Gravitation and Quantum Gravity
Gravity as curved spacetime, black holes and gravitational waves, and the search for a quantum theory of gravity.
arXiv: gr-qc
12 topics
-
Black holes should glow faintly (Hawking radiation), but the calculation traces each emitted particle back to wavelengths far shorter than any known physics can describe. Flowing ultracold gases (Bose-Einstein condensates) and other fluids with sonic horizons, places where the flow outruns sound, let experiments test whether this glow survives changes at short distances.
-
Black hole information and the Page curve
4 problemsA black hole slowly evaporates by emitting faint heat radiation, called Hawking radiation. Quantum mechanics says the information about whatever fell in must come back out in that radiation, but Hawking's original calculation says it does not.
-
Einstein's gravity says nothing special happens when you fall through the horizon of a large black hole. Quantum information arguments suggest that the horizon of an old black hole might instead be a wall of high-energy particles, a firewall.
-
A black hole has an entropy equal to a quarter of its horizon area in Planck units, which means it has an enormous number of hidden internal states. String theory counts these states exactly for special charged, zero-temperature black holes, but not for ordinary ones.
-
Black hole evaporation endpoint
4 problemsAs a black hole radiates it loses mass and gets hotter, so evaporation speeds up toward a final burst. What is left at the end, and how the radiation reshapes spacetime along the way, requires quantum gravity or careful approximations.
-
Stability and uniqueness of Kerr black holes
7 problemsThe Kerr solution describes a spinning black hole in Einstein's theory. To be the endpoint of real collapses it must be the only settled black hole and must be stable: when slightly disturbed, it should settle back to another Kerr black hole.
-
Many extensions of Einstein's theory add new fields or extra curvature terms. They must pass all precise tests so far, which already exclude many, yet could still differ near black holes and in mergers.
-
The problem of time in quantum gravity
5 problemsIn quantum mechanics time is an external clock, but in general relativity time is part of the dynamical geometry, and the basic quantum equation for the universe, the Wheeler-DeWitt equation, contains no time at all. How ordinary time-dependent physics emerges from a timeless description is not settled.
-
Quantum fields such as light can carry pockets of negative energy and act back on the curvature of spacetime. Whether this back-reaction forbids time machines, preserves the singularity theorems, or destabilizes an expanding universe is not settled.
-
Einstein's equations predict places inside black holes and at the Big Bang where curvature becomes infinite and the theory stops making sense. Cosmic censorship is the conjecture that such breakdowns are hidden behind horizons and do not spoil predictability.
-
Gravitational waves from merging black holes and radio images of black holes test Einstein's theory where gravity is strongest. A key target is the ringing of a newly formed black hole, whose tones should depend only on its mass and spin.
-
General relativity treated as a quantum field theory works at low energies but fails near the Planck energy, about $10^{19}\,\mathrm{GeV}$. Several candidate completions exist; the problem is to find which is right and how it could show up in experiments.