358 research outputs found
Excursion Set Halo Mass Function and Bias in a Stochastic Barrier Model of Ellipsoidal Collapse
We use the Excursion Set formalism to compute the properties of the halo mass
distribution for a stochastic barrier model which encapsulates the main
features of the ellipsoidal collapse of dark matter halos. Non-markovian
corrections due to the sharp filtering of the linear density field in real
space are computed with the path-integral technique introduced by Maggiore &
Riotto (2010). Here, we provide a detailed derivation of the results presented
in Corasaniti & Achitouv (2011) and extend the mass function analysis to higher
redshift. We also derive an analytical expression for the linear halo bias. We
find the analytically derived mass function to be in remarkable agreement with
N-body simulation data from Tinker et al. (2008) with differences smaller than
~5% over the range of mass probed by the simulations. The excursion set
solution from Monte Carlo generated random walks shows the same level of
agreement, thus confirming the validity of the path-integral approach for the
barrier model considered here. Similarly the analysis of the linear halo bias
shows deviations no greater than 20%. Overall these results indicate that the
Excursion Set formalism in combination with a realistic modeling of the
conditions of halo collapse can provide an accurate description of the halo
mass distribution.Comment: 16 pages, 9 figures; companion paper published in PRL 106 (2011)
241302. To appear on PR
Constraining the quintessence equation of state with SnIa data and CMB peaks
Quintessence has been introduced as an alternative to the cosmological
constant scenario to account for the current acceleration of the universe. This
new dark energy component allows values of the equation of state parameter
, and in principle measurements of cosmological distances to
Type Ia supernovae can be used to distinguish between these two types of
models. Assuming a flat universe, we use the supernovae data and measurements
of the position of the acoustic peaks in the Cosmic Microwave Background (CMB)
spectra to constrain a rather general class of Quintessence potentials,
including inverse power law models and recently proposed Supergravity inspired
potentials. In particular we use a likelihood analysis, marginalizing over the
dark energy density , the physical baryon density
and the scalar spectral index , to constrain the slopes of our Quintessence
potential. Considering only the first Doppler peak the best fit in our range of
models gives . However, including the SnIa data and the three
peaks, we find an upper limit on the present value of the equation of state
parameter, at , a result that appears to
rule out a class of recently proposed potentials.Comment: Accepted for pubblication in PRD, 7 pages, 6 figures. New revised
analysi
Signals of primordial phase transitions on CMB maps
The analysis of the CMB anisotropies is a rich source of cosmological
informations. In our study, we simulated the signals produced by the relics of
a first order phase transition occured during an inflationary epoch in the
early Universe. These relics are bubbles of true vacuum that leave a
characteristic non-Gaussian imprint on the CMB. We use different statistical
estimators in order to evaluate this non-Gaussianity. We obtain some limits on
the allowed values of the bubble parameters comparing our results with the
experimental data.
We also predict the possibility to detect this signal with the next high
resolution experiments.Comment: 2 pages, submitted to Proceedings of 9th Marcel Grossmann meetin
Present limits to cosmic bubbles from the COBE-DMR three point correlation function
The existence of large scale voids in several galaxy surveys suggests the
occurence of an inflationary first order phase transition. This process
generates primordial bubbles that, before evolving into the present voids,
leave at decoupling a non-Gaussian imprint on the CMB. I this paper we evaluate
an analytical expression of the collapsed three point correlation function from
the bubble temperature fluctuations. Comparing the results with COBE-DMR
measures, we obtain upper limits on the allowed non-Gaussianity and hence on
the bubble parameters.Comment: 4 pages, 3 figures; submitted to MNRA
Extending the Coyote emulator to dark energy models with standard - parametrization of the equation of state
We discuss an extension of the Coyote emulator to predict non-linear matter
power spectra of dark energy (DE) models with a scale factor dependent equation
of state of the form w = w_0 + ( 1 - a )w_a . The extension is based on the
mapping rule between non-linear spectra of DE models with constant equation of
state and those with time varying one originally introduced in ref. [40]. Using
a series of N-body simulations we show that the spectral equivalence is
accurate to sub-percent level across the same range of modes and redshift
covered by the Coyote suite. Thus, the extended emulator provides a very
efficient and accurate tool to predict non-linear power spectra for DE models
with w_0 - w_a parametrization. According to the same criteria we have
developed a numerical code, and we have implemented in a dedicated module for
the CAMB code, that can be used in combination with the Coyote Emulator in
likelihood analyses of non-linear matter power spectrum measurements. All codes
