arXiv subjects

Levon Pogosian

Publications and source records attributed to Levon Pogosian.

At least 55 records · Page 3Linked to original sources

arXiv2016

What can Cosmology tell us about Gravity? Constraining Horndeski with Sigma and Mu

Phenomenological functions $Σ$ and $μ$ (also known as $G_{\rm light}/G$ and $G_{\rm matter}/G$) are commonly used to parameterize possible modifications of the Poisson equation relating the matter density contrast to the lensing and the Newtonian potentials, respectively. They will be well constrained by future surveys of large scale structure. But what would the implications of measuring particular values of these functions be for modified gravity theories? We ask this question in the context of general Horndeski class of single field scalar-tensor theories with second order equations of motion. We find several consistency conditions that make it possible to rule out broad classes of theories based on measurements of $Σ$ and $μ$ that are independent of their parametric forms. For instance, a measurement of $Σ\ne 1$ would rule out all models with a canonical form of kinetic energy, while finding $Σ-1$ and $μ-1$ to be of opposite sign would strongly disfavour the entire class of Horndeski models. We separately examine the large and the small scale limits, the possibility of scale-dependence, and the consistency with bounds on the speed of gravitational waves. We identify sub-classes of Horndeski theories that can be ruled out based on the measured difference between $Σ$ and $μ$.

astro-ph.CO↗

arXiv2016

Constraints on Primordial Magnetic Fields from Planck combined with the South Pole Telescope CMB B-mode polarization measurements

A primordial magnetic field (PMF) present before recombination can leave specific signatures on the cosmic microwave background (CMB) fluctuations. Of particular importance is its contribution to the B-mode polarization power spectrum. Indeed, vortical modes sourced by the PMF can dominate the B-mode power spectrum on small scales, as they survive damping up to a small fraction of the Silk length. Therefore, measurements of the B-mode polarization at high-$\ell$ , such as the one recently performed by the South Pole Telescope (SPT), have the potential to provide stringent constraints on the PMF. We use the publicly released SPT B-mode polarization spectrum, along with the temperature and polarization data from the Planck satellite, to derive constraints on the magnitude, the spectral index and the energy scale at which the PMF was generated. We find that, while Planck data constrains the magnetic amplitude to $B_{1 \, \text{Mpc}} < 3.3$ nG at 95\% confidence level (CL), the SPT measurement improves the constraint to $B_{1 \, \text{Mpc}} < 1.5$ nG. The magnetic spectral index, $n_B$, and the time of the generation of the PMF are unconstrained. For a nearly scale-invariant PMF, predicted by simplest inflationary magnetogenesis models, the bound from Planck+SPT is $B_{1 \, \text{Mpc}} < 1.2$ nG at 95% CL. For PMF with $n_B=2$, expected for fields generated in post-inflationary phase transitions, the 95% CL bound is $B_{1 \, \text{Mpc}} < 0.002$ nG, corresponding to the magnetic fraction of the radiation density $Ω_{Bγ} < 10^{-3}$ or the effective field $B_{\rm eff} < 100$ nG. The patches for the Boltzmann code CAMB and the Markov Chain Monte Carlo engine CosmoMC, incorporating the PMF effects on CMB, are made publicly available.

astro-ph.CO↗

arXiv2016

CMB-S4 Science Book, First Edition

This book lays out the scientific goals to be addressed by the next-generation ground-based cosmic microwave background experiment, CMB-S4, envisioned to consist of dedicated telescopes at the South Pole, the high Chilean Atacama plateau and possibly a northern hemisphere site, all equipped with new superconducting cameras. CMB-S4 will dramatically advance cosmological studies by crossing critical thresholds in the search for the B-mode polarization signature of primordial gravitational waves, in the determination of the number and masses of the neutrinos, in the search for evidence of new light relics, in constraining the nature of dark energy, and in testing general relativity on large scales.

astro-ph.CO↗

arXiv2016

The extended Baryon Oscillation Spectroscopic Survey (eBOSS): a cosmological forecast

