Recent Postings from Cosmology and Extragalactic

On How to Extend the NIR Tully-Fisher Relation to be Truly All-Sky

Dust extinction and stellar confusion by the Milky Way reduce the efficiency of detecting galaxies at low Galactic latitudes, creating the so-called Zone of Avoidance. This stands as a stumbling block in charting the distribution of galaxies and cosmic flow fields, and therewith our understanding of the local dynamics in the Universe (CMB dipole, convergence radius of bulk flows). For instance, ZoA galaxies are generally excluded from the whole-sky Tully-Fisher Surveys ($|b| \leq 5^\circ$) even if catalogued. We show here that by fine-tuning the near-infrared TF relation, there is no reason not to extend peculiar velocity surveys deeper into the ZoA. Accurate axial ratios ($b/a$) are crucial to both the TF sample selection and the resulting TF distances. We simulate the effect of dust extinction on the geometrical properties of galaxies. As expected, galaxies appear rounder with increasing obscuration level, even affecting existing TF samples. We derive correction models and demonstrate that we can reliably reproduce the intrinsic axial ratio from the observed value up to extinction level of about $A_J\simeq3$ mag ($A_V\sim11$ mag), we also recover a fair fraction of galaxies that otherwise would fall out of an uncorrected inclination limited galaxy sample. We present a re-calibration of the 2MTF relation in the NIR $J$, $H$, and $K_s$-bands for isophotal rather than total magnitudes, using their same calibration sample. Both TF relations exhibit similar scatter at high Galactic latitudes. However, the isophotal TF relation results in a significant improvement in the scatter for galaxies in the ZoA, and low surface brightness galaxies in general, because isophotal apertures are more robust in the face of significant stellar confusion.

Constraining condensate dark matter in galaxy clusters [Cross-Listing]

We constrain scattering length parameters in a Bose-Einstein condensate dark matter model by using galaxy clusters radii, with the implementation of a method previously applied to galaxies. At the present work, we use a sample of 114 clusters radii in order to obtain the scattering lengths associated with a dark matter particle mass in the range $10^{-6}-10^{-4}\, {\rm eV}$. We obtain scattering lengths that are 5 orders of magnitude larger than the ones found in the galactic case, even when taking into account the cosmological expansion in the cluster scale by means of the introduction of a small cosmological constant. We also construct and compare curves for the orbital velocity of a test particle in the vicinity of a dark matter cluster in both the expanding and the non-expanding cases.

Constraining condensate dark matter in galaxy clusters

We constrain scattering length parameters in a Bose-Einstein condensate dark matter model by using galaxy clusters radii, with the implementation of a method previously applied to galaxies. At the present work, we use a sample of 114 clusters radii in order to obtain the scattering lengths associated with a dark matter particle mass in the range $10^{-6}-10^{-4}\, {\rm eV}$. We obtain scattering lengths that are 5 orders of magnitude larger than the ones found in the galactic case, even when taking into account the cosmological expansion in the cluster scale by means of the introduction of a small cosmological constant. We also construct and compare curves for the orbital velocity of a test particle in the vicinity of a dark matter cluster in both the expanding and the non-expanding cases.

Refining intermediate inflation in the light of Planck 2013 and BICEP2 results [Cross-Listing]

Here, we first study the intermediate inflation in the standard canonical framework and conclude that it is not compatible with observational results deduced from the Planck 2013 and BICEP2. Then, we consider the intermediate inflation in a non-canonical context with a power-like Lagrangian. We obtain that within this framework, the intermediate inflation can be consistent with the observations of Planck 2013 and BICEP2. Also, we estimate the non-Gaussianity parameter in our model and we see that it lies in the range predicted by Planck 2013. Furthermore, we propose an idea in our non-canonical model to overcome the central drawback of intermediate inflation which is the fact that intermediate inflation never ends. We show explicitly that this modification doesn’t alter the nature of intermediate inflation until the time of horizon exit.

Refining intermediate inflation in the light of Planck 2013 and BICEP2 results [Cross-Listing]

Here, we first study the intermediate inflation in the standard canonical framework and conclude that it is not compatible with observational results deduced from the Planck 2013 and BICEP2. Then, we consider the intermediate inflation in a non-canonical context with a power-like Lagrangian. We obtain that within this framework, the intermediate inflation can be consistent with the observations of Planck 2013 and BICEP2. Also, we estimate the non-Gaussianity parameter in our model and we see that it lies in the range predicted by Planck 2013. Furthermore, we propose an idea in our non-canonical model to overcome the central drawback of intermediate inflation which is the fact that intermediate inflation never ends. We show explicitly that this modification doesn’t alter the nature of intermediate inflation until the time of horizon exit.

Refining intermediate inflation in the light of Planck 2013 and BICEP2 results

Here, we first study the intermediate inflation in the standard canonical framework and conclude that it is not compatible with observational results deduced from the Planck 2013 and BICEP2. Then, we consider the intermediate inflation in a non-canonical context with a power-like Lagrangian. We obtain that within this framework, the intermediate inflation can be consistent with the observations of Planck 2013 and BICEP2. Also, we estimate the non-Gaussianity parameter in our model and we see that it lies in the range predicted by Planck 2013. Furthermore, we propose an idea in our non-canonical model to overcome the central drawback of intermediate inflation which is the fact that intermediate inflation never ends. We show explicitly that this modification doesn’t alter the nature of intermediate inflation until the time of horizon exit.

Influence of adaptive mesh refinement and the hydro solver on shear-induced mass stripping in a minor-merger scenario

We compare two different codes for simulations of cosmological structure formation to investigate the sensitivity of hydrodynamical instabilities to numerics, in particular, the hydro solver and the application of adaptive mesh refinement (AMR). As a simple test problem, we consider an initially spherical gas cloud in a wind, which is an idealized model for the merger of a subcluster or galaxy with a big cluster. Based on an entropy criterion, we calculate the mass stripping from the subcluster as a function of time. Moreover, the turbulent velocity field is analyzed with a multi-scale filtering technique. We find remarkable differences between the commonly used PPM solver with directional splitting in the Enzo code and an unsplit variant of PPM in the Nyx code, which demonstrates that different codes can converge to systematically different solutions even when using uniform grids. For the test case of an unbound cloud, AMR simulations reproduce uniform-grid results for the mass stripping quite well, although the flow realizations can differ substantially. If the cloud is bound by a static gravitational potential, however, we find strong sensitivity to spurious fluctuations which are induced at the cutoff radius of the potential and amplified by the bow shock. This gives rise to substantial deviations between uniform-grid and AMR runs performed with Enzo, while the mass stripping in Nyx simulations of the subcluster is nearly independent of numerical resolution and AMR. Although many factors related to numerics are involved, our study indicates that unsplit solvers with advanced flux limiters help to reduce grid effects and to keep numerical noise under control, which is important for hydrodynamical instabilities and turbulent flows.

