Scientists have confirmed for the first time that the very fabric of space-time “finally collapses” at the edge of a black hole.
The observation of this falling region around black holes was made by astrophysicists at Oxford University Physics and helps 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 binaries with companion stars located relatively close to Earth. The researchers used X-ray data collected from 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 the hot ionized gas and plasma, stripped of its companion star, that was last ejected at the very edge of the associated black hole. The findings showed that these so-called plunging regions around the black hole are the location of some of the strongest gravitational pulls ever observed in our Milky Way galaxy.
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“This is the first look at how plasma, stripped from the outer edge of a star, undergoes its final fall into the center of a black hole, a process occurring in a system about 10,000 light-years away,” team leader and Oxford University physicist. scientist Andrew Mummery said in a statement. “Einstein’s theory predicted that this final collapse would occur, but this is the first time we have been able to demonstrate that it has occurred.
“Think of it as a river turning into a waterfall – until now we’ve been looking at a river. This is our first look at a waterfall.”
Where does black hole collapse come from?
Einstein’s theory of general relativity suggests that objects with mass cause a distortion of the very fabric of space and time, unified 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 balls of increasing weight on a stretched rubber sheet. A golf ball would make a tiny, almost imperceptible dent; a cricket ball would result in a larger dent; and bowling ball a massive dent. This is analogous to the moons, planets and stars “deepening” 4D spacetime. As the mass of the object increases, so does the curvature they cause, thus increasing their gravitational influence. A black hole would be like a cannonball on that analog rubber sheet.
With masses equivalent to tens or even hundreds of suns squeezed into the width around Earth, the curvature of space-time 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 huge massive, with masses equivalent to millions or even billions of suns, surpassing even their stellar counterparts.
Returning to general relativity, Einstein proposed that this curvature of spacetime leads to another interesting physics. For example, he said, there must be a point just beyond the boundary of a black hole at which the particles would not be able to follow a circular or stable path. Instead, matter entering this region would be hurtled toward the black hole at near-light speeds.
Understanding the physics of matter in this hypothetical plunging region of a black hole has been a goal of astrophysicists for some time. To solve this problem, 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 the star is slightly inflated, the black hole’s gravitational pull can pull away the stellar material. Because this plasma comes with angular momentum, it cannot fall directly into the black hole—instead, it forms a flattened rotating cloud called an accretion disk around the black hole.
From this accretion disk, matter is gradually fed into the black hole. According to models of black hole feeding, there should be a point called the innermost stable circular orbit (ISCO) — the last point at which matter can remain stably rotating in an accretion disk. Any matter beyond this is in the “submergence region” and begins its inevitable descent into the maw of the black hole. The debate over whether this infalling region could ever be detected was settled when the Oxford team found emission just beyond the ISCO of accretion disks around a Milky Way black hole binary named MAXI J1820+070.
The black hole component of MAXI J1820+070, located about 10,000 light-years from Earth with a mass of about eight suns, is pulling material from its companion star while firing 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 in a “soft” flare, which represents emission from an accretion disk surrounding a rotating or “Kerr” black hole – a full accretion disk, including the dipping edge.
The researchers say this scenario represents the first robust detection of emission from a subduction region at the inner edge of a black hole’s accretion disk; they call such signals “ISCO emissions”. These emissions within the ISCO confirm the accuracy of general relativity in describing the regions immediately around black holes.
To build 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 enable probing the sinking 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 is published in the journal Monthly Notices of the Royal Astronomical Society.