Text by J. Richard Gott

The 2011 Nobel Prize in Physics has been awarded to Adam Riess, Brian Schmidt, and Saul Perlmutter for their discovery of the accelerating expansion of the universe. It is one of the times when astronomers have won the Nobel Prize in Physics. It is perhaps the most important scientific discovery of the last quarter of the 20th Century. (The most important scientific discovery of the previous quarter century? Watson and Crick’s discovery of the structure of DNA.)

What does this discovery mean? Why is the accelerated expansion of the universe so important? Einstein showed, in his theory of general relativity, that gravity is due to the curvature of spacetime. His theory has been tested many times. Most spectacularly, he predicted that the Sun should bend light rays from distant stars passing near it. This effect was observed in 1919 and Einstein’s prediction was verified, whereas the prediction Newton’s theory would have made was shown to be wrong. Einstein overturned Newton. To use a sports analogy, it was the play of the century in physics–Einstein stole the

ball from Newton and sunk the basket. And the theory of gravity was the most important thing Newton ever did. This catapulted Einstein to world fame. He entered the ranks with Newton and Darwin. In developing his theory, he realized that, for the laws to look the same for all moving observers (i.e., for them to be relativistically invariant), the source of the gravitational field had to include pressure as well as mass-energy density. This is something Newton wouldn’t have thought of. Pressure gravitates as well as mass-energy density. This is encompassed in something called *stress-energy*. Einstein postulated that stress-energy causes curvature. If you knew the amount of stress-energy at a given location at a given time, you could calculate the curvature of spacetime there. Planets and light beams simply followed the straightest possible trajectories in this curved spacetime–like an airliner follows a straight great-circle route when flying over the curved surface of the Earth from New York to Tokyo. When Einstein worked out the equations, they showed how stress-energy caused spacetime to curve. His equations guaranteed local energy conservation (in the same way that Maxwell’s equations of electricity and magnetism guaranteed charge conservation).

Einstein applied his new equations to cosmology. He liked the idea of a static universe (one that neither expands nor contracts)–but he found that his equations would not produce one. So he added a term to the curvature side of the equation called the *cosmological constant*. This added term was designed to preserve the property of local energy conservation. This made a static universe possible–the repulsive effects of the cosmological constant exactly balanced the gravitational attraction of the matter, keeping the model static. However, in 1922 Friedmann published a paper where he used Einstein’s original equations without the cosmological constant term to show that the universe must be dynamical. It starts with a big bang and expands. Space itself expands–a remarkable concept. Galaxies move apart as the space between them expands. Again, this is something Newton would not have thought of. In 1929, Hubble discovered the expansion of the universe when he observed that galaxies really were moving apart just as Friedmann had predicted. Einstein then called the cosmological constant his “biggest blunder.” If he had never invented it, he might have written Friedmann’s paper and predicted an expanding universe ahead of Hubble.

The Friedmann big bang model was triumphant. In 1948 Gamow, Herman, and Alpher used it to predict that the early universe should be hot and that the hot radiation present back then should still be batting around today as a microwave background. In 1965 Penzias and Wilson discovered this cosmic microwave background radiation, for which they later won the Nobel Prize in Physics in 1978. But there were problems. The thermal radiation was very uniform and there was not enough time in the early universe for the different parts we see to reach the same temperature. In 1981, Alan Guth proposed a theory of inflation to explain this. Move the cosmological constant from the curvature side of Einstein’s equations to the stress-energy side and it becomes a quantum vacuum state that has a positive energy density and a negative pressure. The negative pressure has a negative gravitational effect–it causes a gravitational repulsion. Since pressure operates in three directions in space (up-down, left-right, front-back), the gravitational repulsive effects of the negative pressure outweigh the gravitational attraction of the energy density of the vacuum by a factor of three to one, making the overall effect of this vacuum energy density and vacuum pressure repulsive.

Even if the universe starts off at rest, this gravitational repulsion starts the universe expanding. Then it expands faster and faster. It is an accelerated expansion. The universe doubles in size every 10^{-35} seconds. Inflation causes the big bang explosion and explains why the universe is so large today. This period of inflation creates a tiny extra amount of time when the universe was very small which allows the different parts we see to come into contact with each other, thereby explaining the isotropy of the universe we observe. Eventually, the quantum vacuum state decays, dumping its energy into the form of thermal radiation–and the big bang Friedmann model takes over. Random quantum fluctuations occur, and this leads, through the action of gravity operating over 13.7 billion years, to the formation of the galaxies and clusters of galaxies we see today.

