A Cosmic Gravitational Wave Background?

by Paul Gilster on November 24, 2010

A gravitational wave is a ripple in spacetime, one that follows naturally from the theory of general relativity — Einstein did, in fact, predict the existence of such waves back in 1916. Yet so far we have had nothing but an indirect detection in the form of the Hulse-Taylor binary (PSR B1913+16), a pulsar in a binary system that includes a second neutron star, the two orbiting around a common center of mass. The 1993 Nobel Prize in physics went to Richard Hulse and Joseph Hooton Taylor (Princeton University), who showed that the system’s orbital decay corresponds with the loss of energy due to the kind of gravitational waves Einstein predicted.

What we now need is a direct detection, but these waves have proven to be a tricky catch. Consider this: The distance between two spacecraft flying five million kilometers apart would be changed by about a picometer by the effects of gravitational waves. That’s a distance 100 million times smaller than the width of a human hair, some .000000000005 meters. Yet it’s the distance scientists hope to measure with the LISA mission (Laser Interferometer Space Antenna), in which three spacecraft will fly in a triangle connected by laser beams, a formation flight orbiting the Sun roughly 20 degrees behind the Earth.

The plan: Aboard each of the spacecraft will be a cube of platinum and gold that floats freely in space. Passing gravitational waves should cause the distance between the cubes to vary, and the good news out of the Jet Propulsion Laboratory this week is that after six years of working on the LISA technology, scientists have tuned its phase meters (laser beam detectors) to the point where such a detection should be possible, the laser ‘noise’ dropping sufficiently to allow the evanescent wave signature through. Thus JPL physicist Bill Klipstein:

“In order to detect gravitational waves, we have to make extremely precise measurements. Our lasers are much noisier than what we want to measure, so we have to remove that noise carefully to get a clear signal; it’s a little like listening for a feather to drop in the middle of a heavy rainstorm.”

A feather in a rainstorm indeed, and even that seems to understate the case. JPL is now demonstrating that its instruments are sensitive enough to make detecting gravitational waves a possibility. The principle should sound familiar, as it’s basically interferometry (though with a time delay), often discussed here in terms of pooling the resources of multiple telescopes so as to produce an effective aperture equal to the separation of the telescopes. In LISA’s case, the three spacecraft are affected by passing gravitational waves so that the distances between the test masses changes, a fact revealed by the distances traveled by the laser beams of light.

Ground-based data processing will then tell us whether the light detected by the onboard phase meters shows any variation in distance between the spacecraft. By introducing artificial noise into their detectors, the JPL team has been able to show that its data processing techniques can filter it out, highlighting those one-picometer distance changes scientists hope to see. The LISA mission is a joint project between the European Space Agency and NASA that would launch around 2020 if selected. The National Research Council’s decadal report, which more or less put an end to the Space Interferometry Mission, has given LISA a high recommendation.

Image: The gravitational-wave sky, as observed by LISA. The plane of the Galaxy is visible as the white horizontal band of emission from millions of Galactic binaries. The dots and squares mark the locations of a small fraction of the black-hole mergers and capture events that LISA will observe, while the purple background represents the relic gravitational radiation that LISA may detect from the very early Universe. Credit: NASA/ESA.

If it succeeds, the LISA mission will open up a new way of observing the universe, looking for low frequency gravitational waves (0.03 milliHertz to 0.1 Hertz), a band thought to contain the emission from massive black hole binaries of the kind that form after the merger of entire galaxies. Gravitational waves are all about huge astronomical events like this, generated by compact objects like stellar remnants falling into galactic black holes and neutron stars in tight binaries. The behavior of spacetime when pushed to extremes may teach us not only about dense matter and stellar remnants, but about the expansion history of the universe itself.

What would a Cosmic Gravitational Wave Background look like, and what could it tell us about the earliest moments of the universe? Remember that the Cosmic Microwave Background emerged about 400,000 years after the Big Bang, and represents the oldest light we can see via electromagnetic astronomy. Gravitational waves should be able to propagate to us from before that era, making them a probe of the early universe that could identify the existence of new fundamental objects like cosmic superstrings, if they exist, thus offering a window into the various models of string theory and sketching the history of the phase transitions that shaped the cosmos.

