每日科技报告 第68期 Einstein's Theory Fights Off Challengers

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Einstein's Theory Fights Off Challengers
                                                                              
Two new and independent studies have put Einstein's General Theory ofRelativity to the test like never before. These results, made usingNASA's Chandra X-ray Observatory, show Einstein's theory is still thebest game in town.

This composite image of the galaxy cluster Abell 3376 shows X-raydata from the Chandra X-ray Observatory and the ROSAT telescope ingold, an optical image from the Digitized Sky Survey in red, green andblue, and a radio image from the VLA in blue. The "bullet-like"appearance of the X-ray data is caused by a merger, as material flowsinto the galaxy cluster from the right side. The giant radio arcs onthe left side of the image may be caused by shock waves generated bythis merger.

Each team of scientists took advantage of extensive Chandraobservations of galaxy clusters, the largest objects in the Universebound together by gravity. One result undercuts a rival gravity modelto General Relativity, while the other shows that Einstein's theoryworks over a vast range of times and distances across the cosmos.
The first finding significantly weakens a competitor to General Relativity known as "f(R) gravity."
"If General Relativity were the heavyweight boxing champion, thisother theory was hoping to be the upstart contender," said FabianSchmidt of the California Institute of Technology in Pasadena, who ledthe study. "Our work shows that the chances of its upsetting the champare very slim."
In recent years, physicists have turned their attention to competingtheories to General Relativity as a possible explanation for theaccelerated expansion of the universe. Currently, the most popularexplanation for the acceleration is the so-called cosmologicalconstant, which can be understood as energy that exists in empty space.This energy is referred to as dark energy to emphasize that it cannotbe directly detected.
In the f(R) theory, the cosmic acceleration comes not from an exoticform of energy but from a modification of the gravitational force. Themodified force also affects the rate at which small enhancements ofmatter can grow over the eons to become massive clusters of galaxies,opening up the possibility of a sensitive test of the theory.
Schmidt and colleagues used mass estimates of 49 galaxy clusters inthe local universe from Chandra observations, and compared them withtheoretical model predictions and studies of supernovas, the cosmicmicrowave background, and the large-scale distribution of galaxies.
They found no evidence that gravity is different from GeneralRelativity on scales larger than 130 million light years. This limitcorresponds to a hundred-fold improvement on the bounds of the modifiedgravitational force's range that can be set without using the clusterdata.
"This is the strongest ever constraint set on an alternative toGeneral Relativity on such large distance scales," said Schmidt. "Ourresults show that we can probe gravity stringently on cosmologicalscales by using observations of galaxy clusters."
The reason for this dramatic improvement in constraints can betraced to the greatly enhanced gravitational forces acting in clustersas opposed to the universal background expansion of the universe. Thecluster-growth technique also promises to be a good probe of othermodified gravity scenarios, such as models motivated byhigher-dimensional theories and string theory.
A second, independent study also bolsters General Relativity bydirectly testing it across cosmological distances and times. Up untilnow, General Relativity had been verified only using experiments fromlaboratory to Solar System scales, leaving the door open to thepossibility that General Relativity breaks down on much larger scales.
To probe this question, a group at Stanford University comparedChandra observations of how rapidly galaxy clusters have grown overtime to the predictions of General Relativity. The result is nearlycomplete agreement between observation and theory.
"Einstein's theory succeeds again, this time in calculating how manymassive clusters have formed under gravity's pull over the last fivebillion years," said David Rapetti of the Kavli Institute for ParticleAstrophysics and Cosmology (KIPAC) at Stanford University and SLACNational Accelerator Laboratory, who led the new study. "Excitingly andreassuringly, our results are the most robust consistency test ofGeneral Relativity yet carried out on cosmological scales."
Rapetti and his colleagues based their results on a sample of 238clusters detected across the whole sky by the now-defunct ROSAT X-raytelescope. These data were enhanced by detailed mass measurements for71 distant clusters using Chandra, and 23 relatively nearby clustersusing ROSAT, and combined with studies of supernovas, the cosmicmicrowave background, the distribution of galaxies and distanceestimates to galaxy clusters.
Galaxy clusters are important objects in the quest to understand theUniverse as a whole. Because the observations of the masses of galaxyclusters are directly sensitive to the properties of gravity, theyprovide crucial information. Other techniques such as observations ofsupernovas or the distribution of galaxies measure cosmic distances,which depend only on the expansion rate of the universe. In contrast,the cluster technique used by Rapetti and his colleagues measure inaddition the growth rate of the cosmic structure, as driven by gravity.
"Cosmic acceleration represents a great challenge to our modernunderstanding of physics," said Rapetti's co-author Adam Mantz ofNASA's Goddard Space Flight Center in Maryland. "Measurements ofacceleration have highlighted how little we know about gravity atcosmic scales, but we're now starting to push back our ignorance."
The pa-per by Fabian Schmidt was published in Physics Review D,Volume 80 in October 2009 and is co-authored by Alexey Vikhlinin of theHarvard-Smithsonian Center for Astrophysics in Cambridge,Massachusetts, and Wayne Hu of the University of Chicago, Illinois. The pa-per by David Rapetti was recently accepted for publication in the Monthly Notices of the Royal Astronomical Society and is co-authored by Mantz, Steve Allen of KIPAC at Stanford and Harald Ebeling of the Institute for Astronomy in Hawaii.NASA's Marshall Space Flight Center in Huntsville, Ala., manages theChandra program for NASA's Science Mission Directorate in Washington.The Smithsonian Astrophysical Observatory controls Chandra's scienceand flight operations from Cambridge, Mass.

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