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BREAKTHROUGH OF THE YEAR:
Illuminating the Dark Universe

#1 The Winner

Five years ago, Science's cover sported the visage of Albert Einstein looking shocked by 1998's Breakthrough of the Year: the accelerating universe. Two teams of astronomers had seen the faint imprint of a ghostly force in the death rattles of dying stars. The apparent brightness of a certain type of supernova gave cosmologists a way to measure the expansion of the universe at different times in its history. The scientists were surprised to find that the universe was expanding ever faster, rather than decelerating, as general relativity--and common sense--had led astrophysicists to believe. This was the first sign of the mysterious "dark energy," an unknown force that counteracts the effects of gravity and flings galaxies away from each other.

Although the supernova data were compelling, many cosmologists hesitated to embrace the bizarre idea of dark energy. Teams of astronomers across the world rushed to test the existence of this irresistible force in independent ways. That quest ended this year. No longer are scientists trying to confirm the existence of dark energy; now they are trying to find out what it's made of, and what it tells us about the birth and evolution of the universe.

Lingering doubts about the existence of dark energy and the composition of the universe dissolved when the WMAP satellite took the most detailed picture ever of the cosmic microwave background (CMB). The CMB is the most ancient light in the universe, the radiation that streamed from the newborn universe when it was still a glowing ball of plasma. This faint microwave glow surrounds us like a distant wall of fire. The writing on the wall--tiny fluctuations in the temperature (and other properties) of the ancient light--reveals what the universe is made of.

Through a glass, darkly. Microwave data observed by the WMAP satellite (top), supernovae (middle), and galaxy clusters (bottom) all reveal a universe dominated by dark energy.

CREDITS (TOP TO BOTTOM): NASA/WMAP SCIENCE TEAM; GSFC/NASA; SDSS COLLABORATION

Long before there were stars and galaxies, the universe was made of a hot, glowing plasma that roiled under the competing influences of gravity and light. The big bang had set the entire cosmos ringing like a bell, and pressure waves rattled through the plasma, compressing and expanding and compressing clouds of matter. Hot spots in the background radiation are the images of compressed, dense plasma in the cooling universe, and cold spots are the signature of rarefied regions of gas.

Just as the tone of a bell depends on its shape and the material it's made of, so does the "sound" of the early universe--the relative abundances and sizes of the hot and cold spots in the microwave background--depend on the composition of the universe and its shape. WMAP is the instrument that finally allowed scientists to hear the celestial music and figure out what sort of instrument our cosmos is.

The answer was disturbing and comforting at the same time. The WMAP data confirmed the incredibly strange picture of the universe that other observations had been painting. The universe is only 4% ordinary matter, the stuff of stars and trees and people. Twenty-three percent is exotic matter: dark mass that astrophysicists believe is made up of an as-yet-undetected particle. And the remainder, 73%, is dark energy.

The tone of the cosmic bell also reveals the age of the cosmos and the rate at which it is expanding, and WMAP has nearly perfect pitch. A year ago, a cosmologist would likely have said that the universe is between 12 billion and 15 billion years old. Now the estimate is 13.7 billion years, plus or minus a few hundred thousand. Similar calculations based on WMAP data have also pinned down the rate of the universe's expansion--71 kilometers per second per megaparsec, plus or minus a few hundredths--and the universe's "shape": slate flat. All the arguments of the last few decades about the basic properties of the universe--its age, its expansion rate, its composition, its density--have been settled in one fell swoop.

As important as WMAP is, it is not this year's only contribution to cosmologists' understanding of the history of the universe. The Sloan Digital Sky Survey (SDSS) is mapping out a million galaxies. By analyzing the distribution of those galaxies, the way they clump and spread out, scientists can figure out the forces that cause that clumping and spreading--be they the gravitational attraction of dark matter or the antigravity push of dark energy. In October, the SDSS team revealed its analysis of the first quarter-million galaxies it had collected. It came to the same conclusion that the WMAP researchers had reached: The universe is dominated by dark energy.

This year scientists got their most direct view of dark energy in action. In July, physicists superimposed the galaxy-clustering data of SDSS on the microwave data of WMAP and proved--beyond a reasonable doubt--that dark energy must exist. The proof relies on a phenomenon known as the integrated Sachs-Wolfe effect. The remnant microwave radiation acted as a backlight, shining through the gravitational dimples caused by the galaxy clusters that the SDSS spotted. Scientists saw a gentle crushing--apparent as a slight shift toward shorter wavelengths--of the microwaves shining near those gravitational pits. In an uncurved universe such as our own, this can happen only if there is some antigravitational force--a dark energy--stretching out the fabric of spacetime and flattening the dimples that galaxy clusters sit in.

Some of the work of cosmology can now turn to understanding the forces that shaped the universe when it was a fraction of a millisecond old. After the universe burst forth from a cosmic singularity, the fabric of the newborn universe expanded faster than the speed of light. This was the era of inflation, and that burst of growth--and its abrupt end after less than 10^-30 seconds--shaped our present-day universe.

