Search results for " database"
showing 10 items of 684 documents
On the accretion history of galaxy clusters: temporal and spatial distribution
2020
We analyse the results of an Eulerian AMR cosmological simulation in order to quantify the mass growth of galaxy clusters, exploring the differences between dark matter and baryons. We have determined the mass assembly histories (MAHs) of each of the mass components and computed several proxies for the instantaneous mass accretion rate (MAR). The mass growth of both components is clearly dominated by the contribution of major mergers, but high MARs can also occur during smooth accretion periods. We explored the correlations between MARs, merger events and clusters' environments, finding the mean densities in $1 \leq r/R_{200m} \leq 1.5$ to correlate strongly with $\Gamma_{200m}$ in massive …
The Extreme Energy Cosmic Rays and Cosmic Neutrinos as Probes for the Distant Universe. Astrophysics Involved and Experimental Approach
2001
The Cosmic Radiation, with its charged and neutral components, can be considered the second fundamental channel for the investigation of the Universe complementing the “Electromagnetic” specific of the conventional Astronomy. The experimental approach considered here is based on the observation from a Low Orbit Satellite of the UV fluorescence signal produced in the Earth atmosphere by the incoming radiation. A general description is given of the AIR WATCH/OWL experiment; some detail will also be given of “EUSO”: Extreme Universe Space Observatory” a proposal which is being submitted to the European Space Agency in response to an Announcement of Opportunity (AO) issued on October 1999.
The Large-Scale Structure in the Universe: From Power Laws to Acoustic Peaks
2008
The most popular tools for analysing the large scale distribution of galaxies are second-order spatial statistics such as the two-point correlation function or its Fourier transform, the power spectrum. In this review, we explain how our knowledge of cosmic structures, encapsulated by these statistical descriptors, has evolved since their first use when applied on the early galaxy catalogues to the present generation of wide and deep redshift surveys, incorporating the most challenging discovery in the study of the galaxy distribution: the detection of Baryon Acoustic Oscillations.
Observational Constraints on Undulant Cosmologies
2005
In an undulant universe, cosmic expansion is characterized by alternating periods of acceleration and deceleration. We examine cosmologies in which the dark-energy equation of state varies periodically with the number of e-foldings of the scale factor of the universe, and use observations to constrain the frequency of oscillation. We find a tension between a forceful response to the cosmic coincidence problem and the standard treatment of structure formation.
Feasibility of 1 arcmin resolution gamma-ray air-Čerenkov multiple telescope experiment
1995
Abstract In this paper we discuss the feasibility of a ground based detector exploiting the technique of Cerenkov light stereo imaging of air showers for the detection of discrete cosmic sources of γ-rays with energy threshold of 100 GeV, angular resolution (HWHM) of 1 arcmin ( 1 3 mrad), cosmic ray background rejection ≥ 99%. This experiment will make possible a deep survey of the sky with sensitivity of 1 100 of the Crab in 45 h of exposure.
Measurement of the strong coupling constant using τ decays
1993
The strong coupling constant is determined from the leptonic branching ratios, the lifetime, and the invariant mass distribution of the hadronic final state of the tau lepton, using data accumulated at LEP with the ALEPH detector. The strong coupling constant measurement, alpha(s), (m(tau)2) = 0.330 +/-0.046, evolved to the Z mass, yields alpha(s)(M(Z)2) = 0. 1 18 +/- 0.005. The error includes experimental and theoretical uncertainties, the latter evaluated in the framework of the Shifman, Vainshtein and Zakharov (SVZ) approach. The method allows the non-perturbative contribution to the hadronic decay rate to be determined to be 0.3 +/- 0.5% . RI Perrier, Frederic/A-5953-2011; ANTONELLI, AN…
Post-Newtonian constraints onf(R)cosmologies in metric and Palatini formalism
2005
We compute the complete post-Newtonian limit of both the metric and Palatini formulations of $f(R)$ gravities using a scalar-tensor representation. By comparing the predictions of these theories with laboratory and solar system experiments, we find a set of inequalities that any lagrangian $f(R)$ must satisfy. The constraints imposed by those inequalities allow us to find explicit bounds to the possible nonlinear terms of the lagrangian. We conclude that in both formalisms the lagrangian $f(R)$ must be almost linear in $R$ and that corrections that grow at low curvatures are incompatible with observations. This result shows that modifications of gravity at very low cosmic densities cannot b…
Flavor of cosmic neutrinos preserved by ultralight dark matter
2019
Within the standard propagation scenario, the flavor ratios of high-energy cosmic neutrinos at neutrino telescopes are expected to be around the democratic benchmark resulting from hadronic sources, $\left( 1 : 1 : 1 \right)_\oplus$. We show how the coupling of neutrinos to an ultralight dark matter complex scalar field would induce an effective neutrino mass that could lead to adiabatic neutrino propagation. This would result in the preservation at the detector of the production flavor composition of neutrinos at sources. This effect could lead to flavor ratios at detectors well outside the range predicted by the standard scenario of averaged oscillations. We also present an electroweak-in…
Metric-Palatini gravity unifying local constraints and late-time cosmic acceleration
2011
We present a novel approach to modified theories of gravity that consists of adding to the Einstein-Hilbert Lagrangian an f(R) term constructed a la Palatini. Using the respective dynamically equivalent scalar-tensor representation, we show that the theory can pass the Solar System observational constraints even if the scalar field is very light. This implies the existence of a long-range scalar field, which is able to modify the cosmological and galactic dynamics, but leaves the Solar System unaffected. We also verify the absence of instabilities in perturbations and provide explicit models which are consistent with local tests and lead to the late-time cosmic acceleration.
On cosmic quantum tunneling from “nothing”
2015
We extend to a general Λ-Eriedmann-Lemaitre-Robertson-Walker (ΛFLRW) a previous result by Vilenkin and others according to which a closed de Sitter universe could be created from "nothing". More specifically, our main result is that only the closed ΛFLRW universe (but not the open and flat ones) could be created from a corresponding instanton, that is, from the corresponding solution with signature +4 of the Einstein field equations. Before getting this result the suitable corresponding instantons are calculated. The result is in accordance with previous results by another authors obtained by different methods.