Skins! Several binary black holes may roll around each other in egg-shaped orbits

Black hole week ends today (May 10), and there’s no better way to mark the occasion than with some “egg-normal” black hole science.

Using gravitational wave measurements at the Laser Gravitational-Wave-Gravitational Observatory (LIGO), based in the US, and the Virgo and KAGRA detectors, based in Italy and Japan, respectively, scientists discovered that some orbits could Binary black holes have an egg. -shaped and show a strange wobble.

This research is just out of curiosity (and an “egg cuse” to break some bad egg-related puns). The discovery of these oval-shaped orbits in binary black hole systems could help researchers determine how each of these systems formed.

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“We expect most binary black holes to be in so-called ‘quasi-circular’ orbits. The ‘quasi’ means that the separation of the black holes is decreasing over time due to the emission of gravitational waves,” Lead study author Nihar Gupte, from the Max Planck Institute for Gravitational Physics in Germany and the University of Maryland, told

“Our study shows that some of the observed binary holes may be in ‘eccentric’ orbits,” said Gupte. “This means that black holes orbit in an oval or ‘egg’ shape.”

​​​​The team also found that the top of that oval orbit could rotate as the black holes orbit each other, the researcher said.

“​​​​We also found that if you analyze these events using a decentralized model, you will
an overestimation of the mass of black holes,” Gupte said.

What can we learn from the orbit of an oval black hole?

Gupte and his colleagues examined 57 black hole binary pairs detected through gravitational waves by the LIGO-Virgo-KAGRA collaboration. Gravitational waves are a barrier in spacetime first predicted by Albert Einstein in his famous 1915 theory of general relativity.

General relativity suggests that objects with mass create a curvature in the fabric of space and time, unified as a four-dimensional entity called “space-time.” Gravity results from this curve, which becomes more extreme as the mass of the object increases. That’s why stars have more gravitational influence than planets, and galaxies have more gravitational influence than stars.

Einstein also predicted in this revolutionary theory of gravity that when objects accelerate, they send tiny ripples out through space-time — gravitational waves. However, these fluctuations are insignificant until the domain of ultradense objects such as neutron stars and black holes is reached.

When binary neutron stars or black holes orbit each other, they constantly release gravitational waves, which carry energy away from the system in the form of angular momentum. When angular momentum is lost the orbits of these bodies tighten, pulling them together until their gravitational influence becomes complete. Eventually, they collide and merge, sending out a final high-pitched scream of gravitational waves.

Einstein thought that even these gravitational waves would be too small to be noticed on Earth. Fortunately, in September 2015, LIGO proved the scientist very wrong, relying on GW150914, a gravitational wave signal from a black hole binary merger more than 1 billion light years away.

Related: The Laser Interferometer Gravitational-Wave Observatory (LIGO): Detecting cracks in space-time

diagram showing two small black balls surrounded by increasing gray spirals

diagram showing two small black balls surrounded by increasing gray spirals

As the detection of gravitational waves continues to grow, scientists like Gupta are learning how to use them to reveal details about what creates them, as this new research shows.

Gupta explained that using gravitational waves to understand binary hole orbits is like paleontologists studying bones to reconstruct how dinosaurs lived. Therefore, physicists can study the properties of merging binary holes to understand how binary black holes come together in the first place.

This can happen in two distinct ways. Dynamical interactions occur when a binary black hole exists and interacts with another black hole, or even another black hole binary system.

Binaries, on the other hand, could be isolated and formed more simply from two stars that are already orbiting each other and become black holes, or from a single black hole wandering too close to each other and becoming binaries before they collide and merge.

diagram showing how black holes, represented by orange and black balls, orbit each other and mergediagram showing how black holes, represented by orange and black balls, orbit each other and merge

diagram showing how black holes, represented by orange and black balls, orbit each other and merge

“The main idea is that if we look at a binary with eccentricity, it’s likely to come from a dynamical interaction,” Gupta said. “These chaotic interactions can break the binary apart and eject its constituent black holes from their host galaxies and galaxy clusters. But sometimes, they can also reduce the distance between the two black holes, foster avoidance and merge them on short time scales.”

In addition to using orbital eccentricity to tell the story of the black hole binary, the scientist and his team are also interested in considering what the elliptical nature of the orbits does to the gravitational emissions of these systems.

“When you have eccentricity, this means that at some points in the orbit, the black holes are closer together,” Gupta explained. “When black holes are closer together, they have more acceleration, which means they emit more gravitational waves. On the other hand, if they are far away, they have less acceleration, which which means they emit fewer gravitational waves.

“So you see little blips in the amplitude of the waveform [the total pattern of gravitational waves]which occurs from the black holes moving closer and further apart!”

diagram showing five stars near black holes, each pair represented by a red and white figure-8 patterndiagram showing five stars near black holes, each pair represented by a red and white figure-8 pattern

diagram showing five stars near black holes, each pair represented by a red and white figure-8 pattern

Determining the nature and history of binary black holes would be extremely difficult without the use of gravitational waves. Another method of understanding the origin of binary black holes is to look for so-called “common envelope” events with standard light-based astronomy.

These events begin with a star and a black hole orbiting each other, with that star growing into a red giant. The outer layers of the swollen puffed star form a common envelope around the two occupants of the binary, generating friction between the black hole and the star. This shrinks the binary’s orbit, and eventually, after the red giant becomes a black hole, this results in a black hole binary merger.

“The problem is that it is difficult to observe this critical period with electromagnetic observations. This is because massive stars are rare and short-lived, so only a small fraction of these systems are critical evolutionary steps of compact object fusion,” said Gupta. “By studying gravitational waves, on the other hand, we can understand the final moments of the binary merger. This can allow us to trace back the history of the merger and hypothesize what might have happened. “

He added that gravitational waves are particularly useful in this regard because they are “extremely clean storage” or distant events. This refers to the fact that these ripples through space-time can travel vast distances without any interference between the binary and Earth.

Related: The universe is humming with gravitational waves. This is why scientists are so excited about the discovery


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“While we do not claim that these are definitive detections of eccentric binary holes, these results point to the eccentricity. [in the] the existing population,” said Gupte. “This is an important consideration for the Earth-based gravity sensor observation run, as well as future ground and space-based gravity detectors.

“Currently, we do not have enough data to definitively determine the origin of binary black holes. However, if we observe more binary black holes in the future, we can begin to place constraints on the mechanisms that form the systems this.”

The team’s paper has not yet been published in a peer-reviewed journal. You can read a preprint of it at the arXiv online repository.

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