New Constraints on Quantum Gravity from X-ray and Gamma-Ray Observations [Cross-Listing]

One aspect of the quantum nature of spacetime is its "foaminess" at very small scales. We reassess previous proposals to use astronomical observations of distant quasars and AGN to test models of spacetime foam. We show explicitly how wavefront distortions on small scales cause the image intensity to decay to the point where distant objects become undetectable when the path-length fluctuations become comparable to the wavelength of the radiation. We use X-ray observations from {\em Chandra} to constrain on the parameter $\alpha$ in the expression for cosmic phase shifts to be $\gtrsim 0.58$, which rules out the random walk model (with $\alpha = 1/2$). Here $\alpha$ is defined by the expression for the path-length fluctuations, $\delta \ell$, of a source at distance $\ell$, wherein $\delta \ell \simeq \ell^{1 – \alpha} \ell_P^{\alpha}$, with $\ell_P$ being the Planck length. Much firmer constraints can be set utilizing detections of quasars at GeV energies with {\em Fermi}, and at TeV energies with ground-based Cherenkov telescopes: $\alpha \gtrsim 0.67$ and $\alpha \gtrsim 0.72$, respectively. These limits on $\alpha$ seem to rule out predictions of $\alpha = 2/3$ for the holographic model. We also discuss the less certain possibility that spacetime foam may directly blur images by deflecting photon trajectories. We show that these are not essential to constraining models of space-time foam. However, we demonstrate that X-ray microlensing studies of quadruply lensed quasars indicate that any blurring in the X-ray band must be at levels smaller than a micro-arcsecond, even when accumulated over cosmological distances.

New Constraints on Quantum Gravity from X-ray and Gamma-Ray Observations

One aspect of the quantum nature of spacetime is its "foaminess" at very small scales. We reassess previous proposals to use astronomical observations of distant quasars and AGN to test models of spacetime foam. We show explicitly how wavefront distortions on small scales cause the image intensity to decay to the point where distant objects become undetectable when the path-length fluctuations become comparable to the wavelength of the radiation. We use X-ray observations from {\em Chandra} to constrain on the parameter $\alpha$ in the expression for cosmic phase shifts to be $\gtrsim 0.58$, which rules out the random walk model (with $\alpha = 1/2$). Here $\alpha$ is defined by the expression for the path-length fluctuations, $\delta \ell$, of a source at distance $\ell$, wherein $\delta \ell \simeq \ell^{1 – \alpha} \ell_P^{\alpha}$, with $\ell_P$ being the Planck length. Much firmer constraints can be set utilizing detections of quasars at GeV energies with {\em Fermi}, and at TeV energies with ground-based Cherenkov telescopes: $\alpha \gtrsim 0.67$ and $\alpha \gtrsim 0.72$, respectively. These limits on $\alpha$ seem to rule out predictions of $\alpha = 2/3$ for the holographic model. We also discuss the less certain possibility that spacetime foam may directly blur images by deflecting photon trajectories. We show that these are not essential to constraining models of space-time foam. However, we demonstrate that X-ray microlensing studies of quadruply lensed quasars indicate that any blurring in the X-ray band must be at levels smaller than a micro-arcsecond, even when accumulated over cosmological distances.

GMRT observations of the radio source 4C 35.06: precessing jets from a cD galaxy under assembly?

We report GMRT observation of the strong radio source 4C 35.06, an extended (z=0.047) radio-loud AGN at the center of galaxy cluster Abell 407. The radio map at 610 MHz reveal a striking, helically twisted jet system emanating from an optically faint AGN host. The radio morphology closely resembles the precessing jets of the galactic microquasar SS 433. The optical SDSS images of central region show a complex ensemble of nine galactic condensations within 1 arc minute, embedded in a faint, diffuse stellar halo. This system presents a unique case for studying the formation of a giant elliptical galaxy (cD) at the cluster center.

Evolution of spherical overdensities in holographic dark energy models

In this work we investigate the spherical collapse model in flat FRW dark energy universes. We consider the Holographic Dark Energy (HDE) model as a dynamical dark energy scenario with a slowly time-varying equation-of-state (EoS) parameter $w_{\rm de}$ in order to evaluate the effects of the dark energy component on structure formation in the universe. We first calculate the evolution of density perturbations in the linear regime for both phantom and quintessence behavior of the HDE model and compare the results with standard Einstein-de Sitter (EdS) and $\Lambda$CDM models. We then calculate the evolution of two characterizing parameters in the spherical collapse model, i.e., the linear density threshold $\delta_{\rm c}$ and the virial overdensity parameter $\Delta_{\rm vir}$. We show that in HDE cosmologies the growth factor $g(a)$ and the linear overdensity parameter $\delta_{\rm c}$ fall behind the values for a $\Lambda$CDM universe while the virial overdensity $\Delta_{\rm vir}$ is larger in HDE models than in the $\Lambda$CDM model. We also show that the ratio between the radius of the spherical perturbations at the virialization and turn-around time is smaller in HDE cosmologies than that predicted in a $\Lambda$CDM universe. Hence the growth of structures starts earlier in HDE models than in $\Lambda$CDM cosmologies and more concentrated objects can form in this case. It has been shown that the non-vanishing surface pressure leads to smaller virial radius and larger virial overdensity $\Delta_{\rm vir}$. We compare the predicted number of halos in HDE cosmologies and find out that in general this value is smaller than for $\Lambda$CDM models at higher redshifts and we compare different mass function prescriptions. Finally, we compare the results of the HDE models with observations.

