Welcome to the Cosmology FAQ
The purpose of this entry is to shine light on the questions I was often asked about, and that made me look like a jackass because I couldn’t answer them. I thought it would be nice to share No seriously. When my passion for astronomy started, I learned a lot of things I didn’t know at an extraordinarily rapid rate. But these questions are the questions you always think about, the ones that make you dream, and the ones that no one talks about! Even if you quickly jot a keyword in google and try looking for an answer, you’ll see that it’s harder than you think. Ok here we go.

Is the universe finite or infinite?

That depends whether the universe is flat or not. If the universe is round, or curved, it means that the universe is a sphere, and so that the universe is finite. You could be either inside the sphere, or outside it. In this case, you’ve got a finite universe with no borders. If the universe is flat, we would have an infinite universe.
It also depends how dense the universe is. There are three scenarios for the destiny of the universe, defined by the Friedman Equations:

First, the Big Crunch. If the universe is dense enough to counterbalance the expansion, eventually gravity will be stronger than expansion, and matter will start falling back on itself: It’s the opposite of the Big Bang.

Second, the not-named critical density end. If the density of the universe is equivalent to the critical density, the universe will gradually slow down, untill it starts crunching back on itself in an infinite amount of time (basically, it continues its expansion, and stops at time +∞). The critical density corresponds to about 10 protons per cubic meter. The density of the universe today is of about 3 protons per cubic meter.

Third, the Big Rip. The universe continues expanding forever, until the particles that make it up cannot hold it as a whole: the universe gets teared to pieces.

According to the latest observations by the leading satellites, the universe seems to be flat, and we seem to be in the Big Rip Scenario. Thus, evidence tends to prove that the universe is infintely large (although this might not be true, as science always puts its results in question in time).

Was the universe originally a point?

It’s important to set a difference between the universe we can observe, and the universe as a whole: that’s where the cosmological horizon comes joins the game. The big bang, having taken place, according to the latest estimations, 13.7 billion years ago, we cannot see any light coming from a source at a distance that is greater than 13.7 billion light-years from us. So as we go back in time, that horizon gets smaller. According to the theory, the universe was originally confined in a tiny tiny space. And I guess there are more galaxies, stars, nebula, and quasars lying behind that horizon. But no one knows for sure.

When I was in my training course at the Space and Astrophysics Laboratory in Marseille, France, I spoke with Cosmologist José Donas, head of the GALEX project (a project to learn more about stellar evolution using the ultraviolet spectrum), we had an interesting debate about this. He told me that if the universe is infinitely large, there is a good chance that the amount of matter is infinite, and that thus the amount of energy is infinite. If the portion of the universe we can see is a point, and supposing that the amount of matter is infinite, the density of the universe would have been infinite, without meaning that the universe the size of a point. So according to him, the universe was not a point, but still infinite in size, and infinite in density.
Originally, i used to think that the universe was a point, and someone asked me where that point was when the big bang occured. Wouldn’t it be possible to know where that point was in space. Now I realize that the correct answer would have been that you position something relative to something else. How do you position something in an infinitely large container? Anyway, you make up your own opinion…

If the universe is expanding, where is all the matter going?

If the universe has a finite size, its volume is constantly growing larger, and so the density goes down with time.
If the universe is infinite in size, the amount of matter is infinite, and in an infinite universe, you’ve got plenty of space!

I heard that the universe is in expansion. Are stars and galaxies themselves in expansion?

After a short expansion period after the big bang occured, the universe stabilized, which allowed the formation of stars, galaxies, atoms, etc… The internal forces of stars and galaxies (mostly gravity, but also strong interaction, within atoms, and electromagnetism), annihilate completely the internal expansion. If these forces did not exist, the stars would be growing like a pudding in the oven!
Today, these forces are only weak on huge scales, such as between galaxies and clusters of galaxies, which allows expansion to overpower the 4 forces that make up fundamental interaction.

Got any question? ask them here!

Edit: I guess I do have some influence in the astronosphere, because just after posting this, the APOD became a picture illustrating the Big Rip… Jesus ( )