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

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

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 $c$ models have very limited ability to resolve such problems. For a constantly decreasing $c$ 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

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

Quantum creation and inflationary universes: a critical appraisal

We contrast the possibility of inflation starting a) from the universe's inception or b) from an earlier non-inflationary state. Neither case is ideal since a) assumes quantum mechanical reasoning is straightforwardly applicable to the early universe; while case b) requires that a singularity still be present. Further, in agreement with Vachaspati and Trodden [1] case b) can only solve the horizon problem if the non-inflationary phase has equation of state $\gamma<4/3$.Comment: 21 pages Late