The Incredible Expanding Accelerating Universe

Sunil Golwala

We live in a very dynamic universe. Not only is it full of all kinds of exotic objects, but the universe itself is a dynamic, expanding thing. What’s most fascinating about this expansion is that it is an expansion of space-time itself – the universe isn’t expanding into some void around it, but rather the fabric of space-time that is the universe is stretching as time goes on!

This incredible fact was discovered by the astronomer Edwin Hubble in the 1920s. When Hubble looked at galaxies outside of our own, he saw that they were all racing away from us. That might suggest there was an initial explosion that we sat at the center of. But it would be unreasonable (and certainly not very humble) to assume that our galaxy is the center of the universe! So, instead, we deduce that the fabric of space-time, the sheet in which all these galaxies sit, is itself expanding. You can visualize the universe as the surface of a rubber sphere. Paint grid lines on the sphere and put galaxies down on them. Now, inflate the sphere, making its surface expand. The distance along the surface of the sphere between any two points on the sphere increases as the sphere is inflated. But there is no center to the expansion – every point sees all the other points racing away from it, but that’s just an illusion.

What causes the universe to expand? That’s actually two different interesting questions. The first question is: What gave the universe its initial kick? Basically, what was the Big Bang that gave rise to the current expansion of the universe? It really lies at the intersection of the physics of gravity and of quantum mechanics, an interface we just don’t understand well today.

But the other question, what makes the universe keep expanding, is pretty simple to answer. It’s just inertia, or the momentum of the expansion. It’s a lot like shooting a rocket up in the air. If there’s no gravity, it will just keep going forever. But, like that rocket, if there is gravity, the motion is slowed down. In the universe, it is the gravity of the mass and energy that counteracts the expansion. And, just like a rocket, if there is enough gravity, the universe’s expansion could halt at some point and it could recollapse.

What’s been discovered in the last few years is that this expansion is not slowing down, but instead it is accelerating. Normal mass and energy cannot cause this – their self gravity always counteracts expansion. Instead, it is hypothesized that there is some form of “dark energy” that is not diluted by the expansion of the universe like most mass and energy. Such an energy density can help the expansion continue, and possible even accelerate it. Understanding the nature of this dark energy is one of most important problems in astronomy today.

One way to understand the dark energy is to indirectly measure how much there is and how it evolves with the age of the universe. One technique for doing this is to see how big structures in the universe evolve. Large structures, like clusters of galaxies, are created when a small matter density fluctuation grows by gravity – basically accretes surrounding matter – and finally “breaks away” from the expansion to collapse. How long it takes for this happened depends on two things: how much mass there is (how big the overdensities are) and how fast the universe is expanding (which the overdensities have to break away from). We want to measure the variation of the number of clusters of galaxies with the age of the universe in order to see the effect of the dark energy on how fast fluctuations break away from the expansion and collapse.

We use a unique technique to do this. There is a cosmic “backlight”, the cosmic microwave background, that was created when the universe was only 300,000 years old. This light is very uniform and comes from all directions. When it passes through a galaxy cluster, it is made slightly “hotter” because it picks up energy from the free electrons in the cluster; this is called the Sunyaev-Zeldovich effect. So one sees fluctuations in this backlight where there are clusters. This background has cooled with the universe, so it was hotter earlier in time. The fluctuation clusters produce in this background is always a fixed fraction of how hot it is, and so all clusters produce about the same fractional fluctuation. The fraction is preserved even as the background cools, so clusters that are very far away (and hence existed when the universe was young) produce the same size signal as those that are nearby. This provides a method of detecting these clusters that is independent of how far away they are, letting one see all the clusters all the way back to the time they began to form.

We have built a camera for use on the Caltech Submillimeter Observatory to do these observations. Mauna Kea provides one of the best astronomical sites in the world. At the wavelengths we operate at, 1 to 2 mm (a few thousand times longer than the light you see), the transparency of the atmosphere is limited by water vapor. Because of the inversion layer that forms around Mauna Kea every day, most of the atmospheric water vapor sits below the summit and so does not interfere with observations. In addition to absorbing, the water vapor emits light. Fluctuations in the amount of water vapor thus look like noise to us. The inversion layer also ensures a very quiet atmosphere above Mauna Kea, reducing this noise enough to let us do observations in spite of it. With good weather and a little luck, we hope to use the CSO to observe these very distant galaxy clusters and thus get a bit of insight into the mysterious dark energy that is accelerating our universe.