The centre of giant black holes could shed light on the origins of the universe
Black holes, which suck in stars and all other matter, and which previously could not be seen because they emit no light have in recent years been observed with gravitational telescopes. EU-funded scientists under the BHLOC project have been extending the theoretical basis for understanding what is at their centre. “Scientists have now identified that at the centre of the galaxy, beyond reasonable doubt, there is a massive black hole. But we know very little. What we know is that a black hole is spinning at some rate, it has a mass, energy and an electric charge. And that’s it,” says project coordinator Nick Halmagyi, research professor at Paris-Sorbonne University’s Laboratory of Theoretical and High Energy Physics (LPTHE). The BHLOC project worked on computing the corrections to gravity that could be important in understanding the story of the early big bang which created the universe. “If you change the amount of energy, which means going earlier and earlier into the Big Bang, there are some moments where the entire thing can change, and when that happens we expect that just studying gravity is not enough. There will be some additional corrections,” explains Professor Halmagyi. With so little known, “whatever we manage to compute explicitly improves understanding of black holes, and that is progress,” says Shailesh Lal, previously a researcher at South Korea’s Seoul National University, and in India, before taking up a Marie Curie grant at LPTHE. Quantum properties Armed with an understanding of the statistical properties and thus the entropy (a measure of disorder at a very fine level) of the extended gravitational theories, “the idea was to understand the quantum properties of black holes,” says Dr Lal. Late Cambridge University astrophysicists Stephen Hawking and Jacob Beckenstein previously devised a theory on the entropy of black holes. “They uncovered that a black hole behaves exactly as if it is made up of lots of little bits,” says Prof. Halmagyi, who calls the particles ‘gravitational molecules.’ Hence, what is inside a black hole can be studied using quantum theory, the theory of tiny particles. “We have this hint that inside the black hole is a bunch of ‘molecules’, but they are not molecules like we’ve ever imagined, they are some weird effective gravitation that we don’t really understand,” Prof. Halmagyi elaborates. “So we are trying to understand [theoretical] details of these ‘molecules’ and generalise them towards a type of black hole that we can now actually observe in the universe.” New aspects of combined theories The challenge for theoretical physicists is to find what Professor Halmagyi calls the ‘sweet spot’ between simplified theoretical models which provide some results and generalised theories which, although huge and complex, are closer to explaining the real world. String theory, a popular theory of quantum gravity, has been used to predict what is inside in a black hole. The BHLOC project “was about a particular way string theory works to teach us about the ‘molecules’ that make up black holes’ using quantum field theory- a quantum theory that does not contain gravity,” Dr Lal explains. Theoretical physicists studied the smallest rotating black holes, known as extremal black holes, using a simplified predictive mathematical model known as a ‘toy model.’ “Then, if we simplify [the model] a tiny bit more, we get some precise descriptions of ‘molecules’. This was the giant victory of string theory,” Professor Halmagyi explains. “Quantum field theory and gravity are the building blocks of physics. And this theory joins them together in a way that was not done before,” he concludes.
Keywords
BHLOC, cosmology, astrophysics, string theory, quantum physics, quantum theory, gravity, black holes