Evolution of spherical overdensities in holographic dark energy models [Cross-Listing]

In this work we investigate the spherical collapse model in flat FRW dark energy universes. We consider the Holographic Dark Energy (HDE) model as a dynamical dark energy scenario with a slowly time-varying equation-of-state (EoS) parameter $w_{\rm de}$ in order to evaluate the effects of the dark energy component on structure formation in the universe. We first calculate the evolution of density perturbations in the linear regime for both phantom and quintessence behavior of the HDE model and compare the results with standard Einstein-de Sitter (EdS) and $\Lambda$CDM models. We then calculate the evolution of two characterizing parameters in the spherical collapse model, i.e., the linear density threshold $\delta_{\rm c}$ and the virial overdensity parameter $\Delta_{\rm vir}$. We show that in HDE cosmologies the growth factor $g(a)$ and the linear overdensity parameter $\delta_{\rm c}$ fall behind the values for a $\Lambda$CDM universe while the virial overdensity $\Delta_{\rm vir}$ is larger in HDE models than in the $\Lambda$CDM model. We also show that the ratio between the radius of the spherical perturbations at the virialization and turn-around time is smaller in HDE cosmologies than that predicted in a $\Lambda$CDM universe. Hence the growth of structures starts earlier in HDE models than in $\Lambda$CDM cosmologies and more concentrated objects can form in this case. It has been shown that the non-vanishing surface pressure leads to smaller virial radius and larger virial overdensity $\Delta_{\rm vir}$. We compare the predicted number of halos in HDE cosmologies and find out that in general this value is smaller than for $\Lambda$CDM models at higher redshifts and we compare different mass function prescriptions. Finally, we compare the results of the HDE models with observations.

Phenomenological approaches of inflation and their equivalence

In this work, we analyse two possible alternative and model-independent approaches to describe the inflationary period. The first one assumes a general equation of state during inflation due to Mukhanov, while the second one is based on the slow-roll hierarchy suggested by Hoffman and Turner. We find that, remarkably, the two approaches are equivalent, as they single out the same areas in the parameter space, and agree with the inflationary attractors where successful inflation occurs. Rephrased in terms of the familiar picture of a slowly rolling canonically-normalized scalar field, the resulting inflaton excursions in these two approaches are almost identical. Furthermore, once the galactic dust polarization data from Planck are included in the numerical fits, inflaton excursions can safely take sub-Planckian values.

Consistency Relations for Large Field Inflation: Non-minimal Coupling [Cross-Listing]

We derive the consistency relations for chaotic inflation model with a non-minimal coupling to gravity. For a quadratic or quartic potential in the limit of a small non-minimal coupling parameter $\xi$, we give consistency relations among spectral index $n_s$, the tensor-to-scalar ratio $r$ and the running of the spectral index $\alpha$. We find that unlike $r$, $\alpha$ is less sensitive to $\xi$. If $r<0.1$, then $\xi$ is constrained to $\xi<0$ and $\alpha$ is predicted to be $\alpha\simeq -8\times 10^{-4}$ for a quadratic or quartic potential. For general monomial potential, $\alpha$ is constrained in the range $-2.7\times 10^{-3}<\alpha< -6\times 10^{-4}$ as long as $|\xi|\leq 10^{-3}$ if $r<0.1$.

Consistency Relations for Large Field Inflation: Non-minimal Coupling

We derive the consistency relations for chaotic inflation model with a non-minimal coupling to gravity. For a quadratic or quartic potential in the limit of a small non-minimal coupling parameter $\xi$, we give consistency relations among spectral index $n_s$, the tensor-to-scalar ratio $r$ and the running of the spectral index $\alpha$. We find that unlike $r$, $\alpha$ is less sensitive to $\xi$. If $r<0.1$, then $\xi$ is constrained to $\xi<0$ and $\alpha$ is predicted to be $\alpha\simeq -8\times 10^{-4}$ for a quadratic or quartic potential. For general monomial potential, $\alpha$ is constrained in the range $-2.7\times 10^{-3}<\alpha< -6\times 10^{-4}$ as long as $|\xi|\leq 10^{-3}$ if $r<0.1$.

The Linear Perturbation Theory of Reionization in Position-Space: Cosmological Radiative Transfer Along the Light-Cone

The linear perturbation theory of inhomogeneous reionization (LPTR) has been developed as an analytical tool for predicting the global ionized fraction and large-scale power spectrum of ionized density fluctuations during reionization. In the original formulation of the LPTR, the ionization balance and radiative transfer equations are linearized and solved in Fourier space. However, the LPTR’s approximation to the full solution of the radiative transfer equation is not straightforward to interpret, since the latter is most intuitively conceptualized in position space. To bridge the gap between the LPTR and the language of numerical radiative transfer, we present a new, equivalent, position-space formulation of the LPTR that clarifies the approximations it makes and facilitates its interpretation. We offer a comparison between the LPTR and the excursion-set model of reionization (ESMR), and demonstrate the built-in capability of the LPTR to explore a wide range of reionization scenarios, and to go beyond the ESMR in exploring scenarios involving X-rays.

Unraveling the nature of Gravity through our clumpy Universe

We propose a new probe to test the nature of gravity at various redshifts through large-scale cosmological observations. We use our void catalog, extracted from the Sloan Digital Sky Survey (SDSS, DR10), to trace the distribution of matter along the lines of sight to SNe Ia that are selected from the Union 2 catalog. We study the relation between SNe Ia luminosities and convergence and also the peculiar velocities of the sources. We show that the effects, on SNe Ia luminosities, of convergence and of peculiar velocities predicted by the theory of general relativity and theories of modified gravities are different and hence provide a new probe of gravity at various redshifts. We show that the present sparse large-scale data does not allow us to determine any statistically- significant deviation from the theory of general relativity but future more comprehensive surveys should provide us with means for such an exploration.

Lecture notes on non-Gaussianity

We discuss how primordial non-Gaussianity of the curvature perturbation helps to constrain models of the early universe. Observations are consistent with Gaussian initial conditions, compatible with the predictions of the simplest models of inflation. Deviations are constrained to be at the sub percent level, constraining alternative models such as those with multiple fields, non-canonical kinetic terms or breaking the slow-roll conditions. We introduce some of the most important models of inflation which generate non-Gaussian perturbations and provide practical tools on how to calculate the three-point correlation function for a popular class of non-Gaussian models. The current state of the field is summarised and an outlook is given.

