14-01-11

Harvard-Smithsonian Center For Astrophysics - Better Measuring Dark Energy

 

 

Press Release

Release No.: 2011-04

For Release: Thursday, January 13, 2011 09:00:00 AM EST

 

Dark Energy.jpg

 

Copyright : http://commons.wikimedia.org/wiki/File:Cosmological_compo...

 


The Best Way to Measure Dark Energy Just Got Better

 

Seattle, WA

 

Dark energy is a mysterious force that pervades all space, acting as a "push" to accelerate the Universe's expansion.

 

Despite being 70 percent of the Universe, dark energy was only discovered in 1998 by two teams observing Type Ia supernovae.

 

A Type 1a supernova is a cataclysmic explosion of a white dwarf star.

 

These supernovae are currently the best way to measure dark energy because they are visible across intergalactic space.

 

Also, they can function as "standard candles" in distant galaxies since the intrinsic brightness is known.

 

Just as drivers estimate the distance to oncoming cars at night from the brightness of their headlights, measuring the apparent brightness of a supernova yields its distance (fainter is farther).

 

Measuring distances tracks the effect of dark energy on the expansion of the Universe.

 

The best way of measuring dark energy just got better, thanks to a new study of Type Ia supernovae led by Ryan Foley of the Harvard-Smithsonian Center for Astrophysics. 

 

He has found a way to correct for small variations in the appearance of these supernovae, so that they become even better standard candles.

 

The key is to sort the supernovae based on their color.

 

"Dark energy is the biggest mystery in physics and astronomy today.

 

Now, we have a better way to tackle it," said Foley, who is a Clay Fellow at the Center.

 

He presented his findings in a press conference at the 217th meeting of the American Astronomical Society.

 

The new tool also will help astronomers to firm up the cosmic distance scale by providing more accurate distances to faraway galaxies.

 

Type Ia supernovae are used as standard candles, meaning they have a known intrinsic brightness.

 

However, they're not all equally bright.

 

Astronomers have to correct for certain variations.

 

In particular, there is a known correlation between how quickly the supernova brightens and dims (its light curve) and the intrinsic peak brightness.

 

Even when astronomers correct for this effect, their measurements still show some scatter, which leads to inaccuracies when calculating distances and therefore the effects of dark energy.

 

Studies looking for ways to make more accurate corrections have had limited success until now.

 

"We've been looking for this sort of 'second-order effect' for nearly two decades," said Foley.

 

Foley discovered that after correcting for how quickly Type Ia supernovae faded,

they show a distinct relationship between the speed of their ejected material and their color: the faster ones are slightly redder and the slower ones are bluer.

 

Previously, astronomers assumed that redder explosions only appeared that way because of intervening dust, which would also dim the explosion and make it appear farther than it was.

 

Trying to correct for this, they would incorrectly calculate that the explosion was closer than it appeared.

 

Foley's work shows that some of the color difference is intrinsic to the supernova itself.

 

The new study succeeded for two reasons.

 

First, it used a large sample of more than 100 supernovae.

 

More importantly, it went back to "first principles" and reexamined the assumption that Type Ia supernovae are one average color.

 

The discovery provides a better physical understanding of Type Ia supernovae and their intrinsic differences.

 

It also will allow cosmologists to improve their data analysis and make better measurements of dark energy - an important step on the road to learning what this mysterious force truly is, and what it means for the future of the cosmos.

 

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

 


Copyright : http://www.cfa.harvard.edu/news/2011/pr201104.html

 

07-02-10

Distant Galaxies Unlock New Secrets of Dark Matter

dark matterTo weigh the universe, scientists use two kinds of cosmic scales: one to measure all the regular matter out there, and another to deduce how much invisible dark matter remains hidden underneath.These calculations have been taken further than ever before by a new study that tallied both types of mass in smaller and more distant groups of galaxies than any previous projects. The project found that these faraway galactic clusters have roughly the same proportion of dark matter to regular matter as the closer galaxy groups do.The findings could help astronomers understand more about dark matter, as well as its even stranger sibling – dark energy.Invisible universeDark matter is a form of stuff that does not interact with light, so cannot be seen, but makes its presence felt by exerting a gravitational pull on normal matter.Astronomers measure how much dark matter lies in galaxies by a fluke of physics called gravitational lensing. This phenomenon, predicted by Einstein's theory of general relativity, causes light to curve as it flies through space-time that has been dented by the gravity of large bodies of mass. For example, groups of massive galaxies will gravitationally warp the space-time around them, forcing light to bend as it passes through, and causing them to look distorted when their light reaches our telescopes. Scientists can tell how much total mass there is by how much of this distortion occurs.Next, researchers calculate how much normal matter is in a cluster of galaxies by looking at its X-ray light, since the light must be coming from only the regular stars and gas that make up the cluster.Comparing these two calculations — the total matter to just the regular matter — gives a ratio astronomers call the mass-luminosity relation. So far, the mass-luminosity relation has been measured well for nearby, large galaxy clusters, but there has not been good enough X-ray data to probe farther or smaller, dimmer clusters of galaxies."We can map out the big cities, but no one's been able to map out the villages yet," said Alexie Leauthaud of the Lawrence Berkeley National Laboratory in Berkeley, Calif., leader of the new study.New rangesAstronomers used observations from the European Space Agency's XMM-Newton satellite and from NASA's Chandra satellite, as well as data from the Hubble Space Telescope's Cosmic Evolution Survey (COSMOS). These ultra-high resolution photos allowed the scientists to extend the mass-luminosity relation further than ever before.With such dim objects, the gravitational lensing wasn't immediately apparent. So researchers used a statistical analysis to measure the orientation and shape of the galaxies to find small distortions due to so-called weak lensing.They found that the same general ratio of dark matter to normal matter prevailed in these distant, small clusters as for nearby, larger clusters."We didn't know what to expect going down to lower masses or [farther distances], and we find this nice simple relationship," Leauthaud told SPACE.com. "Now the aim is to find out why we find this nice, simple relationship."Dark energy enigmaThe finding may help shed light on an even more bizarre aspect of the universe — dark energy. Dark energy is the name given to whatever mysterious force is causing the universe to accelerate as it expands."We want to try to understand the properties of dark energy," Leauthaud said. "One way to measure properties of dark energy is to measure the number of structures that have formed for a given amount of dark matter."Dark energy basically works against gravity in a tug-of-war. While gravity constantly pulls mass inward, encouraging things to clump together and condense into smaller space, dark energy does the opposite. This force somehow pulls everything apart, causing everything in the universe to move away from everything else at ever-increasing speeds.When mass clumps together enough to form galaxies, it means that gravity has won on those scales, helping things to stick together despite the pull of dark energy. So the more astronomers can measure when and how structures formed in the universe, the better they can understand just how far dark energy's pull reaches.Copyright http://www.space.com/scienceastronomy/dark-matter-galaxy-clusters-100126.html?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+spaceheadlines+%28SPACE.com+Headline+Feed%29&utm_content=Google+Reader