Galaxy clusters
September 29, 2011
Einstein was right about gravity – according to a newly published study from the University of Copenhagen.
In a paper published today in the journal Nature, researchers at the Dark Cosmology Center at the Neils Bohr Institute presented new findings examining how clusters of galaxies affect the light that leaves them.
Galaxy clusters are groupings of galaxies, held together by gravity. The combined gravitational force of such galaxies affects the wavelength of photons, a phenomenon known as "redshift."
In addition to upholding Einstein's ideas about gravity, the findings also add ammunition to the concept of "dark energy," the force behind the accelerating expansion of the universe. While Einstein's theories have continued to hold up over the last century - despite last week's highly unusual reports of possible faster-than-light neutrinos - they have never before been confirmed on such a large scale.
Statistical redshift analysis
Henk Hoekstra, a professor at the Leiden Observatory in the Netherlands, explained redshift through something with which we are all familiar: the Doppler effect.
If an ambulance is moving away from you, the pitch of siren goes down. Light does the same thing – moving away, it appears red. This is known as redshift.
"In this case, what they're measuring is light leaving a cluster," said Hoekstra, who was not involved with the study.
The international team of researchers in Copenhagen used data on 8,000 galaxy clusters, gathered by the Sloan Digital Sky Survey.
Researchers analyzed publicly available data on galaxy spectrums, compiled by the Sloan Foundation from optical telescopes in New Mexico, in the United States.
By stacking different classes of data, the researchers were able to show the effects of redshift.
"The important thing was statistics," said Radek Wojtak, a Polish post-doctoral researcher and lead author of the report.
"So far, we've had tests on white dwarves or black holes, also in our solar system. But these are extremely small scales," Wotjak told Deutsche Welle.
This statistical method – the first test of its kind to be applied on a cosmological scale – reflects both strength and weakness, said Hoekstra.
"It's a very elegant thing they've done - but the statistical uncertainties are fairly large," Hoekstra told Deutsche Welle.
The limited data set constrains its statistical power, Hoekstra said. "It's consistent with general relativity, but you can't rule out the other models."
Kick it like Einstein
Albert Einstein's theory of general relativity, developed in the early part of last century, describes gravity as a metric property governed by certain equations.
Dark matter and dark energy are two building blocks of Einstein's theory.
Based on measurements of the universe's mass, scientists largely agree about the existence of dark matter, so-called because it neither emits nor scatters light, nor is detectable in any direct manner.
Wotjak said the Dark Cosmology Centre's research has contributed to proving Einstein's theory, providing "very strong evidence for the existence of dark matter."
"Probably we can answer this question in 10 to 20 years," Wotjak said. Hoekstra added that scientists at the European Organization for Nuclear Research (CERN) in Switzerland may actually create a small amount of dark matter soon.
To make his equations work out, Einstein inserted a constant to balance gravity. This constant, known as "dark energy," has not yet been measured, and remains one of the biggest questions to date in the field of astrophysics.
"Nobody predicted the existence of dark energy, it's a big problem," Hoekstra said, adding that Einstein later regretted inserting this constant.
Dark energy plays a critical role in the universe's expansion – something which is now believed to be accelerating.
Other theories get around the issue of dark energy, for example research indicating that what looks like acceleration of an expanding universe is actually an illusion.
More data needed
Hoekstra said that Euclid, one model of satellite the European Space Agency is considering building, is to further test this new result by looking at how structures such as galaxies form in the universe.
Hoekstra added that he's been putting considerable energy into determining what the Euclid satellite would require. A decision on it is expected next week.
Author: Sonya Angelica Diehn
Editor: Cyrus Farivar