"Long before it's in the papers"
October 04, 2012

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Scientists measure cosmic “exit door”

Oct. 4, 2012
Courtesy of Jennifer Chu / MIT News Office
and World Science staff

The point of no re­turn: in as­tron­o­my, it’s called a black hole — a place where gra­vity’s pull is so strong that noth­ing, not even light, can es­cape. Black holes are ob­jects some­times bil­lions of times heav­i­er than our sun, and they are be­lieved to lie at the hearts of most ga­lax­ies. Such “su­per­mas­sive” black holes are so pow­er­ful that ac­ti­vity at their bound­aries can rip­ple through­out their host ga­lax­ies.

This im­age, cre­at­ed us­ing com­put­er mod­els, shows how the ex­treme grav­i­ty of the black hole in M87 dis­torts the ap­pear­ance of the je­t near the event ho­ri­zon. Part of the ra­di­a­tion from the je­t is bent by grav­i­ty in­to a ring that is known as the 'shad­ow' of the black hole.
(Im­age: Avery E. Brod­er­ick / Pe­rim­e­ter In­sti­tute & U. of Wa­ter­loo)

Now, re­search­ers have for the first time meas­ured the clos­est dis­tance at which mat­ter can ap­proach be­fore be­ing ir­re­trievably pulled in­to the black hole at the cen­ter of a dis­tant gal­axy. The find­ing could shed light on how ga­lax­ies evolve and on the cor­rect­ness of Ein­stein’s the­o­ries.

Led by re­search­ers at the Mas­sachus­etts In­sti­tute of Tech­nol­o­gy, the sci­en­tists linked to­geth­er ra­di­o tele­scopes in Ha­waii, Ar­i­zo­na and Cal­i­for­nia to cre­ate a tel­e­scope ar­ray called the “Event Ho­ri­zon Tele­scope.” The mega-in­stru­ment can see de­tails 2,000 times fin­er than what’s vis­i­ble to the Hub­ble Space Tel­e­scope, they said. The ar­ray was trained on M87, a gal­axy some 50 mil­lion light years from the Milky Way. A light year is the dis­tance light trav­els in a year.

M87 har­bors a black hole es­ti­mat­ed to weigh the equiv­a­lent of six bil­lion of our suns. Us­ing the ar­ray, the team watched the glow of mat­ter near the edge of this black hole — a re­gion known as the “e­vent hori­zon.”

“Once ob­jects fall through the event ho­ri­zon, they’re lost forever,” said Shep Doele­man, as­sis­tant di­rec­tor at the MIT Hay­stack Ob­serv­a­to­ry and re­search as­so­ci­ate at the Smith­so­nian As­t­ro­phys­i­cal Ob­serv­a­to­ry in Cam­bridge, Mass. “It’s an ex­it door from our uni­verse. You walk through that door, you’re not com­ing back.”

Doele­man and his col­leagues have pub­lished the re­sults of their study this week in the jour­nal Sci­ence.

Jets at the edge of a black hole

Su­per­mas­sive black holes are the most ex­treme ob­jects pre­dicted by Al­bert Ein­stein’s the­o­ry of gra­vity — where, Doele­man said, “gra­vity com­pletely goes hay­wire and crushes an enor­mous mass in­to an in­credibly close space.” At the edge of a black hole, the gravita­t­ional force is so strong that it pulls in eve­ry­thing from its sur­round­ings. How­ev­er, not eve­ry­thing can cross the event ho­ri­zon to squeeze in­to a black hole. The re­sult is a “cos­mic traf­fic jam” in which gas and dust build up, cre­at­ing a flat pan­cake of mat­ter known as an ac­cre­tion disk. This disk of mat­ter or­bits the black hole at nearly the speed of light, feed­ing the black hole a steady di­et of su­per­heated ma­te­ri­al. Over time, this disk can cause the black hole to spin in the same di­rec­tion as the or­bit­ing ma­te­ri­al.