Revised Lens Model for the Multiply-Imaged Lensed Supernova, "SN Refsdal", in MACS J1149+2223

We present a revised lens model of MACS J1149+2223, in which the first resolved multiply-imaged lensed supernova was discovered. The lens model is based on the model of Johnson et al. (2014) with some modifications. We include more lensing constraints from the host galaxy of the newly discovered supernova, and increase the flexibility of the model in order to better reproduce the lensing signal in the vicinity of this galaxy. The revised model accurately reconstructs the positions of the lensed supernova, provides magnifications, and predicts the time delay between the instances of the supernova. Finally, we reconstruct the source image of the host galaxy, and position the supernova on one of its spiral arms. Products of this lens model are available to the community through MAST.

Heavy neutralino relic abundance with Sommerfeld enhancements - a study of pMSSM scenarios

We present a detailed discussion of Sommerfeld enhancements in neutralino dark matter relic abundance calculations for several popular benchmark scenarios in the general MSSM. Our analysis is focused on models with heavy wino- and higgsino-like neutralino LSP and models interpolating between these two scenarios. This work is the first phenomenological application of effective field theory methods that we have developed in earlier work and that allow for the consistent study of Sommerfeld enhancements in non-relativistic neutralino and chargino co-annihilation reactions within the general MSSM, away from the pure-wino and pure-higgsino limits.

Heavy neutralino relic abundance with Sommerfeld enhancements - a study of pMSSM scenarios [Cross-Listing]

We present a detailed discussion of Sommerfeld enhancements in neutralino dark matter relic abundance calculations for several popular benchmark scenarios in the general MSSM. Our analysis is focused on models with heavy wino- and higgsino-like neutralino LSP and models interpolating between these two scenarios. This work is the first phenomenological application of effective field theory methods that we have developed in earlier work and that allow for the consistent study of Sommerfeld enhancements in non-relativistic neutralino and chargino co-annihilation reactions within the general MSSM, away from the pure-wino and pure-higgsino limits.

Non-relativistic pair annihilation of nearly mass degenerate neutralinos and charginos III. Computation of the Sommerfeld enhancements

This paper concludes the presentation of the non-relativistic effective field theory formalism designed to calculate the radiative corrections that enhance the pair-annihilation cross sections of slowly moving neutralinos and charginos within the general minimal supersymmetric standard model (MSSM). While papers I and II focused on the computation of the tree-level annihilation rates that feed into the short-distance part, here we describe in detail the method to obtain the Sommerfeld factors that contain the enhanced long-distance corrections. This includes the computation of the potential interactions in the MSSM, which are provided in compact analytic form, and a novel solution of the multi-state Schr\"odinger equation that is free from the numerical instabilities generated by large mass splittings between the scattering states. Our results allow for a precise computation of the MSSM neutralino dark matter relic abundance and pair-annihilation rates in the present Universe, when Sommerfeld enhancements are important.

Non-relativistic pair annihilation of nearly mass degenerate neutralinos and charginos III. Computation of the Sommerfeld enhancements [Cross-Listing]

This paper concludes the presentation of the non-relativistic effective field theory formalism designed to calculate the radiative corrections that enhance the pair-annihilation cross sections of slowly moving neutralinos and charginos within the general minimal supersymmetric standard model (MSSM). While papers I and II focused on the computation of the tree-level annihilation rates that feed into the short-distance part, here we describe in detail the method to obtain the Sommerfeld factors that contain the enhanced long-distance corrections. This includes the computation of the potential interactions in the MSSM, which are provided in compact analytic form, and a novel solution of the multi-state Schr\"odinger equation that is free from the numerical instabilities generated by large mass splittings between the scattering states. Our results allow for a precise computation of the MSSM neutralino dark matter relic abundance and pair-annihilation rates in the present Universe, when Sommerfeld enhancements are important.

Fifty Years of Quasars: Physical Insights and Potential for Cosmology

Last year (2013) was more or less the 50th anniversary of the discovery of quasars. It is an interesting time to review what we know (and don’t know) about them both empirically and theoretically. These compact sources involving line emitting plasma show extraordinary luminosities extending to one thousand times that of our Milky Way in emitting volumes of a few solar system diameters (bolometric luminosity log L$_{bol} \sim $ 44-48 [erg s$^{-1}$]: D=1-3 light months $\sim$ $10^3$ – $10^4$ gravitational radii). The advent of 8-10 meter class telescopes enables us to study them spectroscopically in ever greater detail. In 2000 we introduced a 4D Eigenvector 1 parameters space involving optical, UV and X-Ray measures designed to serve as a 4D equivalent of the 2D Hertzsprung-Russell diagram so important for depicting the diversity of stellar types and evolutionary states. This diagram has revealed a principal sequence of quasars distinguished by Eddington ratio (proportional to the accretion rate per unit mass). Thus while stellar differences are primarily driven by the mass of a star, quasar differences are apparently driven by the ratio of luminosity-to-mass. Out of this work has emerged the concept of two quasars populations A and B separated at Eddington ratio around 0.2 which maximizes quasar multispectral differences. The mysterious 8% of quasars that are radio-loud belong to population B which are the lowest accretors with the largest black hole masses. Finally we consider the most extreme population A quasars which are the highest accretors and in some cases are among the youngest quasars. We describe how these sources might be exploited as standard candles for cosmology.

Strongly Coupled Cosmologies

Models including an energy transfer from CDM to DE are widely considered in the literature, namely to allow DE a significant high-z density. Strongly Coupled cosmologies assume a much larger coupling between DE and CDM, together with the presence of an uncoupled warm DM component, as the role of CDM is mostly restricted to radiative eras. This allows us to preserve small scale fluctuations even if the warm particle, possibly a sterile neutrino, is quite light, O(100 eV). Linear theory and numerical simulations show that these cosmologies agree with LCDM on supergalactic scales; e.g., CMB spectra are substantially identical. Simultaneously, simulations show that they significantly ease problems related to the properties of MW satellites and cores in dwarfs. SC cosmologies also open new perspectives on early black hole formation, and possibly lead towards unificating DE and inflationary scalar fields.

The Sun and stars: Giving light to dark matter

During the last century, with the development of modern physics in such diverse fields as thermodynamics, statistical physics, and nuclear and particle physics, the basic principles of the evolution of stars have been successfully well understood. Nowadays, a precise diagnostic of the stellar interiors is possible with the new fields of helioseismology and astroseismology. Even the measurement of solar neutrino fluxes, once a problem in particle physics, is now a powerful probe of the core of the Sun. These tools have allowed the use of stars to test new physics, in particular the properties of the hypothetical particles that constitute the dark matter of the Universe. Here we present recent results obtained using this approach.

