Tuesday February 8th 2022
Jeremy Auffinger, Univ. Lyon
Polymerized black hole constraints from Hawking radiation
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Black holes come in different kinds. The ones people are most familiar with are those that result when a star, with a mass a few times that of the Sun, collapses. Their mass is therefore a multiple of the mass of the Sun and are called "stellar sized" black holes. There are much larger black holes, with a mass a million to a billion times the mass of the Sun. These black holes are at the center of galaxies and presumably they formed by absorbing surrounding stars and other black holes. They are called supermassive black holes. A third class of black holes, which has not been observed directly yet, would consist of much smaller black holes. They are produced in the early universe from fluctuations in the density of matter.
The way stars rotate around when they are in galaxies suggests that the latter contain more matter than what is visible. This is the origin of the conjectured "dark matter" present in them. There are various proposals for what could constitute dark matter, ranging from elementary particles of various kinds to primordial black holes.
In the 1970's Hawking showed theoretically that black holes emit radiation much like a heated piece of metal does, with a characteristic temperature. The latter is inversely proportional to the black hole mass. For stellar-sized black holes, the temperature is very small, a millionth of a degree. As a result, the Hawking radiation from such black holes is unobservable in practice. Primordial black holes, as they are much smaller, would have a much higher temperature and therefore more Hawking radiation. This radiation has not been observed (yet) so it can be used to constrain the presence of these types of black holes. If one takes them as a candidate for dark matter, it helps put constraints on the resulting dark matter models.
Loop quantum gravity has been applied over the last decade to spherically symmetric situations, including black holes. Various different models have emerged, differing in the assumptions and provide a picture that differs in some details. But the main feature is they eliminate the infinities that appear in classical general relativity inside black holes and are known as the singularity. This was a long expected result of any successful approach to quantum gravity, as infinities do not exist in reality.
This talk analyzed the modifications to the calculations of Hawking radiation from primordial black holes, and consequently in dark matter models that involve them, if one considers the black holes that stem from loop quantum gravity. If the radiation from these black holes is ever observed it could therefore provide valuable experimental information about loop quantum gravity.
