Scientists have confirmed for the first time that the very fabric of space-time is making a “final dive” at the edge of a black hole.
The observation of this sinking region around black holes was made by Oxford University Physics astrophysicists and helped confirm a key prediction of Albert Einstein’s 1915 theory of gravity: general relativity.
The Oxford team made the discovery by focusing on regions surrounding stellar-mass black holes in binary systems with companion stars located 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 International Space Station’s Neutron Star Interior Composition Explorer (NICER).
This data allowed them to determine the fate of hot ionized gas and plasma torn away from a companion star, taking a final dive at the very edge of its associated black hole. The findings show that these so-called sinking regions around a black hole are the locations of some of the strongest points of gravitational influence ever seen in our Milky Way galaxy.
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“This is the first look at how plasma detached 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,” team leader and Oxford University physicist Andrew Mummery said in a statement. “Einstein’s theory predicted that this final descent would exist, but this is the first time we’ve been able to demonstrate that it happens.
“Think of it as a river turning into a waterfall – so far we’ve been looking at the river. This is our first view of the falls.”
Where does black hole diving come from?
Einstein’s theory of general relativity suggests that objects with mass cause a distortion of the very fabric of space and time, united as a single four-dimensional entity called “space-time.” Gravity arises from the resulting curvature.
Although general relativity works in 4D, it can be vaguely illustrated by a crude 2D analogy. Imagine placing spheres of increasing mass on a stretched rubber sheet. A golf ball would cause a small, almost imperceptible dent; a cricket ball would make a bigger dent; and bowling ball massive dent. This is analogous to moons, planets, and stars “denting” 4D space-time. As an object’s mass increases, so does the curvature they cause, and thus their gravitational influence increases. A black hole would be like a cannonball on this analogous rubber sheet.
With masses equivalent to tens or even hundreds of suns compressed into a width around that of 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 completely different story. They are very much massive, with masses equivalent to millions or even billions of suns, surpassing even their stellar-mass counterparts.
Returning to general relativity, Einstein hypothesized that this curvature of spacetime leads to other interesting physics. For example, he said, there must be a point just outside the boundary of the black hole where the particles could not follow a circular or stable orbit. Instead, matter entering this region will plunge toward the black hole at near the speed of light.
Understanding the physics of matter in this hypothetical black hole submersion region has been a goal of astrophysicists for some time. To address this, the Oxford team looked at what happens when black holes exist in a binary system with an “ordinary” star.
If the two are close enough, or if that star is slightly bulging, the black hole’s gravitational influence can tear away the stellar material. Because this plasma comes with angular momentum, it can’t fall straight toward the black hole—so instead, it forms a flattened rotating cloud around the black hole called an accretion disk.
From this accretion disk, matter is gradually fed to the black hole. According to black hole power models, there should be a point called the innermost stable circular orbit (ISCO)—the last point at which matter can remain stably rotating in the accretion disk. Any matter beyond that is in the “submergence region” and begins its inevitable descent into the mouth of the black hole. The debate over whether this sinking region could ever be detected was resolved when the Oxford team detected emission just beyond the ISCO of accretion discs around a binary black hole in the Milky Way called MAXI J1820+070.
Located about 10,000 light-years from Earth with a mass of about eight suns, the black hole component of MAXI J1820+070 pulls material from its companion star as it fires 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 burst in a “soft state”, which represents emission from an accretion disk surrounding a rotating black hole, or “Kerr” – a complete accretion disk, including the dipping region.
The researchers say this scenario represents the first robust detection of emission from a plunging region at the inner edge of a black hole’s accretion disk; they refer to such signals as “intra-ISCO emissions”. These emissions within ISCO confirm the accuracy of general relativity in describing the regions immediately around black holes.
To follow up on this research, a separate team from Oxford’s Department of Physics is collaborating with a European initiative to build the African Millimeter Telescope. This telescope should improve scientists’ ability to take direct images of black holes and allow the sinking regions of more distant black holes to be studied.
“What’s really exciting is that there are a lot of black holes in the galaxy, and we now have a powerful new technique for using them to study the strongest known gravitational fields,” Mummery concluded.
The team’s research is published in the journal Monthly Notices of the Royal Astronomical Society.