Polytropic dark matter flows illuminate dark energy and accelerated expansion

Currently, a large amount of data implies that the matter constituents of the cosmological dark sector might be collisional. An attractive feature of such a possibility is that, it can reconcile dark matter (DM) and dark energy (DE) in terms of a single component, accommodated in the context of a polytropic-DM fluid. Accordingly, we explore the time evolution and the dynamical characteristics of a spatially-flat cosmological model, in which, in principle, there is no DE at all. Instead, in this model, the DM itself possesses some sort of fluid-like properties, i.e., the fundamental units of the Universe matter-energy content are the volume elements of a DM fluid, performing polytropic flows. In this case, the energy of this fluid’s internal motions is also taken into account as a source of the universal gravitational field. This form of energy can compensate for the extra energy needed to compromise spatial flatness, namely, to justify that, today, the total-energy density parameter is exactly unity. The polytropic cosmological model, depends on only one free parameter, the corresponding exponent, \Gamma. What makes this model particularly interesting, is that, for \Gamma < 0.541, the (conventional) pressure becomes negative enough, so that the Universe accelerates its expansion at cosmological redshifts below a transition value. Several physical reasons impose further constraints on the value of \Gamma, which, eventually, is settled down to the range -0.089 < \Gamma < 0. Such a cosmological model does not suffer either from the age problem or from the coincidence problem. At the same time, this model reproduces to high accuracy the distance measurements performed with the aid of the supernovae Type Ia standard candles, and most naturally interprets, not only when, but also, why the Universe transits from deceleration to acceleration, thus arising as a mighty contestant for a DE model.

Homogeneous Instantons in Bigravity [Cross-Listing]

We study homogeneous gravitational instantons, conventionally called the Hawking-Moss (HM) instantons, in bigravity theory. The HM instantons describe the amplitude of quantum tunneling from a false vacuum to the true vacuum. Corrections to General Relativity (GR) are found in a closed form. Using the result, we discuss the following two issues: reduction to the de Rham-Gabadadze-Tolley (dRGT) massive gravity and the possibility of preference for a large $e$-folding number in the context of the Hartle-Hawking (HH) no-boundary proposal. In particular, concerning the dRGT limit, it is found that the tunneling through the so-called self-accelerating branch is exponentially suppressed relative to the normal branch, and the probability becomes zero in the dRGT limit.

Homogeneous Instantons in Bigravity [Cross-Listing]

We study homogeneous gravitational instantons, conventionally called the Hawking-Moss (HM) instantons, in bigravity theory. The HM instantons describe the amplitude of quantum tunneling from a false vacuum to the true vacuum. Corrections to General Relativity (GR) are found in a closed form. Using the result, we discuss the following two issues: reduction to the de Rham-Gabadadze-Tolley (dRGT) massive gravity and the possibility of preference for a large $e$-folding number in the context of the Hartle-Hawking (HH) no-boundary proposal. In particular, concerning the dRGT limit, it is found that the tunneling through the so-called self-accelerating branch is exponentially suppressed relative to the normal branch, and the probability becomes zero in the dRGT limit.

Homogeneous Instantons in Bigravity

We study homogeneous gravitational instantons, conventionally called the Hawking-Moss (HM) instantons, in bigravity theory. The HM instantons describe the amplitude of quantum tunneling from a false vacuum to the true vacuum. Corrections to General Relativity (GR) are found in a closed form. Using the result, we discuss the following two issues: reduction to the de Rham-Gabadadze-Tolley (dRGT) massive gravity and the possibility of preference for a large $e$-folding number in the context of the Hartle-Hawking (HH) no-boundary proposal. In particular, concerning the dRGT limit, it is found that the tunneling through the so-called self-accelerating branch is exponentially suppressed relative to the normal branch, and the probability becomes zero in the dRGT limit.

Reheating processes after Starobinsky inflation in old-minimal supergravity

We study reheating processes and its cosmological consequences in the Starobinsky model embedded in the old-minimal supergravity. First, we consider minimal coupling between the gravity and matter sectors in the higher curvature theory, and transform it to the equivalent standard supergravity coupled to additional matter superfields. We then discuss characteristic decay modes of the inflaton and the reheating temperature $T_{\rm R}$. Considering a simple model of supersymmetry breaking sector, we estimate gravitino abundance from inflaton decay, and obtain limits on the masses of gravitino and supersymmetry breaking field. We find $T_{\rm R}\simeq 1.0\times10^9$ GeV and the allowed range of gravitino mass as $10^4$ GeV $\lesssim m_{3/2} \lesssim 10^5$ GeV, assuming anomaly-induced decay into the gauge sector as the dominant decay channel.

Reheating processes after Starobinsky inflation in old-minimal supergravity [Cross-Listing]

We study reheating processes and its cosmological consequences in the Starobinsky model embedded in the old-minimal supergravity. First, we consider minimal coupling between the gravity and matter sectors in the higher curvature theory, and transform it to the equivalent standard supergravity coupled to additional matter superfields. We then discuss characteristic decay modes of the inflaton and the reheating temperature $T_{\rm R}$. Considering a simple model of supersymmetry breaking sector, we estimate gravitino abundance from inflaton decay, and obtain limits on the masses of gravitino and supersymmetry breaking field. We find $T_{\rm R}\simeq 1.0\times10^9$ GeV and the allowed range of gravitino mass as $10^4$ GeV $\lesssim m_{3/2} \lesssim 10^5$ GeV, assuming anomaly-induced decay into the gauge sector as the dominant decay channel.

Reheating processes after Starobinsky inflation in old-minimal supergravity [Cross-Listing]

We study reheating processes and its cosmological consequences in the Starobinsky model embedded in the old-minimal supergravity. First, we consider minimal coupling between the gravity and matter sectors in the higher curvature theory, and transform it to the equivalent standard supergravity coupled to additional matter superfields. We then discuss characteristic decay modes of the inflaton and the reheating temperature $T_{\rm R}$. Considering a simple model of supersymmetry breaking sector, we estimate gravitino abundance from inflaton decay, and obtain limits on the masses of gravitino and supersymmetry breaking field. We find $T_{\rm R}\simeq 1.0\times10^9$ GeV and the allowed range of gravitino mass as $10^4$ GeV $\lesssim m_{3/2} \lesssim 10^5$ GeV, assuming anomaly-induced decay into the gauge sector as the dominant decay channel.

