"Long before it's in the papers"
June 03, 2013


Explosion might have rocked space itself, scientists claim

Oct. 4, 2011
Courtesy of 
and World Science staff

As­tro­no­mers search­ing for an ex­ot­ic type of rip­ple in the very fab­ric of space and time say a dis­tant blast cre­at­ing such waves may have al­ready been de­tected.

Al­though no in­stru­ments ex­isted at the time that could have pick­ed up the much-sought “gra­vit­a­tional waves,” the ex­plo­sion re­ported in 1987 may none­the­less have cre­at­ed them, two sci­en­tists claim. They the­o­rize that in the fu­ture, ob­serv­ations of si­m­i­lar ex­plo­sions could help con­firm pro­posed find­ings of gra­vit­at­ional waves.

A sim­u­la­tion of ma­te­ri­al being ejected from a star merg­er. (Cred­it: Stephan Ross­wog )

Al­bert Ein­stein pro­posed the ex­ist­ence of the waves, and we’re still look­ing for them: rip­ples in space-time that are in a sense the sounds of our uni­verse. Just as sound—which con­sists of rip­ples in the air or oth­er ma­te­ri­al—com­ple­ments vi­sion in dai­ly life, gra­vit­at­ional  waves are ex­pected to com­ple­ment our view of the uni­verse tak­en by stand­ard tele­scopes. Ad­vanced gra­vit­at­ional wave de­tec­tors are be­ing built in the U.S., Eu­rope, Ja­pan and Aus­tral­ia.

In the­o­ry, any mo­tion pro­duces gra­vit­at­ional waves. But a sig­nal loud enough to be de­tected re­quires two huge mass­es to col­lide very, very fast. The prime can­di­date sources are merg­ers of two neu­tron stars: ob­jects, each with a weight com­pa­ra­ble to that of our sun, that spir­al around each oth­er, then crash to­geth­er at nearly light speed.

Such events are cal­cu­lat­ed to take place only once every sev­er­al hun­dred thou­sand years in a giv­en gal­axy. So to de­tect a sig­nal with­in our life­time, the de­tec­tors must be sen­si­tive enough to de­tect them out to dis­tances of a bil­lion light years away from Earth, re­search­ers say. A light year is the dis­tance light trav­els in a year. 

Such a search poses an im­mense tech­no­log­i­cal chal­lenge: at such dis­tances, the gravit­at­ional wave sig­nal would sound like a faint knock on our door when a TV set is turned on and a phone rings at the same time. Com­pet­ing noise sources are many, rang­ing from seis­mic noise pro­duce by ti­ny quakes or even a dis­tant ocean wave. How can we know we’ve de­tected a gravit­at­ional wave from space rath­er than a fall­ing tree or a ram­bling truck?

Thus as­tro­no­mers have been look­ing for years for a light sig­nal that might accompany or fol­low gra­vit­at­ional waves. This sig­nal would al­low us to “look through the peep­hole” af­ter hear­ing the faint knock on the door, and ver­i­fy that in­deed some­thing is there, ac­cord­ing to the two Is­rae­li re­search­ers who de­scribe their in­vest­igations in­to the top­ic in a new new pa­per in the re­search jour­nal Na­ture.

The sci­en­tists, Tsvi Pi­ran of He­brew Uni­versity of Je­ru­sa­lem and Ehud Nakar of Tel Aviv Uni­versity, sug­gest that such a sig­nal may al­ready have been no­ticed years ago.

Pi­ran and Nakar con­tend that that gas and dust sur­round­ing the col­lid­ing ob­jects would slow de­bris ejected at ve­locities close to light speed dur­ing neu­tron star merg­ers. Heat gen­er­at­ed dur­ing this pro­cess would ra­di­ate out­ward as ra­di­o waves. The re­sult­ing ra­di­o flare would last a few months and would be de­tectable with cur­rent ra­di­o tele­scopes from a bil­lion light years away, ac­cord­ing to the re­search­ers. 

