Self-similar cosmological solutions with dark energy. II: black holes, naked singularities and wormholes

We use a combination of numerical and analytical methods, exploiting the equations derived in a preceding paper, to classify all spherically symmetric self-similar solutions which are asymptotically Friedmann at large distances and contain a perfect fluid with equation of state $p=(\gamma -1)\mu$ with $0<\gamma<2/3$. The expansion of the Friedmann universe is accelerated in this case. We find a one-parameter family of self-similar solutions representing a black hole embedded in a Friedmann background. This suggests that, in contrast to the positive pressure case, black holes in a universe with dark energy can grow as fast as the Hubble horizon if they are not too large. There are also self-similar solutions which contain a central naked singularity with negative mass and solutions which represent a Friedmann universe connected to either another Friedmann universe or some other cosmological model. The latter are interpreted as self-similar cosmological white hole or wormhole solutions. The throats of these wormholes are defined as two-dimensional spheres with minimal area on a spacelike hypersurface and they are all non-traversable because of the absence of a past null infinity.Comment: 12 pages, 19 figures, 1 table, final version to appear in Physical Review

Growth of primordial black holes in a universe containing a massless scalar field

The evolution of primordial black holes in a flat Friedmann universe with a massless scalar field is investigated in fully general relativistic numerical relativity. A primordial black hole is expected to form with a scale comparable to the cosmological apparent horizon, in which case it may go through an initial phase with significant accretion. However, if it is very close to the cosmological apparent horizon size, the accretion is suppressed due to general relativistic effects. In any case, it soon gets smaller than the cosmological horizon and thereafter it can be approximated as an isolated vacuum solution with decaying mass accretion. In this situation the dynamical and inhomogeneous scalar field is typically equivalent to a perfect fluid with a stiff equation of state $p=\rho$. The black hole mass never increases by more than a factor of two, despite recent claims that primordial black holes might grow substantially through accreting quintessence. It is found that the gravitational memory scenario, proposed for primordial black holes in Brans-Dicke and scalar-tensor theories of gravity, is highly unphysical.Comment: 24 pages, accepted for publication in Physical Review

Evaluating candidate reactions to selection practices using organisational justice theory

Objectives: This study aimed to examine candidate reactions to selection practices in postgraduate medical training using organisational justice theory. Methods: We carried out three independent cross-sectional studies using samples from three consecutive annual recruitment rounds. Data were gathered from candidates applying for entry into UK general practice (GP) training during 2007, 2008 and 2009. Participants completed an evaluation questionnaire immediately after the short-listing stage and after the selection centre (interview) stage. Participants were doctors applying for GP training in the UK. Main outcome measures were participants’ evaluations of the selection methods and perceptions of the overall fairness of each selection stage (short-listing and selection centre). Results: A total of 23 855 evaluation questionnaires were completed (6893 in 2007, 10 497 in 2008 and 6465 in 2009). Absolute levels of perceptions of fairness of all the selection methods at both the short-listing and selection centre stages were consistently high over the 3 years. Similarly, all selection methods were considered to be job-related by candidates. However, in general, candidates considered the selection centre stage to be significantly fairer than the short-listing stage. Of all the selection methods, the simulated patient consultation completed at the selection centre stage was rated as the most job-relevant. Conclusions: This is the first study to use a model of organisational justice theory to evaluate candidate reactions during selection into postgraduate specialty training. The high-fidelity selection methods are consistently viewed as more job-relevant and fairer by candidates. This has important implications for the design of recruitment systems for all specialties and, potentially, for medical school admissions. Using this approach, recruiters can systematically compare perceptions of the fairness and job relevance of various selection methods

Holes in the walls: primordial black holes as a solution to the cosmological domain wall problem

We propose a scenario in which the cosmological domain wall and monopole problems are solved without any fine tuning of the initial conditions or parameters in the Lagrangian of an underlying filed theory. In this scenario domain walls sweep out (unwind) the monopoles from the early universe, then the fast primordial black holes perforate the domain walls, change their topology and destroy them. We find further that the (old vacuum) energy density released from the domain walls could alleviate but not solve the cosmological flatness problem.Comment: References added; Published in Phys. Rev.

Evolution of primordial black holes in Jordan-Brans-Dicke cosmology

We consider the evolution of primordial black holes in a generalyzed Jordan-Brans-Dicke cosmological model where both the Brans-Dicke scalar field and its coupling to gravity are dynamical functions determined from the evolution equations. The evaporation rate for the black holes changes compared to that in standard cosmology. We show that accretion of radiation can proceed effectively in the radiation dominated era. The black hole lifetime shortens for low initial mass, but increases for high initial mass, and is thus considerably modified compared to the case of standard cosmology. We derive a cut-off value for the initial black hole mass, below which primordial black holes evaporate out in the radiation dominated era, and above which they survive beyond the present era.Comment: 5 pages, Latex; uses MNRAS stylefiles; minor changes; accepted for publication in MNRA

Stability criterion for self-similar solutions with a scalar field and those with a stiff fluid in general relativity

A stability criterion is derived in general relativity for self-similar solutions with a scalar field and those with a stiff fluid, which is a perfect fluid with the equation of state $P=\rho$. A wide class of self-similar solutions turn out to be unstable against kink mode perturbation. According to the criterion, the Evans-Coleman stiff-fluid solution is unstable and cannot be a critical solution for the spherical collapse of a stiff fluid if we allow sufficiently small discontinuity in the density gradient field in the initial data sets. The self-similar scalar-field solution, which was recently found numerically by Brady {\it et al.} (2002 {\it Class. Quantum. Grav.} {\bf 19} 6359), is also unstable. Both the flat Friedmann universe with a scalar field and that with a stiff fluid suffer from kink instability at the particle horizon scale.Comment: 15 pages, accepted for publication in Classical and Quantum Gravity, typos correcte

Intermediate inflation in light of the three-year WMAP observations

The three-year observations from the Wilkinson Microwave Anisotropy Probe have been hailed as giving the first clear indication of a spectral index n_s<1. We point out that the data are equally well explained by retaining the assumption n_s=1 and allowing the tensor-to-scalar ratio r to be non-zero. The combination n_s=1 and r>0 is given (within the slow-roll approximation) by a version of the intermediate inflation model with expansion rate H(t) \propto t^{-1/3}. We assess the status of this model in light of the WMAP3 data.Comment: 4 pages RevTeX4 with one figure. Minor changes to match PRD accepted versio

MACHOs, White Dwarfs, and the Age of the Universe

(Abridged Abstract) A favored interpretation of recent microlensing measurements towards the Large Magellanic Cloud implies that a large fraction (i.e. 10--50%) of the mass of the galactic halo is composed of white dwarfs. We compare model white dwarf luminosity functions to the data from the observational surveys in order to determine a lower bound on the age of any substantial white dwarf halo population (and hence possibly on the age of the Universe). We compare various theoretical white dwarf luminosity functions, in which we vary hese three parameters, with the abovementioned survey results. From this comparison, we conclude that if white dwarfs do indeed constitute more than 10% of the local halo mass density, then the Universe must be at least 10 Gyr old for our most extreme allowed values of the parameters. When we use cooling curves that account for chemical fractionation and more likely values of the IMF and the bolometric correction, we find tighter limits: a white dwarf MACHO fraction of 10% (30%) requires a minimum age of 14 Gyr (15.5 Gyr). Our analysis also indicates that the halo white dwarfs almost certainly have helium-dominated atmospheres.Comment: Final version accepted for publication, straight TeX formate, 6 figs, 22 page