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When gravitational waves are released from the merger of two black holes, the combined mass of the resultant black hole is less than sum of its component black

by mudil 10y ago

When gravitational waves are released from the merger of two black holes, the combined mass of the resultant black hole is less than sum of its component black holes, because some of the mass was released in the form gravitational waves. Questions. What are the implications of this on black hole entropy and temperature? Can black holes evaporate from gravitational radiation alone without Hawkins radiation? What are the implications of this on the mass of any object in the universe, since all objects are related to each other gravitationally and the universe is expanding? Does it mean objects are constantly loosing mass and the universe is filled with energy from this release? Can dark energy be related to this process? Can universe be expanding because matter is constantly lost into the gravitational waves? What are the implications of gravitational waves on the fabric of spacetime, if objects are constantly leaking gravitational waves in a nonstatic universe?

akuchling 10y ago
My understanding is that gravitational waves are only radiated when there's a dipole moment: two (or more, I guess) objects in rotation around each other. A black hole just sitting on its own doesn't radiate any gravitational waves at all. According to http://www.ast.cam.ac.uk/public/ask/2519 http://www.ast.cam.ac.uk/public/ask/2519 , the Earth and Moon should also be radiating gravitational waves (and therefore losing a little bit of energy) but this is surely dwarfed by other effects and not measurable.
- mudil 10y ago
  But in a nonstatic, expanding universe any two objects, no matter how far apart, are gravitationally related and have dipole moment. Isn't it true?
- ISL 10y ago
  Important pedantic point: There are no gravitational dipoles, as there is no repulsive gravitational interaction. A dumbbell-shaped mass is the sum of a gravitational monopole and a gravitational quadrupole. That gravitational radiation originates only from quadrupolar sources (which are far less-efficient radiators than an equal-sized dipole) is one reason that gravitational waves are hard to detect.
cyphar 10y ago
> What are the implications of this on black hole entropy and temperature? Can black holes evaporate from gravitational radiation alone without Hawkins radiation? No, gravitational waves are only generated when you have large masses moving. When two black holes merge, they become a single spinning mass which then becomes uniform very quickly. Once the mass is uniform, it doesn't generate gravitational waves from rotation. I have yet to study cosmology (I did some research in astroseismology), so I can't answer general cosmology questions.
raattgift 10y ago
Cosmologically we turn to the conservation of energy-mass-momentum wherein the matter fields at one point in spacetime can donate energy-mass-momentum into the gravitational field, and can pick some of that up again at a distance from the gravitational field, typically changing local matter field content's (linear) momentum. At cosmological scales the "objects" are the various matter fields (in the most general sense, everything that is neither the cosmological constant nor the gravitational field itself) and we concern ourselves with energy-density, which drops with the metric expansion of space. Sean Carroll goes into some detail here: http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ http://www.preposterousuniverse.com/blog/2010/02/22/energy-i... Back to GWs: these change the peculiar motions of matter field content, at cosmological scales giving tiny nudges to galaxy clusters compared to what is imparted by Dark Energy. So even though we lose the conservation of energy, we get back the conservation of energy-momentum.

by mudil 10y ago

akuchling 10y ago

My understanding is that gravitational waves are only radiated when there's a dipole moment: two (or more, I guess) objects in rotation around each other. A black hole just sitting on its own doesn't radiate any gravitational waves at all. According to http://www.ast.cam.ac.uk/public/ask/2519 http://www.ast.cam.ac.uk/public/ask/2519 , the Earth and Moon should also be radiating gravitational waves (and therefore losing a little bit of energy) but this is surely dwarfed by other effects and not measurable.

mudil 10y ago
But in a nonstatic, expanding universe any two objects, no matter how far apart, are gravitationally related and have dipole moment. Isn't it true?
ISL 10y ago
Important pedantic point: There are no gravitational dipoles, as there is no repulsive gravitational interaction. A dumbbell-shaped mass is the sum of a gravitational monopole and a gravitational quadrupole. That gravitational radiation originates only from quadrupolar sources (which are far less-efficient radiators than an equal-sized dipole) is one reason that gravitational waves are hard to detect.

cyphar 10y ago

> What are the implications of this on black hole entropy and temperature? Can black holes evaporate from gravitational radiation alone without Hawkins radiation? No, gravitational waves are only generated when you have large masses moving. When two black holes merge, they become a single spinning mass which then becomes uniform very quickly. Once the mass is uniform, it doesn't generate gravitational waves from rotation. I have yet to study cosmology (I did some research in astroseismology), so I can't answer general cosmology questions.

raattgift 10y ago

Cosmologically we turn to the conservation of energy-mass-momentum wherein the matter fields at one point in spacetime can donate energy-mass-momentum into the gravitational field, and can pick some of that up again at a distance from the gravitational field, typically changing local matter field content's (linear) momentum. At cosmological scales the "objects" are the various matter fields (in the most general sense, everything that is neither the cosmological constant nor the gravitational field itself) and we concern ourselves with energy-density, which drops with the metric expansion of space. Sean Carroll goes into some detail here: http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ http://www.preposterousuniverse.com/blog/2010/02/22/energy-i... Back to GWs: these change the peculiar motions of matter field content, at cosmological scales giving tiny nudges to galaxy clusters compared to what is imparted by Dark Energy. So even though we lose the conservation of energy, we get back the conservation of energy-momentum.