Einstein passes cosmic test
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By Zeeya Merali
It's another victory for Einstein -- albeit not a resounding one. General relativity has been confirmed at the largest scale yet. But the galactic tests used to put the theory through its paces cannot rule out all rival theories of gravity.
General relativity has been rigorously tested within the Solar System, where it explains the motion of planets with precision. But its reach between galaxies has been harder to verify and should not be taken for granted, says cosmologist Alexie Leauthaud, at the Lawrence Berkeley National Laboratory in California. "It's actually a tremendous extrapolation to assume that general relativity works on cosmic scales," she says.
If general relativity does break down at large scales, it could help cosmologists to explain away one of their biggest headaches: dark energy. In the 1990s, astronomers were surprised to discover that the expansion of the Universe is accelerating. That runs counter to the predictions of general relativity, which suggests that gravity's grip should be slowing the expansion. To explain this, cosmologists now invoke a 'dark energy', a force that makes up almost three-quarters of the matter and energy in the Universe and pushes it apart. But the origin of dark energy remains a mystery.
The accelerated expansion could be explained without dark energy, however, if general relativity is wrong and gravity weakens at cosmic scales. Several candidate 'modified gravity' theories take this line but, until now, no one has come up with a way to test them at large scales.
Relative success
Now, Reina Reyes at Princeton University in New Jersey and her colleagues have compared some of these models using data on the position, velocity and apparent shape of 70,000 distant galaxies mapped by the Sloan Digital Sky Survey1. The rival theories make different predictions about the degree to which light travelling to us from distant galaxies will be bent by the gravity of intermediate galaxies. This process, called 'gravitational lensing', distorts the apparent shape of the galaxies. The theories also make different predictions for both how fast galaxies grow and how they cluster together.
No single prediction can be used compare the theories directly, says Reyes. Because all the models include assumptions about whether dark matter -- the invisible substance thought to make up the bulk of matter in the Universe -- exists and if so, how it clumps together, relative to visible matter. Instead, the team had to combine all three measures -- gravitational lensing, growth and clustering -- into one ratio, called EG, in such a way that any uncertainty introduced by dark matter assumptions cancels out. The team found a value for EG of about 0.39 - a good match to the general relativistic prediction of around 0.4.
Scott Dodelson at the Fermilab Center for Particle Astrophysics in Batavia, Illinois, who was part of the team that proposed2 the EG test in 2007, says that the work is "exciting and important". "It's impressive that the team has shown we can rigorously test general relativity at large scales with data we have now," he says.
Open question
General relativity's rivals, however, returned mixed results. The team's test rules out a version of the tensor-vector-scalar, or TeVeS, model, proposed in 2004, which modifies gravity using a set of interacting fields to mimic dark matter. This model was already struggling to explain observations of galactic collisions that seem to show direct evidence of dark matter3, says Reyes. "Our test is another blow to TeVeS," says Reyes. However, she notes, more complicated versions of TeVeS could well pass the test.
The team also failed to rule out the set of so-called 'f(R) models' that slightly tweak the parameters of general relativity at large scales to explain away dark energy.
"The question of whether modified gravity or general relativity will ultimately prevail is still very much open," says Dodelson.
Glenn Starkman, a cosmologist at Case Western Reserve University in Cleveland, Ohio, says that EG "will be a useful addition to the battery of tests that modified gravity theories are already subjected to". Starkman and his colleagues have been testing the viability of a third modified gravity model, dubbed 'Einstein-Aether' theory, which uses a single new field to replace both dark matter and dark energy. Their analysis concludes that Einstein aether cannot explain the patterns seen in the cosmic microwave background4.
Leauthaud thinks that within the next two decades various planned experiments will have collected enough observational data to discriminate between general relativity and its competitors using EG. "Either we'll find general relativity is wrong, or we'll discover a new type of physics to explain dark energy," she says. "We're looking at a paradigm shift, either way."
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25 Comments
Add CommentWhat Dark Matter?
