Einstein validated on cosmic scale

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Einstein validated on cosmic scale

#1  Postby RichardPrins » Mar 11, 2010 12:10 am

Einstein validated on cosmic scale
UC BERKELEY (US)—An analysis of more than 70,000 galaxies demonstrates that the universe—at least up to a distance of 3.5 billion light years from Earth—plays by the rules set out 95 years ago by Albert Einstein in his General Theory of Relativity.

By calculating the clustering of these galaxies, which stretch nearly one-third of the way to the edge of the universe, and analyzing their velocities and distortion from intervening material, the researchers have shown that Einstein’s theory explains the nearby universe better than alternative theories of gravity.

One major implication of the new study is that the existence of dark matter is the most likely explanation for the observation that galaxies and galaxy clusters move as if under the influence of some unseen mass, in addition to the stars astronomers observe.

“The nice thing about going to the cosmological scale is that we can test any full, alternative theory of gravity, because it should predict the things we observe,” says coauthor Uros Seljak, a professor of physics and of astronomy at the University of California, Berkeley, and a faculty scientist at Lawrence Berkeley National Laboratory who is on leave at the Institute of Theoretical Physics at the University of Zurich. “Those alternative theories that do not require dark matter fail these tests.”

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Above, a partial map of the distribution of galaxies in the Sloan Digital Sky Survey, going out to a distance of 7 billion light years. The amount of galaxy clustering that we observe today is a signature of how gravity acted over cosmic time, and allows as to test whether general relativity holds over these scales. (Credit: M. Blanton/Sloan Digital Sky Survey)

In particular, the tensor-vector-scalar gravity (TeVeS) theory, which tweaks general relativity to avoid resorting to the existence of dark matter, fails the test.

The result conflicts with a report late last year that the very early universe, between 8 and 11 billion years ago, did deviate from the general relativistic description of gravity.

Seljak and colleagues report their findings in the journal Nature.

Einstein’s General Theory of Relativity holds that gravity warps space and time, which means that light bends as it passes near a massive object, such as the core of a galaxy. The theory has been validated numerous times on the scale of the solar system, but tests on a galactic or cosmic scale have been inconclusive.

“There are some crude and imprecise tests of general relativity at galaxy scales, but we don’t have good predictions for those tests from competing theories,” Seljak says.

Such tests have become important in recent decades because the idea that some unseen mass permeates the universe disturbs some theorists and has spurred them to tweak general relativity to get rid of dark matter.

TeVeS, for example, says that acceleration caused by the gravitational force from a body depends not only on the mass of that body, but also on the value of the acceleration caused by gravity.

The discovery of dark energy, an enigmatic force that is causing the expansion of the universe to accelerate, has led to other theories, such as one dubbed f(R), to explain the expansion without resorting to dark energy.

Tests to distinguish between competing theories are not easy, Seljak says. A theoretical cosmologist, he noted that cosmological experiments, such as detections of the cosmic microwave background, typically involve measurements of fluctuations in space, while gravity theories predict relationships between density and velocity, or between density and gravitational potential.

“The problem is that the size of the fluctuation, by itself, is not telling us anything about underlying cosmological theories. It is essentially a nuisance we would like to get rid of,” Seljak explains. “The novelty of this technique is that it looks at a particular combination of observations that does not depend on the magnitude of the fluctuations. The quantity is a smoking gun for deviations from general relativity.”

Physicists from the University of California, Berkeley, University of Zurich, and Princeton University contributed to the study.
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Re: Einstein validated on cosmic scale

#2  Postby Farsight » Mar 12, 2010 1:16 am

This is an interesting one - I was talking about it earlier. I'm all for general relativity, but actually, it says energy causes gravity, not matter. See The Foundation of the General Theory of Relativity" (3.6 Mbytes). On page 185 Einstein says the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy". A gravitational field is a region that contains energy and in itself causes gravity, hence an integration approach is required, as per page 201. But we don't consider a gravitational field to be dark matter, at least not in the usual sense. However it is "dark", and the mass of a system is a measure of its energy content, so if you defined the space around a planet as a system, it has a mass of sorts. See http://en.wikipedia.org/wiki/Mass for "..mass is added to systems when energy is added.." and http://en.wikipedia.org/wiki/Mass_in_general_relativity for "..the mass of a system in general relativity may not even be defined..". All very interesting.
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Re: Einstein validated on cosmic scale

#3  Postby Joe09 » Mar 12, 2010 9:55 pm

E=mc2
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Foiling an attack on general relativity

#4  Postby RichardPrins » Mar 12, 2010 10:25 pm

Foiling an attack on general relativity
Einstein's General Theory of Relativity explains gravity in terms of the curvature of space by mass. Dating from the second decade of the 20th century, after more than 90 years it is still the basis of our understanding of how gravity works to shape the cosmos.

But as evidence for a universe filled with dark matter and dark energy has mounted, General Relativity's ability to explain the structure and expansion of the universe has faced new challenges.

