465 research outputs found
Explicit form of the Mann-Marolf surface term in (3+1) dimensions
The Mann-Marolf surface term is a specific candidate for the "reference
background term" that is to be subtracted from the Gibbons-Hawking surface term
in order make the total gravitational action of asymptotically flat spacetimes
finite. That is, the total gravitational action is taken to be:
(Einstein-Hilbert bulk term) + (Gibbons-Hawking surface term) - (Mann-Marolf
surface term).
As presented by Mann and Marolf, their surface term is specified implicitly
in terms of the Ricci tensor of the boundary. Herein I demonstrate that for the
physically interesting case of a (3+1) dimensional bulk spacetime, the
Mann-Marolf surface term can be specified explicitly in terms of the Einstein
tensor of the (2+1) dimensional boundary.Comment: 4 pages; revtex4; V2: Now 5 pages. Improved discussion of the
degenerate case where some eigenvalues of the Einstein tensor are zero. No
change in physics conclusions. This version accepted for publication in
Physical Review
Rastall gravity is equivalent to Einstein gravity
Rastall gravity, originally developed in 1972, is currently undergoing a
significant surge in popularity. Rastall gravity purports to be a modified
theory of gravity, with a non-conserved stress-energy tensor, and an unusual
non-minimal coupling between matter and geometry, the Rastall stress-energy
satisfying nabla_b [T_R]^{ab} = {\lambda/4} g^{ab} nabla_b R. Unfortunately, a
deeper look shows that Rastall gravity is completely equivalent to Einstein
gravity --- usual general relativity. The gravity sector is completely
standard, based as usual on the Einstein tensor, while in the matter sector
Rastall's stress-energy tensor corresponds to an artificially isolated part of
the physical conserved stress-energy.Comment: V1: 5 pages. V2: 6 pages; 5 added references, some added discussion,
no changes in physics conclusions. V3: 7 pages, 2 added references, some
added discussion, no changes in physics conclusion
Hawking radiation: a particle physics perspective
It has recently become fashionable to regard black holes as elementary
particles. By taking this suggestion seriously it is possible to cobble
together an elementary particle physics based estimate for the decay rate
.
This estimate of the spontaneous emission rate contains two free parameters
which may be fixed by demanding that the high energy end of the spectrum of
emitted quanta match a blackbody spectrum at the Hawking temperature. The
calculation, though technically trivial, has important conceptual implications:
(1) The existence of Hawking radiation from black holes is ultimately dependent
only on the fact that massless quanta (and all other forms of matter) couple to
gravity. (2) The thermal nature of the Hawking spectrum depends only on the
fact that the number of internal states of a large mass black hole is enormous.
(3) Remarkably, the resulting formula for the decay rate gives meaningful
answers even when extrapolated to low mass black holes. The analysis strongly
supports the scenario of complete evaporation as the endpoint of the Hawking
radiation process (no naked singularity, no stable massive remnant).Comment: (15 pages) RevTe
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