The Universe in the Classroom

No. 37 - Winter 1997

© 1997, Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, CA 94112.

The Biggest Bang of Them All

Philippe Brieu
University of Michigan

George Musser
Astronomical Society of the Pacific


When did time begin? Where did we come from? What is our destiny? Three very fundamental questions that have always been on human minds.

What was there before the beginning of time? What is there beyond the edge of the universe? Is it possible to travel back in time? Three more bewildering questions that are bound to fascinate youngsters and stump teachers.

Both of us remember asking ourselves these kinds of questions as kids. Unfortunately, nobody around either of us seemed to have the answers. Yet we had to know! We should become astronomers and figure it all out, we thought.

Well, we found out it's not that easy. Scientists have made significant progress in recent years, but plenty of issues remain controversial. This is often illustrated by newspaper headlines: "Scientist sees face of God, confirms Big Bang model" (a reference to results from the Cosmic Background Explorer satellite), "Is the cosmos younger than some of the stars it contains? Cosmology is in chaos" (attempts to measure the expansion rate of the universe), "Astronomers discover billions of new galaxies they did not expect" (Hubble Space Telescope observations), "Physicists describe the grim end of the world" (speculations on the fate of the universe).

All these issues, and many more, make cosmology an exciting field. By definition, cosmology is the study of the universe as a whole, its history, and its overall contents. Cosmology attempts to answer some of the most basic questions we have about the reality we inhabit.

The Lone Universe
The Mother of All Bangs
How to Make an Element
The Growing Universe
In the Beginning
The Fate of the Universe

The Lone Universe

Cosmologists are in a unique situation among scientists. There is only one specimen for us to study: the one and only universe we live in. We cannot reproduce it; we cannot take another one and see what happens to it; we cannot compare it to anything else, as astronomers routinely do with planets, stars, and galaxies. Nor can we experiment with what we are studying. We can only observe it passively.

Moreover, we are inside the universe (by definition). That makes it even more difficult to study. Cosmologists are like dentists trying to operate on their own teeth. We are trapped inside the object of our study. In science, it always helps to look at things from the outside. Zoologists try not to be seen by the animals they study, in order not to influence their behavior. Meteorologists look at storm systems from above using weather satellites. But we cannot escape from the universe. We cannot go outside it and take a look.

This means that we have to be careful about the meaning we give to what we see nearby and far away. If you look around you, you can probably see a wall, a door, a window, a closet. If you look farther away, you can see a street, other buildings, maybe mountains in the distance. In between could be a farm, a lake, a forest, a highway. So you can see very different things in different directions and at different distances.

If you could keep looking farther away, you would eventually see the ocean. At that point, you could talk in terms of land vs. water: here a continent, there an ocean, another continent, and so on. Someone else could do the same from anywhere on Earth, and while his or her immediate neighborhood could be quite different from yours, on a large enough scale ­ that of the whole Earth ­ the two of you would have the same basic description: land vs. water.

In cosmology, galaxies are the land and the space between galaxies is the water. It is only on the scales of galaxies and larger groupings that we can talk about the universe in general. On that scale, your exact position in the universe does not matter, because it looks roughly the same from any location in any direction: galaxies all.

This observation is the starting point of cosmology. Just as Earth is not the center of the solar system, it is not the center of the universe, but rather some random location equivalent to any other location. In technical terms, cosmologists say the universe is "homogeneous" and "isotropic" on its largest scales. Homogeneous means that the universe has the same basic composition and structure everywhere. Isotropic means that it looks basically the same in every direction.

Cosmologists also expect that the same laws of physics ­ gravity, motion, electricity and magnetism, and so on ­ apply everywhere at all times. This, too, is supported by observations. If the laws varied even slightly, distant stars would refuse to shine, orbits would go haywire, light would look sickly. We see none of this. The remarkable uniformity of the universe is what allows us to study it in its entirety.

From town to Kuiper Belt
From Oort Cloud to Cosmic Horizon
How big is the universe, anyway? Each of these bubbles is about 300 times as wide as the previous one. If you start with your hometown and zoom out nine times, you reach the edge of the observable universe. This diagram is a simple example of a logarithmic scale. Incidentally, if you went the other direction, shrinking each bubble by a factor of 300, you would end up with a speck smaller than the smallest known subatomic particle. So, human beings are at the middle of the range of cosmic scales. We are about as small in comparison to the universe as subatomic particles are in comparison to us. Diagram by Kathleen L. Blakeslee for the ASP.

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