We present a science forecast for the eBOSS survey, part of the SDSS-IV project, which is a spectroscopic survey using multiple tracers of large-scale structure, including luminous red galaxies (LRGs), emission line galaxies (ELGs) and quasars (both as a direct probe of structure and through the Ly-$α$ forest). Focusing on discrete tracers, we forecast the expected accuracy of the baryonic acoustic oscillation (BAO), the redshift-space distortion (RSD) measurements, the $f_{\rm NL}$ parameter quantifying the primordial non-Gaussianity, the dark energy and modified gravity parameters. We also use the line-of-sight clustering in the Ly-$α$ forest to constrain the total neutrino mass. We find that eBOSS LRGs ($0.6 0.6$), ELGs ($0.6<z<1.2$) and Clustering Quasars (CQs) ($0.6<z<2.2$) can achieve a precision of 1%, 2.2% and 1.6% precisions, respectively, for spherically averaged BAO distance measurements. Using the same samples, the constraint on $fσ_8$ is expected to be 2.5%, 3.3% and 2.8% respectively. For primordial non-Gaussianity, eBOSS alone can reach an accuracy of $σ(f_{\rm NL})\sim10-15$, depending on the external measurement of the galaxy bias and our ability to model large-scale systematic errors. eBOSS can at most improve the dark energy Figure of Merit (FoM) by a factor of $3$ for the Chevallier-Polarski-Linder (CPL) parametrisation, and can well constrain three eigenmodes for the general equation-of-state parameter (Abridged).

astro-ph.CO↗

arXiv2016

POLARBEAR Constraints on Cosmic Birefringence and Primordial Magnetic Fields

We constrain anisotropic cosmic birefringence using four-point correlations of even-parity $E$-mode and odd-parity $B$-mode polarization in the cosmic microwave background measurements made by the POLARization of the Background Radiation (POLARBEAR) experiment in its first season of observations. We find that the anisotropic cosmic birefringence signal from any parity-violating processes is consistent with zero. The Faraday rotation from anisotropic cosmic birefringence can be compared with the equivalent quantity generated by primordial magnetic fields if they existed. The POLARBEAR nondetection translates into a 95% confidence level (C.L.) upper limit of 93 nanogauss (nG) on the amplitude of an equivalent primordial magnetic field inclusive of systematic uncertainties. This four-point correlation constraint on Faraday rotation is about 15 times tighter than the upper limit of 1380 nG inferred from constraining the contribution of Faraday rotation to two-point correlations of $B$-modes measured by Planck in 2015. Metric perturbations sourced by primordial magnetic fields would also contribute to the $B$-mode power spectrum. Using the POLARBEAR measurements of the $B$-mode power spectrum (two-point correlation), we set a 95% C.L. upper limit of 3.9 nG on primordial magnetic fields assuming a flat prior on the field amplitude. This limit is comparable to what was found in the Planck 2015 two-point correlation analysis with both temperature and polarization. We perform a set of systematic error tests and find no evidence for contamination. This work marks the first time that anisotropic cosmic birefringence or primordial magnetic fields have been constrained from the ground at subdegree scales.

astro-ph.CO↗

arXiv2015

Reconstruction of the dark matter-vacuum energy interaction

An interaction between the vacuum energy and dark matter is an intriguing possibility which may offer a way of solving the cosmological constant problem. Adopting a general prescription for momentum exchange between the two dark components, we reconstruct $α(a)$, the temporal evolution of the coupling strength between dark matter and vacuum energy, in a nonparametric Bayesian approach using combined observational data sets from the cosmic microwave background, supernovae and large scale structure. An evolving interaction between the vacuum energy and dark matter removes some of the tensions between different data sets. However, it is not preferred over $Λ$CDM in the Bayesian sense, as improvement in the fit is not sufficient to compensate for the increase in the volume of the parameter space.

astro-ph.CO↗

arXiv2015

Searching for scalar gravitational interactions in current and future cosmological data

Modified gravity theories often contain a scalar field of gravitational strength which interacts with matter. We examine constraints on the range and the coupling strength of a scalar gravitational degree of freedom using a subset of current data that can be safely analyzed within the linear perturbation theory. Using a model-independent implementation of scalar-tensor theories in MGCAMB in terms of two functions of the scale factor describing the mass and the coupling of the scalar degree of freedom, we derive constraints on the $f(R)$, generalized chameleon, Symmetron and Dilaton models. Since most of the large scale structure data available today is from relatively low redshifts, only a limited range of observed scales is in the linear regime, leading to relatively weak constraints. We then perform a forecast for a future large scale structure survey, such as Large Synoptic Survey Telescope (LSST), which will map a significant volume at higher redshifts, and show that it will produce much stronger constraints on scalar interactions in specific models. We also perform a principal component analysis and find that future surveys should be able to provide tight constraints on several eigenmodes of the scalar mass evolution.

