Do things in the universe get farther apart or bigger? (Amateur)

Question

To start off I would like to say I am a complete amateur and have no experience in physics! I am just a sophomore in high school with some questions.

Now that that is out of the way. There are many questions on here concerning the idea of the expansion of the universe but what I would like to clear up is by the universe "expanding" does this imply that the universe's space we can fill is infinite and that galaxies and everything inside it are being pushed away from the "center" at a constant rate to try and "fill up" this ever expanding infinite universal space?

Maybe I am off base with my whole conception of the universe expanding so could someone please explain?

Yes I did but what I am wondering is if by the universe expanding are people imply that things are getting larger in size or that things are moving away from each other to reach the "end of the universe" (I know there is no end but for sake of the question just assume) — zzirrgrizz, Dec 04 '14 at 21:32
I'm still sure we've answered your question somewhere around here :) Perhaps Why does space expansion not expand matter? or Why space expansion affects matter?? — ACuriousMind, Dec 04 '14 at 21:36
Also related: http://physics.stackexchange.com/q/7359/2451 and links therein. — Qmechanic, Dec 04 '14 at 21:49

score 2 · Accepted Answer · answered Dec 04 '14 at 21:37

So, we first have to understand what it actually means to be 'expanding.' The FLRW Metric describes our universe as homogenous (physics works the same at all points), isotropic (looks basically the same everywhere on a large scale), and expanding. In simplest terms, the further back in time we look, the closer things generally seem. Basically, objects that are not gravitationally bound to one another will get further apart as time progresses (as per the metric).

Now, you seem to harbor a very common misconception in that the Universe itself has a 'center.' This is not the case. There is neither a center nor an edge to the Universe so far as we can tell. Expansion was not a single point moving outward but rather a uniform expansion at all points in the Universe simultaneously. The best way I've seen it explained (in an intuitive sense) is as follows:

Picture an infinite sheet of graph paper in the sense that it has no edge. The lines are getting farther apart from one another, but it is still infinite. If we roll back time far enough, there will have been 0 distance between any 2 polar coordinates, and thus our modern physics breaks down. The lines are not getting larger, really, but they are moving apart from one another. At any given reference frame, lines at twice the distance are moving at twice the velocity.

Hopefully this was of some help to you.

Yes this helped quite a bit. Thanks for the graph paper explanation, it put in perspective! — zzirrgrizz, Dec 04 '14 at 22:19

score 1 · Answer 2 · answered Dec 04 '14 at 21:48

The universe was infinite at its creation, and continued to be so. It just got bigger during inflation. There may be a misconception that the universe begun as a "infinitesimal point", but singularity just means that it was non-defined.

During inflation, the space between matter became wider, analogous in 2-dimensions, to the "spots on a balloon's surface" getting further apart as the balloon is inflated. Just like a balloon, as no spot is the centre-most spot, to place in the universe is the centre either.

Unlike a balloon's surface which "stretches" as it inflates, space itself contains no matter to "stretch". It just widens, and would give the illusion of matter getting further apart. This is important, because the illusion is that inflation appears to occur faster than the speed of light. What really happens is that the inflating space gives rise to greater differences, without the light having to travel the extra distance.

Do things in the universe get farther apart or bigger? (Amateur)

2 Answers2