New Search for Dark Energy Goes Back in Time

Baryon acoustic oscillation (BAO) sounds like it could be technobabble from a Star Trek episode. BAO is real, but astronomers are searching for these particle fluctuations to do what seems like science fiction: look back in time to find clues about dark energy. The Baryon Oscillation Spectroscopic Survey(BOSS), a part of the Sloan Digital Sky Survey III (SDSS-III), took its “first light” of astronomical data last month, and will map the expansion history of the Universe.

“Baryon oscillation is a fast-maturing method for measuring dark energy in a way that’s complementary to the proven techniques of supernova cosmology,” said David Schlegel from the Lawrence Berkeley National Laboratory (Berkeley Lab), the Principal Investigator of BOSS. “The data from BOSS will be some of the best ever obtained on the large-scale structure of the Universe.”

BOSS uses the same telescope as the original Sloan Digital Sky Survey — 2.5-meter telescope
at Apache Point Observatory in New Mexico — but equipped with new, specially-built spectrographs to measure the spectra.

Senior Operations Engineer Dan Long loads the first cartridge of the night into the Sloan Digital Sky Survey telescope. The cartridge holds a “plug-plate” at the top which then holds a thousand optical fibers shown in red and blue. These cartridges are locked into the base of the telescope and are changed many times during a night. Photo credit: D. Long
Senior Operations Engineer Dan Long loads the first cartridge of the night into the Sloan Digital Sky Survey telescope. The cartridge holds a “plug-plate” at the top which then holds a thousand optical fibers shown in red and blue. These cartridges are locked into the base of the telescope and are changed many times during a night. Photo credit: D. Long

Baryon oscillations began when pressure waves traveled through the early universe. The same density variations left their mark as the Universe evolved, in the periodic clustering of visible matter in galaxies, quasars, and intergalactic gas, as well as in the clumping of invisible dark matter.

Comparing these scales at different eras makes it possible to trace the details of how the Universe has expanded throughout its history – information that can be used to distinguish among competing theories of dark energy.

“Like sound waves passing through air, the waves push some of the matter closer together as they travel” said Nikhil Padmanabhan, a BOSS researcher who recently moved from Berkeley Lab to Yale University. “In the early universe, these waves were moving at half the speed of light, but when the universe was only a few hundred thousand years old, the universe cooled enough to halt the waves, leaving a signature 500 million light-years in length.”

“We can see these frozen waves in the distribution of galaxies today,” said Daniel Eisenstein of the University of Arizona, the Director of the SDSS-III. “By measuring the length of the baryon oscillations, we can determine how dark energy has affected the expansion history of the universe. That in turn helps us figure out what dark energy could be.”

“Studying baryon oscillations is an exciting method for measuring dark energy in a way that’s complementary to techniques in supernova cosmology,” said Kyle Dawson of the University of Utah, who is leading the commissioning of BOSS. “BOSS’s galaxy measurements will be a revolutionary dataset that will provide rich insights into the universe,” added Martin White of Berkeley Lab, BOSS’s survey

On Sept. 14-15, 2009, astronomers used BOSS to measure the spectra of a thousand galaxies and quasars. The goal of BOSS is to measure 1.4 million luminous red galaxies at redshifts up to 0.7 (when the Universe was roughly seven billion years old) and 160,000 quasars at redshifts between 2.0 and 3.0 (when the Universe was only about three billion years old). BOSS will also measure variations in the density of hydrogen gas between the galaxies. The observation program will take five years.

Source: Sloan Digital Sky Survey

Variability in Type 1A Supernovae Has Implications for Studying Dark Energy


The discovery of dark energy, a mysterious force that is accelerating the expansion of the universe, was based on observations of type 1a supernovae, and these stellar explosions have long been used as “standard candles” for measuring the expansion. But not all type 1A supernovae are created equal. A new study reveals sources of variability in these supernovae, and to accurately probe the nature of dark energy and determine if it is constant or variable over time, scientists will have to find a way to measure cosmic distances with much greater precision than they have in the past.