Reheating processes after Starobinsky inflation in old-minimal supergravity [Cross-Listing]

We study reheating processes and its cosmological consequences in the Starobinsky model embedded in the old-minimal supergravity. First, we consider minimal coupling between the gravity and matter sectors in the higher curvature theory, and transform it to the equivalent standard supergravity coupled to additional matter superfields. We then discuss characteristic decay modes of the inflaton and the reheating temperature $T_{\rm R}$. Considering a simple model of supersymmetry breaking sector, we estimate gravitino abundance from inflaton decay, and obtain limits on the masses of gravitino and supersymmetry breaking field. We find $T_{\rm R}\simeq 1.0\times10^9$ GeV and the allowed range of gravitino mass as $10^4$ GeV $\lesssim m_{3/2} \lesssim 10^5$ GeV, assuming anomaly-induced decay into the gauge sector as the dominant decay channel.

Dust Content, Galaxy Orientations, and Shape Noise in Imaging Surveys

We show that dust absorption in disk galaxies leads to a color- and orientation-dependent centroid shift which is expected to be observable in multi-band imaging surveys. This centroid shift is an interesting new probe which contains astrophysically and cosmologically relevant information: it can be used to probe the dust content of a large sample of galaxies, and to reduce the shape noise due to inclination of disk galaxies for weak lensing shear. Specifically, we find that data sets comparable to CFHTLenS, the Dark Energy Survey (DES) or the Hyper Suprime-Cam (HSC) survey should provide a dust measurement for several hundred galaxies per square degree. Conversely, given knowledge of the dust optical depth, this technique will significantly lower the shape noise for the brightest galaxies in the sample (signal-to-noise greater than a few hundred), thereby increasing their relative importance for the weak lensing shear measurement.

An Unbiased Estimator of Peculiar Velocity with Gaussian Distributed Errors for Precision Cosmology

We introduce a new estimator of the peculiar velocity of a galaxy or group of galaxies from redshift and distance estimates. This estimator results in peculiar velocity estimates which are statistically unbiased and that have errors that are Gaussian distributed, thus meeting the assumptions of analyses that rely on individual peculiar velocities. We apply this estimator to the SFI++ and the Cosmicflows-2 catalogs of galaxy distances and, using the fact that peculiar velocity estimates of distant galaxies are error dominated, examine their error distributions, The adoption of the new estimator significantly improves the accuracy and validity of studies of the large-scale peculiar velocity field and eliminates potential systematic biases, thus helping to bring peculiar velocity analysis into the era of precision cosmology. In addition, our method of examining the distribution of velocity errors should provide a useful check of the statistics of large peculiar velocity catalogs, particularly those that are compiled out of data from multiple sources.

Zeldovich pancakes in observational data are cold

The present day universe consists of galaxies, galaxy clusters, one-dimensional filaments and two-dimensional sheets or pancakes, all of which combine to form the cosmic web. The so called "Zeldovich pancakes", are very difficult to observe, because their overdensity is only slightly greater than the average density of the universe. Falco et al (2014) presented a method to identify Zeldovich pancakes in observational data, and these were used as a tool for estimating the mass of galaxy clusters. Here we expand and refine that observational detection method. We study two pancakes on scales of 10 Mpc, identified from spectroscopically observed galaxies near the Coma cluster, and compare with twenty numerical pancakes. We find that the observed structures have velocity dispersion about 100 km/sec, which is relatively low compared to typical groups and filaments. These velocity dispersions are consistent with those found for the numerical pancakes. We also confirm that the identified structures are in fact two-dimensional structures. Finally, we estimate the stellar to total mass of the observational pancakes to be 2 x 10^{-4}, within one order of magnitude, which is smaller than that of clusters of galaxies.

Primordial star clusters at extreme magnification

Gravitationally lensed galaxies with magnification ~10-100 are routinely detected at high redshifts, but magnifications significantly higher than this are hampered by a combination of low probability and large source sizes. Magnifications of ~1000 may nonetheless be relevant in the case of intrinsically small, high-redshift objects with very high number densities. Here, we explore the prospects of detecting compact (< 10 pc), high-redshift (z > 7) Population III star clusters at such extreme magnifications in large-area surveys with planned telescopes like Euclid, WFIRST and WISH. We find that the planned WISH 100 sq. deg ultradeep survey may be able to detect a small number of such objects, provided that the total stellar mass of these star clusters is > 10000 solar masses. If candidates for such lensed Population III star clusters are found, follow-up spectroscopy of the surrounding nebula with the James Webb Space Telescope or groundbased Extremely Large Telescopes should be able to confirm the Population III nature of these objects. Multiband photometry of these objects with the James Webb Space Telescope also has the potential to confirm that the stellar initial mass function in these Population III star clusters is top-heavy, as supported by current simulations.

Zeldovich pancakes at redshift zero: the equilibration state and phase space properties

One of the components of the cosmic web are sheets, which are commonly referred to as Zeldovich pancakes. These are structures which have only collapsed along one dimension, as opposed to filaments or galaxies and cluster, which have collapsed along two or three dimensions. These pancakes have recently received renewed interest, since they have been shown to be useful tools for an independent method to determine galaxy cluster masses. We consider sheet-like structures resulting from cosmological simulations, which were previously used to establish the cluster mass determination method, and we show through their level of equilibration, that these structures have indeed only collapsed along the one dimension. We also extract the density profiles of these pancake, which agrees acceptably well with theoretical expectations. We derive the observable velocity distribution function (VDF) analytically by generalizing the Eddington method to one dimension, and we compare with the distribution function from the numerical simulation.

A possible indication of momentum-dependent asymmetric dark matter in the Sun [Cross-Listing]

Broad disagreement persists between helioseismological observables and predictions of solar models computed with the latest surface abundances. Here we show that most of these problems can be solved by the presence of asymmetric dark matter coupling to nucleons as the square of the momentum $q$ exchanged in the collision. We compute neutrino fluxes, small frequency separations, surface helium abundances, sound speed profiles and convective zone depths for a number of models, showing more than a $6\sigma$ preference for $q^2$ models over others, and over the Standard Solar Model. The preferred mass (3 GeV) and reference dark matter-nucleon cross-section ($10^{-37}$ cm$^2$ at $q_0 = 40$ MeV) are within the region of parameter space allowed by both direct detection and collider searches.