Reply | Report Abuse | Link to thisIn the late 1960s, when Vera Rubin first began studying the rotation of our neighboring Andromeda galaxy, she and other astronomers expected the orbital velocities of stars to diminish with their distance from the galactic bulge, just like the planets in our Solar system. Its gravitational characteristics are effectively dominated by the enormous spherical mass of its central Sun. That the stars of the spiral Andromeda galaxy do not behave as do planets in our Solar system created the mystery of the Galaxy Rotation Problem, and led to general acceptance of the Dark Matter hypothesis.
However, if the vast differences in their relative distributions of mass had been properly considered it should not have been expected that in these disparate gravitational systems would behave even similarly, much less identically. More recent efforts to represent the galactic disc of spiral galaxies collectively as a fluid distribution of mass have successfully predicted the orbital velocities of their stars.
The primary cause of the Galaxy Rotation Problem is astronomers’ application of the sparse planetary system mass distribution model to represent proportionately much denser spiral galaxies. More fundamentally, non-spherical aggregations of massive objects have been implicitly represented as spherically symmetrical distributions of mass.
Please refer to the comment/essay: "Dark Matter as Gravitational Estimation Error" posted with the article at:
http://www.scientificamerican.com/article.cfm?id=dark-matter-cdms
Subsequent ‘confirmation’ of dark matter’s existence, from prior identification of excessive galactic cluster velocities (for spiral galaxies) and galactic lensing (for spiral galaxies) suffer the same erroneous methods of assessing gravitational affects.
Dark Energy is the unidentified energy necessary to produce the proposed acceleration of the universe, beginning about 5M years ago, identified interpreting observations of type Ia Supernovae and their host galaxies. The observations leading to it indicated that a set of galaxies about 10B light years away were further away than was predicted by standard cosmological models, whereas a set of galaxies about 5B light years away were not. The astronomers reporting these observations apparently concluded that the periphery of the universe is receding away from us at an accelerating rate.
Reply | Report Abuse | Link to thisI can only interpret those observations to indicate that it is the light from more distant galaxies emitted into the earlier universe that had been subjected to the greater expansion rates. As a result, I can only conclude that expansion has decelerated in accordance with the second law of thermodynamics. Based on this interpretation, the observation results were fundamentally misinterpreted due to the researcher’s improper spatial perspective of the temporally developing universe.
It should be expected that extraordinary proof would be required of extraordinary explanations that are inconsistent with fundamental laws of physics. In practice, however, now that Dark Energy seems to have become well established, since it has withstood the test of time (for 10 years), alternative explanations are no longer under consideration.
Are the "'" entries the result of the writers' using word processors that automatically put in "curly quotes" (or apostrophes) that get distorted on conversion to web pages?
Reply | Report Abuse | Link to thisrhodinsthinker - Using the free Firefox web browser from Mozilla.com, I don't see the WP artifact. MS IE and other products don't work so well with this site...
Reply | Report Abuse | Link to thiscorrection: the proposed acceleration of universal expansion is expected to have begun 5B years ago, not 5M.
Reply | Report Abuse | Link to thisI think gravity being weaker at larger cosmic scales still cannot explain the acceleration of expansion of the Universe. Because the expansion requires an energy source to continue and that is because unit volume of space-time must have a unit amount of zero-point energy to be created. So if there is no Dark Energy to be used then the expansion cannot continue.
Reply | Report Abuse | Link to thisjtdwyer
Reply | Report Abuse | Link to thisGreat comments. I wrote an article in 8/07 that stated:
"The other thing is that the Hubble Constant apparently demonstrates that the farther away a galaxy is, the faster it is receding. Well, since the universe is expanding, score one for the acceleration theory right? Not so fast. All that tells me is that the galaxies that are really far away have a greater red shift and therefore a greater velocity than the galaxies that are closer. But the light from those far away galaxies is really old. We are getting light signals that are collectively saying (at least to me) that galaxies moved faster billions of years ago than galaxies a few hundred million years ago did. And the galaxies a few hundred million years ago moved faster than the ones whose light is less than a hundred million years old. Wouldnt this be argument for an apparent deceleration?"
