Cosmology Primer: The Expanding Universe

In thinking about the expanding universe, it is tempting to appeal to some sort of simile: distant galaxies are like raisins in a baking loaf of bread, or dots drawn on the surface of a balloon. But the universe is a unique place, and similes tend not to do it justice (or worse, to suggest something misleading). It's actually best just to think about the universe itself, and what it looks like.

So imagine standing outside on a clear night and looking into the sky. Imagine further that you have perfect vision, including not only ordinary light but all other kinds of radiation (radio waves, infrared and ultraviolet light, X-rays and gamma-rays). The first thing you notice are stars; each star is much like our Sun, but further away and correspondingly fainter. But the stars aren't distributed equally throughout the sky; they are arranged into a disk, and our solar system is near one edge of the disk. This disk of stars, orbiting slowly under their mutual gravitational attraction, is our galaxy, the Milky Way. There are almost one trillion stars in the Milky Way; in the night sky it shows up as a faint band stretching from one horizon to the other.

But stars aren't the only thing we see; there are tiny patches of fuzzy light, which stand out in contrast to the pointlike stars. The patches are "nebulae", and were a source of controversy earlier in the twentieth century -- were they clouds of gas and dust within our galaxy, or separate galaxies in their own right? Eventually Edwin Hubble showed that many (although not all) of the nebulae were in fact distant systems of stars, comparable in size to our own Milky Way galaxy. In our observable universe, there are approximately one hundred billion such galaxies. For more discussion, see the page on the luminous universe.

Galaxy map from the Sloan Digital Sky Survey.
(Click image to enlarge.)

Cosmology, as the study of the entire universe, would be a completely intractable subject if it weren't for one crucially simplifying feature: if we look over large enough distances, the distribution of galaxies is basically the same everywhere. In technical terms, we say that the universe is both "homogeneous" (the same at every point) and "isotropic" the same in every direction). Of course these statements are not strictly true; the center of a galaxy has a higher average density than the space in between galaxies. But as we look over larger and larger distances, the deviations from place to place become smaller and smaller; once we are considering distances of hundreds of millions of light years and more, the universe looks extremely uniform.

However, although galaxies are spread evenly throughout space, they are not static as a function of time; Hubble's second great discovery is that the universe is expanding. The concept of an expanding universe can be a tricky one, so it is worth being careful about what we mean. It is best to think of space itself stretching, so that the amount of space between any two distant galaxies is increasing. The observable phenomenon that leads us to this conclusion is the redshift: as light travels from one galaxy to another, its wavelength is stretched as the universe expands, reaching the second galaxy having been shifted to the red (longer wavelengths). We therefore see relatively nearby galaxies slightly redshifted, and very distant galaxies extremely redshifted. The cosmological redshift is similar in result (although different in underlying cause) to the well-known Doppler shift that results when an object is moving away from you. It is therefore convenient to assign a "velocity" to this redshift; Hubble's Law states that this apparent velocity is directly proportional to the distance to the galaxy, with the constant of proportionality being the Hubble constant. (At a deep level, the expansion of space is different than the motion of objects through a static space; however, the differences are negligible when the apparent velocities are much less than the speed of light, so the abuse of language is acceptable.)

Since we see distant galaxies moving away from us, it is tempting to think that we are in the center of something big. But that's an incorrect impression; if we were living on any one of the other galaxies, we would still see all the galaxies moving directly away from us, as a consequence of the general expansion. There is no center to the universe, nor any preferred location; it's basically the same everywhere. Likewise, the universe is not (so far as we know) expanding "into" anything; it's just that the amount of space in our single universe is growing with time.

Einstein's general theory of relativity, which states that spacetime is curved and that curvature is what we perceive as "gravity," provides a dynamical framework for understanding the expansion of the universe. In cosmology, the curvature of spacetime comes from two contributions: the curvature of space by itself, and the expansion rate of the universe. The curvature of space is the same throughout space, and can be positive, negative, or zero; recent observations of the Cosmic Microwave Background indicate that it is close to zero. General relativity then relates the expansion rate and spatial curvature to the energy density of the universe -- the amount of energy in each volume of space. If we know the spatial curvature, and we know the energy density, and we know how the energy density changes as the universe expands, we can reconstruct the entire expansion history of the universe.

In particular, we can extrapolate backwards from our current situation to describe the very early universe. Since it is expanding now, it was smaller in the past; galaxies were closer together, and the universe was both hotter and more dense. Going back sufficiently far, the universe was so hot that galaxies and stars could not exist; further back, individual atoms could not exist; further still, it was too hot for atomic nuclei themselves to exist. If we extend ourselves fearlessly all the way back, the universe was of essentially zero size about 13.7 billion years ago -- the Big Bang. Of course there is a lot we don't know about this period, although there is some that we do; see the pages on the early universe and the really early universe for details.

If we can extrapolate into the past, we can also extrapolate into the future. The problem there, of course, is that we have no observational data to check our speculations. Strictly speaking, therefore, we really don't know what the far future history of the universe will bring. Our best current models, in which the universe is dominated by dark matter and dark energy, predict that it is likely for the universe to continue expanding forever, becoming increasingly cold and dark as time goes by. See the page on the dark universe for more specifics. However, our current ideas are still speculative, so it pays to keep an open mind.

Next: The Evolving Universe