Caught up in this spi­ral­ing flow are mag­net­ic fields, which ac­cel­er­ate hot ma­te­ri­al along pow­er­ful beams above the ac­cre­tion disk. The re­sulting high-speed je­t, launched by the black hole and the disk, shoots out across the gal­axy, ex­tend­ing for hun­dreds of thou­sands of light-years. These je­ts can in­flu­ence many ga­lac­tic pro­cesses, in­clud­ing how fast stars form.

“Is Ein­stein right?”

A je­t’s tra­jec­to­ry may help sci­en­tists un­der­stand the dy­nam­ics of black holes in the re­gion where their gra­vity is the dom­i­nant force. Doele­man said such an ex­treme en­vi­ron­ment is per­fect for con­firm­ing Ein­stein’s the­o­ry of gen­er­al rel­a­ti­vity — to­day’s de­fin­i­tive de­scrip­tion of gravita­t­ion.

“Ein­stein’s the­o­ries have been ver­i­fied in low-gravita­t­ional field cases, like on Earth or in the so­lar sys­tem,” Doele­man said. “But they have not been ver­i­fied pre­cisely in the only place in the uni­verse where Ein­stein’s the­o­ries might break down — which is right at the edge of a black hole.”

Ac­cord­ing to Ein­stein’s the­o­ry, a black hole’s mass and its spin de­ter­mine how closely ma­te­ri­al can or­bit be­fore becom­ing un­sta­ble and fall­ing in to­ward the event ho­ri­zon. Be­cause M87’s je­t is mag­net­ic­ally launched from this small­est or­bit, as­tro­no­mers can es­ti­mate the black hole’s spin through care­ful meas­ure­ment of the je­t’s size as it leaves the black hole. Un­til now, no tel­e­scope has had the mag­ni­fy­ing pow­er re­quired for this kind of ob­serva­t­ion.

“We are now in a po­si­tion to ask the ques­tion, ‘Is Ein­stein right?’” Doele­man said. “We can iden­ti­fy fea­tures and sig­na­tures pre­dicted by his the­o­ries, in this very strong gravita­t­ional field.”

The team used a tech­nique called Very Long Base­line In­ter­fer­om­etry, which links da­ta from ra­di­o dishes lo­cat­ed thou­sands of miles apart. Sig­nals from the var­i­ous dishes, tak­en to­geth­er, cre­ate a “vir­tual tel­e­scope” with the re­solv­ing pow­er of a sin­gle tel­e­scope as big as the space be­tween the dishes.

Us­ing the tech­nique, Doele­man and his team meas­ured the in­ner­most or­bit of the ac­cre­tion disk to be only 5.5 times the size of the black hole event ho­ri­zon. Ac­cord­ing to the laws of phys­ics, this size sug­gests that the ac­cre­tion disk is spin­ning in the same di­rec­tion as the black hole — the first di­rect ob­serva­t­ion to con­firm the­o­ries of how black holes pow­er je­ts from the cen­ters of ga­lax­ies.

The team plans to ex­pand its tel­e­scope ar­ray, adding ra­di­o dishes in Chil­e, Eu­rope, Mex­i­co, Green­land and Ant­arc­ti­ca, in or­der to ob­tain even more de­tailed pic­tures of black holes in the fu­ture.

Chris­to­pher Reyn­olds, a pro­fes­sor of as­tron­o­my at the Uni­vers­ity of Mar­y­land, said the group’s re­sults pro­vide the first ob­serva­t­ional da­ta that will help sci­en­tists un­der­stand how a black hole’s je­ts be­have.

“The bas­ic na­ture of je­ts is still mys­te­ri­ous,” Reyn­olds said. “Many as­t­ro­phys­i­cists sus­pect that je­ts are pow­ered by black hole spin ... but right now, these ideas are still en­tirely in the realm of the­o­ry. This meas­ure­ment is the first step in put­ting these ideas on a firm ob­serva­t­ional ba­sis.”