I thought I was the only one on the planet who thought about these things without influence. It's nice to know there is someone else who questions authority. Check out the relativity section and the article links on www.sciencewithoutfiction.com I have a feeling you might like what you read.
space mark - Thanks! I will look at the link.
Reply | Report Abuse | Link to thisYour similar thoughts predate mine.
What astronomers don't realize is that the location in the sky from which distant light emissions can be detected has no direct bearing on the current location of the objects that emitted that light. They're processing the characteristic properties of ancient light emissions, not observing current objects.
I think the universe is quite different than astrophysicists currently imagine. I no longer hope to convince them of this, but maybe someday one will have a thought...
Newton gave us a gravity theory based on his famous equation of gravitational force. But though it has been a great tool for astronomical observation but nature and source of gravity was not discussed. Einstein modified newtons gravity by introducing space-time continuum and distortion concept and was again successful with his field equations but was again not very much with the concept and source of gravity. He made mistakes in considering time as dimensions which is purely entropical thermodynamics . He considered from Michelsons experiment that ether is not detectable as such not present. He postulated a few things like light velocity etc which has been found not correct. As such the basics of his relativity theory or general theory of gravitation are not conforming to our observation except a few pockets here and there. Because again the field equations are helping us to point out some accidental proof in isolated manner but unless we go back to wave theory discarding particle theory of light or matter, we will not be in a position to come nearer to reality. We do not want any proof to support or reject the gravity theory of Einstein but some more refinement to get clear vision with our universe where ether is theory of everything in un particle quantum matter universe surrounded by antimatter universe. We have to study as our matter universe is increasing entropy and outer antimatter universe is decreasing entropy ,then when this decrease tends to zero, how thermodynamically triggers a cycle reverse and then the re bounce progresses etc etc. May be a smart spherical string gravitoethertons theory of ether in spherical co ordinates taking time as entropical not dimension is proposed to explain our universe instead of wasting time by adhering to Einstein and quarrelling on eleven dimensions. We do not know how this ether which is gravity and dark energy is being produced by annihilation of matter and antimatter at the common spherical boundary of our balloon inside balloon unverses. We know that there is a very interesting relation with ether and quarks by a factor 1/r.r which is reflected in Bohrs model of atom or even planetary orbits . We have to study the application in Avogadros law or Mendeleefs table etc etc . But we are instead trying to prove Einstein right and wasting money in gravity probe and things like that. I published my ether-gravity-dark energy theory of gravitoethertons and balloon inside balloon theory in ASTRONOMY.NET in year 2002 and given a new direction .
Reply | Report Abuse | Link to thisdebu - Just as Einstein rejected Newton's reliance on an imaginary attractive gravitational force, I have to wonder about mass 'curving' spacetime as a dimensional construct: I don't think mathematical constructs can have physical affects. Dimensional constructs only describe physical affects. Something seems to still be missing.
Reply | Report Abuse | Link to thisI don’t know, but it is interesting that (for spherically symmetrical distributions of mass) a point directed accelerating external force could produce all observed affects of gravitation, including the curvature of light. The intersection of two opposingly directed accelerating force vectors could even be effectively described as an attractive force.
Reply | Report Abuse | Link to this"Because all the models include assumptions about whether dark matter -- the invisible substance thought to make up the bulk of matter in the Universe -- exists and if so, how it clumps together, relative to visible matter."
Reply | Report Abuse | Link to thisSentence not this is a.
I prefer English.
