1,713 research outputs found
Modelling the relative velocities of isolated pairs of galaxies
We study the comoving relative velocities, v12, of model isolated galaxy
pairs at z=0.5. For this purpose, we use the predictions from the GALFORM
semi-analytical model of galaxy formation and evolution based on a Lambda cold
dark matter cosmology consistent with the results from WMAP7. In real space, we
find that isolated pairs of galaxies are predicted to form an angle t with the
line-of-sight that is uniformily distributed as expected if the Universe is
homogeneous and isotropic. We also find that isolated pairs of galaxies
separated by a comoving distance between 1 and 3 Mpc/h are predicted to have
=0. For galaxies in this regime, the distribution of the angle t is
predicted to change minimally from real to redshift space, with a change
smaller than 5% in . However, the distances defining the comoving
regime strongly depends on the applied isolation criteria.Comment: 4 pages, 4 figures, SF2A 2013 Proceedin
Catastrophic Photo-z Errors and the Dark Energy Parameter Estimates with Cosmic Shear
We study the impact of catastrophic errors occurring in the photometric
redshifts of galaxies on cosmological parameter estimates with cosmic shear
tomography. We consider a fiducial survey with 9-filter set and perform photo-z
measurement simulations. It is found that a fraction of 1% galaxies at
z_{spec}~0.4 is misidentified to be at z_{phot}~3.5. We then employ both chi^2
fitting method and the extension of Fisher matrix formalism to evaluate the
bias on the equation of state parameters of dark energy, w_0 and w_a, induced
by those catastrophic outliers. By comparing the results from both methods, we
verify that the estimation of w_0 and w_a from the fiducial 5-bin tomographic
analyses can be significantly biased. To minimize the impact of this bias, two
strategies can be followed: (A) the cosmic shear analysis is restricted to
0.5<z<2.5 where catastrophic redshift errors are expected to be insignificant;
(B) a spectroscopic survey is conducted for galaxies with 3<z_{phot}<4. We find
that the number of spectroscopic redshifts needed scales as N_{spec} \propto
f_{cata}\times A where f_{cata}=1% is the fraction of catastrophic redshift
errors (assuming a 9-filter photometric survey) and A is the survey area. For
A=1000 {deg}^2, we find that N_{spec}>320 and 860 respectively in order to
reduce the joint bias in (w_0,w_a) to be smaller than 2\sigma and 1\sigma. This
spectroscopic survey (option B) will improve the Figure of Merit of option A by
a factor \times 1.5 thus making such a survey strongly desirable.Comment: 25 pages, 9 figures. Revised version, as accepted for publication in
Ap
Prospects for Dark Energy Evolution: a Frequentist Multi-Probe Approach
A major quest in cosmology is the understanding of the nature of dark energy.
It is now well known that a combination of cosmological probes is required to
break the underlying degeneracies on cosmological parameters. In this paper, we
present a method, based on a frequentist approach, to combine probes without
any prior constraints, taking full account of the correlations in the
parameters. As an application, a combination of current SNIa and CMB data with
an evolving dark energy component is first compared to other analyses. We
emphasise the consequences of the implementation of the dark energy
perturbations on the result for a time varying equation of state. The impact of
future weak lensing surveys on the measurement of dark energy evolution is then
studied in combination with future measurements of the cosmic microwave
background and type Ia supernovae. We present the combined results for future
mid-term and long-term surveys and confirm that the combination with weak
lensing is very powerful in breaking parameter degeneracies. A second
generation of experiment is however required to achieve a 0.1 error on the
parameters describing the evolution of dark energy.Comment: Submitted to Astronomy & Astrophysics 14 pages, 8 figure
On the determination of the deceleration parameter from Supernovae data
Supernovae searches have shown that a simple matter-dominated and
decelerating universe should be ruled out. However a determination of the
present deceleration parameter through a simple kinematical description
is not exempt of possible drawbacks. We show that, with a time dependent
equation of state for the dark energy, a bias is present for : models
which are very far from the so-called Concordance Model can be accommodated by
the data and a simple kinematical analysis can lead to wrong conclusions. We
present a quantitative treatment of this bias and we present our conclusions
when a possible dynamical dark energy is taken into account.Comment: 4 pages, 3 figures, submitte
Jackknife resampling technique on mocks: an alternative method for covariance matrix estimation
We present a fast and robust alternative method to compute covariance matrix
in case of cosmology studies. Our method is based on the jackknife resampling
applied on simulation mock catalogues. Using a set of 600 BOSS DR11 mock
catalogues as a reference, we find that the jackknife technique gives a similar
galaxy clustering covariance matrix estimate by requiring a smaller number of
mocks. A comparison of convergence rates show that 7 times fewer
simulations are needed to get a similar accuracy on variance. We expect this
technique to be applied in any analysis where the number of available N-body
simulations is low.Comment: 11 pages, 11 figures, 2 table
Probing Dark Energy with Supernovae : a concordant or a convergent model?
