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September 24, 2015

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“Missing” space-time waves leave scientists puzzled

Sept. 24, 2015
Courtesy of CSIRO
and World Science staff

Al­bert Ein­stein’s the­o­ry of rel­a­ti­vity—which uses ge­om­e­try to de­scribe how gra­vity shapes the uni­verse and ce­les­tial mo­tions—has with­stood eve­ry test sci­en­tists have thrown at it.

But one fes­ter­ing prob­lem is start­ing to raise eye­brows. 

Ein­stein’s el­e­gant the­o­ry pre­dicts a type of cos­mic fluctua­t­ion known as gravita­t­ional waves. But these haven’t di­rectly shown them­selves, de­spite the con­struc­tion of sev­er­al elab­o­rate in­stru­ments meant to de­tect them, and strong cir­cum­stanti­al ev­i­dence for their ex­ist­ence.

An image from a si­mu­la­tion of two black holes merg­ing. (© Mich­ael Kop­pitz / Al­bert Ein­stein Inst.)


This week, sci­en­tists an­nounced that an 11-year search us­ing the Parkes tel­e­scope in Aus­tral­ia has re­vealed an ex­pected “back­ground” rum­ble of waves to be mis­sing. This must force sci­en­tists to re­vise their think­ing about fun­da­men­tal cos­mic ob­jects like black holes and ga­lax­ies, ac­cord­ing to the re­search­ers.

Gravita­t­ional waves ex­ert a pow­er­ful ap­peal on sci­en­tists. Con­ceived as rip­ples in space and time it­self, they’re thought to car­ry in­forma­t­ion al­low­ing us to look back in­to the very be­gin­nings of the uni­verse. 

The uni­verse, how­ev­er, does­n’t seem to be co­op­er­at­ing.

“It seems to be all qui­et on the cos­mic front” as far as those waves go, said Ryan Shan­non of CSIRO, Aus­tral­ia’s na­t­ional sci­ence agen­cy, and of the In­terna­t­ional Cen­tre for Ra­di­o As­tron­o­my Re­search. Shan­non led the new re­search, pub­lished Sept. 24 in the re­search jour­nal Sci­ence.

On the pos­i­tive side, he said, “by push­ing our tel­e­scopes to the lim­its re­quired for this sort of cos­mic search we’re mov­ing in­to new fron­tiers, forc­ing our­selves to un­der­stand how ga­lax­ies and black holes work.” 

Ga­lax­ies grow by merg­ing, and eve­ry large gal­axy is thought to be cen­tered on a su­per­mas­sive black hole—a su­premely com­pact ob­ject whose gra­vity cap­tures an­y­thing that strays too close. 

When two ga­lax­ies un­ite, the black holes at­tract each oth­er and even­tu­ally merge. This vi­o­lent event is ex­pected to send rip­ples known as gravita­t­ional waves through space-time, the very fab­ric of the uni­verse. While a black hole is theor­ized to bend space-time on its own, it takes a merg­er of the two for the distort­ions to spread outward, which they do, in theory, at light speed.

To look for the waves, Shan­non’s team used the Parkes tel­e­scope to mon­i­tor a group of “mil­lisec­ond pul­sars”—s­mall stars that send out reg­u­lar ra­di­o pulses and act like clocks in space. The pas­sage of a gravita­t­ional wave should throw off the tim­ing of these pulses just slight­ly.

The study went on for 11 years, which should have been long enough to re­veal gravita­t­ional waves. Noth­ing.

So why haven’t they turned up? There could be a few rea­sons, but the sci­en­tists sus­pect it’s be­cause black holes merge very fast, spend­ing lit­tle time gener­at­ing gravita­t­ional waves.

“There could be gas sur­round­ing the black holes that cre­ates fric­tion and car­ries away their en­er­gy, let­ting them come to the clinch quite quick­ly,” said team mem­ber Paul Lasky, a post­doc­tor­al re­search fel­low at Monash Un­ivers­ity in Aus­tral­ia.