Search af­ter such a ra­di­o sig­nal would take place fol­lowing a fu­ture de­tection, or even a ten­ta­tive de­tection of gra­vit­at­ional waves. But even be­fore the ad­vanced gra­vit­at­ional wave de­tec­tors be­come oper­at­ional, as ex­pected in 2015, ra­di­o as­tro­no­mers are geared to look­ing for these unique flares.

Nakar and Pi­ran say an uniden­ti­fied “ra­di­o tran­sient” source re­ported in 1987 by Geof­frey Bow­er of the Uni­versity of Cal­i­for­nia, Berke­ley, and col­leagues has all the char­ac­teris­tics of such a flare. A ra­di­o tran­sient is an un­usu­al as­tro­nom­i­cal ob­ject or event that gives off ra­di­o emis­sions for a few months or less, and is gen­er­ally be­lieved to mark some sort of ex­plo­sion or vi­o­lent pro­cess. The one seen by Bow­er’s group may have been the first di­rect de­tection of a neu­tron star bi­na­ry merg­er re­sult­ing in gra­vit­at­ional waves, ac­cord­ing to Pi­ran and col­leagues.

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Astronomers searching for an exotic type of ripple in the very fabric of space and time say a distant blast creating such waves may have already been detected. Although no instruments existed at the time that could have picked up the much-sought “gravitational waves,” the explosion reported in 1987 may nonetheless have created them, two scientists claim. They theorize that in the future, observations of similar explosions could help confirm proposed findings of gravitational waves. Albert Einstein proposed the existence of gravitational waves, and we’re still looking for them: ripples in space-time that are in a sense the sounds of our universe. Just as sound—which consists of ripples in the air or other material—complements vision in daily life, gravitational waves are expected to complement our view of the universe taken by standard telescopes. Einstein predicted their existence in 1918. Today, advanced gravitational wave detectors are being built in the US, Europe, Japan and Australia to search for them. In theory, any motion produces gravitational waves. But a signal loud enough to be detected requires two huge masses to collide very, very fast. The prime candidate sources are mergers of two neutron stars: objects, each with a weight comparable to that of our sun, that spiral around each other, then crash together at nearly light speed. Such events are calculated to take place only once every several hundred thousand years in a given galaxy. So to detect a signal within our lifetime, the detectors must be sensitive enough to detect them out to distances of a billion light years away from Earth, researchers say—a light year being the distance light travels in a year. Such a search poses an immense technological challenge: at such distances, the gravitational waves signal would sound like a faint knock on our door when a TV set is turned on and a phone rings at the same time. Competing noise sources are many, ranging from seismic noise produce by tiny quakes or even a distant ocean wave. How can we know we’ve detected a gravitational wave from space rather than a falling tree or a rambling truck? Thus astronomers have been looking for years for a light signal that might accompany or follow gravitational waves. This signal would allow us to “look through the peephole” after hearing the faint knock on the door, and verify that indeed “someone” is there, according to the two Israeli researchers who describe their investigations into the topic in a new new paper in the research journal Nature The scientists, Tsvi Piran of Hebrew University of Jerusalem and Ehud Nakar of Tel Aviv University, suggest that such a signal may already have been noticed years ago. Piran and Nakar contend that that gas and dust surrounding the colliding objects would slow debris ejected at velocities close to light speed during neutron star mergers. Heat generated during this process would radiate outward as radio waves. The resulting radio flare would last a few months and would be detectable with current radio telescopes from a billion light years away, according to the researchers, Search after such a radio signal would certainly take place following a future detection, or even a tentative detection of gravitational waves. But even before the advanced gravitational wave detectors become operational, as expected in 2015, radio astronomers are geared to looking for these unique flares. Nakar and Piran say an unidentified “radio transient” source reported in 1987 by Geoffrey Bower of the University of California, Berkeley, and colleagues has all the characteristics of such a flare. A radio transient is an unusual astronomical object or event that gives off radio emissions for a month or less, and is generally believed to mark some sort of explosion or violent process. The one seen by Bower’s group may have been the first direct detection of a neutron star binary merger resulting in gravitational waves, according to Piran and colleagues.