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Homing in on dark energy

Largest set of type Ia supernovae studied by the hubble space telescope

DOE/Lawrence Berkeley National Laboratory

BERKELEY, CA -- A unique set of 11 distant Type Ia supernovae studied with the Hubble Space Telescope sheds new light on dark energy, according to the latest findings of the Supernova Cosmology Project, recently posted at http://www.arxiv.org/abs/astro-ph/0309368, and soon to appear in the Astrophysical Journal.

Light curves and spectra from the 11 distant supernovae constitute "a strikingly beautiful data set, the largest such set collected solely from space," says Saul Perlmutter, an astrophysicist at Lawrence Berkeley National Laboratory and leader of the Supernova Cosmology Project (SCP). The SCP is an international collaboration of researchers from the United States, Sweden, France, the United Kingdom, Chile, Japan, and Spain.

Type Ia supernovae are among astronomy's best "standard candles," so similar that their brightness provides a dependable gauge of their distance, and so bright they are visible billions of light years away.

The new study reinforces the remarkable discovery, announced by the Supernova Cosmology Project early in 1998, that the expansion of the universe is accelerating due to a mysterious energy that pervades all space. That finding was based on data from over three dozen Type Ia supernovae, all but one of them observed from the ground. A competing group, the High-Z Supernova Search Team, independently announced strikingly consistent results, based on an additional 14 supernovae, also predominantly observed from the ground.

Because the Hubble Space Telescope (HST) is unaffected by the atmosphere, its images of supernovae are much sharper and stronger and provide much better measurements of brightness than are possible from the ground. Robert A. Knop, assistant professor of physics and astronomy at Vanderbilt University in Nashville, Tenn., led the Supernova Cosmology Project's data analysis of the 11 supernovae studied with the HST and coauthored the Astrophysical Journal report with the 47 other members of the SCP.

"The HST data also provide a strong test of host-galaxy extinction," Knop says, referring to concerns that measurements of the true brightness of supernovae could be thrown off by dust in distant galaxies, which might absorb and scatter their light. But dust would also make a supernova's light redder, much as our sun looks redder at sunset because of dust in the atmosphere. Because the data from space show no anomalous reddening with distance, Knop says, the supernovae "pass the test with flying colors."

"Limiting such uncertainties is crucial for using supernovae -- or any other astronomical observations -- to explore the nature of the universe," says Ariel Goobar, a member of SCP and a professor of particle astrophysics at Stockholm University in Sweden. The extinction test, says Goobar, "eliminates any concern that ordinary host-galaxy dust could be a source of bias for these cosmological results at high-redshifts." (See "What is Host-Galaxy Extinction?" under additional information, below.)

The term for the mysterious "repulsive gravity" that drives the universe to expand ever faster is dark energy. The new data are able to provide much tighter estimates of the relative density of matter and dark energy in the universe: under straightforward assumptions, 25 percent of the composition of the universe is matter of all types and 75 percent is dark energy. Moreover, the new data provides a more precise measure of the "springiness" of the dark energy, the pressure that it applies to the universe's expansion per unit of density.

Among the numerous attempts to explain the nature of dark energy, some are allowed by these new measurements -- including the cosmological constant originally proposed by Albert Einstein -- but others are ruled out, including some of the simplest models of the theories known as quintessence. (See "What is Dark Energy?" under additional information, below.)

High-redshift supernovae are the best single tool for measuring the properties of dark energy -- and eventually determining what dark energy is. As supernova studies with the HST demonstrate, the best place to study high-redshift supernovae is with a telescope in space, unaffected by the atmosphere.

Nevertheless, "to make the best use of a telescope in space, it's essential to make the best use of the finest telescopes on the ground," says SCP member Chris Lidman of the European Southern Observatory.

For the supernovae in the present study, the SCP team invented a strategy whereby the Hubble Space Telescope could quickly respond to discoveries made from the ground, despite the need to schedule HST time long in advance. Working together, the SCP and the Space Telescope Science Institute implemented the strategy to superb effect.

The current study, based on HST observations of 11 supernovae, points the way to the next generation of supernova research: in the future, the SuperNova/Acceleration Probe, or SNAP satellite, will discover thousands of Type Ia supernovae and measure their spectra and their light curves from the earliest moments, through maximum brightness, until their light has died away.

SCP's Perlmutter is now leading an international group of collaborators based at Berkeley Lab who are developing SNAP with the support of the U.S. Department of Energy's Office of Science. It may be that the best candidate for a correct theory of dark energy will be identified soon after SNAP begins operating. A world of new physics will open as a result.

"New constraints on omega-m, omega-lambda, and w from an independent set of eleven high-redshift supernovae observed with the HST," by Robert A. Knop and 47 others (the Supernova Cosmology Project), will appear in the Astrophysical Journal and is currently available online at http://www.arxiv.org/abs/astro-ph/0309368.

For more about the Supernova Cosmology Project visit http://supernova.lbl.gov/. For more about the Hubble Space Telescope and the Space Telescope Science Institute visit http://www.stsci.edu/resources/. For more about the SNAP satellite visit http://snap.lbl.gov/.

The Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.

Additional information:

"What is Host-Galaxy Extinction?"
Type Ia supernovae are among the best standard candles known to astronomy -- objects whose distance can be determined because their intrinsic brightness is known or can be computed, just as the distance to a 100-watt bulb can be calculated by comparing its apparent brightness with its actual brightness.

Determining the expansion rate of the universe by comparing the brightness and redshift of far-off Type Ia supernovae therefore critically depends on accurate measurements of both.