What­ev­er the ex­plana­t­ion, it means that if as­tro­no­mers want to de­tect gravita­t­ional waves by tim­ing pul­sars they’ll have to rec­ord them for many more years. 

Higher-frequency waves might prove eas­i­er to de­tect, they added, and the highly sen­si­tive Square Kil­o­me­tre Ar­ray tel­e­scope, set to start con­struc­tion in 2018, might help. The fail­ure to find waves us­ing the pul­sar meth­od, they added, does­n’t af­fect prospects for anoth­er, ground-based gravita­t­ional de­tector that started ob­serva­t­ions last week—the Ad­vanced LIGO, or La­ser In­ter­fer­om­eter Gravita­t­ional-Wave Ob­serv­a­to­ry.

“Ground-based de­tectors are look­ing for higher-frequency gravita­t­ional waves gen­er­at­ed by oth­er sources [than black holes], such as co­a­lesc­ing neu­tron stars,” said Vikram Ravi, a mem­ber of Shan­non’s team who is now at the Cal­i­for­nia In­sti­tute of Tech­nol­o­gy.


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Albert Einstein’s theory of relativity—which uses geometry to describe how gravity shapes the universe and celestial motions—has withstood every test scientists have thrown at it. But one festering problem is starting to raise eyebrows. Einstein’s elegant theory predicts a type of cosmic fluctuation known as gravitational waves. But these haven’t directly shown themselves, despite the construction of several elaborate instruments meant to detect them, and strong circumstantial evidence for their existence. This week, scientists announced that an 11-year search using the Parkes telescope in Australia has revealed an expected “background” rumble of waves to be missing. This must force scientists to revise their thinking about fundamental cosmic objects like black holes and galaxies, according to the researchers. Gravitational waves exert a powerful appeal on scientists. Conceived as ripples in space and time itself, they’re thought to carry information allowing us to look back into the very beginnings of the universe. The universe, however, doesn’t seem to be cooperating. “It seems to be all quiet on the cosmic front” as far as those waves go, said Ryan Shannon of CSIRO, Australia’s national science agency, and of the International Centre for Radio Astronomy Research. Shannon led the new research, published Sept. 24 in the research journal Science. On the positive side, he said, “by pushing our telescopes to the limits required for this sort of cosmic search we’re moving into new frontiers, forcing ourselves to understand how galaxies and black holes work.” Galaxies grow by merging, and every large galaxy is thought to be centered on a supermassive black hole—a supremely compact object whose gravity captures anything that strays too close. When two galaxies unite, the black holes attract each other and eventually merge. This violent event is expected to send ripples known as gravitational waves through space-time, the very fabric of the universe. To look for the waves, Shannon’s team used the Parkes telescope to monitor a group of “millisecond pulsars”—small stars produce regular trains of radio pulses and act like clocks in space. The passage of a gravitational wave should throw off the regular timing of these pulses just slightly. The study went on for 11 years, which should have been long enough to reveal gravitational waves. Nothing. So why haven’t they turned up? There could be a few reasons, but the scientists suspect it’s because black holes merge very fast, spending little time generating gravitational waves. “There could be gas surrounding the black holes that creates friction and carries away their energy, letting them come to the clinch quite quickly,” said team member Paul Lasky, a postdoctoral research fellow at Monash University in Australia. Whatever the explanation, it means that if astronomers want to detect gravitational waves by timing pulsars they’ll have to record them for many more years. Higher-frequency waves might prove easier to detect, they added, and the highly sensitive Square Kilometre Array telescope, set to start construction in 2018, might help. The failure to find waves using the pulsar method, they added, doesn’t affect prospects for another, ground-based gravitational detector that started observations last week—the Advanced LIGO, or Laser Interferometer Gravitational-Wave Observatory. “Ground-based detectors are looking for higher-frequency gravitational waves generated by other sources [than black holes], such as coalescing neutron stars,” said Vikram Ravi, a member of Shannon’s team who is now at the California Institute of Technology.