Scientists were anxious to determine the energy density of the universe, which according to Friedmann would determine the future fate of the universe. If the density were above a critical density, the universe would continue slowing down in its expansion due to the gravitational attraction of the galaxies for each other, and the whole universe would collapse in a big crunch at some future time. If the universe was below that critical density, it would continue expanding forever. The fate of the universe could thus be determined today by measuring how fast the expansion of the universe was decelerating–how fast its expansion rate was slowing down. Two teams set out to measure this using distant supernovae. Saul Perlmutter led one team at Berkeley. When Bob Kirshner arrived at Harvard from the University of Michigan, he was already a world expert on supernovae and got people there interested in using supernovae to measure distances. Brian Schmidt formed a team at Harvard, including Bob Kirschner and Adam Riess, to use large-redshift supernovae to measure the deceleration of the universe, like the Perlmutter team at Berkeley. They determined the distances to supernovae by measuring their brightness. They used Type Ia supernovae, which were caused by the collapse of white dwarf stars. Since such white dwarf stars always collapse when they reach a critical Chandrasekhar mass limit of 1.4 solar masses, all these supernovae were quite alike and could be used as standard candles. If you know the intrinsic brightness of a supernova, and how bright it appears in the telescope, you can determine how far away it is. Bill Press came up with an algorithm to further improve these brightness estimates. If you measure the redshift of lines in the specturm of a supernova, you can find out how big the universe was (relative to today) when the supernova exploded (because wavelengths of light coming from the supernova stretch with space as the universe expands.) Chart the distances of supernovae as a function of their redshift and you can construct a graph of the expansion history of the universe. When Adam Riess first looked at the results, he was quite surprised–the expansion of the universe was not decelerating, but *accelerating*–it was expanding faster and faster! The most likely explanation was that old cosmological constant term of Einstein! The two teams came to the same conclusion and reported their results in 1998. The editors of *Science* magazine put a cartoon of Einstein on the cover happily blowing bubbles from his pipe to celebrate the event.

Today, most physicists, influenced by inflation, would put this cosmological constant term over on the stress-energy side of Einstein’s equations and call it *dark energy*. It’s called dark because you can’t see it and energy because it is an energy density that fills the vacuum of space. And it has a negative pressure.

Later Adam Riess used the Hubble Space Telescope to observe very distant supernovae and found that at earlier times the universe was decelerating just as expected. As the universe expands, dark energy stays at nearly constant energy density and, as the matter in the universe thins out, the dark energy begins to dominate. Once that occurs, the universe goes from an expansion that is slowing down to an expansion that is becoming faster and faster. In the future we expect the universe to begin to double in size approximately every 10 billion years. It will be an exponential expansion, just like we had in the early universe in inflation. The repulsive effects of dark energy seem to guarantee that the universe will continue to expand forever. No big crunch. The Nobel press release emphasized this–saying if things continue this way the world will end not in fire but in ice. Today dark energy comprises 73% of the universe. It is most of the stuff in the universe. To discover most of the stuff in the universe at this late date is quite an amazing accomplishment.

The Nobel committee awarded half the prize to Perlmutter and one quarter of the prize each to Schmidt and Riess. Perlmutter and Schmidt were the two team leaders. They effectively split the prize equally between the two teams.

For the Harvard team the prize was split between the team leader and Riess who made particularly crucial contributions. For example, he was the first author on the Harvard team’s discovery paper. The rules say that the Noble Prize can only be split among at most three people. Since many people work together in large groups today, in many years when the Nobel Prize is awarded there are usually additional people who participate in the discovery and come close to winning as well. When describing this discovery in our National Geographic book *Sizing Up the Universe,* co-authored by Bob Vanderbei, we gave credit to “Adam Riess, Saul Perlmutter, Brian Schmidt, Robert Kirshner, Alex Filippenko and their colleagues” for the discovery. Congratulations to them all!