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Antonio Tavani November 24, 2010 at 19:04

Una domanda: ma attraverso l’analisi delle “onde gravitazionali”, saebbe possibile, in futuro, verificare l’eventuale esistenza di un Universo, precedente al nostro?

A questo proposito(cioè la possibile esistenza di un’altro universo, precedente) porgo all’attenzione di tutti, questo articolo: http://arxiv.org/ftp/arxiv/papers/1011/1011.3706.pdf

Saluti da Antonio Tavani.

Paul Gilster November 24, 2010 at 21:35

Antonio’s post via Google Translate:

One question: Through the analysis of “gravitational waves”, would it be possible in future, to check whether there is a Universe before ours?

In this respect (ie the possible existence of another universe, above) I offer to everyone’s attention, this article:

http://arxiv.org/ftp/arxiv/papers/1011/1011.3706.pdf

Freederick November 25, 2010 at 5:53

I see the LISA mission as a white elephant: an extremely costly* outlay of taxpayer money that is incapable of telling us anything of importance. Consider:

1) If LISA detects gravitational waves, it will only serve as yet another confirmation of currently well-accepted theory. No new physics will follow from this.

2) if LISA fails to detect gravitational waves, the result will be inconclusive, because it is operating at the borderline of resolution. The only thing that will follow is a proposal for a yet more elaborate and costlier project to doublecheck the results.

LISA ought to be shelved at least until such time as we have the technology to make a conclusive non-detection. In the meantime, the funds should be applied towards developing cheaper launch technology and a more permanent presence in space. Once this is achieved, costs of missions such as LISA will plummet even as the technology moves forward. And last but not least, an Earth-based detection method matching LISA’s sensitivity is probably not far off, and would be a better choice for too many reasons to enumerate, the chief being ease of modification/correction/retrofitting.

____________
*We’re talking about THREE spacecraft lifted past the GEO to orbit the Sun, not to mention the extremely sensitive detection apparatus that has to be designed from scratch for that one mission, tested, checked, and finetuned, and guidance systems likewise one-shot and likewise of extreme precision; and there is no guarantee whatsoever that the whole setup will work as intended. What has a trunk, a pair of tusks, and high albedo?
Answer: VSI7

Oliver Jennrich November 25, 2010 at 15:28

@Frederick:

ad 1) You are right. Most likely there will be no new physics coming out of a GW detection. But neither is there new physics in the detection of photons, and yet few astronomers would debate the value of e.g. HST.

LISA is not so much about detection or non-detection of GW, but about GW astronomy. LISA will see objects that no electromagnetic telescope will ever be able to see, and most likely lots of them.

ad 2) The signals that LISA will likely detect (e.g. from coalescing super-massive black holes) have a signal-to-noise ratio of thousands – they will be observable out to very high redshifts, all the way back to the formation of the first galaxies. Hardly borderline. Now, you might argue that we don’t know, if SMBH coalescences actually occur in nature. Granted, but then, failure to detect those will provide us with important astrophysical information as well.

If you are looking for guaranteed signals, galactic binaries come to the rescue. We know of a handful of binaries (through electro-magnetic observations) that have the right mass, distance and frequency to be right in the LISA band. Those will have a SNR of 50-100, again hardly borderline. A far cry from the SNR~1000 of the SMBH-monsters, but *guaranteed*.

So I’m afraid, no tusks, no trunk, and a rather low albedo.

Freederick November 26, 2010 at 4:47

@Oliver:

If this is an observatory rather than a detector (like LIGO), and if the SNR is really this good, then you’re absolutely right, and I withdraw my reservations. However, this is not what the article says, quote:

“What we now need is a direct detection…”
“…after six years of working on the LISA technology, scientists have tuned its phase meters to the point where such a detection should be possible, the laser ‘noise’ dropping sufficiently to allow the evanescent wave signature through…”

Note that they are talking about detection that “should be possible”, not about imaging. Also that bit about noise barely letting the wave signature through refers to a borderline SNR; it would be a stretch to interpret that sentence differently.