A possible indication of momentum-dependent asymmetric dark matter in the Sun

Broad disagreement persists between helioseismological observables and predictions of solar models computed with the latest surface abundances. Here we show that most of these problems can be solved by the presence of asymmetric dark matter coupling to nucleons as the square of the momentum $q$ exchanged in the collision. We compute neutrino fluxes, small frequency separations, surface helium abundances, sound speed profiles and convective zone depths for a number of models, showing more than a $6\sigma$ preference for $q^2$ models over others, and over the Standard Solar Model. The preferred mass (3 GeV) and reference dark matter-nucleon cross-section ($10^{-37}$ cm$^2$ at $q_0 = 40$ MeV) are within the region of parameter space allowed by both direct detection and collider searches.

Constraining Hybrid Natural Inflation with recent CMB data

We study the {\it Hybrid Natural Inflation} ({\it HNI}) model and some of its realisations in the light of recent CMB observations, mainly Planck temperature and WMAP-9 polarization, and compare with the recent release of BICEP2 dataset. The inflationary sector of {\it HNI} is essentially given by the potential $V(\phi) = V_0(1+a\cos (\frac{\phi}{f} ) )$, where $a$ is a positive constant smaller or equal to one and $f$ is the scale of (pseudo Nambu-Goldstone) symmetry breaking. We show that to describe the {\it HNI} model realisations we only need two observables; the spectral index $n_s$, the tensor-to-scalar ratio, and a free parameter in the amplitude of the cosine function $a$. We find that in order to make the {\it HNI} model compatible with the BICEP2 observations, we require a large positive running of the spectra. We find that this could over-produce PBHs in the most theoretically consistent case of the model. This situation could be aleviated if, as recently argued, the BICEP2 data do not correspond to primordial gravitational waves.

Constraining Hybrid Natural Inflation with recent CMB data [Cross-Listing]

We study the {\it Hybrid Natural Inflation} ({\it HNI}) model and some of its realisations in the light of recent CMB observations, mainly Planck temperature and WMAP-9 polarization, and compare with the recent release of BICEP2 dataset. The inflationary sector of {\it HNI} is essentially given by the potential $V(\phi) = V_0(1+a\cos (\frac{\phi}{f} ) )$, where $a$ is a positive constant smaller or equal to one and $f$ is the scale of (pseudo Nambu-Goldstone) symmetry breaking. We show that to describe the {\it HNI} model realisations we only need two observables; the spectral index $n_s$, the tensor-to-scalar ratio, and a free parameter in the amplitude of the cosine function $a$. We find that in order to make the {\it HNI} model compatible with the BICEP2 observations, we require a large positive running of the spectra. We find that this could over-produce PBHs in the most theoretically consistent case of the model. This situation could be aleviated if, as recently argued, the BICEP2 data do not correspond to primordial gravitational waves.

Constraining Hybrid Natural Inflation with recent CMB data [Cross-Listing]

We study the {\it Hybrid Natural Inflation} ({\it HNI}) model and some of its realisations in the light of recent CMB observations, mainly Planck temperature and WMAP-9 polarization, and compare with the recent release of BICEP2 dataset. The inflationary sector of {\it HNI} is essentially given by the potential $V(\phi) = V_0(1+a\cos (\frac{\phi}{f} ) )$, where $a$ is a positive constant smaller or equal to one and $f$ is the scale of (pseudo Nambu-Goldstone) symmetry breaking. We show that to describe the {\it HNI} model realisations we only need two observables; the spectral index $n_s$, the tensor-to-scalar ratio, and a free parameter in the amplitude of the cosine function $a$. We find that in order to make the {\it HNI} model compatible with the BICEP2 observations, we require a large positive running of the spectra. We find that this could over-produce PBHs in the most theoretically consistent case of the model. This situation could be aleviated if, as recently argued, the BICEP2 data do not correspond to primordial gravitational waves.

Near optimal bispectrum estimators for large-scale structure

Clustering of large-scale structure provides significant cosmological information through the power spectrum of density perturbations. Additional information can be gained from higher-order statistics like the bispectrum, especially to break the degeneracy between the linear halo bias $b_1$ and the amplitude of fluctuations $\sigma_8$. We propose new simple, computationally inexpensive bispectrum statistics that are near optimal for the specific applications like bias determination. Corresponding to the Legendre decomposition of nonlinear halo bias and gravitational coupling at second order, these statistics are given by the cross-spectra of the density with three quadratic fields: the squared density, a tidal term, and a shift term. For halos and galaxies the first two have associated nonlinear bias terms $b_2$ and $b_{s^2}$, respectively, while the shift term has none in the absence of velocity bias (valid in the $k \rightarrow 0$ limit). Thus the linear bias $b_1$ is best determined by the shift cross-spectrum, while the squared density and tidal cross-spectra mostly tighten constraints on $b_2$ and $b_{s^2}$ once $b_1$ is known. Since the form of the cross-spectra is derived from optimal maximum-likelihood estimation, they contain the full bispectrum information on bias parameters. Perturbative analytical predictions for their expectation values and covariances agree with simulations on large scales, $k\lesssim 0.09h/\mathrm{Mpc}$ at $z=0.55$ with Gaussian $R=20h^{-1}\mathrm{Mpc}$ smoothing, for matter-matter-matter, and matter-matter-halo combinations. For halo-halo-halo cross-spectra the model also needs to include corrections to the Poisson stochasticity.

