![]() ![]() Under certain conditions, particles in the ergoregion can escape and carry with them energy-and spin-from the black hole. Around rapidly rotating black holes, general relativity predicts the existence of a region outside the event horizon, named the ergoregion, in which spacetime is “dragged” around the black hole. Here’s where black holes enter the story. However, these bounds are not yet completely robust because of uncertainties, and they cannot easily be extended to larger values of the dark matter mass. These studies, which analyzed small galaxies at very high redshifts and density fluctuations of the intergalactic medium, provided lower bounds of around 1 0 − 2 1 eV ∕ c 2 for the mass of dark matter. So far, researchers have studied the implications of the FDM models on the structure of the Universe at the million-light-year scale. ![]() The potential to resolve such issues is one of the main motivations for investigating FDM. On the other hand, if the dark matter particles have a mass around or slightly above 1 0 − 2 2 eV ∕ c 2, the large spatial extension of their quantum wave function could actually solve some problems at subgalactic scales with the standard cosmological model, which assumes a heavy dark matter particle. This spread in position would lead to a smooth density distribution, which is inconsistent with the large-scale galactic structure that is observed today. The reason for this lower bound is that dark matter particles lighter than this value would have had a very large quantum uncertainty in their position during the early Universe. The particles associated with these fields have a wide range of masses, but in order for them to constitute a large fraction of dark matter, their mass must be above roughly 1 0 − 2 2 eV ∕ c 2 (see 9 August 2000 Focus story). In particular, the existence of light bosonic fields (specifically, pseudoscalars) is a generic prediction of string theory. Such ultralight particles appear in several scenarios beyond the standard model of particle physics. The FDM scenario assumes that dark matter particles are many orders of magnitude lighter than other particles. It’s at the light end of this spectrum that the recent EHT image (Fig. As a consequence, researchers are turning their attention to a large variety of dark matter models, with masses in the range from around 1 0 − 2 2 eV ∕ c 2 to several times a solar mass ( 1 0 6 6 eV ∕ c 2). Several ongoing experiments are searching for WIMPs, but nothing has turned up yet. A popular model is the WIMP or weakly interacting massive particle, which has an expected mass on the order of 1 0 0 GeV ∕ c 2 ( 1 0 1 1 eV ∕ c 2 ). However, its fundamental nature is still completely unknown. As current observations suggest that the black hole is rapidly spinning, Davoudiasl and Denton were able to place new bounds on acceptable masses for dark matter.ĭark matter, a key ingredient of modern cosmology, is a physical object whose gravitational effects are observed in a wide range of systems, from small, nearby galaxies to the primordial plasma as probed by the cosmic microwave background. The researchers studied the implications of the EHT black hole’s estimated rotation rate on the so-called superradiance effect-which is an enhanced particle emission process that can slow the rotation rate of a black hole. Hooman Davoudiasl and Peter Denton from Brookhaven National Laboratory in New York have now interpreted the EHT observations in the context of an appealing ultralight dark matter scenario, referred to as fuzzy dark matter (FDM). Black-hole-related observations can provide tests of new, exotic physical processes and, in particular, they can point toward or exclude some dark matter models. Recently, astronomers obtained the first image of the accretion disk around the horizon of a supermassive black hole using a world-wide network of radio observatories called the Event Horizon Telescope (EHT). EHT Collaboration Figure 1: Image of the supermassive black hole M87*, taken by the Event Horizon Telescope.
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