We present a revised interpretation of recent analysis of supernovae data. We
evaluate the effect of the priors on the extraction of the dark energy equation
of state. We find that the conclusions depend strongly on the prior
value and on its uncertainty, and show that a biased fitting procedure applied
on non concordant simulated data can converge to the "concordance model".
Relaxing the prior on points to other sets of solutions, which are
not excluded by observational data.Comment: 1+4 pages, 6 figures, submitted to Phys. Re
Torsion, an alternative to dark matter?
We confront Einstein-Cartan's theory with the Hubble diagram. An affirmative
answer to the question in the title is compatible with today's supernovae data.Comment: 14 pp, 3 figures. Version 2 matches the version published in Gen.
Rel. Grav., references added. Version 3 corrects a factor 3 in Cartan's
equations to become
Constraining Type Ia supernova models: SN 2011fe as a test case
The nearby supernova SN 2011fe can be observed in unprecedented detail.
Therefore, it is an important test case for Type Ia supernova (SN Ia) models,
which may bring us closer to understanding the physical nature of these
objects. Here, we explore how available and expected future observations of SN
2011fe can be used to constrain SN Ia explosion scenarios. We base our
discussion on three-dimensional simulations of a delayed detonation in a
Chandrasekhar-mass white dwarf and of a violent merger of two white
dwarfs-realizations of explosion models appropriate for two of the most
widely-discussed progenitor channels that may give rise to SNe Ia. Although
both models have their shortcomings in reproducing details of the early and
near-maximum spectra of SN 2011fe obtained by the Nearby Supernova Factory
(SNfactory), the overall match with the observations is reasonable. The level
of agreement is slightly better for the merger, in particular around maximum,
but a clear preference for one model over the other is still not justified.
Observations at late epochs, however, hold promise for discriminating the
explosion scenarios in a straightforward way, as a nucleosynthesis effect leads
to differences in the 55Co production. SN 2011fe is close enough to be followed
sufficiently long to study this effect.Comment: Accepted for publication in The Astrophysical Journal Letter
Beyond Einstein-Cartan gravity: Quadratic torsion and curvature invariants with even and odd parity including all boundary terms
Recently, gravitational gauge theories with torsion have been discussed by an
increasing number of authors from a classical as well as from a quantum field
theoretical point of view. The Einstein-Cartan(-Sciama-Kibble) Lagrangian has
been enriched by the parity odd pseudoscalar curvature (Hojman, Mukku, and
Sayed) and by torsion square and curvature square pieces, likewise of even and
odd parity. (i) We show that the inverse of the so-called Barbero-Immirzi
parameter multiplying the pseudoscalar curvature, because of the topological
Nieh-Yan form, can only be appropriately discussed if torsion square pieces are
included. (ii) The quadratic gauge Lagrangian with both parities, proposed by
Obukhov et al. and Baekler et al., emerges also in the framework of Diakonov et
al.(2011). We establish the exact relations between both approaches by applying
the topological Euler and Pontryagin forms in a Riemann-Cartan space expressed
for the first time in terms of irreducible pieces of the curvature tensor.
(iii) Only in a Riemann-Cartan spacetime, that is, in a spacetime with torsion,
parity violating terms can be brought into the gravitational Lagrangian in a
straightforward and natural way. Accordingly, Riemann-Cartan spacetime is a
natural habitat for chiral fermionic matter fields.Comment: 12 page latex, as version 2 an old file was submitted by mistake,
this is now the real corrected fil
Light-particle emission from the fissioning nuclei 126Ba, 188Pt and (266,272,278)/110: theoretical predictions and experimental results
We present a comparison of our model treating fission dynamics in conjunction
with light-particle (n, p, alpha) evaporation with the available experimental
data for the nuclei 126Ba, 188Pt and three isotopes of the element Z=110. The
dynamics of the symmetric fission process is described through the solution of
a classical Langevin equation for a single collective variable characterizing
the nuclear deformation along the fission path. A microscopic approach is used
to evaluate the emission rates for pre-fission light particles.
Entrance-channel effects are taken into account by generating an initial spin
distribution of the compound nucleus formed by the fusion of two deformed
nuclei with different relative orientations
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