astro-ph.CO↗

arXiv2015

Magnetic monopole - domain wall collisions

Interactions of different types of topological defects can play an important role in the aftermath of a phase transition. We study interactions of fundamental magnetic monopoles and stable domain walls in a Grand Unified theory in which $SU(5) \times Z_2$ symmetry is spontaneously broken to $SU(3)\times SU(2)\times U(1)/Z_6$. We find that there are only two distinct outcomes depending on the relative orientation of the monopole and the wall in internal space. In one case, the monopole passes through the wall, while in the other it unwinds on hitting the wall.

hep-th↗

arXiv2015

Cosmic Strings

This article, written for Scolarpedia, provides a brief introduction into the subject of cosmic strings, together with a review of their main properties, cosmological evolution and observational signatures.

astro-ph.CO↗

arXiv2014

Domain walls coupled to matter: the symmetron example

We study properties of domain walls in the symmetron model, in which the scalar gravitational degree of freedom decouples from matter in regions of high density, and exhibits a spontaneously broken $Z_2$ symmetry at low densities. The non-minimal coupling of the scalar to matter leads to a host of interesting properties of the domain walls that are not present in minimally coupled theories. We estimate the cosmological energy fraction in domain walls and find that this leads to an upper bound on the redshift of the symmetry breaking. We also show that a spherical symmetron wall can remain stable if it is "pinned" on matter halos and derive a criterion for the stability. In addition, we present results of numerical simulations of representative interactions between domain walls and matter over-densities.

astro-ph.CO↗

arXiv2014

Did BICEP2 see vector modes? First B-mode constraints on cosmic defects

Scaling networks of cosmic defects, such as strings and textures, actively generate scalar, vector and tensor metric perturbations throughout the history of the universe. In particular, {\em vector} modes sourced by defects are an efficient source of the CMB B-mode polarization. We use the recently released BICEP2 and POLARBEAR B-mode polarization spectra to constrain properties of a wide range of different types of cosmic strings networks. We find that in order for strings to provide a satisfactory fit on their own, the effective inter-string distance needs to be extremely large -- spectra that fit the data best are more representative of global strings and textures. When a local string contribution is considered together with the inflationary B-mode spectrum, the fit is improved. We discuss implications of these results for theories that predict cosmic defects.

astro-ph.CO↗

arXiv2014

Observable physical modes of modified gravity

At linear order in cosmological perturbations, departures from the growth in the cosmological standard model can be quantified in terms of two functions of redshift $z$ and Fourier number $k$. Previous studies have performed principal component forecasts for several choices of these two functions, based on expected capabilities of upcoming large structure surveys. It is typically found that there will be many well-constrained degrees of freedom. However, not all and, probably most, of these degrees of freedom were physical if the parametrization had allowed for an arbitrary $k$-dependence. In this paper, we restrict the $k$-dependence to that allowed in local theories of gravity under the quasi-static approximation, i.e. ratios of polynomials in $k$, and identify the best constrained features in the ($z$,$k$)-dependence of the commonly considered functions $μ$ and $γ$ as measured by an LSST-like weak lensing survey. We estimate the uncertainty in the measurements of the eigenmodes of modified growth. We find that imposing the theoretical prior on $k$-dependence reduces the number of degrees of freedom and the covariance between parameters. On the other hand, imaging surveys like LSST are not as sensitive to the $z$-dependence as they are to the $k$-dependence of the modified growth functions. This trade off provides us with, more or less, the same number of well-constrained eigenmodes (with respect to our prior) as found before, but now these modes are physical.

astro-ph.CO↗

arXiv2013

Searching for primordial magnetism with multi-frequency CMB experiments

Bounds on the amplitude of a scale-invariant stochastic primordial magnetic field (PMF) can be significantly improved by measurements of the Faraday Rotation (FR) of CMB polarization. The mode-coupling correlations induced by FR make it possible to extract it from cross-correlations of the B-mode polarization with the E-mode and the temperature anisotropy. In this paper, we construct an estimator of the rotation measure that appropriately combines measurements of the FR from multiple frequency channels. We study the dependence of the signal-to-noise in the PMF detection on the resolution and the noise of the detectors, as well as the removal of the weak lensing contribution and the galactic FR. We show that a recently proposed space-based experiment PRISM can detect magnetic fields of 0.1 nano-Gauss strength at a 2 sigma level. Higher detection levels can be achieved by reducing the detector noise and improving the resolution or increasing the number of channels in the 30-70 GHz frequency range.