As fish live in water, we live in ether which we call space. As fish notices whirling logs around a pole in river flow ,we also notice planets rotating around sun . But water is not very much changing in density where as ether is taking a density as per mass around which is why gravity directly proportional to mass and neutron stars have very high gravity. But I think we live in different colour ether syrup and each colour is its field strength. That is why time dilates as the rate of change of entropy in a process varies due to vicinity apart from energy level gradient. Time never dimension but it distorts. Whether gravitoethertons actually vary in frequency also as per location is not known and what even happens to quark in very high density neutron star is to be discussed. It is a complicated fluid dynamics as well as thermodynamics also not only Einstein relativity and quantum physics. Gravitoetherons are exotic in nature and much smaller than quark and very high frequency with speed greater than light and we failed to catch a glimpse even except gravity or casimir effect or at the centre of earth as hot iron core due to focus of gravitoethertons. But magnetism on earth is due to flow of electrons in iron core causing dynamic law magnetism and we know that. Even our outward galaxies are flying faster as if something from outside pulling is known to us. So we know outside antimatter universe is causing extra pull effect on our galaxies and the inflow of gravitoethertons due to annihilation causing our universe to expand. This kind of explanation may be necessary to concentrate our approach of planning cosmos study. But suitable mathematics in refined string theory also may predict our further observation. Another thing is entropical tug of war of these two universes in unstable equilibrium on opposite entropy path creating cycles with new laws in each re bounce and by chance suitable laws to create life etc after many barren cycles may seem philosophical but not science fiction. Dr. Roger Penrose declared physics wrong and we also feel a new begining is necessary but I am a retired power engineer now studying physics and mathematics and trying to formulate a theory of everything in spherical vibrating gravitoethertons in spherical co ordinates when I get encouraged by recent observations as predicted in my balloon inside balloon and ether-gravity-dark energy theory of gravitoethertons published in ASTRONOMY.NET IN YEAR 2002.
Reply | Report Abuse | Link to thisI prefer spatial imagery.
Reply | Report Abuse | Link to thisdebu - I you consider my original comments indicating that the expansion of the universe is actually decelerating, as had been expected prior to the observations I assert were misinterpreted and the erroneous accelerating of the universe was proposed, isn't the requirement for your proposed antimatter external universe eliminated? Isn't it necessary only to produce the acceleration of our own universe's expansion?
Reply | Report Abuse | Link to thisI suspect that the interactions and energy exchanges of accelerating and decelerating objects can produce the fluidic flow like affects of motion without requiring the hypothetical material substance of ether.
As an aside, acceleration and deceleration actually only describe the relative motions of objects. If two objects are considered in isolation, any variation in their velocities produces a differential relative motion. It is impossible to distinguish one's acceleration from the other's deceleration: all that can be determined is their relative motion.
Expansion describes the detected common differential relative motion of all observed distant objects. Its variance in time establishes the acceleration or deceleration of universal expansion.
Expansion of our universe is due to pumping in of gravitoethertons which we call dark energy also and accelerating of external galaxies are due to extra pull by out side antimatter universe around our universe which in our calculation appear as dark matter also. So read my theories by opening --durgadas datta face book where links will be available.
Reply | Report Abuse | Link to thisDoesn't it all boil down to a very simple concept? Have we wondered if the universe accelaration is caused by space increasingly losing mass as friction (resistance/inertia) diminishes causing space speed to increase. Wound't it all end up in an eventual loss of friction whereby that laws of physics would colapse transforming matter to a nonmeasurable existence.
Reply | Report Abuse | Link to thisjt,
Reply | Report Abuse | Link to thisYes, if (a long time ago) our galaxy was moving at the same speed as GS-109-7 and they both slow due to the effects of gravity but ours starts to decelerate first, or at least decelerates at a higher rate, (possibly because the light from GS-109-7 is so old it hadn't begun to decelerate yet) then we would probably observe GS-109-7 as accelerating away from us right? Is that what you mean??
space mark - My comments were very general in nature. To answer your question: generally, yes, that what I'm getting at.
Reply | Report Abuse | Link to thisObserving stars within our own galaxy, the spectrum shift of light indicates the relative velocity between Earth and the star observed. The blue or red shift indicates whether the direction of motion is towards or away from us and the amount of spectrum shift indicates the star's speed relative to Earth, which can be generally considered to be stationary.
