3,037 research outputs found
Cosmological perturbations and the reionization epoch
We investigate the dependence of the epoch of reionization on the properties
of cosmological perturbations, in the context of cosmologies permitted by WMAP.
We compute the redshift of reionization using a simple model based on the
Press-Schechter approximation. For a power-law initial spectrum we estimate
that reionization is likely to occur at a redshift ,
consistent with the WMAP determination based on the temperature-polarization
cross power spectrum. We estimate the delay in reionization if there is a
negative running of the spectral index, as weakly indicated by WMAP. We then
investigate the dependence of the reionization redshift on the nature of the
initial perturbations. We consider chi-squared probability distribution
functions with various degrees of freedom, motivated both by non-standard
inflationary scenarios and by defect models. We find that in these models
reionization is likely occur much earlier, and to be a slower process, than in
the case of initial gaussian fluctuations. We also consider a hybrid model in
which cosmic strings make an important contribution to the seed fluctuations on
scales relevant for reionization. We find that in order for that model to agree
with the latest WMAP results, the string contribution to the matter power
spectrum on the standard scale is likely to be at most at the
level of one percent, which imposes tight constraints on the value of the
string mass per unit length.Comment: 6 pages LaTeX file with 3 figures incorporate
Cosmic reionization constraints on the nature of cosmological perturbations
We study the reionization history of the Universe in cosmological models with non-Gaussian density fluctuations, taking them to have a renormalized probability distribution function parametrized by the number of degrees of freedom, . We compute the ionization history using a simple semi-analytical model, considering various possibilities for the astrophysics of reionization. In all our models we require that reionization is completed prior to , as required by the measurement of the Gunn--Peterson optical depth from the spectra of high-redshift quasars. We confirm previous results demonstrating that such a non-Gaussian distribution leads to a slower reionization as compared to the Gaussian case. We further show that the recent WMAP three-year measurement of the optical depth due to electron scattering, , weakly constrains the allowed deviations from Gaussianity on the small scales relevant to reionization if a constant spectral index is assumed. We also confirm the need for a significant suppression of star formation in mini-halos, which increases dramatically as we decrease
The Cluster Abundance in Flat and Open Cosmologies
We use the galaxy cluster X-ray temperature distribution function to
constrain the amplitude of the power spectrum of density inhomogeneities on the
scale corresponding to clusters. We carry out the analysis for critical density
universes, for low density universes with a cosmological constant included to
restore spatial flatness and for genuinely open universes. That clusters with
the same present temperature but different formation times have different
virial masses is included. We model cluster mergers using two completely
different approaches, and show that the final results from each are extremely
similar. We give careful consideration to the uncertainties involved, carrying
out a Monte Carlo analysis to determine the cumulative errors. For critical
density our result agrees with previous papers, but we believe the result
carries a larger uncertainty. For low density universes, either flat or open,
the required amplitude of the power spectrum increases as the density is
decreased. If all the dark matter is taken to be cold, then the cluster
abundance constraint remains compatible with both galaxy correlation data and
the {\it COBE} measurement of microwave background anisotropies for any
reasonable density.Comment: Uuencoded package containing LaTeX file (uses mn.sty) plus 7
postscript figures incorporated using epsf. Total length 10 pages. Final
version, to appear MNRAS. COBE comparison changed to 4yr data. No change to
results or conclusion
Layzer-Irvine equation: new perspectives and the role of interacting dark energy
We derive the Layzer-Irvine equation in the presence of a homogeneous (or
quasi-homogeneous) dark energy component with an arbitrary equation of state.
