Featured Research

from universities, journals, and other organizations

Searching for dark energy with the whole world's supernova dataset

April 24, 2010
DOE/Lawrence Berkeley National Laboratory
The Supernova Cosmology Project's Union2 compilation and reanalysis of decades of supernova surveys from the world's leading researchers, with the addition of six high-redshift supernovae, puts new bounds on possible values for the nature of dark energy. Einstein's cosmological constant comfortably fits the data, but there's still plenty of room at the top for dynamical theories.

Two views of one of the six new distant supernovae in the Supernova Cosmology Project's just-released Union2 survey, which among other refinements compares ground-based infrared observations (in this case by Japan's Subaru Telescope on Mauna Kea) with follow-up observations by the Hubble Space Telescope.
Credit: Image courtesy of DOE/Lawrence Berkeley National Laboratory

The international Supernova Cosmology Project (SCP), based at the U.S. Department of Energy's Lawrence Berkeley National Laboratory, has announced the Union2 compilation of hundreds of Type Ia supernovae, the largest collection ever of high-quality data from numerous surveys. Analysis of the new compilation significantly narrows the possible values that dark energy might take -- but not enough to decide among fundamentally different theories of its nature.

"We've used the world's best-yet dataset of Type Ia supernovae to determine the world's best-yet constraints on dark energy," says Saul Perlmutter, leader of the SCP. "We've tightened in on dark energy out to redshifts of one" -- when the universe was only about six billion years old, less than half its present age -- "but while at lower redshifts the values are perfectly consistent with a cosmological constant, the most important questions remain."

That's because possible values of dark energy from supernovae data become increasingly uncertain at redshifts greater than one-half, the range where dark energy's effects on the expansion of the universe are most apparent as we look farther back in time. Says Perlmutter of the widening error bars at higher redshifts, "Right now, you could drive a truck through them."

As its name implies, the cosmological constant fills space with constant pressure, counteracting the mutual gravitational attraction of all the matter in the universe; it is often identified with the energy of the vacuum. If indeed dark energy turns out to be the cosmological constant, however, even more questions will arise.

"There is a huge discrepancy between the theoretical prediction for vacuum energy and what we measure as dark energy," says Rahman Amanullah, who led SCP's Union2 analysis; Amanullah is presently with the Oskar Klein Center at Stockholm University and was a postdoctoral fellow in Berkeley Lab's Physics Division from 2006 to 2008. "If it turns out in the future that dark energy is consistent with a cosmological constant also at early times of the universe, it will be an enormous challenge to explain this at a fundamental theoretical level."

A major group of competing theories posit a dynamical form of dark energy that varies in time. Choosing among theories means comparing what they predict about the dark energy equation of state, a value written w. While the new analysis has detected no change in w, there is much room for possibly significant differences in w with increasing redshift (written z).

"Most dark-energy theories are not far from the cosmological constant at z less than one," Perlmutter says. "We're looking for deviations in w at high z, but there the values are very poorly constrained."

In their new analysis to be published in the Astrophysical Journal, the Supernova Cosmology Project reports on the addition of several well-measured, very distant supernovae to the Union2 compilation.

Dark energy fills the universe, but what is it?

Dark energy was discovered in the late 1990s by the Supernova Cosmology Project and the competing High-Z Supernova Search Team, both using distant Type Ia supernovae as "standard candles" to measure the expansion history of the universe. To their surprise, both teams found that expansion is not slowing due to gravity but accelerating.

Other methods for measuring the history of cosmic expansion have been developed, including baryon acoustic oscillation and weak gravitational lensing, but supernovae remain the most advanced technique. Indeed, in the years since dark energy was discovered using only a few dozen Type Ia supernovae, many new searches have been mounted with ground-based telescopes and the Hubble Space Telescope; many hundreds of Type Ia's have been discovered; techniques for measuring and comparing them have continually improved.

In 2008 the SCP, led by the work of team member Marek Kowalski of the Humboldt University of Berlin, created a way to cross-correlate and analyze datasets from different surveys made with different instruments, resulting in the SCP's first Union compilation. In 2009 a number of new surveys were added.

The inclusion of six new high-redshift supernovae found by the SCP in 2001, including two with z greater than one, is the first in a series of very high-redshift additions to the Union2 compilation now being announced, and brings the current number of supernovae in the whole compilation to 557.

