Black Hole Week ends today (May 10), and there’s no better way to mark the occasion than with some “weird-as-egg” black hole science.
Using gravitational wave measurements from the US-based Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo and KAGRA detectors located in Italy and Japan respectively, scientists have discovered that the orbits of some binary black holes may be egg-shaped and show a curious wobble.
This research is more than just curiosity (and an “egg claim” to break some bad egg-related puns). Finding these oval-shaped orbits in binary systems with black holes can help researchers determine how each of these systems formed.
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“We find that the majority of binary black holes are expected to be in so-called ‘quasi-circular’ orbits. “‘Quasi’ simply means that the separation of black holes decreases with time due to the emission of gravitational waves,” lead study author Nihar Gupte of the Max Planck Institute for Gravitational Physics in Germany and the University of Maryland told Space.com.
“Our research suggests that some of the observed binary black holes may be in ‘eccentric’ orbits,” added Gupte. “This means that black holes orbit in an oval or ‘egg’ shape.
The team also found that the tip of this egg-shaped oval orbit can rotate as the black holes rotate around each other, the researcher said.
“We also found that if you analyze these events using a non-eccentric model, you will
overestimate the masses of black holes,” Gupte added.
What we can learn from the ovoid orbits of a black hole
Gupte and his colleagues studied 57 binary pairs of black holes detected by gravitational waves from the LIGO-Virgo-KAGRA collaboration. Gravitational waves are ripples in spacetime that were first predicted by Albert Einstein in his famous theory of general relativity in 1915.
General relativity suggests that objects with mass create a curvature in the very fabric of space and time, united as a four-dimensional unit called “space-time.” Gravity arises from this curvature, which becomes more extreme as the objects’ masses increase. This is why stars have a greater gravitational influence than planets, and galaxies have a greater gravitational influence than stars.
Einstein also predicted in this revolutionary theory of gravity that when objects accelerate, they send tiny ripples radiating through space-time—gravitational waves. However, these waves are negligible until the region of superdense objects such as neutron stars and black holes is reached.
When binary neutron stars or black holes spin around each other, they continuously emit gravitational waves that carry energy away from the system in the form of angular momentum. The loss of angular momentum causes the orbits of these bodies to tighten, pulling them together until their gravitational influence takes over. Eventually, they collide and merge, sending out a final high-pitched screech of gravitational waves.
Einstein thought that even these gravitational waves would be too weak to be detected on Earth. Fortunately, in September 2015, LIGO proved the great scientist wrong by detecting GW150914, a gravitational wave signal from a double black hole merger more than 1 billion light years away.
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As the detection of gravitational waves continues to emerge, scientists like Gupta are learning how to use them to reveal details about the objects that create them, as this new study shows.
Gupta explained that using gravitational waves to understand the orbits of binary black holes is similar to paleontologists studying bones to reconstruct how dinosaurs may have lived. In this way, physicists can study the properties of binary black hole mergers to understand how binary black holes come together in the first place.
This can be done in two different ways. Dynamical interactions occur when a binary black hole encounters and interacts with another black hole or even another black hole binary system.
Binaries, on the other hand, can be isolated and form more simply from two stars already orbiting each other and becoming black holes, or from one black hole moving too close to another and forming a binary system before they collide and merge.
“The key idea is that if we observe a binary system with an eccentricity, it probably comes from a dynamical interaction,” Gupta said. “These chaotic interactions can disrupt the binary system and eject the constituent black holes from the galaxies and galaxy clusters that host them. But sometimes they can also reduce the distance between the two black holes, induce eccentricity and cause them to merge on short timescales. “
In addition to using orbital eccentricity to tell the story of black hole binaries, the scientist and his team are also interested in looking at what the oval nature of the orbits does to the gravitational wave emissions of these systems.
“When you have eccentricity, it means that at some points in the orbit the black holes are closer together,” Gupta explained. “When black holes are closer together, they have a greater acceleration, which means they emit more gravitational waves. On the other hand, if they are far away, they have less acceleration, which means they emit less gravitational waves.
“So you end up seeing little blips in the amplitude of the waveform [the total pattern of gravitational waves]which arise from black holes moving closer and further apart!”
The nature and history of binary black holes would be incredibly difficult to determine without the use of gravitational waves. An alternative method for understanding the origin of binary black holes is to search for so-called “common envelope” events with standard light astronomy.
These events begin with a star and a black hole orbiting each other, with that star becoming a red giant. The outer layers of the swollen bulge star create a common envelope around the two inhabitants of the binary system, generating friction between the black hole and the star. This shrinks the orbit of the binary system and eventually, after the red giant has become a black hole, it leads to a binary black hole merger.
“The problem is that observing this critical period is difficult with electromagnetic observations. This is because massive stars are rare and short-lived, so the critical evolutionary phases of merging compact objects occupy a small fraction of these systems,” Gupta said. “By studying gravitational waves, on the other hand, we can understand the final moments of the binary merger. This may allow us to trace the history of the merger and speculate on what might have formed it.”
He added that gravitational waves are particularly useful in this regard because they are “extremely clean probes” or distant events. This refers to the fact that these waves through space-time can travel vast distances without interference from anything between the binary system and Earth.
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“While we do not claim that these are definitive detections of eccentric binary black holes, these results point to eccentricity [in the] existing population,” Gupte said. “This is an important consideration for current ground-based gravitational wave detector observations, as well as for future ground-based and space-based gravitational wave detectors.
“Currently, we do not have enough data to definitively determine the origin of binary black holes. However, if we observe more eccentric binary black holes in the future, we can begin to place constraints on which mechanisms form these systems.”
The team’s report has not yet been published in a peer-reviewed journal. You can read a reprint of it in the arXiv online repository.