One worrisome possible source of error in measuring distant supernovae has been host-galaxy extinction, the filtering effect of dust peculiar to the galaxy in which the supernova occurs. Dust occurs in our own galaxy too, but has been so extensively studied that it is of less concern in supernova distance measurements.

The concern is that distant supernovae appear dimmer not because of the accelerating effects of dark energy but, more prosaically, because of dust. There is a straightforward way to distinguish these effects, however, since dust normally reddens the light passing through it. Shorter, bluer wavelengths are absorbed and scattered more readily than longer, redder wavelengths.

"When you want to measure a supernova's brightness you can measure the light that was blue when it left, or the light that was red," says Greg Aldering, a member of the Supernova Cosmology Project and leader of the Nearby Supernova Factory program, which concentrates on studying the intrinsic properties of Type Ia supernovae. "Both measurements are valid, but what you want is to make sure you get the same answer on both sides of the spectrum. If the blue is fainter, you've got a dust problem."

The extraordinarily high quality of photometric data from the 11 distant supernovae studied by the Hubble Space Telescope in this study allowed their intrinsic brightness to be determined and compared in both bands.

The study determined that no anomalous effects of host-galaxy extinction occur at great distance; distant supernovae are comparable to nearby supernovae in this respect, underlining their utility as standard candles.

"What is Dark Energy?"
When SCP researchers initially set out to measure the expansion rate of the universe, they expected to find that distant supernovae appeared brighter than their redshifts would suggest, indicating a slowing rate of expansion. Instead they found the opposite: at a given redshift, distant supernovae were dimmer than expected. Expansion was accelerating.

Not only did this discovery mean that the universe would never come to an end, more fundamentally it implied that a large part of the universe is made of something we know nothing about -- the mysterious whatever-it-is that goes by the name "dark energy."

Later, new measurements of cosmic microwave background (CMB) radiation provided strong evidence that the universe is flat (having an overall geometry of space like Euclid's, in which parallel lines never meet or diverge) -- and because there is not enough matter in the universe, whether visible or dark, to produce flatness, the difference can be attributed to dark energy, providing a strong confirmation of the supernova measurements.

The first attempt to explain the nature of dark energy was by invoking Albert Einstein's notorious "cosmological constant," an extra term he introduced early in the the equations of the theory of general relativity in the 20th century under the mistaken impression, shared by astronomers and cosmologists of the time, that the universe was static. The cosmological constant, which Einstein signified by the Greek letter lambda, made it so.

Einstein happily abandoned the cosmological constant when, in 1929, Edwin Hubble found the universe was not static but expanding. However, lambda came back strong -- albeit 70 years later! -- when supernova studies led to the discovery that expansion was accelerating.

"For the cosmological constant, the vacuum -- space itself -- possesses a certain springiness," says Eric Linder, a cosmologist at Berkeley Lab and director of the Center for Cosmology and Spacetime Physics at Florida Atlantic University. "As you stretch it, you don't lose energy, you store extra energy in it just like a rubber band."

Such springiness, whether of matter, energy, or space itself, is described mathematically by a term called the equation-of-state parameter (w). For lambda, the value of this parameter is minus one, corresponding to a component of the universe that has "negative pressure" -- unlike matter or radiation, which have zero or positive pressure. True to its name, the cosmological constant doesn't change over time: the energy stored by lambda scales uniformly, increasing exactly as the volume of the universe increases.

The problem is that the most obvious source for lambda's stored energy is what quantum theory calls the energy of the vacuum ?? so much more powerful (10 to the 120th power!) than what's been observed for lambda, Linder says, that if this were the dark energy "it would overwhelm the expansion of the universe. It would have brought the universe to a swift end a miniscule fraction of a second after it was created in the big bang."

Other explanations of dark energy, called "quintessence," originate from theoretical high-energy physics. In addition to baryons, photons, neutrinos, and cold dark matter, quintessence posits a fifth kind of matter (hence the name), a sort of universe-filling fluid that acts like it has negative gravitational mass. The new constraints on cosmological parameters imposed by the HST supernova data, however, strongly discourage at least the simplest models of quintessence.

Quite different "topological defect" models attribute dark energy to defects created as the early universe cooled, during the phase changes that precipitated different forces and particles from a highly symmetrical early state.

Certain of these theoretical defects, known as domain walls, could have partitioned space into distinct cells whose boundaries would have repulsive gravity, thus filling the role of dark energy. But the new HST supernovae study rules out -- with 99 percent certainty -- domain walls as the source of dark energy.

While the case for the cosmological constant looks strong by comparison to these alternatives, many other exciting possibilities remain. Some even propose a cosmos in which our universe, having three dimensions of space, is afloat in a higher-dimensional world, with gravity free to interact among the dimensions.

Or there could be a time-varying form of dark energy that only temporarily mimics lambda. If it becomes less gravitationally repulsive in the future, it could bring acceleration to a halt, perhaps even causing expansion to reverse and triggering the collapse of the universe.

The opposite is also possible: superaccelerating dark energy. These models have w, the equation-of-state parameter, less than minus one -- unlike lambda, stored energy would not scale uniformly as the universe expands but increase faster than the increase in volume.

"One of the goals of the SuperNova/Acceleration Probe satellite is to determine whether w may be changing with time," says Saul Perlmutter, coprincipal investigator of the SNAP satellite now under development. "This will help us narrow the possibilities for the nature of dark energy. That's an exciting prospect for physicists, because understanding dark energy will be crucial to finding a final, unified picture of physics."

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