astro-ph.CO↗

arXiv2013

CMB Faraday rotation as seen through the Milky Way

Faraday Rotation (FR) of CMB polarization, as measured through mode-coupling correlations of E and B modes, can be a promising probe of a stochastic primordial magnetic field (PMF). While the existence of a PMF is still hypothetical, there will certainly be a contribution to CMB FR from the magnetic field of the Milky Way. We use existing estimates of the Milky Way rotation measure (RM) to forecast its detectability with upcoming and future CMB experiments. We find that the galactic RM will not be seen in polarization measurements by Planck, but that it will need to be accounted for by CMB experiments capable of detecting the weak lensing contribution to the B-mode. We then discuss prospects for constraining the PMF in the presence of FR due to the galaxy under various assumptions that include partial de-lensing and partial subtraction of the galactic FR. We find that a realistic future sub-orbital experiment, covering a patch of the sky near the galactic poles, can detect a scale-invariant PMF of 0.1 nano-Gauss at better than 95% confidence level, while a dedicated space-based experiment can detect even smaller fields.

astro-ph.CO↗

arXiv2013

Principal Component Analysis of Modified Gravity using Weak Lensing and Peculiar Velocity Measurements

We perform a principal component analysis to assess ability of future observations to measure departures from General Relativity in predictions of the Poisson and anisotropy equations on linear scales. In particular, we focus on how the measurements of redshift-space distortions (RSD) observed from spectroscopic galaxy redshift surveys will improve the constraints when combined with lensing tomographic surveys. Assuming a Euclid-like galaxy imaging and redshift survey, we find that adding the 3D information decreases the statistical uncertainty by a factor between 3 and 10 compared to the case when only observables from lensing tomographic surveys are used. We also find that the number of well-constrained modes increases by a factor between 3 and 7. Our study indicates the importance of joint galaxy imaging and redshift surveys such as SuMIRe and Euclid to give more stringent tests of the ΛCDM model and to distinguish between various modified gravity and dark energy models.

astro-ph.CO↗

arXiv2013

A practical approach to cosmological perturbations in modified gravity

The next generation of large scale surveys will not only measure cosmological parameters within the framework of General Relativity, but will also allow for precision tests of the framework itself. At the order of linear perturbations, departures from the growth in the LCDM model can be quantified in terms of two functions of time and Fourier number k. We argue that in local theories of gravity, in the quasi-static approximation, these functions must be ratios of polynomials in k, with the numerator of one function being equal to the denominator of the other. Moreover, the polynomials are even and of second degree in practically all viable models considered today. This means that, without significant loss of generality, one can use data to constraint only five functions of a single variable, instead of two functions of two variables. Furthermore, since the five functions are expected to be slowly varying, one can fit them to data in a non-parametric way with the aid of an explicit smoothness prior. We discuss practical application of this parametrization to forecasts and fits.

astro-ph.CO↗

arXiv2013

Probing Primordial Magnetism with Off-Diagonal Correlators of CMB Polarization

Primordial magnetic fields (PMF) can create polarization $B$-modes in the cosmic microwave background (CMB) through Faraday rotation (FR), leading to non-trivial 2-point and 4-point correlators of the CMB temperature and polarization. We discuss the detectability of primordial magnetic fields using different correlators and evaluate their relative merits. We have fully accounted for the contamination by weak lensing, which contributes to the variance, but whose contribution to the 4-point correlations is orthogonal to that of FR. We show that a Planck-like experiment can detect scale-invariant PMF of nG strength using the FR diagnostic at 90GHz, while realistic future experiments at the same frequency can detect 10^{-10} G. Utilizing multiple frequencies will improve on these prospects, making FR of CMB a powerful probe of scale-invariant PMF.

astro-ph.CO↗

arXiv2012

Practical solutions for perturbed f(R) gravity

We examine the accuracy of the quasi-static approximation and a parametric form commonly employed for evolving linear perturbations in f(R) gravity theories and placing cosmological constraints. In particular, we analyze the nature and the importance of the near horizon effects that are often neglected. We find that for viable models, with a small present value of the scalaron field, such corrections are entirely negligible with no impact on observables. We also find that the one-parameter form, commonly used to represent the modified equations for linear perturbations in f(R), leads to theoretical systematic errors that are relatively small and, therefore, is adequate for placing constraint on f(R) models.

astro-ph.CO↗