What would it look like if dark matter fell into a black hole? What might you see while it was happening?
Mike Follows, Sutton Coldfield, West Midlands, UK
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There would be no discernible difference to the appearance of a black hole. Besides, a small amount of dark matter probably already falls into black holes.
Dark matter doesn鈥檛 interact electromagnetically 鈥� so cannot emit, absorb or scatter light 鈥� but it does respond to the force of gravity, so would fall into a black hole in the same way that ordinary matter does. However, while ordinary matter collapses to form a galactic disc, emitting electromagnetic radiation in the process, dark matter stays in the form of a spherical halo around the visible component. This means that it is mainly ordinary matter that falls into a black hole.
鈥淧eople would be exposed to the 鈥榯reatment鈥� simply by standing next to the wrong person at the supermarket鈥�
Dark matter probably paved the way for black holes to exist by creating the texture of the universe, as well as the location of galaxies. Until the universe was about 300,000 years old, it was too hot for neutral atoms to form. The early universe was composed of plasma (protons and electrons). These charged particles were scattered by photons and this prevented ordinary matter clumping together gravitationally.
Meanwhile, any fluctuations in the density of dark matter were accentuated by gravity, whose fingerprint can be seen today as ripples in the cosmic microwave background radiation. As soon as neutral atoms formed, they could gravitate towards areas where clumps of dark matter built up, leading to the creation of galaxies and the spawning of stars, including black holes.
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