In our brief look at dark matter, you found more than 80% of the matter in the universe is a type of particle or particles that emit no light, interact very weakly with matter in our everyday world, yet exert profound gravitational influence on the rotation of galaxies and the movement of galaxy clusters. Although particle physicists have a few good ideas, no one knows for sure what this dark matter might be, which is a little unsettling. But it gets even stranger because astronomers have since discovered most of the universe is made not of matter but a strange and unidentified type of energy– “dark energy”– that accelerates the expansion of the universe and may one day carry distant galaxies forever out of view.
It all started with an attempt to weigh the universe.
In the late 1980s and into the 1990s, two teams of astronomers, one led by Saul Perlmutter and another by Brian P. Schmidt, set about to measure the rate at which the universe was expanding. They expected, as did many astronomers, to find the expansion, first discovered by Edwin Hubble, was slowing down as a consequence of the gravitational pull of ordinary and dark matter. Einstein’s general theory of relativity suggest the rate at which expansion slows gives an indirect way to “weigh” all the mass in the universe. The result could be compared to a “bottom up” approach where the total mass of the universe is estimated by the adding the mass of all the observable stars and galaxies, along with estimates of dark matter. The two measurements should agree within their fairly large margins of uncertainty.
To measure the expansion rate of the universe, the two teams searched for particular type of exploding stars, called Type Ia supernovae, in distant galaxies. These stars are enormously useful tools for measuring distance to faraway galaxies because they are extremely bright, and they all explode at about the same brightness. By comparing the apparent brightness, which depends on distance, to true brightness, which is known, astronomers can measure distance to the supernova. Once the distance is known, the redshift of its spectrum is measured to determine how fast the star and its home galaxy are moving away.
Type Ia supernovae have been observed for decades. But the Perlmutter and Schmidt teams looked out farther for these supernovae than anyone before them. And they calibrated the slight differences in brightness between each exploding star to get far more accurate distance measurements. In all, the two teams detected about 50 of these exploding stars out to a distance of a few billion light years. Their results, published in 1998 and 1999, were totally unexpected. Instead slowing down, the expansion of the universe was accelerating.
This was an amazing result, as if you tossed a ball in the air and, instead of falling back to Earth, it took off through the ceiling. Scientists are a careful bunch, so it took some time to corroborate the results of the two supernova teams. But the results appear correct, and three key members of the teams, Saul Perlmutter, Brian P. Schmidt, and Adam Reiss, won the Nobel Prize for Physics in 2011.
The accelerating expansion can not be explained by any type of missing mass, because mass implies gravity and more gravity should put the brakes on expansion. So astronomers proposed a new type of energy to explain the observations. They call it “dark energy”, though it may or may not be energy as we understand it.
Naming it is one thing. But explaining it is quite another, and astronomers have no idea what dark energy might be. It’s unlike any sort of energy we understand. It’s repulsive. Its density remains constant even as the universe expands. The more space expands, the greater the effect of dark energy. It is very strange stuff.
An obvious explanation is that space itself has intrinsic energy. A candidate for this energy is the “vacuum point” energy of space. This is predicted by well-known ideas in quantum physics. Unfortunately, the predicted energy from quantum physics is a factor of 10100 too large to explain dark energy, which some joke is the worst-ever prediction in all of science. There may be ways to explain this discrepancy. Or maybe dark energy is something completely different, perhaps a new type of field called “quintessence”. No one knows. Which is what makes it interesting.
In each piece of space, there’s not a lot of dark energy. Our entire solar system would contain the mass equivalent of just 6 tons of the stuff, about the mass of a small school bus. But there’s a lot of space in the universe, so the dark energy adds up. In fact it seems the universe is made mostly of dark energy, about 68%, compared to 27% dark matter and just under 5% ordinary matter. We have no idea about the nature of 95% of the universe.
Whatever the nature of dark energy, the resulting accelerating expansion, if it continues, has implications for the fate of the universe. As space expands faster and faster, galaxies beyond our own Local Supercluster, which is bound together by gravity, will eventually be carried away so quickly by expanding space they will become invisible to us, and astronomers in the far future will be unable to see more than a few thousand galaxies in the night sky. The rest of the universe will appear to us as a black void.
But that’s a discussion for another time.