Astronomy 162: Professor Barbara Ryden

Tuesday, March 4


``The past is a foreign country; they do things differently there.'' - Lesley Poles Hartley

Key Concepts

(1) The Hubble Law implies that the universe is expanding uniformly.

With this lecture, I am starting our investigation of cosmology. Cosmology is generally defined as the study of the structure and evolution of the universe, or cosmos, regarded as a whole. Thus, cosmologists aren't greatly concerned with tiny objects like galaxies, or even clusters of galaxies.

In the 1920's, Edwin Hubble uncovered the Hubble Law, which states that distant galaxies are receding from us with a speed proportional to their distance. As I mentioned in the lecture for Tuesday, February 25, this observational result implies that the universe is expanding uniformly.

We see distant galaxies all moving away from us. This does not imply that we are at the center of expansion. In uniform expansion, every galaxy sees all the other galaxies moving away. (Consider, as an analogy, an expanding loaf of raisin bread. As the loaf expands, the distance between every pair of raisins increases. Thus, each raisin -- if you can imagine a raisin with eyes -- sees all the other raisins moving away from it.) The expansion of the universe has no center. As far as we can tell, the universe has no edge, either. (This is where the ``raisin bread'' analogy breaks down; there is no crust or boundary to the universe.)

Please note, as a clarifying point, that you are not expanding with the universal expansion. You are strongly bonded together by electromagnetic forces. Our galaxy is not expanding either; it's strongly bonded together by gravitational forces. It's only on large scales -- scales larger than clusters of galaxies -- that the universe is expanding.

(2) The universe began in a very hot, dense state, then cooled as it expanded.

As distances increase, the average density of the universe decreases. (The same amount of stuff is being spread over a larger and larger volume.) Since matter cools down as it expands (think of the outer layers of an expanding red giant, for instance), the universe is going from a hot dense state to a cool low-density state.

What do I mean when I talk about a ``cooling universe''? Consider, as an example, blackbody radiation from a star in a distant galaxy. The star's temperature is identical to that of the Sun: T=5800 Kelvin. Thus, the wavelength of maximum emission is 500 nanometers (well within the range of visible light). As the universe expands, however, the wavelength of light traveling through the universe expands as well.

For instance, if distances double due to the universal expansion, the light from the distant star will be doubled in wavelength. The wavelength of maximum emission will then be 1000 nanometers (in the infrared). The starlight will be cooled, therefore, to a temperature of 2900 Kelvin, half its initial temperature of 5800 Kelvin. The redshift of distant objects, due to the universal expansion, is called the cosmological redshift.

The observed cosmological redshift of a galaxy tells us how much the universe has expanded since the light we observe was emitted. Consider, for instance, a galaxy with a redshift z=1. The wavelength of the galaxy's light has doubled since it was emitted. Thus, a galaxy with a cosmological redshift z=1 is seen as it was when distances between galaxies were half their present value (and hence were more likely to interact and merge with each other).

(3) The moment when the expansion began (also known as the Big Bang) was roughly 14 billion years ago.

How long did it take the universe to reach its present density? For an approximate calculation, let's look at a pair of galaxies which are currently separated by a distance d. From the Hubble Law, we know that they are currently moving apart at a speed v = H0 d. If we assume that the relative speed of the two galaxies has been constant, we can calculate how long it has taken them to reach their current separation, assuming that they were originally very close together when the universe was very dense. Note that this result is independent of the distance between the galaxies.

If no forces have been acting on the galaxies, and they've just been coasting freely apart at a constant speed, then the time required for the expansion of the universe is equal to one over the Hubble constant.

If H0 = 70 km/sec/Mpc (the best current value) then 1/H0 = 14 billion years. (This is just an approximation, of course. If gravity has been slowing the expansion, the universe took less time than this to expand. If some force is currently speeding up the expansion, the universe took more time than this to expand. But it's a good approximation to start with.)

The universe is not infinitely old. The ``Big Bang'' (that is, the explosion that marked the beginning of the expansion) happened about 14 billion years ago. Because the universe is about 14 billion years old, we can't see objects more than about 14 billion light-years away. The light from them simply hasn't had time to reach us yet. As a consequence of the universe's finite age, we are surrounded by a horizon (technically referred to as the ``cosmic particle horizon'') beyond which we cannot yet see. The universe may well be infinite in extent, but we can only see a finite patch of it. The existence of the cosmic particle horizon explains why the night sky is dark. (If the universe were infinitely old, and stars had been shining all that time, then everywhere we looked we would see a star -- the whole sky would look as bright to us as the surface of the Sun.)

The ``Big Bang Theory'' states that the universe was initially very hot and dense, and is currently expanding. This theory is supported by a wide array of observational evidence (we'll be examining some of it in the remaining lectures). The Big Bang Theory also poses many questions which cosmologists are still pondering. For instance:

The short answer to all these questions is, ``Nobody's sure yet.'' They are all hot topics in cosmological research. (The question ``What happened before the Big Bang?'' may even be a meaningless question, according to Steven Hawking's essay at the end of Chapter 28. It may be the equivalent of asking what's north of the North Pole.)
Prof. Barbara Ryden (

Updated: 2003 Feb 4

Copyright 2003, Barbara Ryden