Some theorists deny that dark matter or dark energy exist, suggesting that there's a problem with General Relativity's handling of gravity. They hope to explain away the apparent gravitational effects of dark matter, and the apparent accelerating expansion of the universe caused by dark energy, with appeals to modified gravitational theories.

"One of the first proper theories of modified gravity is called the tensor-vector-scalar theory, or TeVeS," says Uros Seljak, a member of Lawrence Berkeley National Laboratory's Physics Division, who is also a professor of physics and astronomy at the University of California at Berkeley and a professor of physics at the University of Zurich. "By a 'proper' theory, I mean one that makes definite predictions about what we should be able to observe if it is true."

Testing predictions about the shape and growth of the cosmos requires measurements on the scale of the cosmos itself. Only in recent years have surveys like the Sloan Digital Sky Survey (SDSS), which has collected spectra of well over a million distant stars, quasars, and galaxies since it began operation in 1998, made such universe-spanning measurements possible.

Now Seljak and a group of colleagues including some of his current and former students, as well as James E. Gunn, founder of SDSS, have analyzed some 70,000 red luminous galaxies from SDSS's collection to test the TeVeS theory of modified gravity, and with it another modified theory of gravity called f(R), which seeks to explain the accelerating universe without recourse to dark energy.

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A collection of galaxies from the Sloan Digital Sky Survey. Red galaxies composed of older stars are more luminous; the recent study used a sample of 70,000 red luminous galaxies to combine galaxy clustering, weak gravitational lensing, and redshifts to compare and test competing theories of gravity. Credit: Image by Michael Blanton for SDSS


"Our measurement combines gravitational lensing, galaxy clustering, and the growth rate of the large-scale structure of the universe," Seljak explains. "No one of these by itself could test modified gravity theories because of large uncertainties in the observations at cosmological distances."

The collaborators report their findings in the March 11, 2010 issue of Nature. First authors of the paper are Princeton University graduate student Reinabelle Reyes and recent Princeton Ph.D. Rachel Mandelbaum. With Seljak and Gunn, the other authors are Tobias Baldauf, Lucas Lombriser, and Robert E. Smith of the University of Zurich.

Combining measurements to reduce uncertainty

An important source of uncertainty in cosmological measurements is so-called "galaxy bias," which can be observed as the change in the way galaxies cluster according to what type of galaxies they are, for example blue galaxies or more luminous red ones. One explanation is that galaxy bias is due to the difference between the distribution of galaxies and the distribution of the invisible dark matter that underlies them — but this doesn't help when testing a theory that says there's no such thing as dark matter.

"Galaxy bias is one of those 'nuisance parameters' that tells us nothing by itself," says Seljak. "Because it tells us nothing on its own about dark matter or dark energy or other cosmological ideas, we'd like to get it out of the way."

Galaxy bias can essentially be bypassed by combining measures of gravitational lensing – the way intervening mass bends the light from more distant luminous objects, making them appear distorted – with galaxy clustering and the growth of structure. The three together yield a quantity called EG, originally proposed by Pengjie Zhang of Shanghai Observatory and his collaborators as a way to test cosmological models.

Modified gravitational theories don't predict the same value of EG as General Relativity (with dark matter thrown in) when it comes to comparing the mass density of the universe to the growth of its structure. In general, modified theories predict faster growth of structure, making EG smaller.

Growth rate can be calculated from redshift surveys, which measure velocities of galaxies. Galaxy clustering and weak gravitational lensing – which must be used at cosmological distances, where the distorted shapes of background galaxies can't be measured directly but have to be derived statistically – can be used to estimate mass density.

The value of EG that Seljak's colleagues obtained from their deeper-than-ever probe of cosmological growth still has a large uncertainty, some 16 percent. Even with wide error bars, however, the value is enough to exclude the predictions of the TeVeS "no dark matter" theory.

The best fit of the value of EG from this survey assumes that dark matter exists and General Relativity is correct. The uncertainty is still too great to rule out f(R) theories that modify gravity so as to exclude dark energy, however.

"To test theories that do away with dark energy, we'll need much larger data sets for better control of systematic errors," says Seljak. "Fortunately, SDSS-III is now underway, with most of its telescope time devoted to BOSS."

BOSS, the Baryon Oscillation Spectroscopic Survey led by David Schlegel of Berkeley Lab's Physics Division, will collect data from over a million and a half luminous red galaxies and quasars. Though BOSS's main purpose is to provide an independent measure of dark energy through the technique called baryon acoustic oscillation, the data from BOSS will be some of the best ever obtained on the large-scale structure of the universe and can be used to narrow the uncertainty of measuring EG.

As to whether or not Einstein needs to be updated, the final answer may have to await BigBOSS, the joint Berkeley Lab/National Science Foundation proposal to survey some 50 million galaxies in both the northern and southern hemispheres over a 10-year period. BigBOSS would produce an astonishingly wide and deep survey of the sky, enough to tighten the error bars around the best gravitational theory of all.

Will it be General Relativity after all? Stay tuned.
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