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There are several effects. Accelerating black holes, or actually any accelerated matter, emits gravitational waves. This are disturbances of space-time itself

by danbruc 11y ago

There are several effects. Accelerating black holes, or actually any accelerated matter, emits gravitational waves. This are disturbances of space-time itself and unlike electromagnetic waves mediated by photons moving through space-time. At least in a classical picture of gravity, not sure how that would work out within quantum gravity when gravitational waves are mediated by gravitons. But I guess you can just think of the gravitational waves being emitted at the event horizon. Then there is electromagnetic radiation emitted from matter falling towards the black hole but still outside of the event horizon. The gravitational forces of the black hole heat up this matter to the point that it emits electromagnetic radiation in a wide spectrum. Finally there is Hawking radiation which is a suspected electromagnetic radiation due to quantum effects near the event horizon. The associated black-body temperature is tiny, two billionth of a Kelvin for a 30 solar mass black hole and decreasing with increasing mass. The temperature of the cosmic microwave background with 2.7 Kelvin is in comparison gigantic and therefore black holes absorb much more cosmic microwave background radiation than they emit Hawking radiation. The universe has to become a lot cooler before Hawking radiation can start to evaporate black holes.

raattgift 11y ago
Gravitational waves (GWs) propagate a _change_ in the gravitational field. Consequently, GWs are only shed by accelerating matter where the change in motion is not rotationally or spherically symmetric. Spherically symmetric gravitational collapses (of dust or gas, for example) are certainly systems of accelerated matter, but do not shed gravitational waves. Rotating spherical bodies also do not shed GWs. To first order, this means the rotation of stars and planets do not shed GWs, although a planet revolving around the star in an essentially Kepler orbit will shed GWs, as will a pair of stars in close orbit around each other. GWs solve the apparent removal of momentum-energy from a system where there is no removal of mass-energy by non-gravitational radiation. For most systems the energy loss is negligible. That's why the first GW detection involved a late stage inspiral of two compact massive objects with no way to ditch their angular momentum electromagnetically or via neutrino (or other dark matter) emission. GWs themselves have spin 2 symmetry; in theories with gravitons, those are spin-2 gauge bosons, typically massless (because they are long range), and you get large numbers of them forming the classical GWs, just as you will find large numbers of photons forming a bright flicker of light, although you can detect tiny numbers of photons because electromagnetism is relatively strong -- tiny numbers of gravitons are not going to be detectable by humanity any time soon. It's not curvature (and certainly not gravitational forces) heating up accretion disks. The matter in the disks move on intersecting geodesics and occasionally collide. Such collisions tend to ditch the particles' momentum into emitted photons. Hawking radiation is an effect of accelerated observers seeing more (and more energetic) particles than non-accelerated ones, and gravitational collapses producing an acceleration between past and future observers. Where a past observer sees a tiny number of low-frequency particles in the "vacuum" outside the collapsing object, a future observer (after the horizon has formed) will see a larger number of higher frequency ones. The acceleration is curvature dependent, and curvature just outside the horizon depends on black hole mass -- more massive black holes have less curvature just outside the horizon (which is further away from the gravitational singularity or whatever is at the centre of a black hole). These particles near but outside the horizon can escape, which takes mass-energy away from the dynamical spacetime just outside the horizon. That in turn causes the horizon to retract and that exposes stronger curvature. That in turn produces a stronger acceleration for further future observers, and the process continues -- that's black hole evaporation. Hawking radiation is always emitted, for SMBHs that only grow by incidental interception of CMB radiation, and even for stellar mass black holes. Infalling matter simply replaces (and then some) the outgoing "fossil" infrared radiation from the space just outside the black hole as the horizon was forming.

by danbruc 11y ago

raattgift 11y ago

Gravitational waves (GWs) propagate a _change_ in the gravitational field. Consequently, GWs are only shed by accelerating matter where the change in motion is not rotationally or spherically symmetric. Spherically symmetric gravitational collapses (of dust or gas, for example) are certainly systems of accelerated matter, but do not shed gravitational waves. Rotating spherical bodies also do not shed GWs. To first order, this means the rotation of stars and planets do not shed GWs, although a planet revolving around the star in an essentially Kepler orbit will shed GWs, as will a pair of stars in close orbit around each other. GWs solve the apparent removal of momentum-energy from a system where there is no removal of mass-energy by non-gravitational radiation. For most systems the energy loss is negligible. That's why the first GW detection involved a late stage inspiral of two compact massive objects with no way to ditch their angular momentum electromagnetically or via neutrino (or other dark matter) emission. GWs themselves have spin 2 symmetry; in theories with gravitons, those are spin-2 gauge bosons, typically massless (because they are long range), and you get large numbers of them forming the classical GWs, just as you will find large numbers of photons forming a bright flicker of light, although you can detect tiny numbers of photons because electromagnetism is relatively strong -- tiny numbers of gravitons are not going to be detectable by humanity any time soon. It's not curvature (and certainly not gravitational forces) heating up accretion disks. The matter in the disks move on intersecting geodesics and occasionally collide. Such collisions tend to ditch the particles' momentum into emitted photons. Hawking radiation is an effect of accelerated observers seeing more (and more energetic) particles than non-accelerated ones, and gravitational collapses producing an acceleration between past and future observers. Where a past observer sees a tiny number of low-frequency particles in the "vacuum" outside the collapsing object, a future observer (after the horizon has formed) will see a larger number of higher frequency ones. The acceleration is curvature dependent, and curvature just outside the horizon depends on black hole mass -- more massive black holes have less curvature just outside the horizon (which is further away from the gravitational singularity or whatever is at the centre of a black hole). These particles near but outside the horizon can escape, which takes mass-energy away from the dynamical spacetime just outside the horizon. That in turn causes the horizon to retract and that exposes stronger curvature. That in turn produces a stronger acceleration for further future observers, and the process continues -- that's black hole evaporation. Hawking radiation is always emitted, for SMBHs that only grow by incidental interception of CMB radiation, and even for stellar mass black holes. Infalling matter simply replaces (and then some) the outgoing "fossil" infrared radiation from the space just outside the black hole as the horizon was forming.