arXiv subjects

Philippe Jetzer

Publications and source records attributed to Philippe Jetzer.

At least 73 records · Page 4Linked to original sources

arXiv2010

Limits on decaying dark energy density models from the CMB temperature-redshift relation

The nature of the dark energy is still a mystery and several models have been proposed to explain it. Here we consider a phenomenological model for dark energy decay into photons and particles as proposed by Lima (J. Lima, Phys. Rev. D 54, 2571 (1996)). He studied the thermodynamic aspects of decaying dark energy models in particular in the case of a continuous photon creation and/or disruption. Following his approach, we derive a temperature redshift relation for the CMB which depends on the effective equation of state $w_{eff}$ and on the "adiabatic index" $γ$. Comparing our relation with the data on the CMB temperature as a function of the redshift obtained from Sunyaev-Zel'dovich observations and at higher redshift from quasar absorption line spectra, we find $w_{eff}=-0.97 \pm 0.034$, adopting for the adiabatic index $γ=4/3$, in good agreement with current estimates and still compatible with $w_{eff}=-1$, implying that the dark energy content being constant in time.

astro-ph.CO↗

arXiv2010

Spin effects in the phasing of gravitational waves from binaries on eccentric orbits

We compute here the spin-orbit and spin-spin couplings needed for an accurate computation of the phasing of gravitational waves emitted by comparable-mass binaries on eccentric orbits at the second post-Newtonian (PN) order. We use a quasi-Keplerian parametrization of the orbit free of divergencies in the zero eccentricity limit. We find that spin-spin couplings induce a residual eccentricity for coalescing binaries at 2PN, of the order of $10^{-4}$-$10^{-3}$ for supermassive black hole binaries in the LISA band. Spin-orbit precession also induces a non-trivial pattern in the evolution of the eccentricity, which could help to reduce the errors on the determination of the eccentricity and spins in a gravitational wave measurement.

gr-qc↗

arXiv2010

On the 1/c Expansion of f(R) Gravity

We derive for applications to isolated systems - on the scale of the Solar System - the first relativistic terms in the $1/c$ expansion of the space time metric $g_{μν}$ for metric $f(R)$ gravity theories, where $f$ is assumed to be analytic at $R=0$. For our purpose it suffices to take into account up to quadratic terms in the expansion of $f(R)$, thus we can approximate $f(R) = R + aR^2$ with a positive dimensional parameter $a$. In the non-relativistic limit, we get an additional Yukawa correction with coupling strength $G/3$ and Compton wave length $\sqrt{6a}$ to the Newtonian potential, which is a known result in the literature. As an application, we derive to the same order the correction to the geodetic precession of a gyroscope in a gravitational field and the precession of binary pulsars. The result of the Gravity Probe B experiment yields the limit $a \lesssim 5 \times 10^{11} \, \mathrm{m}^2$, whereas for the pulsar B in the PSR J0737-3039 system we get a bound which is about $10^4$ times larger. On the other hand the Eöt-Wash experiment provides the best laboratory bound $a \lesssim 10^{-10} \, \mathrm{m}^2$. Although the former bounds from geodesic precession are much larger than the laboratory ones, they are still meaningful in the case some type of chameleon effect is present and thus the effective values could be different at different length scales.

gr-qc↗

arXiv2010

Effects of Interplanetary Dust on the LISA drag-free Constellation

The analysis of non-radiative sources of static or time-dependent gravitational fields in the Solar System is crucial to accurately estimate the free-fall orbits of the LISA space mission. In particular, we take into account the gravitational effects of Interplanetary Dust (ID) on the spacecraft trajectories. The perturbing gravitational field has been calculated for some ID density distributions that fit the observed zodiacal light. Then we integrated the Gauss planetary equations to get the deviations from the LISA keplerian orbits around the Sun. This analysis can be eventually extended to Local Dark Matter (LDM), as gravitational fields are expected to be similar for ID and LDM distributions. Under some strong assumptions on the displacement noise at very low frequency, the Doppler data collected during the whole LISA mission could provide upper limits on ID and LDM densities.

