When multiple black holes appear in the same vicinity as one another, they will interact with their environment via dynamical friction. As the matter gets either swallowed or expelled, the black holes become more tightly gravitationally bound. If the black holes are of unequal masses, the smaller one will lose more orbital energy than the larger one, and when they approach each other closely enough, their orbits will decay via the emission of gravitational waves before ultimately leading to a merger. (Credit: Mark Garlick/SPL)
Two supermassive black holes on an inevitable death spiral push the limits of Einstein’s relativity. New observations reveal even more.
Out there in the Universe, black holes abound.
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When two black holes merge, a significant portion of their mass can get converted into energy, in the form of gravitational waves, in one very short time interval. Over a much longer period of time, there’s an earlier stage where these black holes orbit with periods of 1–10 years, and pulsar timing can be sensitive to the cumulative effects of those systems throughout the cosmos. Different types of detectors are needed to observe gravitational waves produced by sources of various masses. (Credit: NASA’s Goddard Space Flight Center)
Most are relatively low-mass: just a few solar masses.
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When the two arms of an optical interferometer are of exactly equal length and there is no gravitational wave passing through, the signal is null and the interference pattern is constant. As the arm lengths change, the signal is real and oscillatory, and the interference pattern changes with time in a predictable fashion. This technique is what is used to directly reveal the presence of gravitational waves. (Credit: NASA’s The Space Place)
However, supermassive varieties, at millions or billions of solar masses, exist within galaxies.
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The three different sets of approaches to gravitational waves, ground-based laser interferometers, space-based laser interferometers, and pulsar timing arrays, are all sensitive to different classes of gravitational wave signals. While LIGO was the first collaboration to detect gravitational waves at very high frequencies, the NANOGrav collaboration sees strong evidence at very low (nanohertz) frequencies. Pulsar timing is one of the best methods for probing the longest-wavelength gravitational waves of all, produced primarily by the heaviest orbiting supermassive black hole pairs. (Credit: NANOGrav Collaboration)
The image above shows an illustration of the three future LISA, or Laser Interferometer Space Antennae, spacecraft, in a trailing orbit behind the Earth. LISA will be our first space-based gravitational wave detector, sensitive to objects thousands of times as massive than the ones LIGO can detect. An array of LISA detectors set up at various points in Earth orbit would be able to detect even merging supermassive black holes: a proposal known as Big Bang Observer. (Credit: University of Florida/NASA)