Apparent horizon and gravitational thermodynamics of the Universe: The temperature confusion, first and second laws, and extensions to modified gravity [Cross-Listing]

The thermodynamics of the Universe is re-studied by requiring its compatibility with the holographic-style gravitational equations which govern the dynamics of both the cosmological apparent horizon and the entire Universe. We start from the Lambda Cold Dark Matter ($\Lambda$CDM) cosmology of general relativity (GR) to establish a framework for the gravitational thermodynamics. The Clausius equation $T_AdS_A=-A_A \psi_t$ for the isochoric process of an instantaneous apparent horizon indicates that, the Universe and its horizon entropies encode the Positive Out thermodynamic sign convention, which encourage us to adjust the traditional positive-heat-in Gibbs equation into the positive-heat-out version $dE_m=-T_mdS_m-P_mdV$. It turns out that the standard and the generalized second laws (GSLs) of nondecreasing entropies are always respected by the event-horizon system as long as the expanding Universe is dominated by nonexotic matter $-1\leq w_m\leq 1$, while for the apparent-horizon simple open system the two second laws hold if $-1\leq w_m<-1/3$; also, the artificial local equilibrium assumption is abandoned in the GSL. All constraints regarding entropy evolution are expressed by the equation of state parameters, which show that from a thermodynamic perspective the phantom dark energy is less favored than the cosmological constant and the quintessence. Finally, the whole framework is extended from GR and $\Lambda$CDM to modified gravities with field equations $R_{\mu\nu}-Rg_{\mu\nu}/2=8\pi G_{\text{eff}} T_{\mu\nu}^{\text{(eff)}}$. Furthermore, this paper also proposes a solution to the \textit{temperature confusion} and argues that the Cai-Kim temperature is more suitable than Hayward-Kodama, and both temperatures are independent of the inner or outer trappedness of the cosmological apparent horizon.

Apparent horizon and gravitational thermodynamics of the Universe: The temperature confusion, first and second laws, and extensions to modified gravity

The thermodynamics of the Universe is re-studied by requiring its compatibility with the holographic-style gravitational equations which govern the dynamics of both the cosmological apparent horizon and the entire Universe. We start from the Lambda Cold Dark Matter ($\Lambda$CDM) cosmology of general relativity (GR) to establish a framework for the gravitational thermodynamics. The Clausius equation $T_AdS_A=-A_A \psi_t$ for the isochoric process of an instantaneous apparent horizon indicates that, the Universe and its horizon entropies encode the Positive Out thermodynamic sign convention, which encourage us to adjust the traditional positive-heat-in Gibbs equation into the positive-heat-out version $dE_m=-T_mdS_m-P_mdV$. It turns out that the standard and the generalized second laws (GSLs) of nondecreasing entropies are always respected by the event-horizon system as long as the expanding Universe is dominated by nonexotic matter $-1\leq w_m\leq 1$, while for the apparent-horizon simple open system the two second laws hold if $-1\leq w_m<-1/3$; also, the artificial local equilibrium assumption is abandoned in the GSL. All constraints regarding entropy evolution are expressed by the equation of state parameters, which show that from a thermodynamic perspective the phantom dark energy is less favored than the cosmological constant and the quintessence. Finally, the whole framework is extended from GR and $\Lambda$CDM to modified gravities with field equations $R_{\mu\nu}-Rg_{\mu\nu}/2=8\pi G_{\text{eff}} T_{\mu\nu}^{\text{(eff)}}$. Furthermore, this paper also proposes a solution to the \textit{temperature confusion} and argues that the Cai-Kim temperature is more suitable than Hayward-Kodama, and both temperatures are independent of the inner or outer trappedness of the cosmological apparent horizon.

Morphology parameters: substructure identification in X-ray galaxy clusters

In recent years multi-wavelength observations have shown the presence of substructures related to merging events in a high fraction of galaxy clusters. Clusters can be roughly grouped into two categories — relaxed and non-relaxed — and a proper characterisation of the dynamical state of these systems is of crucial importance both for astrophysical and cosmological studies. In this paper we investigate the use of a number of morphological parameters (Gini, $M_{20}$, Concentration, Asymmetry, Smoothness, Ellipticity and Gini of the second order moment, $G_{M}$) introduced to automatically classify clusters as relaxed or dynamically disturbed systems. We apply our method to a sample of clusters at different redshifts extracted from the {\it Chandra} archive and we investigate possible correlations between morphological parameters and other X-ray gas properties. We conclude that a combination of the adopted parameters is a very useful tool to properly characterise the X-ray cluster morphology. According to our results three parameters — Gini, $M_{20}$ and Concentration — are very promising for identifying cluster mergers. The Gini coefficient is a particularly powerful tool, especially at high redshift, being independent from the choice of the position of the cluster centre. We find that high Gini ($>$ 0.65), high Concentration ($>$ 1.55) and low $M_{20}$ ($<$ -2.0) values are associated with relaxed clusters, while low Gini ($<$ 0.4), low Concentration ($<$ 1.0) and high $M_{20}$ ($>$ -1.4) characterise dynamically perturbed systems. We also estimate the X-ray cluster morphological parameters in the case of {\it radio loud} clusters. In excellent agreement with previous analyses we confirm that diffuse intracluster radio sources are associated with major mergers.

Predicted properties of multiple images of the strongly lensed supernova SN Refsdal

We construct a mass model of the cluster MACS J1149.6+2223 to study the expected properties of SN Refsdal, the first example of a gravitationally lensed supernova with resolved multiple images recently reported by Kelly et al. We find that the best-fit model predicts six supernova images in total, i.e., two extra images in addition to the observed four Einstein cross supernova images S1–S4. One extra image is predicted to have appeared about 17 years ago, whereas the other extra image is predicted to appear in about one year from the appearance of S1–S4, which is a testable prediction with near future observations. The predicted magnification factors of individual supernova images range from $\sim 18$ for the brightest image to $\sim 4$ for the faint extra images. Confronting these predictions with future observations should provide an unprecedented opportunity to improve our understanding of cluster mass distributions.

String Theory clues for the low-$\ell$ CMB ?

"Brane Supersymmetry Breaking" is a peculiar string-scale mechanism that can unpair Bose and Fermi excitations in orientifold models. It results from the simultaneous presence, in the vacuum, of collections of D-branes and orientifolds that are not mutually BPS, and is closely tied to the scale of string excitations. It also leaves behind, for a mixing of dilaton and internal breathing mode, an exponential potential that is just too steep for a scalar to emerge from the initial singularity while descending it. As a result, in this class of models the scalar can generically bounce off the exponential wall, and this dynamics brings along, in the power spectrum, an infrared depression typically followed by a pre-inflationary peak. We elaborate on a possible link between this type of bounce and the low-$\ell$ end of the CMB angular power spectrum. For the first 32 multipoles, one can reach a 50 % reduction in $\chi^{\,2}$ with respect to the standard $\Lambda$CDM setting.

 

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