340 research outputs found

### Properties of Black Hole Radiation From Tunnelling

We consider the space-time associated with the evaporation of a black hole by
quantum mechanical tunnelling events. It is shown that the surface through
which tunnelling occurs is distinct from the global event horizon, and that
this has consequences for the radiation reaching future null infinity. A
spherical collapse process is modelled, and the radiation expected to be
observed at future null infinity is calculated. It is shown that external
observers witness an evaporation process that begins as the tunnelling surface
is exposed, and ends as the collapsing object passes behind its event horizon.
The sensitivity of emitted radiation to the collapse process is illustrated.Comment: 14 pages, 6 figure

### FRW cosmology in Milgrom's bimetric theory of gravity

We consider spatially homogeneous and isotropic Friedmann-Robertson-Walker
(FRW) solutions of Milgrom's recently proposed class of bimetric theories of
gravity. These theories have two different regimes, corresponding to high and
low acceleration. We find simple power-law matter dominated solutions in both,
as well as solutions with spatial curvature, and exponentially expanding
solutions. In the high acceleration limit these solutions behave like the FRW
solutions of General Relativity, with a cosmological constant term that is of
the correct order of magnitude to explain the observed accelerating expansion
of the Universe. We find that solutions that remain in the high acceleration
regime for their entire history, however, require non-baryonic dark matter
fields, or extra interaction terms in their gravitational Lagrangian, in order
to be observationally viable. The low acceleration regime also provides some
scope to account for this deficit, with solutions that differ considerably from
their general relativistic counterparts.Comment: 12 page

### Archipelagian Cosmology: Dynamics and Observables in a Universe with Discretized Matter Content

We consider a model of the Universe in which the matter content is in the
form of discrete islands, rather than a continuous fluid. In the appropriate
limits the resulting large-scale dynamics approach those of a
Friedmann-Robertson-Walker (FRW) universe. The optical properties of such a
space-time, however, do not. This illustrates the fact that the optical and
`average' dynamical properties of a relativistic universe are not equivalent,
and do not specify each other uniquely. We find the angular diameter distance,
luminosity distance and redshifts that would be measured by observers in these
space-times, using both analytic approximations and numerical simulations.
While different from their counterparts in FRW, the effects found do not look
like promising candidates to explain the observations usually attributed to the
existence of Dark Energy. This incongruity with standard FRW cosmology is not
due to the existence of any unexpectedly large structures or voids in the
Universe, but only to the fact that the matter content of the Universe is not a
continuous fluid.Comment: 49 pages, 15 figures. Corrections made to description of lattice
constructio

### The Parameterised Post-Newtonian Limit of Bimetric Theories of Gravity

We consider the post-Newtonian limit of a general class of bimetric theories
of gravity, in which both metrics are dynamical. The established parameterised
post-Newtonian approach is followed as closely as possible, although new
potentials are found that do not exist within the standard frame-work. It is
found that these theories can evade solar system tests of post-Newtonian
gravity remarkably well. We show that perturbations about Minkowski space in
these theories contain both massless and massive degrees of freedom, and that
in general there are two different types of massive mode, each with a different
mass parameter. If both of these masses are sufficiently large then the
predictions of the most general class of theories we consider are
indistinguishable from those of general relativity, up to post-Newtonian order
in a weak field, low velocity expansion. In the limit that the massive modes
become massless, we find that these general theories do not exhibit a van
Dam-Veltman-Zakharov-like discontinuity in their gamma parameter, although
there are discontinuities in other post-Newtonian parameters as the massless
limit is approached. This smooth behaviour in gamma is due to the
discontinuities from each of the two different massive modes cancelling each
other out. Such cancellations cannot occur in special cases with only one
massive mode, such as the Isham-Salam-Strathdee theory.Comment: 36 page

### Higher Powers in Gravitation

We consider the Friedmann-Robertson-Walker cosmologies of theories of gravity
that generalise the Einstein-Hilbert action by replacing the Ricci scalar, R,
with some function, f(R). The general asymptotic behaviour of these cosmologies
is found, at both early and late times, and the effects of adding higher and
lower powers of R to the Einstein-Hilbert action is investigated. The
assumption that the highest powers of R should dominate the Universe's early
history, and that the lowest powers should dominate its future is found to be
inaccurate. The behaviour of the general solution is complicated, and while it
can be the case that single powers of R dominate the dynamics at late times, it
can be either the higher or lower powers that do so. It is also shown that it
is often the lowest powers of R that dominate at early times, when approach to
a bounce or a Tolman solution are generic possibilities. Various examples are
considered, and both vacuum and perfect fluid solutions investigated.Comment: 30 pages, 9 figure

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