gr-qc↗

arXiv2010

Microlensing events from galactic globular clusters

We present an analysis of the large set of microlensing events detected so far toward the Galactic center with the purpose of investigating whether some of the dark lenses are located in Galactic globular clusters. We find that in four cases some events might indeed be due to lenses located in the globular clusters themselves. We also give a rough estimate for the average lens mass of the events being highly aligned with Galactic globular cluster centers and find that, under reasonable assumptions, the deflectors could most probably be either brown dwarfs, M-stars or stellar remnants.

astro-ph.CO↗

arXiv2009

Parameter estimation for coalescing massive binary black holes with LISA using the full 2-post-Newtonian gravitational waveform and spin-orbit precession

With one exception, previous analyses of the measurement accuracy of gravitational wave experiments for comparable-mass binary systems have neglected either spin-precession effects or subdominant harmonics and amplitude modulations. Here we give the first explicit description of how these effects combine to improve parameter estimation. We consider supermassive black hole binaries as expected to be observed with the planned space-based interferometer LISA, and study the measurement accuracy for several astrophysically interesting parameters obtainable taking into account the full 2PN waveform for spinning bodies, as well as spin-precession effects. We find that for binaries with a total mass in the range 10^5 M_Sun < M < 10^7 M_Sun at a redshift of 1, a factor ~1.5 is in general gained in accuracy, with the notable exception of the determination of the individual masses in equal-mass systems, for which a factor ~5 can be gained. We also find, as could be expected, that using the full waveform helps increasing the upper mass limit for detection, which can be as high as M = 10^8 M_Sun at a redshift of 1, as well as the redshift limit where some information can be extracted from a system, which is roughly z = 10 for M < 10^7 M_Sun, 1.5-5 times higher than with the restricted waveform. We computed that the full waveform allows to use supermassive black hole binaries as standard sirens up to a redshift of z = 1.6, about 0.4 larger than what previous studies allowed. We found that for lower unequal-mass binary systems, the measurement accuracy is not as drastically improved as for other systems. This suggests that for these systems, adding parameters such as eccentricity or alternative gravity parameters could be achieved without much loss in the accuracy.

astro-ph.CO↗

arXiv2008

Galactic globular clusters contribution to microlensing events?

In this note we perform an analysis of the large set of microlensing events detected so far toward the Galactic center with the purpose of investigating whether some of the dark lenses are located in Galactic globular clusters. We find that in four cases some events might indeed be due to lenses located in the globular clusters themselves. We also give a rough estimate for the average lens mass of the subset of events being highly aligned with Galactic globular cluster centers and find that, under reasonable assumptions, the deflectors could most probably be either brown dwarfs, M-stars or stellar remnants.

astro-ph↗

arXiv2007

Solar system tests of the cosmological constant

We discuss the influence of the cosmological constant $Λ$ on the gravitational equations of motion of bodies with arbitrary masses and eventually solve the two-body problem. Observational constraints are derived from measurements of the periastron advance in stellar systems, in particular binary pulsars and the solar system. For the latter we consider also the change in the mean motion due to $Λ$. Up to now, Earth and Mars data give the best constraint, $Λ\sim 10^{-36} \mathrm{km}^{-2}$. If properly accounting for the gravito-magnetic effect, this upper limit on $Λ$ could greatly improve in the near future thanks to new data from planned or already operating space-missions. Dark matter or modifications of the Newtonian inverse-square law in the solar system are discussed as well. Variations in the $1/r^2$ behavior are considered in the form of either a possible Yukawa-like interaction or a modification of gravity of MOND type.

astro-ph↗

arXiv2007

Substructures in lens galaxies: PG1115+080 and B1555+375, two fold configurations

