Most of us have heard that the universe is expanding, and what does this mean exactly? In 1929 Edwin Hubble measured the distances to galaxies far from the Milky Way and found every single one of them to be moving away from us.
The important point is that all distant galaxies were moving away from us, with no exceptions. We now know the distances to millions of galaxies, and again all the galaxies well outside of Milky Way’s neighborhood show this behavior. Further, in every case the more distant the galaxy is, the faster is its recessional velocity. Note, the term ‘far away’ is selected carefully as galaxies in the Milky Way vicinity are gravitationally-bound to one other and do not participate in this cosmological expansion. Thus for now we will exclude these neighbors from this particular story. So, when we say the distant galaxies are all moving away from us, what is happening physically – are the galaxies running away from Earth? We think not, but rather that that the space between the galaxies is growing with time. Here the analogy of a party balloon becomes relevant. If you draw dots on a balloon and then start inflating the balloon, you will see the spaces between the dots growing with time. It is useful in this exercise to choose one dot and label it ‘MW’ for Milky Way. It gets even more interesting if you now deflate the balloon, label a different dot as MW, and repeat the procedure. What do you notice? In both cases all the objects appear to be moving away from the ‘MW’ dot. Likewise, we expect that if we were magically to be ‘beamed’ to a distant galaxy, we would still see that all distant galaxies are receding from our new far away home. In fact, wherever we would go in the universe, we would observe this cosmological expansion all around us. In this view there is the interesting implication that the universe has no center, and no edges.