“As we begin the next generation of cosmology experiments, we will want to use type 1a supernovae as very sensitive measures of distance,” said lead author Daniel Kasen, of a study published in Nature this week. “We know they are not all the same brightness, and we have ways of correcting for that, but we need to know if there are systematic differences that would bias the distance measurements. So this study explored what causes those differences in brightness.”

Kasen and his coauthors–Fritz Röpke of the Max Planck Institute for Astrophysics in Garching, Germany, and Stan Woosley, professor of astronomy and astrophysics at UC Santa Cruz–used supercomputers to run dozens of simulations of type 1a supernovae. The results indicate that much of the diversity observed in these supernovae is due to the chaotic nature of the processes involved and the resulting asymmetry of the explosions.

For the most part, this variability would not produce systematic errors in measurement studies as long as researchers use large numbers of observations and apply the standard corrections, Kasen said. The study did find a small but potentially worrisome effect that could result from systematic differences in the chemical compositions of stars at different times in the history of the universe. But researchers can use the computer models to further characterize this effect and develop corrections for it.

A type 1a supernova occurs when a white dwarf star acquires additional mass by siphoning matter away from a companion star. When it reaches a critical mass–1.4 times the mass of the Sun, packed into an object the size of the Earth–the heat and pressure in the center of the star spark a runaway nuclear fusion reaction, and the white dwarf explodes. Since the initial conditions are about the same in all cases, these supernovae tend to have the same luminosity, and their “light curves” (how the luminosity changes over time) are predictable.

Some are intrinsically brighter than others, but these flare and fade more slowly, and this correlation between the brightness and the width of the light curve allows astronomers to apply a correction to standardize their observations. So astronomers can measure the light curve of a type 1a supernova, calculate its intrinsic brightness, and then determine how far away it is, since the apparent brightness diminishes with distance (just as a candle appears dimmer at a distance than it does up close).

The computer models used to simulate these supernovae in the new study are based on current theoretical understanding of how and where the ignition process begins inside the white dwarf and where it makes the transition from slow-burning combustion to explosive detonation.

The simulations showed that the asymmetry of the explosions is a key factor determining the brightness of type 1a supernovae. “The reason these supernovae are not all the same brightness is closely tied to this breaking of spherical symmetry,” Kasen said.

The dominant source of variability is the synthesis of new elements during the explosions, which is sensitive to differences in the geometry of the first sparks that ignite a thermonuclear runaway in the simmering core of the white dwarf. Nickel-56 is especially important, because the radioactive decay of this unstable isotope creates the afterglow that astronomers are able to observe for months or even years after the explosion.

“The decay of nickel-56 is what powers the light curve. The explosion is over in a matter of seconds, so what we see is the result of how the nickel heats the debris and how the debris radiates light,” Kasen said.

Kasen developed the computer code to simulate this radiative transfer process, using output from the simulated explosions to produce visualizations that can be compared directly to astronomical observations of supernovae.

The good news is that the variability seen in the computer models agrees with observations of type 1a supernovae. “Most importantly, the width and peak luminosity of the light curve are correlated in a way that agrees with what observers have found. So the models are consistent with the observations on which the discovery of dark energy was based,” Woosley said.

Another source of variability is that these asymmetric explosions look different when viewed at different angles. This can account for differences in brightness of as much as 20 percent, Kasen said, but the effect is random and creates scatter in the measurements that can be statistically reduced by observing large numbers of supernovae.

The potential for systematic bias comes primarily from variation in the initial chemical composition of the white dwarf star. Heavier elements are synthesized during supernova explosions, and debris from those explosions is incorporated into new stars. As a result, stars formed recently are likely to contain more heavy elements (higher “metallicity,” in astronomers’ terminology) than stars formed in the distant past.

“That’s the kind of thing we expect to evolve over time, so if you look at distant stars corresponding to much earlier times in the history of the universe, they would tend to have lower metallicity,” Kasen said. “When we calculated the effect of this in our models, we found that the resulting errors in distance measurements would be on the order of 2 percent or less.”

Further studies using computer simulations will enable researchers to characterize the effects of such variations in more detail and limit their impact on future dark-energy experiments, which might require a level of precision that would make errors of 2 percent unacceptable.

Source: EurekAlert