We study the anomalous flux ratio which is observed in some four-image lens systems, where the source lies close to a fold caustic. In this case two of the images are close to the critical curve and their flux ratio should be equal to unity, instead in several cases the observed value differs significantly. The most plausible solution is to invoke the presence of substructures, as for instance predicted by the Cold Dark Matter scenario, located near the two images. In particular, we analyze the two fold lens systems PG1115+080 and B1555+375, for which there are not yet satisfactory models which explain the observed anomalous flux ratios. We add to a smooth lens model, which reproduces well the positions of the images but not the anomalous fluxes, one or two substructures described as singular isothermal spheres. For PG1115+080 we consider a smooth model with the influence of the group of galaxies described by a SIS and a substructure with mass $\sim 10^{5} M_{\odot}$ as well as a smooth model with an external shear and one substructure with mass $\sim 10^{8} M_{\odot}$ . For B1555+375 either a strong external shear or two substructures with mass $\sim 10^{7} M_{\odot}$ reproduce the data quite well.

astro-ph↗

arXiv2006

Josephson junctions and dark energy

In a recent paper Beck and Mackey [astro-ph/0603397] argue that the argument we gave in our paper [Phys. Lett. B 606, 77 (2005)] to disprove their claim that dark energy can be discovered in the Lab through noise measurements of Josephson junctions is incorrect. In particular, they emphasize that the measured noise spectrum in Josephson junctions is a consequence of the fluctuation dissipation theorem, while our argument was based on equilibrium statistical mechanics. In this note we show that the fluctuation dissipation relation does not depend upon any shift of vacuum (zero-point) energies, and therefore, as already concluded in our previous paper, dark energy has nothing to do with the proposed measurements.

astro-ph↗

arXiv2004

Temperature and entropy profiles of nearby cooling flow clusters observed with XMM-Newton

We investigate temperature and entropy profiles of 13 nearby cooling flow clusters observed with the EPIC cameras of XMM-Newton. When normalized and scaled by the virial radius the temperature profiles turn out to be remarkably similar. At large radii the temperature profiles show a clear decline starting from a break radius at ~ 0.1 r_vir. The temperature decreases by ~30 % between 0.1 r_vir and 0.5 r_vir. As expected for systems where non-gravitational processes are of great importance, the scale length characterizing the central temperature drop is not found to be proportional to the virial radius of the system. The entropy of the plasma increases monotonically moving outwards almost proportional to the radius and the central entropy level is tightly correlated with the core radius of the X-ray emission. The dispersion in the entropy profiles is smaller if the empirical relation S \propto T^{0.65} is used instead of the standard self-similar relation S \propto T and, as expected for cooling flow clusters, no entropy cores are observed.

astro-ph↗

arXiv2004

Has Dark Energy really been discovered in the Lab?

We show that Dark Energy contributions can not be determined from noise measurements of Josephson junctions, as was recently suggested in a paper by C. Beck and M.C. Mackey.

astro-ph↗

arXiv2004

Microlensing towards the LMC

The nature and the location of the lenses discovered in the microlensing surveys done so far towards the LMC remain unclear. Motivated by these questions we computed the optical depth for the different intervening populations and the number of expected events for self-lensing, using a recently drawn coherent picture of the geometrical structure and dynamics of the LMC. By comparing the theoretical quantities with the values of the observed events it is possible to put some constraints on the location and the nature of the MACHOs. Clearly, given the large uncertainties and the few events at disposal it is not yet possible to draw sharp conclusions, nevertheless we find that up to 3-4 MACHO events might be due to lenses in LMC, which are most probably low mass stars, but that hardly all events can be due to self-lensing. The most plausible solution is that the events observed so far are due to lenses belonging to different intervening populations: low mass stars in the LMC, in the thick disk, in the spheroid and some true MACHOs in the halo of the Milky Way and the LMC itself.

