92 research outputs found
Quantum Cosmological Models
We contrast the initial condition requirements of various contemporary
cosmological models, including inflationary and bouncing cosmologies. Various
proposals such as Hartle-Hawking's no boundary, or Tunnelling boundary
conditions are assessed on grounds of naturalness and fine tuning.
Alternatively a quiescent or ``time machine'' state is considered. Extensions
to brane models are also addressed. Further ideas about universe creation from
a meta-universe are outlined.
We compare the recent loop quantum cosmology of Bojowald and coworkers with
these earlier proposals. A number of possible difficulties and limitations are
outlined.Comment: revised: now includes time asymmetry, black hole final state etc, 71
page
Varying c cosmology and Planck value constraints
It has been suggested that by increasing the speed of light during the early
universe various cosmological problems of standard big bang cosmology can be
overcome, without requiring an inflationary phase. However, we find that as the
Planck length and Planck time are then made correspondingly smaller, and
together with the need that the universe should not re-enter a Planck epoch,
the higher models have very limited ability to resolve such problems. For a
constantly decreasing the universe will quickly becomes quantum
gravitationally dominated as time increases: the opposite to standard cosmology
where quantum behaviour is only ascribed to early times.Comment: extended versio
A `warp drive' with more reasonable total energy requirements
I show how a minor modification of the Alcubierre geometry can dramatically
improve the total energy requirements for a `warp bubble' that can be used to
transport macroscopic objects. A spacetime is presented for which the total
negative mass needed is of the order of a few solar masses, accompanied by a
comparable amount of positive energy. This puts the warp drive in the mass
scale of large traversable wormholes. The new geometry satisfies the quantum
inequality concerning WEC violations and has the same advantages as the
original Alcubierre spacetime.Comment: 9 pages, 1 figure; error in calculation correcte
Factor ordering in standard quantum cosmology
The Wheeler-DeWitt equation of Friedmann models with a massless quantum field
is formulated with arbitrary factor ordering of the Hamiltonian constraint
operator. A scalar product of wave functions is constructed, giving rise to a
probability interpretation and making comparison with the classical solution
possible. In general the bahaviour of the wave function of the model depends on
a critical energy of the matter field, which, in turn, depends on the chosen
factor ordering. By certain choices of the ordering the critical energy can be
pushed down to zero.Comment: 15 pages, 3 figure
Pre-big bang model has Planck problem
The pre-big bang's kinetic driven inflationary mechanism is not an adequate
form of inflation: the Planck length grows more rapidly than the scale factor.
In order to explain our large universe, the resulting post-big bang universe
requires the same unnatural constants (Planck problem) as those of any other
non-inflationary big bang model.Comment: figures not include
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