In 1998, two competing groups of astronomers were attempting to measure the rate at which the universe’s expansion was slowing. As Vesto Slipher and Edwin Hubble found in the 1920s, far galaxies are moving away from our galaxy at a rate more-or-less proportional to their distance. That was the beginning of observational cosmology, moving cosmology from philosophy departments into the realm of science. If the universe is expanding, running the clock back far enough would show a beginning point: the Big Bang.
Einstein’s general theory of relativity provided the theoretical understanding of expansion: a universe that looks roughly the same everywhere and in every direction (homogeneous and isotropic being the technical terms) will either expand or contract. However, it could go several ways, depending on the total amount of stuff (matter, light, etc.) in the universe. Expansion battles gravity from the lumps of stuff inside the universe, which overall would have the effect of slowing down the expansion rate, so it was widely assumed that there would be some sort of deceleration going on, even if the universe would expand forever. Many astronomers set out to measure the deceleration rate.
Thus we come to 1998 and the two groups of astronomers: the High-z Supernova Search Team (an international collaboration led by Riess and Schmidt) and the Supernova Cosmology Project (headed by Perlmutter), both involving a lot of researchers to run the telescopes, make the observations, and analyze the data. (It’s truly a shame that the Nobel Prizes are limited to three people. Nobody with an ounce of knowledge of this kind of project would believe that only these three men did the work. I’m not impugning them at all, just saying that the tent should be big enough to encompass everyone involved.) As the names of the groups suggest, they used supernovas of a particular type: the explosion of a white dwarf.
White dwarfs can only grow so large before gravity overwhelms the pressure holding them from collapse. This limit is known as the Chandrasekhar limit (for the great astrophysicist Subrahmanyan Chandrasekhar), and it’s the most massive any white dwarf can become. If enough mass is added to any white dwarf (from a companion star), it will explode in a similar fashion: a supernova of type Ia. The name comes from its spectrum, which makes it possible for scientists to distinguish white dwarf supernovas from the explosions of massive stars, which are not at all uniform in the way they explode. Astronomers know how bright these white dwarf explosions are, so if one happens in a distant galaxy, they can accurately determine how far that galaxy is from us.
If the expansion of the universe was decelerating, then there would be slightly more supernovas at smaller distances than at larger. However, the supernova search teams found that there were slightly more supernovas at larger distances, indicating that the rate of expansion is growing. The universe is accelerating, and nobody has a good answer as to why. The name given to quantify our ignorance is “dark energy”; observations of the cosmic microwave background show that it comprises 72% of the energy content of the universe, compared with about 5% for ordinary matter and 23% for dark matter.
Strange as dark matter is, it still has the characteristics of matter: it collects together and behaves normally under gravitation. Dark energy doesn’t behave like anything normal: if you compress it, its pressure goes down, while if you let it expand, the pressure goes up. For this reason, many people associate it with vacuum energy: the energy of nothing, since the “more nothing” you have, the more energy you would have in that volume. It’s a good idea, and supported by quantum physics, except that the vacuum energy calculated is far too large compared with dark energy.
However, the burden right now is not on the observers to decide what dark energy is, just how it behaves. The identity of dark energy is for theorists, and we’re behind the curve on that. So let’s focus on success today as we celebrate again the discovery of dark energy. Congratulations to Saul Permutter, Adam Riess, Brian Schmidt, and everyone whose hard work discovered one of the strangest phenomena in our wonderful universe!