Mapping the Big Bang

As of today we know better than ever when the universe began, how it behaved in its earliest instants, how it has evolved since then, and everything it contains.

At a NASA press conference in Washington, D.C., scientists released the much-anticipated first results from the Wilkinson Microwave Anisotropy Probe satellite, or WMAP. The big news is actually no news — and to astronomers, that's good news. WMAP has powerfully confirmed the conclusions that many teams of astronomers had already reached in recent years using instruments carried by balloons and on the ground.

Launched in June 2001, WMAP flew to the vicinity of the L2 Lagrangian point, 1.5 million kilometers away from Earth's warmth and noisy interference. From there it has been mapping the cosmic microwave background radiation that fills the sky. The microwave background is the white light from the white-hot early universe 380,000 years after the Big Bang. Since that time the light's wavelength has been redshifted (lengthened by the expansion of space) by a factor of 1,100, bringing it into the microwave region of the spectrum.

The background radiation is very uniform across the sky — but not perfectly so. It is filled with weak ripples, first discovered 11 years ago, that differ in temperature by no more than a few parts per 100,000 from one place to another. These ripples resulted from oscillations and turbulence during the universe's earliest times. Their sizes and strengths as seen on the sky today tell volumes about the origin of the universe, its age, shape, and overall contents.

Astronomers originally thought that the ripples ("anisotropies") could be measured well only from space, and NASA planned the WMAP mission accordingly. In the meantime, however, dozens of teams have been racing to measure them using instruments on mountaintops, on the Antarctic plateau, and in the upper atmosphere. These experiments have covered only small patches of sky, while WMAP has surveyed the entire celestial sphere. And many have lacked WMAP's sensitivity or resolution and its ability to remove foreground contamination, such as from the Milky Way. However, the detector technology has advanced so much in just the last few years, and so many experiments have filled in each other's gaps, that the ground-based experiments beat WMAP to most of its major conclusions. So its results, once expected to be revolutionary, are now mostly confirmations.

Among WMAP's findings:

Space all across the universe is "flat," just like the familiar space right around us. That is, parallel lines will never meet no matter how far they are extended, and other aspects of geometry work normally no matter how large you look. Flat space means that the cosmic inflation theory underlying the Big Bang is right on target. This in turn implies that the familiar scenery of galaxies and galaxy clusters that we see extending across the visible universe also extends infinitely far beyond our cosmic horizon.

Everything in the universe. Normal 'baryonic' matter — in other words everything made of atoms, which are mostly protons and neutrons (baryons) — amounts to only about 4.4 percent of the total cosmic matter-and-energy budget. The other 95.6 percent is made of mysteries.

Courtesy NASA / WMAP Science Team.

The sum total of the universe is a mix of 4.4 ± 0.4 percent normal, baryonic matter (familiar stuff composed of atoms), 23 ± 4 percent nonbaryonic dark matter (probably particles as yet unknown to physicists), and 73 ± 4 percent "dark energy" (about which no one has a clue). These values build on, confirm, and improve previous determinations, some of which were made by completely independent astronomical methods.

The mysterious dark matter is "cold dark matter." The hypothesized hot or warm dark matter — particles moving at high speeds — is ruled out. In particular, neutrinos cannot constitute more than 1.5 percent of the matter and energy in the universe, meaning that the neutrino cannot have a mass of more than 0.23 electron volt — an important constraint for particle physicists.

The universe is 13.7 ± 0.2 billion years old. This is the best age determination ever achieved — accurate to better than 2 percent. It fits perfectly with recent determinations using a variety of independent astronomical methods.

The Hubble constant, the measure of how fast the universe is expanding today, is 71 ± 4 kilometers per second per megaparsec. This closely matches modern astronomical determinations — such as by the Hubble Space Telescope Key Project on the Extragalactic Distance Scale, which tracked Cepheid variable stars in relatively nearby galaxies. The Hubble constant has been a Holy Grail number for cosmologists since the 1920s and has stubbornly resisted refinement for much of that time.

The first stars seem to have formed when the universe was only 100 to 400 million years old (between redshift 30 and 11), generally earlier than astronomers once expected. WMAP found this by measuring polarization patterns in the microwave background. These yielded the date of the "reionization era," when the first starlight began ionizing the cold hydrogen that filled the universe during the "dark age" after the Big Bang cooled.

WMAP is making it possible to distinguish between different versions of the inflation theory itself. Inflation says the largest cosmic structures began as microscopic, random quantum fluctuations in the hot early universe at its very first moments. "The simplest version of inflation can now be rejected," said David Spergel (Princeton) at the press conference. That version predicted equal amounts of clumping on all size scales very close to time zero. Apparently something skewed the clumping, which must be telling us something important about how inflation worked.

"Every astronomer will remember where they were when they first heard the results from WMAP," said John Bahcall (Institute for Advance Study) at the press conference. The currently favored model of the universe "has been confirmed by WMAP with exquisite detail."

Today's results come less than halfway through WMAP's scheduled four-year mission and are based on just its first year of data. The craft's continuing measurements should refine the picture as time goes on.

Already, however, cosmologists are looking toward the next big step: The European Space Agency's Planck mission. It will bring much greater resolution and sensitivity to the task and may detect another type of signal: much more detailed polarization patterns in the microwaves. Planck may map these well enough for cosmologists to determine explicitly how inflation worked, and thereby to get a glimpse into the larger, underlying pre-existence outside of our universe from which the Big Bang sprang. Planck is due for launch in early 2007.