466 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

### Thermality of the Hawking flux

Is the Hawking flux "thermal"? Unfortunately, the answer to this seemingly
innocent question depends on a number of often unstated, but quite crucial,
technical assumptions built into modern (mis-)interpretations of the word
"thermal". The original 1850's notions of thermality --- based on classical
thermodynamic reasoning applied to idealized "black bodies" or "lamp black
surfaces" --- when supplemented by specific basic quantum ideas from the early
1900's, immediately led to the notion of the black-body spectrum, (the
Planck-shaped spectrum), but "without" any specific assumptions or conclusions
regarding correlations between the quanta. Many (not all) modern authors (often
implicitly and unintentionally) add an extra, and quite unnecessary, assumption
that there are no correlations in the black-body radiation; but such usage is
profoundly ahistorical and dangerously misleading. Specifically, the Hawking
flux from an evaporating black hole, (just like the radiation flux from a leaky
furnace or a burning lump of coal), is only "approximately" Planck-shaped over
a bounded frequency range. Standard physics (phase space and adiabaticity
effects) explicitly bound the frequency range over which the Hawking flux is
"approximately" Planck-shaped from both above and below --- the Hawking flux is
certainly not exactly Planckian, and there is no compelling physics reason to
assume the Hawking photons are uncorrelated.Comment: V1: 13 pages. V2: Now 17 pages. 3 references added; other references
updated; new section on the relationship between past and future null
infinity; small edits throughout the text. V3: Now 19 pages. 4 more
references added; extra discussion/small edits. No physics change

- â€¦