The most elusive black holes in the universe aren’t the big ones or the small ones. These are the ways – and a team of astronomers has come up with a new method, using ripples in space-time, hunt for them.
Known black holes come in two general varieties. There are stellar-mass black holes, which range from a few to a few tens of times the mass of the sun. And then there are the supermassive black holes, ranging in mass from a million suns up to 50 billion solar masses.
Astronomers theorize that there could be black holes caught between these two extremes, known as intermediate-mass black holes (IMBH), with masses a few thousand times that of the sun. But there’s a catch: Although we’ve searched for them for decades, we haven’t found any yet. And so a team of astronomers, writing on the preprint server arXiv.org (opens in a new tab)proposed a new method for hunting IMBHs, using the subtle ripples in spacetime caused by the movements of black holes in the center of the Milky Way.
Astronomers are desperate to find an IMBH because it will help them piece together the history of black hole formation. As far as we currently understand, giant black holes weren’t born that way. Instead, they started out as simple stellar-mass black holes. Then, for hundreds of millions of years, they swallowed up any bits of material that wandered too close, and they also found each other and merged, rapidly swelling to their current impressive size. This rapid growth suggests that there should be little IMBH left, as this black hole size represents only a temporary phase on a black hole’s path to supermassiveness.
However, other models of black hole formation suggest that IMBHs may in fact be common and simply difficult to detect. The cramped conditions of the galactic core, for example, could be ripe to potentially produce thousands of large but not supermassive black holes. But these IMBHs could be hidden, because the supermassive black hole in the the very center of the galaxy dominate our observations.
To uncover these possibilities, the authors of the new study hope to use the Laser Interferometer Space Antenna (LISA), a planned gravitational wave detector that the European Space Agency hopes to launch in 2037. The observatory will detect the waves gravitational waves, which are subtle ripples in space-time first predicted by Einstein, as they pass through the solar system. Similar ground-based observatories like the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo have already had tremendous success in detecting stellar-mass black hole mergers, but these instruments are not sensitive to the frequencies needed to detect signals. from something bigger.
LISA will consist of a trio of satellites orbiting the sun that will constantly monitor the distances between them. When a gravitational wave passes, satellites pick up the telltale signature, like buoys in the ocean recognizing a passing tidal wave.
To search for IMBHs, astronomers must hope for a stroke of luck. If an IMBH at the galactic center manages to capture a wandering dense remnant (like a small black hole, a neutron star, or a white dwarf), the process will emit gravitational waves that LISA can potentially detect. Because the IMBH itself will be orbiting the central supermassive black hole, these gravitational waves will experience a Doppler shift (like the frequency change of a passing ambulance) due to the motion of the IMBH.
If enough IMBHs lurk in our galactic core, these silent acts of destruction could leave tricky signatures in gravitational waves. The proposed method is however far from being complete: LISA will only be able to detect IMBHs if they have a mass range between 1,000 and 100,000 solar masses. It’s a long shot to be sure, but it just might work.
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