Ripples in the fabric of spacetime could someday provide observational evidence for the goings-on in the earliest instants of the universe, revealing high-energy processes that currently remain opaque to even the largest particle colliders.

So-called gravitational waves are a prediction of Albert Einstein's theory of general relativity—moving objects perturb spacetime, generating waves like a boat moving across a lake. But the waves tend to be subtle, and only celestial heavyweights are expected to produce detectable effects. To date, only indirect evidence for gravitational waves has been found, although supremely sensitive detectors have been built to hunt for more direct proof in the form of waves emanating from nearby cataclysms such as a collision between two ultradense neutron stars.

A review paper in the May 21 issue of Science presents the prospects for detecting more primordial gravitational waves—those produced in the early universe that may still be detectable by the imprint they left billions of years ago or by the ripples that persist today.

Such primordial waves might offer the best means for testing cosmological models such as inflation, which holds that the newborn universe ballooned from a tiny pocket to something roughly 10²⁶ times larger in just a sliver of a second. "It's hard to imagine a mechanism that's going to give us a direct window to a time closer to the instant of creation," says study co-author Lawrence Krauss, a theoretical physicist at Arizona State University. (Krauss, who writes a monthly column for Scientific American, is a member of the magazine's board of advisers.)

The first place to look for the mark of gravitational waves is in the cosmic microwave background, or CMB, remnant radiation from only 380,000 years after the big bang. The European Space Agency's Planck satellite, launched in 2009, is now following up its NASA predecessor, the Wilkinson Microwave Anisotropy Probe (WMAP), in measuring temperature fluctuations across the CMB. Those temperature fluctuations trace regions of greater and lesser density in the infant universe, providing important clues to how the universe and its component structures formed.

The CMB maps made by WMAP provided a boost to inflation, broadly affirming the inflationary model's predictions for what the early universe should look like, and more precise measurements could provide further confirmation. "The same events that we believe formed the hot spots in the cosmic microwave background could have produced gravitational waves, and we can estimate their magnitude," Krauss says. "It's at least possible that with the next generation of satellites we'll be able to observe their effects."

Gravitational waves passing through space would have left their imprint on the photons of the CMB in subtle polarization patterns. WMAP's measurements set upper bounds for the prevalence of those waves, and with Planck's greater sensitivity the newer spacecraft "might get lucky" and detect polarization from primordial gravitational waves, Krauss says.

Yale University cosmologist Richard Easther notes that CMB measurements are already yielding clues, albeit not huge ones, to the dawn of the universe.* "In fact, some inflationary scenarios are already ruled out because they would produce more gravitational waves than current measurements, mainly from the WMAP mission, would allow," he says. Planck and other experiments are now working to push those limits even lower. "So, if nature has been kind to us, we could have the first evidence for inflationary gravitational waves in the next few years," Easther says. If that evidence escapes Planck and its contemporaries, a more specialized polarization-measurement mission may be needed.

A CMB imprint from primordial gravitational waves would be indirect evidence, like the high-water mark left by a receding tide, but what of detecting the waves themselves? At present, the instruments most sensitive to locally produced gravitational waves are interferometers such as the dual Laser Interferometer Gravitational Wave Observatory (LIGO) installations. Each sends laser light down four-kilometer arms to look for interference effects that would be caused by passing gravitational waves stretching space in one direction and compressing it in the other. In future decades, space-based interferometers could use the same principles on much larger scales, bouncing laser light across the solar system, to directly detect the fainter primordial gravitational waves.

"Those kind of things could potentially—not in the near term but in the next generation—directly detect them," Krauss says. "We'd be able to look at the spectrum, and we'd be able to get some conclusive proof of what's going on." Easther says that the defining feature of a gravitational wave background produced by inflation would be that across all wavelengths, "from perhaps a few meters to the current size of the visible universe," the amplitude of the waves would be roughly the same. "To put this in perspective," he says, "if we could build a piano that produced gravitational waves instead of sound, the keyboard would need about a thousand keys to produce a big enough range of frequencies, and inflation manages to hit each note with almost exactly the same strength."

Although specific predictions of inflation might be vetted by detecting gravitational waves, a nondetection might not paint as clear a picture. "All inflationary scenarios produce gravitational waves, but the signal from some inflationary models is incredibly faint," Easther says. "Consequently, there is no consensus that failing to detect an inflationary gravitational wave background at some level can be taken as proof that inflation itself did not happen—although a huge number of specific inflationary scenarios would be ruled out."

Andrew Jaffe, a cosmologist at Imperial College London, portrays experiment and theory as participants in a sort of cat-and-mouse game. "We are rapidly getting to the point where the simplest versions of inflation may be ruled out, but it's very easy to build models that will evade the coming experimental bounds," he says. "In that case, we'll have to figure out much more clever ways to do our tests, or to attach the predictions of inflation to broader theoretical models of particle physics."

*CORRECTION (5/21/2010): This sentence originally referred to "CMB polarization measurements," but Easther was referring to measurements of the CMB's temperature fluctuations.

