Tiny, ancient black holes that may have formed in the early universe could help us uncover the internal structure of the densest stars. If they exist, these black holes could devour neutron stars from the inside, creating ripples in space-time called gravitational waves that we might be able to detect.
So-called primordial black holes are potentially important as an explanation for some of the effects of dark matter, but are predicted to be small, and so nearly impossible to directly detect. 鈥淎n Earth-mass primordial black hole is only expected to have a radius of about 0.9 centimetres,鈥� says Yong-Feng Huang at Nanjing University in China. 鈥淵ou can imagine how hard it would be to detect such a small object that is completely black.鈥�
However, like any other object, some primordial black holes can at some point be expected to fall into stars. Huang and Ze-Cheng Zou, also at Nanjing University, calculated the effect on space-time聽if a primordial black hole fell聽into a neutron star.
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They found that a black hole captured by a neutron star should quickly spiral towards the centre and swallow it from within. For a聽black hole with an initial mass similar to Earth鈥檚, this would take a聽little over 3 minutes. For a black hole about the mass of Jupiter, it聽would take only 0.2 seconds.
During this brief cosmic carnage, the motion of the black hole through the neutron star would send a unique pattern of gravitational waves propagating outwards across the universe. 鈥淭his sort of gravitational wave emission can potentially help solve three fundamental issues in astrophysics: the existence of primordial black holes, the constitution of dark matter and the internal composition of neutron stars,鈥� says Huang.
The researchers have calculated that the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US and its companion detector, Virgo in Italy, could potentially detect such gravitational waves from millions of light years away. Based on previous constraints on how common primordial black holes could be if they exist, it isn鈥檛 surprising that one of these events hasn鈥檛 been spotted yet.
鈥淚t would take 10,000 to 100,000 years for LIGO-Virgo to detect one such event. This is really very depressing,鈥� says Huang. 鈥淗owever, the next generation gravitational wave detectors will be much more sensitive 鈥� they might be able to detect one event after several years of running.鈥�
The Astrophysical Journal Letters
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