Admittedly the article reads more like popular press than a scientific treatment; perhaps you have access to more accurate information. If so, please cite or link.

Oliver Jennrich November 26, 2010 at 12:28

@Freederick,

The story behind the laser (frequency) noise is that the JPL work neatly demonstrates that the way in which LISA deals with laser frequency noise works to the required precision. Not that many people had significant doubts about that, but demonstration is always good and welcome. The phase meter, which lies at the heart of the LISA measurement system, has been demonstrated to work some while ago (also at JPL among other places) – here we are actually talking about approaching flight hardware, not just a ‘proof-of-principle’ in a laboratory.

LISA is ‘imaging’ only in the broadest meaning of the word as it does not register gravitons on a CCD sensor or suchlike. A better analogy is probably of someone, who sits in the dark, listening to an orchestra. Even if the sound does not create a real picture on her eardrums, she will be able to identify different pieces of music being performed, different instruments played and, fiven a good enough acoustic, might even be able to tell that the violins are sitting to the left of the conductor and the violoncelli to the right.

Unlike ground-based GW detectors, who are limited by the gravity gradient noise on Earth to frequencies above about one hertz, LISA is designed for a frequency band much lower (0.1 mHz to 100 mHz) — and this is where the most powerful sources live. The price to pay is that LISA will have no chance to pick up SN explosions or rapidly spinning neutron stars — they remain the domain of the ground-based detectors at frequencies 1 Hz – 1 kHz.

BTW: Here is the preprint for
de Vine, G et al. (May 2010). Phys. Rev. Lett. 104.21, pp. 211103–+.

http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010PhRvL.104u1103D&link_type=PREPRINT&db_key=AST

And not to neglect some other, very active group, that demonstrated the measurment priciple in a somewhat complementary setup:
Mitryk, SJ et al. (Apr. 2010). Class. Quantum Grav. 27.8, pp. 084012–+.
http://iopscience.iop.org/0264-9381/27/8/084012/

philw1776 November 28, 2010 at 13:15

Color me skeptical regarding LISA. Let’s start with LIGO which was hyped to secure funding and years later has detected nothing. Now we’re onto Advanced LIGO which now promises to d what LIGO was hyped as doing, and more hundreds of millions of dollars. Let’s see first if Advanced LIGO detects these grav waves…or not. Space based LISA is more than an order of magnitude more technically complex and risky and costs far more. It would suck up most of the likely funding for space science observatories with high risk of operational failure never mind lack of detection. I disagree with those recommending the LISA boondoggle which seems to me a bone thrown to the physics post doc community lobby. I favor lower hanging fruit, such as much lower cost high data productivity follow on missions to Kepler.

David Brown February 23, 2011 at 14:17

NASA’s Gravity Probe B (GP-B) mission detected gravitational waves in the form of frame-dragging. I suggest that M-theory predicts the Rañada-Milgrom effect, which is that the -1/2 in the standard form of Einstein’s field equations should be replaced by -1/2 + sqrt(15) * 10**-5. I also suggest that this effect has been confirmed by GP-B, because the GP-B science team overlooked Milgrom’s Law when they calibrated their gyroscopes and falsely attributed “misalignment torques” to electrostatic patches. See the postings “NASA’s Gravity Probe B: patch effects or quantum gravitational effects?” and “Dark matter: why should Rañada and Milgrom win the Nobel prize?” at nks forum applied nks. The LISA mission might prove the existence of quantum gravitational waves from dark matter.

ljk April 22, 2011 at 11:35

Looks like LISA may become one of the newest victims of our sad economic times:

http://blogs.discovermagazine.com/cosmicvariance/2011/04/06/nasa-gives-up-on-lisa/

And another scientific and cultural lobotomy is performed.

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