Dark energy is here to stay. This year's Nobel Prize in Physics was given to a trio of astronomers who made an extraordinary discovery in 1998: that the universe not only is expanding, but it's doing so at an accelerated rate. Nobel Prize winner physicist Frank Wilczek called this "the most fundamentally mysterious thing in basic science." It's an understatement to say that when the accelerated universe was first announced, the physics and astronomy community were completely baffled. To a large extent, we still are, 13 years later. I'd like to use the blog today to put their discovery into context, exploring why dark energy is so bizarre.
But first, some presentations: Saul Perlmutter is an astrophysicist at the Lawrence Berkeley National Laboratory at the University of California, Berkeley; Brian Schmidt, born in Montana, is an astronomer at the Research School of Astronomy and Astrophysics at the Australian National University; and Adam Riess is a professor of Astronomy and Physics at the Johns Hopkins University, and a senior staff member at the Space Telescope Institute. Adam Riess and Brian Schmidt were part of the High-z Supernova Research Team, operating from telescopes in Cerro Tololo, Chile, while Permutter, who got half the prize, headed the Supernova Cosmology Project.
When physicists say the universe is expanding we don't mean that objects are flying off from a central point like shrapnel from an explosion. If that were the case, the universe would have a center, the point where the big bang happened, while — as in the case of the surface of a sphere — no point in the universe is more important than any other. So what do we mean?
The expansion of the universe is actually the expansion, or stretching, of space itself. We can picture this by imagining that space is like a rubber sheet, where galaxies are anchored. As the sheet stretches equally in all directions, it carries the galaxies along with it. If you are an observer in a galaxy, you will see the other galaxies moving away from you and conclude that the universe is expanding.
How, exactly, do we know galaxies are moving away from us? We measure their light (in the visible and other wavelengths, like infrared, ultraviolet, radio ... ) using what is known as "standard candles," sources that have the same emission of light everywhere. For example, if you have 10 identical flashlights and spread them out in a dark field, the light will dim with the square of the distance from you. So, since the flashlights are identical, by measuring the intensity of their emitted light you can infer how far away they are. The biggest challenge for astronomers trying to see far away galaxies is precisely to find reliable standard candles that are powerful enough to be caught by their telescopes.
The triumph of the dark energy trio is to have found standard candles at galaxies really far away. So far, in fact, that their light had left them roughly five billion years ago, around the same time the sun and the earth were being formed from a primeval hydrogen cloud.
The standard candles they found are called supernovae Type Ia, amazingly powerful explosions that happen when one star sucks the matter from a neighboring one in a furious way, to a point where it can't support itself any longer. These supernova explosions are among the most powerful in the universe and, most importantly, show very little variation in brightness, to about 10 percent: just what is needed for a standard candle.
Once in possession of the candles, astronomers can also determine the velocity with which their host galaxies are receding from us. To do that, they use the Doppler shift, the change in the shape of waves that happens when their source moves. For example, when a truck blows its horn while approaching you, you hear a higher pitch (shorter wavelength), while if it blows its horn when moving away, the pitch will be lower (longer wavelength). The same happens with light waves: as the source moves away (as it would in an expanding universe), its light gets shifted toward the red. The faster the movement, the larger the change.
Putting the distance and velocity results together, astronomers can then determine how fast the universe is expanding at different moments of its history. Essentially, this is a cranked-up version of the technique that astronomer Edwin Hubble used in 1929 to establish the expansion of the universe. What the trio found was that the expansion, at about five billion years ago or so, became much faster, as if cosmic turbo engines were turned on. Something capable of accelerating the cosmos became dominant at about that time. The question is what was it?
Cosmology has advanced to such an extent that we now can say with confidence that the universe appeared 13.7 billion years ago and that it has been expanding ever since, fueled by its energy and matter content. More remarkably, there are three main ingredients to the cosmic recipe: ordinary matter, the atoms you and stars are made off, makes up only 4 percent of the total; 23 percent comes in the form of dark matter, probably made of small particles that only interact with ordinary matter via gravity — we can "see" dark matter by the way it makes galaxies spin and how it bends light as it travels through space, but we still don't know what it is; and finally, the rest, 73 percent of the stuff in the universe, is attributed to dark energy, the cause of the baffling cosmic acceleration. So, what the trio discovered in 1998 is the dominant source of energy in the cosmos, powerful enough to make it stretch faster than the speed of light.
"What?" you say. "How can something go faster than light? Are these neutrinos?" Not at all. The laws of physics as we know them prohibit particles of matter (including neutrinos) from traveling faster than light, but not space itself. It can stretch faster than light without any problem.
The challenge now is to determine what could be the source of this expansion, that is, to determine the nature of dark energy. As I mentioned here last week, it could be related to Einstein's cosmological constant which, in turn, could be related to ephemeral energy fluctuations predicted to occur even in empty space (in the vacuum) by quantum theory; or it could be some new force of nature, related to an undiscovered field; or it could point to the need to revise Einstein's theory of general relativity, which encompasses our current understanding of gravity. At this point, all bets are off. Which makes the coming years extremely exciting for cosmology. Whatever dark energy turns out to be, the answer is bound to redefine the way we think about the relation between space, time and matter.
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