Astronomers also refer to distant galaxies as moving away from us because of their redshifted light. However, the Earth, Solar system and Milky Way galaxy have not been stationary since the emission of the observed ancient light.
Since our current observations of light from distant galaxies was emitted long ago, the differential relative motion indicated by its redshift should reflect the emitting object's direction of motion and speed at the moment of emission in relation to our current direction of motion and speed. In addition, the expansion of intervening spacetime traversed by the light on whatever path it took to reach us would also affect the redshift of the observed light, by increasing the distance traversed. I doubt that a rate of universal expansion can be derived from the redshift of observed ancient light.
One other comment regarding your question: I think the role of gravitation in halting expansion is highly overrated. Since its affects diminish with distance, its affect at intergalactic scales of distance should be negligible. I envision expansion to have resulted from the release of initial (big bang) energy that was not converted into matter. As the universe expands, that energy is diminished as it is dispersed. The expansion of the universe should decelerate in time from the affects of entropy, with little or no assistance from any diminished intergalactic gravitational affects.
I hope this explanation helps, although I could not answer your seemingly straightforward question directly.
space mark - Perhaps the best answer is that we perceive all distant galaxies to be accelerating away from us because we presume that we are stationary, and spacetime has expanded during the transmission of all observed distant light, increasing the distance between the emission location of distant light and our current, observation location.
Reply | Report Abuse | Link to thisWe presume that we are stationary only because distant galaxies appear to be surrounding us. However, this is an observational illusion, since it is we who have receded away from the emission location of all observed distant light during its transmission. We can only presume that distant galaxies have also receded away from us, but we have no direct evidence for even their continued existence.
We have no direct evidence of any observed distant galaxy's current directional velocity or that of our own galaxy at the moment of any observed distant light's emission.
The actual differential relative motion of our galaxy in relation to any distant galaxy at any discrete moment in time is indeterminable.
Without correctly considering our actual observational perspective of the expanding universe, it's impossible to correctly interpret our observations of it.
Reply | Report Abuse | Link to thisCrucial factors in correctly considering the affects of our observational perspective:
1. Current observations of distant objects are more precisely only the detection of their ancient light emissions.
2. The variable characteristic properties of ancient light have been altered subsequent to its emission by all of the conditions it has been subjected to during its transmission.
3. Summary conclusions regarding the varying conditions detected light has been subjected to can be indirectly inferred from the observed characteristics of its properties: spectrum and luminosity.
4. The conclusions inferred from detected ancient light emissions represent only the net affect of all conditions encountered since the moment of its emission.
5. Conditions in effect at any moment in time could only be indirectly inferred from statistical analysis of a large sampling of observations of varying ages.
Comparing two sample populations of observations, one of light emitted 10B years ago and one emitted 5B years ago for instance, to the extent that conditions affecting distant light are universal, the conditions affecting the light emitted more recently also affected the more ancient light emission. Any differences in the characteristic properties of the observed sample groups are the result of conditions that existed prior to the more recent light emissions. As a result, if the light emitted earlier (10B years ago) indicates greater universal expansion than does the light emitted more recently (5B years ago), then the universal expansion rates in affect 5-10B years ago were greater than those of the past 5B years. The observations leading to conclusion of an accelerating universe actually indicate a decelerating universe, as had previously been expected based on the second law of thermodynamics.
Reply | Report Abuse | Link to thisThis straightforward interpretation of the observation results is consistent with the established fundamental laws of physics and does not require the action of any unknown form of Dark Energy.
jtdwyer...nice write up...very much agree....
Reply | Report Abuse | Link to thisWayne Williamson - Thank you very much.
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Reply | Report Abuse | Link to thisTHe dark matter at the centre of the Galaxy can readily be resolved by a modification of gravity not too dissimilar to the general theory of relativiity at least as far as the results are concerned.
Available on line at aip.org.
1). An advanced dynamic adaptation of Newtonian equations of gravity. Physics Essays 21: 222-228
2) String quintessence and the formulation of advanced quantum gravity. Physics Essays 22: 364-377