astro-ph↗

arXiv2004

Microlensing towards LMC and M31

astro-ph↗

arXiv2002

Aspherical galaxy clusters: effects on cluster masses and gas mass fractions

We present an investigation of the effects of asphericity on the estimates of total mass and gas mass fraction in galaxy clusters from X-ray observations. We model the aspherical shape of galaxy clusters by a triaxial model and compare the true total mass and the true total gas mass fraction with the corresponding quantities obtained with the assumption of spherical symmetry. In the triaxial model we allow the extent along the line of sight to vary in order to describe elongated and compressed cluster shapes. Using a sample of 10 ROSAT clusters and a recent CHANDRA observation we find the following results. For prolate or oblate shapes the difference between triaxial and spherical model both in the mass and in the gas mass fraction are negligible (less than 3 %). For more aspherical shapes the total mass is underestimated (overestimated) in the centre, if the cluster is compressed (elongated). The gas mass fraction is overestimated for compressed clusters and slightly underestimated for elongated clusters. Comparing X-ray masses with gravitational lensing estimates, we find that elongations along the line of sight can resolve discrepancies of masses determined by the two different methods of up to ~30 %. The combination of Sunyaev-Zel'dovich and X-ray observations is useful to measure the elongation of the cluster along the line of sight. As an application, we estimate the elongation of the cluster CL0016+16 with two different approaches, Sunyaev-Zel'dovich measurements and comparison of weak lensing and X-ray masses, and find reasonable agreement.

astro-ph↗

arXiv2001

Dark matter and gamma rays from the galactic halo

The nature of the dark matter in the halo of our Galaxy is still largely unknown. The microlensing events found so far towards the Large Magellanic Cloud suggest that at most about 20% of the halo dark matter is in the form of MACHOs (Massive Astrophysical Compact Halo Objects). The dark matter could also, at least partially, consist of cold molecular clouds (mainly $H_2$). Another possibility is that WIMPs (Weakly Interacting Massive Particles) make up the dark matter and that, due to annihilition processes, they show up through gamma-ray emission.

hep-ph↗

arXiv1999

Microlensing towards different Galactic targets

We calculate the optical depth and the number of events due to gravitational microlensing towards the Galactic bulge, the spiral arm directions $γ$ Scutum, $β$ Scutum, $γ$ Normae, $\vartheta$ Muscae and some dwarf galaxies in the halo of the Galaxy. Using the events found by the MACHO collaboration during their first year of observation towards Baade's Window we estimate the mass functions for the bulge and disk populations following the mass moment method. We find that the mass function can be described by a decreasing power-law with slope $α\simeq 2.0$ in both cases and a minimal mass of $\sim 0.01$ $M_{\odot}$ for the bulge and $\sim 0.02$ $M_{\odot}$ for the disk, respectively. Assuming that the obtained mass function for the disk is also valid in the spiral arms, we find that the expected number of events towards the spiral arms is in reasonable agreement with the observations. However, the small number of observed events does not yet constrain much the different parameters entering in the computation of the mass function. To study the influence of the Magellanic Clouds on the shape and the velocity dispersion in the halo we perform a N-body simulation. We find that their presence induces a slight flattening of the halo ($q_{H}\simeq 0.8$). As a result the expected number of microlensing events towards some targets in the halo, such as the LMC or the SMC, decreases by about 20%, whereas due to the the modification induced on the velocity dispersion the event duration increases.

astro-ph↗

arXiv1997

Gravitational microlensing research

One of the most important problems in astrophysics concerns the nature of the dark matter in galactic halos, whose presence is implied mainly by the observed flat rotation curves in spiral galaxies. In the framework of a baryonic scenario the most plausible candidates are brown dwarfs, M-dwarfs or white dwarfs and cold molecular clouds (mainly of $H_2$). The former can be detected with the ongoing microlensing experiments, which are rapidly leading to important new results. The French collaboration EROS and the American-Australian collaboration MACHO have reported until August 1997 the observation of $\sim$ 16 microlensing events by monitoring during several years the brightness of millions of stars in the Large Magellanic Cloud and one event towards the Small Magellanic Cloud. In particular, the MACHO team found 8 microlensing candidates by analysing their first 2 years of observations. This implies that the halo dark matter fraction in form of MACHOs (Massive Astrophysical Compact Halo Objects) is of the order of 45-50% assuming a standard spherical halo model. More than 150 microlensing events have been detected in the direction of the galactic bulge by the MACHO, OGLE and DUO teams. The measured optical depth implies the presence of a bar in the galactic centre. Here, we give an overview of microlensing and the main results achieved so far.

astro-ph↗