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The point of no return: in astronomy, it’s called a black hole — a place where gravity’s pull is so strong that nothing, not even light, can escape. Black holes are objects sometimes billions of times heavier than our sun, and they are believed to lie at the hearts of most galaxies. Such “supermassive” black holes are so powerful that activity at their boundaries can ripple throughout their host galaxies. Now, researchers have for the first time measured the closest distance at which matter can approach before being irretrievably pulled into the black hole at the center of a distant galaxy. The finding could shed light on how galaxies evolve and on the correctness of Einstein’s theories. Led by researchers at the Massachussets Institute of Technology, the scientists linked together radio telescopes in Hawaii, Arizona and California to create a telescope array called the “Event Horizon Telescope.” The mega-instrument can see details 2,000 times finer than what’s visible to the Hubble Space Telescope, they said. The array was trained on M87, a galaxy some 50 million light years from the Milky Way. A light year is the distance light travels in a year. M87 harbors a black hole estimated to weigh the equivalent of six billion of our suns. Using the array, the team watched the glow of matter near the edge of this black hole — a region known as the “event horizon.” “Once objects fall through the event horizon, they’re lost forever,” said Shep Doeleman, assistant director at the MIT Haystack Observatory and research associate at the Smithsonian Astrophysical Observatory in Cambridge, Mass. “It’s an exit door from our universe. You walk through that door, you’re not coming back.” Doeleman and his colleagues have published the results of their study this week in the journal Science. Jets at the edge of a black hole Supermassive black holes are the most extreme objects predicted by Albert Einstein’s theory of gravity — where, Doeleman said, “gravity completely goes haywire and crushes an enormous mass into an incredibly close space.” At the edge of a black hole, the gravitational force is so strong that it pulls in everything from its surroundings. However, not everything can cross the event horizon to squeeze into a black hole. The result is a “cosmic traffic jam” in which gas and dust build up, creating a flat pancake of matter known as an accretion disk. This disk of matter orbits the black hole at nearly the speed of light, feeding the black hole a steady diet of superheated material. Over time, this disk can cause the black hole to spin in the same direction as the orbiting material. Caught up in this spiraling flow are magnetic fields, which accelerate hot material along powerful beams above the accretion disk The resulting high-speed jet, launched by the black hole and the disk, shoots out across the galaxy, extending for hundreds of thousands of light-years. These jets can influence many galactic processes, including how fast stars form. “Is Einstein right?” A jet’s trajectory may help scientists understand the dynamics of black holes in the region where their gravity is the dominant force. Doeleman said such an extreme environment is perfect for confirming Einstein’s theory of general relativity — today’s definitive description of gravitation. “Einstein’s theories have been verified in low-gravitational field cases, like on Earth or in the solar system,” Doeleman said. “But they have not been verified precisely in the only place in the universe where Einstein’s theories might break down — which is right at the edge of a black hole.” According to Einstein’s theory, a black hole’s mass and its spin determine how closely material can orbit before becoming unstable and falling in toward the event horizon. Because M87’s jet is magnetically launched from this smallest orbit, astronomers can estimate the black hole’s spin through careful measurement of the jet’s size as it leaves the black hole. Until now, no telescope has had the magnifying power required for this kind of observation. “We are now in a position to ask the question, ‘Is Einstein right?’” Doeleman said. “We can identify features and signatures predicted by his theories, in this very strong gravitational field.” The team used a technique called Very Long Baseline Interferometry, or VLBI, which links data from radio dishes located thousands of miles apart. Signals from the various dishes, taken together, create a “virtual telescope” with the resolving power of a single telescope as big as the space between the disparate dishes. The technique enables scientists to view extremely precise details in faraway galaxies. Using the technique, Doeleman and his team measured the innermost orbit of the accretion disk to be only 5.5 times the size of the black hole event horizon. According to the laws of physics, this size suggests that the accretion disk is spinning in the same direction as the black hole — the first direct observation to confirm theories of how black holes power jets from the centers of galaxies. The team plans to expand its telescope array, adding radio dishes in Chile, Europe, Mexico, Greenland and Antarctica, in order to obtain even more detailed pictures of black holes in the future. Christopher Reynolds, a professor of astronomy at the University of Maryland, said the group’s results provide the first observational data that will help scientists understand how a black hole’s jets behave. “The basic nature of jets is still mysterious,” Reynolds said. “Many astrophysicists suspect that jets are powered by black hole spin ... but right now, these ideas are still entirely in the realm of theory. This measurement is the first step in putting these ideas on a firm observational basis.”