Scientists have confirmed for the first time that the fabric of spacetime takes a “final plunge” at the edge of a black hole.
The observation of this crashing region around black holes was made by astrophysicists at Oxford University Physics, and helps validate a key prediction of Albert Einstein’s 1915 theory of gravity: general relativity.
The Oxford team made this discovery while focusing on regions around stellar-mass black holes in binary stars with companion stars relatively close to Earth. The researchers used X-ray data collected from a range of space telescopes, including NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) and the International Space Station-mounted Neutron Star Interior Composition Explorer (NICER).
This data allowed them to determine the fate of hot ionized gas and plasma, stripped of a companion star, which took a final plunge at the far edge of its associated black hole. The findings showed that these so-called diving regions around a black hole are the locations of some of the strongest spots of gravitational influence ever observed in our Milky Way Galaxy.
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“This is the first look at how plasma, peeled from the outer edge of a star, undergoes its final fall into the center of a black hole, a process that takes place in a system about 10,000 light-years away,” said team leader and physics major University of Oxford. scientist Andrew Mummery said in a statement. ‘Einstein’s theory predicted that this definitive jump would exist, but this is the first time we have been able to demonstrate that this 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 waterfall.”
Where does the black hole’s dive 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. Gravity arises from the resulting curvature.
Although general relativity works in 4D, it can be vaguely illustrated using a rough 2D analogy. Imagine placing spheres of increasing mass on a stretched rubber sheet. A golf ball would make a small, almost unnoticeable dent; a cricket ball would cause a bigger dent; and a bowling ball a huge dent. That’s analogous to moons, planets, and stars “denting” 4D spacetime. As the mass of an object increases, the curvature it causes also increases, and thus the gravitational influence increases. 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 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 enormous enormous, with masses equivalent to millions or even billions of suns, dwarfing even their stellar-mass counterparts.
Returning to general relativity, Einstein suggested that this curvature of spacetime leads to other interesting physics. For example, he said that there must be a point just outside the boundary of the black hole where particles would not be able to follow a circular or stable path. Instead, matter entering this region would plummet toward the black hole at near-light speeds.
Understanding the physics of matter in this hypothetical black hole dive 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 star system with a ‘normal’ star.
If the two are close enough, or if this star is slightly blown out, the black hole’s gravitational influence can pull away stellar material. Because this plasma has angular momentum, it cannot fall directly to the black hole. Instead, it forms a flattened, rotating cloud around the black hole, called an accretion disk.
From that accretion disk, matter is gradually transported to the black hole. According to models of feeding black holes, there should be a point called the innermost stable circular orbit (ISCO) – the last point at which matter can continue to rotate stably in an accretion disk. All matter outside of it is in the ‘dive zone’ and begins its inevitable descent into the maw of the black hole. The debate over whether this plummeting region could ever be detected was settled when the Oxford team discovered emissions from just beyond ISCO from accretion disks around a black hole binary star in the Milky Way called MAXI J1820+070.
Located about 10,000 light-years from Earth and with a mass of about eight suns, the black hole component of MAXI J1820+070 pulls material from its stellar companion while emitting two jets at about 80% the speed of light; it also produces strong X-rays.
The team found that the X-ray spectrum of MAXI J1820+070 is in a ‘soft’ burst, which represents the emission from an accretion disk around a rotating, or ‘Kerr’, black hole – an entire accretion disk, including the collapsing region.
The researchers say this scenario is the first robust detection of emission from a plunging region at the inner edge of a black hole accretion disk; they call such signals ‘intra-ISCO emissions’. These intra-ISCO emissions confirm the accuracy of general relativity in describing the regions immediately surrounding black holes.
To continue this research, a separate team from Oxford’s Department of Physics is working with a European initiative to build the Africa Millimeter Telescope. This telescope should increase scientists’ ability to capture direct images of black holes and make it possible to investigate the deeper regions of more distant black holes.
“What’s really exciting is that there are many black holes in the Milky Way, and we now have a powerful new technique to use them to study the strongest known gravitational fields,” Mummery concluded.
The team’s research has been published in the journal Monthly Notices of the Royal Astronomical Society.