Astrophysicists now have a better idea of how fast the universe is expanding.
A study by an intercollegiate team of researchers has developed a new method of reading measurements taken from exploding stars called supernovae. These measurements are important in measuring the rate of expansion of the universe, and may lead to an understanding of the mysterious force called dark energy, which is hypothesized to be responsible for the expansion of the universe. It has been known since the 1990s that the universe is expanding at an increasing rate, a finding that contradicts the laws of physics as they are currently understood, as one would expect gravity to make the universe’s expansion slow down. The work was published in the Astrophysical Journal.
“The big question in cosmology is dark energy,” said Clare Saunders, graduate student at University of California Berkeley and the study’s lead author.
Currently, many experimental efforts in astrophysics are focused on better measuring the cosmic history of dark energy in order to build better models of the universe’s evolution. One of those efforts — of which Charles Baltay, Yale professor of physics and study co-author, is a member — is focused on studying the light emitted by supernovae.
Data taken from supernovae’s positions and velocities give accurate measurements of cosmic expansion. In order to accurately read those data, scientists must be able to take into account the “Doppler effect.” This effect causes light that comes from supernovae that are moving away from the Earth to have a wavelength that is slightly longer than it would be otherwise.
Fortunately, the light emitted by supernovae comes in only certain wave lengths, so by comparing data taken from actual supernovae to an idealized model, scientists can reason the velocity with which a supernova is moving toward or away from the Earth. This study figured out a way to make those velocity estimates better by comparing snapshot measurements to measurements taken over time.
The challenge of learning the true nature of dark matter is one of the great open problems in physics, Saunders said.
“How do we have to modify the laws of physics to explain the expansion of the universe?” Baltay asked.
Baltay is also working to design a telescope for NASA which will increase the number of observable supernovae. The new telescope, called the Wide Field Infrared Survey Telescope, will constantly survey the sky for supernovae.
The research performed managed to reduce the error in observations by 50 percent, according to Greg Aldering, study co-author and professor of physics at University of California Berkeley. With that decrease in error, scientists can take only one-fourth the amount of data as before without affecting the quality of their conclusions.
“We believe that really new data can come from the space experiment,” said Baltay.
Baltay and another team also submitted for review last week a second paper on supernovae data. In this paper, the researchers analyzed data collected from few different telescopes, comparing data sets across telescopes and calibration methods.
The researchers decided to use a data set of supernovae called the Nearby Supernova Factory. Although other data sets exist, the researchers did not take data over time, eliminating the possibility of this kind of statistical analysis. Because of the quality of the NSNF data set, the team was able to build an empirical model that could be applied to other supernovae data sets collected with alternative methods. Aldering said that because no complete model of supernovae dynamics exist, much of their work is empirical, and this type of analysis could only be done with observed data.
“We don’t know a lot about supernovae, and we have to try to use them as cosmological tools,” he said.
The research was published within a week of another study in the Astrophysical Journal, this one on galactic evolution.
Millions of galaxies have been observed, but only a very small number may hold the key to explaining much of cosmic history. These galaxies — which are far away, moving away fast and very old — are called “high-redshifted” galaxies, so named because they appear red due to the Doppler effect. The team who performed the research, including Priyamvada Natarajan, professor of astrophysics at Yale and study co-author, found a novel method of finding these rare galaxies.
The method, called “gravitational lensing,” is used to spot faint galaxies, or galaxies that are dim from Earth’s perspective. The theory of general relativity postulates that light bends when it passes an extremely massive object, and the team was able to use this fact to look behind large galactic clusters, using their mass as a lens to see faint galaxies behind them.
“People are excited about galaxies at high redshift (z>6) because they tell us something about the beginning of structure formation [how galaxies are formed],” said Harald Ebeling, co-author of the study and professor of astronomy at the University of Hawaii.
These galaxies represent some of the first structures to be visible since light could be transmitted across the universe. However, up until now, only a handful have ever been discovered, preventing scientists from analyzing them.
Johan Richard, study co-author and researcher at Université Lyon, said he plans to use the methods developed in this analysis on data taken from other clusters.
There are 100 billion to 200 billion galaxies in the universe.
Correction: Feb. 23
A previous version of this article misidentified study co-author Johan Richard as a female.
Correction: Feb. 26
A previous version of the story misspelled Clare Saunders’ name.