9 Comments

1. 1. Dan Visser 01:45 PM 5/21/10

   It is sad to notice that stil the only focuss is layed on observing the big bang. If a new cosmological model was taken into account, such as the "Twin-Tori cosmological Model", than more success could be achieved. There is so much money invested in all those projects about the big bang, that blindness holds science away from new fundamental discoveries. There is a real anomaly problem with the abundance of Litium-7 in the big bang , and the one-way timeflow of the big bang, caused by entropy, is not matching the two-way timeflow of quantummechanics, while both are part of the same universe. That does not plead to hold on to the big bang. Moreover, several times the big bang is adapted as a model to new observations. Such methods do not represent trustworthy cosmology. When are cosmologists open there eyes? For more information: www.darkfieldnavigator.com

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2. 2. jtdwyer 02:55 PM 5/21/10

   Finding ‘local’ externally generated gravitational waves from within the local gravitational fields, the goal of the LIGO detectors, is less likely than detecting a triangle rung during the cacophony of an orchestra warming up.
   
   Common representations of CMB data indicate spatial variability that does not support the production of early galaxies: only by intensifying the image contrast is sufficient variation apparent. This adjustment does not properly correct for the actual effect: the dispersal of background radiation signals in the temporally expanding universe. Just as the luminosity of stars diminishes at distance, the luminosity of the microwave background is diminished in time and distance. To better represent the spatial magnitude of the initial UV light emission, the currently detected signal should be spatially compressed using a pixel accumulation method to reconstruct the spatial amplitude of the original signal.

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3. 3. jack.123 08:54 PM 5/22/10

   Has there been any studies of the virtual particle's appearing out of nothing in space-time?Could there be any hidden information in these fluctuations that could show the existence of gravity waves,and other phenomenons?

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4. 4. jtdwyer in reply to jack.123 12:43 PM 5/23/10

   jack.123 - A very interesting suggestion! I have wondered whether the CMB might actually represent the composite continuous emissions of particle-antiparticle annihilations as they occur throughout spacetime.
   
   In that case, temporal variations of CMD data may reveal information of unknown processes and conditions. I don't know whether they might include gravity waves or not, but if gravity waves exist I'd expect them to be most identifiable in intergalactic space.

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5. 5. jimhenson 07:31 PM 5/23/10

   New discoveries of evolved elliptical galaxies 13 1/2 billion years old, and globular clusters 16 billion years old, prove that the big bang models are all wrong about the age of the universe and expansion rates. The early universe expanded far more diversely, likely from a quantum effect straight line singularity emergence. with time space cooled and expanded to form stars, but the initial explosion has produced a visible light energy horizon of billions of light years where the dark flow would be located. Gravity wave satellites should search in the direction of the dark flow, and have a magnetic field detection telescope installed to detect primordial magnetism in the universe.

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6. 6. Wayne Williamson 05:53 PM 5/24/10

   gravity is the weakest force....any gravity waves would be less than that of em....also, it seems that em(ie a photon) is directional where as gravity would be in all directions reducing it even further....good luck in finding them...maybe if a couple of neutron stars collide with in a few light years we could detect a gravity wave about the same time as we are incinerated by the em;-)

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7. 7. mike cook 11:26 PM 6/2/10

   I have been reading Lee Smolin's book THE TROUBLE WITH PHYSICS this week and it recounts a whole lot of radical theories, many of which originate in questions of some pretty basic assumptions that the edifice of contemporary physics rests upon.
   
   Basic assumptions like whether it is really impossible to distinguish rest from motion in a relativistic sense, or whether the speed of light has been slowing down since the Big Bang.
   
   My own problem with the BB is that we are always told that space and time both originated simultaneously and in concert with the expansion of the first matter. Literally, this means that nothing was outside the alleged tiny little point of the very early BB, not even any background of any kind. Nothing.
   
   Which always has raised with me the usefulness of calling something an expansion or an inflation when it has absolutely no point of reference outside itself. Why not just call it an evolution--the universe was always the same "size" but what has been evolving are all the rules and the constants.
   
   We perceive this evolution as an expansion, but perceptions are very deceptive things. For instance, many good scientists now claim that a person on a spaceship nearing the surface of a large Black Hole would not feel like they were being physically rent to shreds by the immense tidal forces, although an outside observer might see that happening. What is happening on the spaceship is that everything in 3-D is being dimensionally converted into 2-D, which would not necessarily be fatal or even noticeable in the spaceship.
   
   Imagine the spaceship is merely doing a high velocity drive by on the Black Hole, swooping close on one lob of a highly elliptical orbit. Passengers on the spaceship look to outside observers to be undergoing tremendous accelerations and ship time is varying wildly relative to observer time, but nobody on the ship notices anything strange at all.
   
   We could view the whole universe as our spaceship. Although we interpret the walls of our container as apparently expanding, what we are really observing are changes in both the speeed of light, the force of gravity, and a lot of other constants.

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8. 8. genetownsend 01:04 PM 11/23/10

   "WMAP's measurements set upper bounds for the prevalence of those waves" as the author states also ruled out the simplist forms of inflation. Planck may lower the limits for detection of B-mode polarization required for measurement of the gravitational waves predicted... thowing more forms of inflation out of favor. Still, the dogma of BB will survive with increasingly complex and contrived explanations of how it all works. Too bad. It was a good theory for many years. Time to move on.

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9. 9. wlminex 10:49 PM 10/4/12

   jack.123, jtdwyer, mike cook: I note that your posts on Scientific American are a couple of years old . . . . . . but I think we share some common ideas. Would you guys please visit my webpage at the link below and comment on my ideas re: the hypothesis and the (continuous) CMB source mechanism? Thanks . . . I get a LOT of flak from Standard Modelers on Sciforums.com. I really appreciate your constructive comments and any ideas you might contribute. Regards, Bill Mansker, Ph.D. wlminex@msn.com
   
   https://sites.google.com/site/eemuhypothesis/
   

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