Black Hole Week comes to a close today (May 10), and what better way to mark the occasion than with some ‘extraordinary egg’ science.
Using gravitational wave measurements by the Laser Interferometer Gravitational-Wave Observatory (LIGO), based in the US, and the Virgo and KAGRA detectors, based in Italy and Japan respectively, scientists have discovered that the orbits of some binary black holes can ovulate are. shaped and exhibit a peculiar wobble.
This investigation is more than just a curiosity (and an “egg-cuse” to crack 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 was formed.
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“We find that the majority of binary black holes are expected to be in so-called ‘quasi-circular’ orbits. The ‘quasi’ just means that the distance between the black holes decreases over time due to the gravitational wave emission.” lead author Nihar Gupte of the Max Planck Institute for Gravitational Physics in Germany and the University of Maryland, told Space.com.
“Our research shows that some of the observed binary black holes may be in ‘eccentric’ orbits,” Gupte added. “This means that the black holes spin around in an oval or ‘egg’ shape.”
The team also found that the tip of that egg-shaped 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 non-eccentric model, you will
overestimates the mass of the black holes,” Gupte added.
What can we learn from egg-shaped orbits of black holes?
Gupte and his colleagues examined 57 binary black hole pairs detected via gravitational waves by the LIGO-Virgo-KAGRA collaboration. Gravitational waves are ripples in spacetime that were 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.” This curvature creates gravity, which becomes more extreme as the mass of the objects increases. That is 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 emit small ripples that radiate through space-time: gravitational waves. However, these ripples are insignificant until the domain of ultra-dense objects such as neutron stars and black holes is reached.
When binary neutron stars or black holes orbit each other, they continuously emit gravitational waves, which 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 scream 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 binary black hole merger more than 1 billion light-years away.
Related: The Laser Interferometer Gravitational-Wave Observatory (LIGO): Detecting ripples in space-time
As detections of gravitational waves continue to pour in, scientists like Gupta are learning how to use them to reveal details about the objects they create, as this new research shows.
Gupta explained that using gravitational waves to understand the orbits of binary black holes is akin to paleontologists studying bones to reconstruct how dinosaurs might have lived. Physicists can thus study the properties of merging black holes to understand how binary black holes come together in the first place.
This can be done in two different ways. Dynamic interactions occur when a black hole binary star encounters and interacts with another black hole or even another black hole binary star system.
On the other hand, binary stars can be isolated and formed more easily from two stars that are already orbiting each other and then become black holes, or from one black hole that gets too close to another and forms a binary star before colliding and merging.
“The key idea is that if we observe a binary with eccentricity, it likely arises from a dynamic interaction,” said Gupta. ‘These chaotic interactions can tear the binary apart and blast the black holes that compose them from their host galaxies and galaxy clusters. But sometimes they can also reduce the distance between the two black holes, causing eccentricity and causing them to merge on short time scales. “
In addition to using orbital eccentricity to tell the story of black hole binaries, the scientist and his team are also interested in what the oval nature of orbits does to the gravitational wave emissions from these systems.
“If you have eccentricity, it means that the black holes are closer together at some points in the orbit,” explains Gupta. ‘When black holes are closer together, they have a greater acceleration, which means they emit more gravitational waves. On the other hand, when they are far away, they have a smaller acceleration, which means they emit fewer gravitational waves.
“So you end up seeing little irregularities in the amplitude of the waveform [the total pattern of gravitational waves]which arise because the black holes are moving further and further away from each other!”
The nature and history of binary black holes would be incredibly difficult to determine without the use of gravitational waves. An alternative method of understanding the origins of binary black holes is to look for so-called ‘common envelope’ events using 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 create a common shell around both inhabitants of the binary star, creating friction between the black hole and the star. This shrinks the orbit of the binary, and eventually, after the red giant becomes a black hole, leads to a merger of the binary black hole.
“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 compact object mergers cover a small fraction of these systems,” said Gupta. “By studying gravitational waves, on the other hand, we can understand the final moments of the binary merger. This could allow us to trace the history of the merger and hypothesize what might have formed it.”
He added that gravitational waves are especially useful in this regard because they are an “extremely clean probe” for distant events. This refers to the fact that these ripples can travel great distances through space-time without interference from whatever is between the binary star pair and Earth.
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‘While we do not claim that these are definitive detections of eccentric binary black holes, these results point towards eccentricity. [in the] existing population,” Gupte said. “This is an important consideration for the current observing run of gravitational wave detectors on Earth, as well as for future gravitational wave detectors on the ground and in space.
“Right now, we don’t have enough data to definitively determine the origins of binary black holes. However, if we observe more eccentric binary black holes in the future, we may begin to place constraints on the mechanisms that form these systems.”
The team’s article has not yet been published in a peer-reviewed journal. You can read a preprint of it in the online repository arXiv.