For decades, inflation provided few testable hypotheses. Now the exquisite precision of the WMAP data is finally allowing scientists to test inflation directly. Each current version of inflation proposes a slightly different scenario about the precise nature of the inflating force, and each makes a concrete prediction about the CMB, the distribution of galaxies, and even the clustering of gas clouds in the later universe. Scientists are just beginning to winnow out a handful of theories and test some make-or-break hypotheses. And as the SDSS data set grows--yielding information on distant quasars and gas clouds as well as the distribution of galaxies--scientists will challenge inflation theories with more boldness.

The properties of dark energy are also now coming under scrutiny. WMAP, SDSS, and a new set of supernova observations released this year are beginning to give scientists a handle on the way dark energy reacts to being stretched or squished. Physicists have already had to discard some of their assumptions about dark energy. Now they have to consider a form of dark energy that might cause all the matter in the universe to die a violent and sudden death. If the dark energy is stronger than a critical value, then it will eventually tear apart galaxies, solar systems, planets, and even atoms themselves in a "big rip." (Not to worry; cosmologists aren't losing sleep about the prospect.)

For the past 5 years, cosmologists have tested whether the baffling, counterintuitive model of a universe made of dark matter and blown apart by dark energy could be correct. This year, thanks to WMAP, the SDSS data, and new supernova observations, they know the answer is yes--and they're starting to ask new questions. It is, perhaps, a sign that scientists will finally begin to understand the beginning.

Papers and Articles

Selected Research Papers C. L. Bennett et al., "First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Preliminary Maps and Basic Results ," Astrophys. J. Suppl. Ser. 148, 1 (2003) [arXiv.org preprint server] D. N. Spergel et al., "First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters,"Astrophys. J. Suppl. Ser. 148, 175 (2003) [arXiv.org preprint server] M. R. Nolta et al., "First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Dark Energy Induced Correlation with Radio Sources" (2003) [arXiv.org preprint server] R. Scranton et al., "Physical Evidence for Dark Energy" (2003) [arXiv.org preprint server] L. M. Krauss, "Implications of the WMAP Age Measurement for Stellar Evolution and Dark Energy," Astrophys. J. 596, L1 (2003) [arXiv.org preprint server] M Tegmark et al., "The 3D Power Spectrum of Galaxies from the SDSS" (2003) [arXiv.org preprint server] J. L. Tonry et al., "Cosmological Results from High-z Supernovae," Astrophys. J. 594, 1 (2003) [arXiv.org preprint server] A. V. Filippenko, "The Accelerating Universe and Dark Energy: Evidence from Type Ia Supernovae" (2003) [arXiv.org preprint server] News and Features C. Seife, "MAP Glimpses Universe's Rambunctious Childhood," Science 299, 990 (2003) C. Seife, "With Its Ingredients MAPped, Universe's Recipe Beckons," Science 300, 730 (2003) R. Irion, "The Warped Side of Dark Matter," Science 300, 1894 (2003) [from Science's Dark Side special issue] C. Seife, "Dark Energy Tiptoes Toward the Spotlight," Science 300, 1896 (2003) [from Science's Dark Side special issue] C. Seife, "Dark Energy Passes Another Test," Science 301, 449 (2003) C. Seife, "Galaxy Maps Support Theory That the Universe Is Flying to Pieces," Science 302, 762 (2003) Breakthrough of the Year: The Accelerating Universe (18 December 1998) We identified the discovery of dark energy, through observations of supernovae, as 1998's top science advance. Reviews and Perspectives Science Special Issue: The Dark Side (20 June 2003) Four reviews on dark matter and dark energy. S. L. Bridle et al., "Precision Cosmology? Not Just Yet . . .," Science 299, 1532 (2003) A. Gangui, "A Preposterous Universe," Science 299, 1333 (2003) K. C. Freeman, "The Hunt for Dark Matter in Galaxies," Science 302, 1902 (2003)

Interesting Web Sites

Special Web Supplement: The Dark Side Collection of Web resources on topics such as the CMB and supernovae, to supplement Science's Dark Side special issue. Wilkinson Microwave Anisotropy Probe (WMAP) Site of the ongoing NASA mission includes a useful educational component, Cosmology 101, an image gallery, and more. Planck European CMB mission, slated for launch in 2007. Sloan Digital Sky Survey Web site of "the most ambitious astronomical survey project ever undertaken." The Physics of Microwave Background Anisotropies Well-known primer on cosmology, CMB, and dark matter and energy issues by Wayne Hu of the University of Chicago. Supernova and Supernova Remnant Pages on the WWW Well-maintained selection of supernova links by Marcos Montes of the Naval Research Laboratory. High-Z Supernova Search Supernova Cosmology Project

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