All of us have seen some news or a video talking about the expanding universe and how it came to be that way. But first, we should ask, what do we actually mean by expansion? Imagine u have a 1-D grid like the one in the figure. Expansion could mean 2 things: (1) the things located at x=1 and x=2 move away from each other. (2) The grid size “a” changes, so that they remain at x=1 and x=2 but the distance between them still changes. In cosmology, expansion is basically the second scenario. Of course, this doesn’t mean that galaxies or stars dont move at all ; they do. In fact, at ‘smaller scales” , we don’t really feel the expansion of the universe because at those scales the relative motion between objects is much more significant. But, if we go to scales of the order of say 10-100 million light years, the expansion is significant.
How do we measure expansion? It involves measuring the brightness of standard objects, mostly type 1- A supernovae. These explosions of stars are very bright, sometimes they can outshine their host galaxy for a brief time! Any two supernovae of this kind basically look the same and so if we measure a change in their frequency/wavelength , it must be due to the expansion of the universe.
So, what causes this expansion? In cosmology, we have the friedmann equation which basically says something like:
“Some quantity associated with the rate of expansion” is proportional to “Some quantity associated with the energy density “ .
Without going into the specific details of what these quantities are, we can still note a few things. The most important thing is that we take the energy density, not the mass density. This means that light, which is massless, still contributes to the equation. We can thus have light, ordinary matter as well as stuff like dark matter and dark energy in the equation.
Light however, “dilutes” faster than matter. For eg. , if u have 2 units of energy of matter (normal matter or dark matter) in unit cube, the density is 2 units m^-3, which reduces by 1/8th if we increase the cube side length by 2. However, for light, the density reduces by 1/16th !. When the universe was young (about 1 lakh years or so old ) , the density of light/radiation was larger than matter and the expansion was driven by radiation. However, after a point in time called the matter –radiation equality, matter density overtook it and the expansion was matter driven. However, dark energy has a density which doesn’t dilute at all, and this meant that the expansion has been dark energy drive for a some time now.
How the universe expands is basically the key to its future. If the universe continues to expand at its present rate, eventually everything would move way from our field of view and all we would be able to see would be stars in our own galaxy! This means that if we forget to store our data for future generations, they’ll not know if there were other galaxies out there!