Scientists have confirmed for the first time that the fabric of spacetime itself is undergoing a “final nosedive” at the edge of a black hole.
The observation of this dipping region around black holes was carried out by astrophysicists at the University of Oxford’s Physics Institute and helps confirm a key prediction of Albert Einstein’s 1915 theory of gravity: general relativity.
The Oxford team made the discovery while focusing on regions around stellar-mass black holes in binary star systems whose companion stars are relatively close to Earth. The researchers used X-ray data collected by a number of space telescopes, including NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) and the Neutron Star Interior Composition Explorer (NICER) mounted on the International Space Station.
This data allowed them to determine the fate of hot ionized gas and plasma that separated from a companion star and made a final plunge into the very edge of its associated black hole. The results showed that these so-called crash regions around a black hole are the locations of some of the strongest gravitational influence points ever observed in our Milky Way Galaxy.
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“This is the first look at how plasma shed from the outer edge of a star undergoes its final fall into the center of a black hole, a process that occurs in a system about 10,000 light-years away,” said team leader and physicist Oxford University scientist Andrew Mummery said in a statement. “Einstein’s theory predicted that this final crash would occur, but this is the first time we have been able to demonstrate that it will happen.”
“Imagine it as a river turning into a waterfall – so far we’ve been looking at the river. This is our first view of the waterfall.”
Where does the black hole crash come from?
Einstein’s theory of general relativity suggests that objects with mass cause the fabric of space and time, unified as a single four-dimensional entity called “spacetime,” to warp. The resulting curvature creates gravity.
Although general relativity works in 4D, it can be vaguely illustrated by a rough 2D analogy. Imagine placing balls of increasing mass on a stretched rubber plate. A golf ball would leave a tiny, barely noticeable dent; a cricket ball would cause a larger dent; and a bowling ball a huge dent. This is comparable to moons, planets and stars “bumping” 4D spacetime. As an object’s mass increases, the curvature it causes also increases, and thus its influence on gravity also increases. A black hole would be like a cannonball on this analog rubber plate.
With masses equivalent to dozens or even hundreds of suns compressed to a width around the Earth, the curvature of spacetime and the gravitational influence of stellar-mass black holes can become quite extreme. Supermassive black holes, on the other hand, are a whole different story. They are enormous massive, with masses equivalent to millions or even billions of suns, dwarfing even their stellar mass counterparts.
Going back to general relativity, Einstein proposed that this curvature of space-time leads to other interesting physical phenomena. For example, there must be a point just outside the boundary of the black hole at which the particles would no longer be able to follow a circular or stable orbit. Instead, matter entering this region would rush toward the black hole at nearly the speed of light.
Understanding the physics of matter in this hypothetical black hole crash region has been a goal of astrophysicists for some time. To tackle this problem, the Oxford team studied what happens when black holes exist in a binary star system with a “normal” star.
If the two are close enough, or if that star is slightly inflated, the black hole’s gravitational influence can pull star material away. Because this plasma has angular momentum, it cannot fall directly onto the black hole, but instead forms a flattened, rotating cloud around the black hole called the accretion disk.
From this accretion disk, matter is gradually fed into the black hole. According to black hole feeding models, there should be a point called the innermost stable circular orbit (ISCO) – the last point at which matter can rotate stably in an accretion disk. Any matter beyond that is in the “crash region” and begins its inevitable descent into the black hole’s maw. The debate over whether this crash region could ever be discovered was settled when the Oxford team discovered emissions just beyond the ISCO of the accretion disks around a black hole binary in the Milky Way called MAXI J1820+070.
The black hole component of MAXI J1820+070 is located about 10,000 light-years from Earth and has a mass of about eight suns. It pulls material from its stellar companion, ejecting twin jets at about 80% the speed of light; it also produces strong X-ray emissions.
The team found that the X-ray spectrum of MAXI J1820+070 is in a “soft-state” burst, which represents the emission of an accretion disk surrounding a rotating or “Kerr” black hole – a complete accretion disk, including the Immersion region.
The researchers say this scenario represents the first reliable evidence of emission from a crash region at the inner edge of a black hole accretion disk; They refer to such signals as “intra-ISCO emissions.” These intra-ISCO emissions confirm the accuracy of general relativity in describing the regions immediately surrounding black holes.
To take this research further, another team from Oxford’s physics department is working with a European initiative to build the Africa Millimeter Telescope. This telescope is designed to allow scientists to take direct images of black holes and study the plummeting regions of more distant black holes.
“What’s really exciting is that there are many black holes in the galaxy and we now have a powerful new technique to use them to study the strongest known gravitational fields,” Mummery concluded.
The team’s research results are published in the journal Monthly Notices of the Royal Astronomical Society.