"Even with the world's premier astronomical observatories, obtaining good quality, time-critical data of supernovae that are beyond a redshift of one is a difficult task," says SCP member Chris Lidman of the Anglo-Australian Observatory near Sydney, a major contributor to the analysis. "It requires close collaboration between astronomers who are spread over several continents and several time zones. Good team work is essential."

Union2 has not only added many new supernovae to the Union compilation but has refined the methods of analysis and in some cases improved the observations. The latest high-z supernovae in Union2 include the most distant supernovae for which ground-based near-infrared observations are available, a valuable opportunity to compare ground-based and Hubble Space Telescope observations of very distant supernovae.

Type Ia supernovae are the best standard candles ever found for measuring cosmic distances because the great majority are so bright and so similar in brightness. Light-curve fitting is the basic method for standardizing what variations in brightness remain: supernova light curves (their rising and falling brightness over time) are compared and uniformly adjusted to yield comparative intrinsic brightness. The light curves of all the hundreds of supernova in the Union2 collection have been consistently reanalyzed.

The upshot of these efforts is improved handling of systematic errors and improved constraints on the value of the dark energy equation of state with increasing redshift, although with greater uncertainty at very high redshifts. When combined with data from cosmic microwave background and baryon oscillation surveys, the "best fit cosmology" remains the so-called Lambda Cold Dark Matter model, or ΛCDM.

ΛCDM has become the standard model of our universe, which began with a big bang, underwent a brief period of inflation, and has continued to expand, although at first retarded by the mutual gravitational attraction of matter. As matter spread and grew less dense, dark energy overcame gravity, and expansion has been accelerating ever since.

To learn just what dark energy is, however, will first require scientists to capture many more supernovae at high redshifts and thoroughly study their light curves and spectra. This can't be done with telescopes on the ground or even by heavily subscribed space telescopes. Learning the nature of what makes up three-quarters of the density of our universe will require a dedicated observatory in space.

This work was supported in part by the U.S. Department of Energy's Office of Science.

Story Source:

The above story is based on materials provided by DOE/Lawrence Berkeley National Laboratory. Note: Materials may be edited for content and length.

Cite This Page:

DOE/Lawrence Berkeley National Laboratory. "Searching for dark energy with the whole world's supernova dataset." ScienceDaily. ScienceDaily, 24 April 2010. <www.sciencedaily.com/releases/2010/04/100421133120.htm>.
DOE/Lawrence Berkeley National Laboratory. (2010, April 24). Searching for dark energy with the whole world's supernova dataset. ScienceDaily. Retrieved July 29, 2014 from www.sciencedaily.com/releases/2010/04/100421133120.htm
DOE/Lawrence Berkeley National Laboratory. "Searching for dark energy with the whole world's supernova dataset." ScienceDaily. www.sciencedaily.com/releases/2010/04/100421133120.htm (accessed July 29, 2014).

Share This

More Space & Time News

Tuesday, July 29, 2014

Featured Research

from universities, journals, and other organizations

Featured Videos

from AP, Reuters, AFP, and other news services

Russia Saves Gecko Sex Satellite, Media Has Some Fun With It

Russia Saves Gecko Sex Satellite, Media Has Some Fun With It

Newsy (July 27, 2014) The satellite is back under ground control after a tense few days, but with a gecko sex experiment on board, the media just couldn't help themselves. Video provided by Newsy
Powered by NewsLook.com


NASA (July 25, 2014) NASA EDGE webcasts live from Vandenberg AFB for the launch of the Oribiting Carbon Observatory-2 (OCO) launch. Video provided by NASA
Powered by NewsLook.com
This Week @ NASA, July 25, 2014

This Week @ NASA, July 25, 2014

NASA (July 25, 2014) Apollo 11 celebration, Next Giant Leap anticipation, ISS astronauts appear in the House and more... Video provided by NASA
Powered by NewsLook.com
Space to Ground: Coming and Going

Space to Ground: Coming and Going

NASA (July 25, 2014) One station cargo ship leaves, another arrives, aquatic research and commercial spinoffs. Questions or comments? Use #spacetoground to talk to us. Video provided by NASA
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.


Breaking News:
from the past week

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News


Free Subscriptions

Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile

Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?

Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins