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Discoverers of the accelerating expansion of the universe share this year's physics Nobel

5 October 2011

Saul Perlmutter, Adam Riess, and Brian Schmidt are the recipients of this year's Nobel Prize in Physics for their momentous 1998 discovery.

Saul Perlmutter, Adam Riess, and Brian Schmidt are the recipients of this year's Nobel Prize in Physics for their discovery in 1998 of the accelerating expansion of the universe.

Perlmutter, who's affiliated with Lawrence Berkeley National Laboratory and the University of California, Berkeley, led one of the two independent—and rival—teams that made the discovery. Half the prize will go to him. Schmidt of the Australian National University in Canberra led the other team, which includes Riess of the Johns Hopkins University in Baltimore, Maryland. Riess and Schmidt will share the other half.

Although the accelerating cosmic expansion came as a surprise to astronomers, its discovery provided an essential ingredient in what has become the prevailing cosmological model. Thanks to dark energy—as the source of the acceleration is loosely labeled—the dynamics of our expanding universe can be reconciled with its modest mass content.

Dark energy's true nature remains a mystery. Theoretical attempts to account for its observed properties have foundered. Its existence, however, appears secure. The new Nobel laureates and their collaborators discovered the accelerated expansion by observing a certain type of reliably uniform supernova. Supporting evidence has since come from three independent sources: large-scale structure, clusters of galaxies, and the cosmic microwave background.

Standard candles

The universe looks roughly the same in all directions—even parts that seem to be separated by more than the distance light has traveled since the Big Bang. To account for that puzzling uniformity, astronomers in the 1980s invoked 'inflation': a period of exponentially rapid expansion in which a tiny pocket of post–Big Bang universe spontaneously blew up to cosmic scales. Quantum fluctuations inside the pockets were amplified along with the universe. From those density fluctuations galaxies eventually formed.

Inflation's repercussions were imprinted on the expansion history of the universe. In principle, one can trace that history back in cosmic time by observing a so-called standard candle, a class of astronomical object whose intrinsic luminosity is uniform. The object's apparent brightness yields the distance; its redshift yields the velocity.

To be useful in probing expansion, a standard candle has to be both luminous (so that it can be seen at great distances and therefore long look-back times) and highly uniform (so that intrinsic scatter doesn't blur the signal). By the late 1980s, a suitable standard candle had been indentified: type Ia supernovae.

Type Ia supernovae are so luminous that they outshine their host galaxies for several weeks. They're also remarkably uniform, thanks to the special circumstances that engender them. Type Ia supernovae occur in binary systems when one of the stars, a white dwarf, accretes just enough material from its companion to tip it over the Chandrasekhar limit, the maximum mass a white dwarf can have before it collapses under its own gravity.

The chance of observing a type Ia supernova in any given galaxy is low. Only one or two are expected to pop off per galaxy per millennium. But if you could monitor the whole night sky, you'd see a type Ia supernova every few seconds.

Creating a distance–redshift diagram from type Ia supernovae requires three main steps: 1) finding the supernovae in the first place; 2) proving that they are indeed type Ia; and 3) measuring their redshifts. The first step can be accomplished at a 4-meter telescope equipped with a wide-field camera. The second and third steps require follow-up observations at an 8-meter or larger telescope or with the Hubble Space Telescope (HST).

Perlmutter and his Berkeley collaborators built a CCD-based wide-field camera to use and test at the Australian Astromical Observatory's 3.9-meter telescope, the AAT, which was built in 1976. Obtaining observing time on one of the newer, more powerful 8-meter telescopes or on the HST was, and remains, more challenging.

To convince time-allocation panels that follow-up observations would pay off, Perlmutter devised an ingenious observing strategy. He and his collaborators would observe at Kitt Peak or other 4-m telescopes just after a new moon, comb through their data to identify new supernovae, and then apply for followup observing time just before the next new moon. The moon-free nights guaranteed that Perlmutter would find candidates at the AAT. The three-week delay guaranteed that he could confirm and characterize the candidates at the 10-m telescope at the Keck Observatory on Mauna Kea, the HST, or another powerful telescope.

Dark energy

By the early 1990s, Perlmutter had acquired a large team of collaborators, called the Supernova Cosmology Project (SCP). In 1994, Schmidt formed a rival team, the High-z Supernova Search Team (HZT), which used methods similar to those developed by SCP. Riess led the analysis of the HZT data.

The first results from HZT (10 supernovae) and SCP (42 supernovae) were published, respectively, in the Astronomical Journal in 1998 and in the Astrophysical Journal in 1999. Both papers reached the same conclusion: Distant supernovae are receding at a slower rate than you'd expect if matter (dark and nondark) was the only source of gravitational action. Rather than gradually slowing down, the expansion was proceeding as if it were being given an extra kick by space itself, by something that constitutes 75% of the mass–energy of the universe.

Theorists, including Albert Einstein, had been analyzing the dynamics of an expanding universe even before Edwin Hubble observed it in 1929. The SCP and HZT results are consistent with a nonzero value of the cosmological constant, Λ. Einstein had introduced Λ into his general relativity to preserve what he presumed to be the static nature of the universe. Ironically, it appears that Λ represents a substance that underlies the dynamic nature of the universe.

What could that substance be? One possibility, the vacuum energy that pervades empty space, is ruled out—at least in the form that is responsible for the Casimir force or the Lamb shift. It's too big by several orders of magnitude. Another possibility is that the accelerated expansion arises not from a substance but from a modified form of gravity, one that behaves differently on cosmic scales of time and space.

Whether dark energy has retained its value throughout the history of the universe is an important clue to its nature. Evidently, dark energy can't have been much stronger than it is today lest it prevent matter from coalescing to form galaxies. The latest observations, which reach a redshift of about 2, are consistent with dark energy having been around in more or less its current form for the past 10 billion years.

If dark energy remains constant or if it increases in strength, then, as the cosmic expansion further reduces the density of matter and with it matter's ability to retard the expansion, the universe will expand forever without the possibility of rebirth.

Despite the uncertainty surrounding the nature of dark energy, its discovery has increased confidence in the so-called ΛCDM model, which provides a broad framework for the evolution of the universe since the Big Bang. Features of the universe—such as the synthesis of light elements, the structure of the cosmic microwave background, and the formation of large-scale structure—all fit within the model, which assumes that Λ is the dark energy and that dark matter is cold (nonrelativistic).

Charles Day

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Charles Day

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