said that a common misunderstanding is that the Big Bang was like an explosion throwning mass away from a central point, to illustrate that it's really more like what gabrielh describes. Is a corollary to that that near the point of the Big Bang, say when everything was still the density of plasma, the universe may still have extended infinitely in every direction?

Many people I've talked to seem to think that at that point in time the universe was like a relatively small sphere in shape, perhaps due to the “Cosmic Egg” analogy to the Hindu creation myths. (Or maybe they're right and I'm wrong...)

⚛​

 

[gabrielh]

Thank you guys for your help. It makes more sense to me now.

 

[Allday]

Yes Captain Quasar, It is possible (and the most likely possibility because measurements show that the Universe is probably spatially flat) that the Universe was spatially infinite when it was a hot dense uniform plasma. Our observable Universe now was most likely a very small patch of the plasma then, but the whole Universe would still be infinite.