We extend the Layzer-Irvine equation to homogeneous and isotropic universes
with an arbitrary number of dimensions and obtain the corresponding virial
relation for sufficiently relaxed objects. We find analogous equations
describing the dynamics of cosmic string loops and other p-branes of arbitrary
dimensionality, discussing the corresponding relativistic and non-relativistic
limits. Finally, we generalize the Layzer-Irvine equation to account for a
non-minimal interaction between dark matter and dark energy, discussing its
practical use as a signature of such an interaction.Comment: 4 page
Cold dark matter models with high baryon content
Recent results have suggested that the density of baryons in the Universe,
OmegaB, is much more uncertain than previously thought, and may be
significantly higher. We demonstrate that a higher OmegaB increases the
viability of critical-density cold dark matter (CDM) models. High baryon
fraction offers the twin benefits of boosting the first peak in the microwave
anisotropy power spectrum and of suppressing short-scale power in the matter
power spectrum. These enable viable CDM models to have a larger Hubble constant
than otherwise possible. We carry out a general exploration of high OmegaB CDM
models, varying the Hubble constant h and the spectral index n. We confront a
variety of observational constraints and discuss specific predictions. Although
some observational evidence may favour baryon fractions as high as 20 per cent,
we find that values around 10 to 15 per cent provide a reasonable fit to a wide
range of data. We suggest that models with OmegaB in this range, with h about
0.5 and n about 0.8, are currently the best critical-density CDM models.Comment: 14 pages, LaTeX, with 9 included figures, to appear in MNRAS. Revised
version includes updated references, some changes to section 4. Conclusions
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Lessons from Youth in Focus
Youth in Focus (YIF) is a Big Lottery Fund initiative aimed at supporting vulnerable young people through difficult changes in their lives.
Beyond Youth Custody (BYC) is one of three England-wide learning and awareness projects funded under the Big Lottery Fund’s YIF programme. BYC has been designed to challenge, advance, and promote better thinking in policy and practice for the effective resettlement of young people after release from custody. BYC brings together Nacro, the social justice charity, with three research and evaluation partners: ARCS (UK), and Salford and Bedfordshire universities, all of which have exceptional track records in action-based research focusing on youth offending and resettlement.
The programme was initially funded for a five-year period ending in April 2017. During that period, the partnership delivered a multi-faceted programme of research, networking, publicity and awareness-raising activities. The BYC team produced a wide range of publications and resources for practitioners, policy-makers and researchers.
The YIF programme also funded service delivery projects across the country to work with three different client groups: young care-leavers, young carers and young custody-leavers. There were 15 individual YIF projects that worked with young custody-leavers, although some of these projects also worked with care-leavers and young carers.
The BYC work focused specifically on young people leaving custody, working alongside these projects and supporting them to evaluate and monitor their own service and compare and contrast different models of resettlement, facilitating young people’s participation and providing ongoing feedback about effective practice and lessons learnt through the research. A key part of BYC’s work involved close and regular involvement with individual YIF projects that worked with young custody-leavers, focusing on issues concerning data collection and evaluation but also on wider practice and policy issues. That involvement with YIF projects generated a substantial set of evidence concerning the implementation and effectiveness of resettlement practice and informed the team’s critical understanding of key resettlement issues
Model selection in cosmology
Model selection aims to determine which theoretical models are most plausible given some data, without necessarily considering preferred values of model parameters. A common model selection question is to ask when new data require introduction of an additional parameter, describing a newly discovered physical effect. We review model selection statistics, then focus on the Bayesian evidence, which implements Bayesian analysis at the level of models rather than parameters. We describe our CosmoNest code, the first computationally efficient implementation of Bayesian model selection in a cosmological context. We apply it to recent WMAP satellite data, examining the need for a perturbation spectral index differing from the scaleinvariant (Harrison–Zel'dovich) case
Apparent and actual galaxy cluster temperatures
The redshift evolution of the galaxy cluster temperature function is a
powerful probe of cosmology. However, its determination requires the
measurement of redshifts for all clusters in a catalogue, which is likely to
prove challenging for large catalogues expected from XMM--Newton, which may
contain of order 2000 clusters with measurable temperatures distributed around
the sky. In this paper we study the apparent cluster temperature, which can be
obtained without cluster redshifts. We show that the apparent temperature
function itself is of limited use in constraining cosmology, and so concentrate
our focus on studying how apparent temperatures can be combined with other
X-ray information to constrain the redshift. We also briefly study the
circumstances in which non-thermal spectral features can give redshift
information.Comment: 7 pages LaTeX file with 13 figures incorporated (uses mn.sty and
epsf). Minor changes to match MNRAS accepted versio
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