can be found at https://github.com/luciano-casarini/PKequalComment: All codes can be found at https://github.com/luciano-casarini/PKequa
An indirect limit on the amplitude of primordial Gravitational Wave Background from CMB-Galaxy Cross Correlation
While large scale cosmic microwave background (CMB) anisotropies involve a
combination of the scalar and tensor fluctuations, the scalar amplitude can be
independently determined through the CMB-galaxy cross-correlation. Using
recently measured cross-correlation amplitudes, arising from the
cross-correlation between galaxies and the Integrated Sachs Wolfe effect in CMB
anisotropies, we obtain a constraint r < 0.5 at 68% confidence level on the
tensor-to-scalar fluctuation amplitude ratio. The data also allow us to exclude
gravity waves at a level of a few percent, relative to the density field, in a
low - Lambda dominated universe(Omega_Lambda~0.5). In future, joining
cross-correlation ISW measurements, which captures cosmological parameter
information, with independent determinations of the matter density and CMB
anisotropy power spectrum, may constrain the tensor-to-scalar ratio to a level
above 0.05. This value is the ultimate limit on tensor-to-scalar ratio from
temperature anisotropy maps when all other cosmological parameters except for
the tensor amplitude are known and the combination with CMB-galaxy correlation
allows this limit to be reached easily by accounting for degeneracies in
certain cosmological parameters.Comment: 5 Pages, 1 Figure, revised discussion on cosmic variance limits on
the tensor-to-scalar ratio from CMB, matches PRD accepted versio
Detection of a dry-frozen boundary inside Martian regolith
The present work investigates the time oscillations of the temperature at several depths of a Martian soil analogue made of two layers of different physical properties. The maximum temperature-time oscillation inside the Martian soil analogue, DT, and its derivative with depth, d(DT)/dz or DDT, can be analysed to understand the presence of a boundary between dry and frozen soil. The maximum temperature-time oscillation, DT, reduces by about one order of magnitude at the boundary between dry and frozen soil if a frozen layer is present. The reduction of DT at the boundary between two dry soils with different porosity is much smaller. DDT decreases by more than one order of magnitude at the boundary between dry and frozen soil if a frozen layer is present. The reduction of DDT at the boundary between two dry soils with different porosity is much smaller. (C) 2008 Elsevier Ltd. All rights reserved
Self-consistency of the Excursion Set Approach
The excursion set approach provides a framework for predicting how the
abundance of dark matter halos depends on the initial conditions. A key
ingredient of this formalism comes from the physics of halo formation: the
specification of a critical overdensity threshold (barrier) which protohalos
must exceed if they are to form bound virialized halos at a later time. Another
ingredient is statistical, as it requires the specification of the appropriate
statistical ensemble over which to average when making predictions. The
excursion set approach explicitly averages over all initial positions, thus
implicitly assuming that the appropriate ensemble is that associated with
randomly chosen positions in space, rather than special positions such as peaks
of the initial density field. Since halos are known to collapse around special
positions, it is not clear that the physical and statistical assumptions which
underlie the excursion set approach are self-consistent. We argue that they are
at least for low mass halos, and illustrate by comparing our excursion set
predictions with numerical data from the DEUS simulations.Comment: 5 pages, 2 figure
Theoretical prediction ofthe thermal conductivity and temperature variation inside mars soil analogues
Mars soil analogues, in dry and frozen conditions, are investigated, as far as the thermal conductivity prediction and the temperature
variation, along its depth, are concerned. The thermal conductivity is theoretically predicted with the cubic cell model, which requires
the knowledge ofthe thermal conductivity ofthe solid particle and ofthe materials present, i.e. atmospheric gas and/or frozen ice, and
the porosity ofthe soil analogue. The soil mineral composition allows to evaluate the thermal conductivity ofthe solid particle. The heat
capacity ofthe soil analogue is evaluated with the knowledge ofits physical properties, the porosity and the speci1c heats ofthe materials
present. The thermal di2usivity is calculated as the ratio ofthe thermal conductivity and heat capacity and results to be a function ofthe
porosity and the ice mass content ofthe soil analogue. The temperature variations, in dry and partially frozen soil analogues, are predicted
during a Martian day. The temperature variation, at di2erent depth, is attenuated, as compared to the surface variation and a phase delay
is present, depending on the soil thermal properties. The temperature variation, as well as the derivative ofthe temperature variation with
the depth, is dependent on the thermal di2usivity ofthe soil analogue. In conclusion, the temperature measurement, along the depth ofa
Martian soil analogue, can be used to verify its physical status, i.e. dry or partially frozen
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