arXiv subjects

G. D. Starkman

Publications and source records attributed to G. D. Starkman.

15 recordsLinked to original sources

arXiv2021

The effects of massive neutrinos on the linear point of the correlation function

The linear point (LP), defined as the mid-point between the dip and the peak of the two-point clustering correlation function (TPCF), has been shown to be an excellent standard ruler for cosmology. In fact, it is nearly redshift-independent, being weakly sensitive to non-linearities, scale-dependent halo bias and redshift-space distortions. So far, these findings were tested assuming that neutrinos are massless; in this paper we extend the analysis to massive-neutrino cosmologies. In particular, we examine if the scale-dependent growth induced by neutrinos affects the LP position and if it is possible to detect the neutrino masses using the shift of the LP compared to the massless-neutrino case. For our purposes, we employ two sets of state-of-the-art $N$-body simulations with massive neutrinos. For each of them we measure the TPCF of cold dark matter (CDM) and halos and, to estimate the LP, fit the TPCF with a model-independent parametric fit in the range of scales of the Baryon Acoustic Oscillations (BAO). Overall, we find that the LP retains its features as a standard ruler even when neutrinos are massive. The cosmic distances measured with the LP can therefore be employed to constrain the neutrino mass.

astro-ph.CO↗

arXiv2018

Exploring suppressed long-distance correlations as the cause of suppressed large-angle correlations

The absence of large-angle correlations in the map of cosmic microwave background temperature fluctuations is among the well-established anomalies identified in full-sky and cut-sky maps over the past three decades. Suppressed large-angle correlations are rare statistical flukes in standard inflationary cosmological models. One natural explanation could be that the underlying primordial density perturbations lack correlations on large distance scales. To test this idea, we replace Fourier modes by a wavelet basis with compact spatial support. While the angular correlation function of perturbations can readily be suppressed, the observed monopole and dipole-subtracted correlation function is not generally suppressed. This suggests that suppression of large-angle temperature correlations requires a mechanism that has both real-space and harmonic-space effects.

astro-ph.CO↗

arXiv2013

Probing Large-Angle Correlations with the Microwave Background Temperature and Lensing Cross Correlation

A lack of correlations in the microwave background temperature between sky directions separated by angles larger than 60 degrees has recently been confirmed by data from the Planck satellite. This feature arises as a random occurrence within the standard LCDM cosmological model less than 0.3 per cent of the time, but so far no other compelling theory to explain this observation has been proposed. Here we investigate the theoretical cross-correlation function between microwave background temperature and the gravitational lensing potential of the microwave background, which in contrast to the temperature correlation function depends strongly on gravitational potential fluctuations interior to our Hubble volume. For standard LCDM cosmology, we generate random sky realizations of the microwave temperature and gravitational lensing, subject to the constraint that the temperature correlation function matches observations, and compare with random skies lacking this constraint. The distribution of large-angle temperature-lensing correlation functions in these two cases is different, and the two cases can be clearly distinguished in around 40 per cent of model realizations. We present an a priori procedure for using similar large-angle correlations between other types of data, to determine whether the lack of large-angle correlations is a statistical fluke or points to a shortcoming of the standard cosmological model.

astro-ph.CO↗

arXiv2013

Large-Angle CMB Suppression and Polarization Predictions

The anomalous lack of large angle temperature correlations has been a surprising feature of the cosmic microwave background (CMB) since first observed by COBE-DMR and subsequently confirmed and strengthened by the Wilkinson Microwave Anisotropy Probe. This anomaly may point to the need for modifications of the standard model of cosmology or may indicate that our Universe is a rare statistical fluctuation within that model. Further observations of the temperature auto-correlation function will not elucidate the issue; sufficiently high precision statistical observations already exist. Instead, alternative probes are required. In this work we explore the expectations for forthcoming polarization observations. We define a prescription to test the hypothesis that the large-angle CMB temperature perturbations in our Universe represent a rare statistical fluctuation within the standard cosmological model. These tests are based on the temperature-$Q$ Stokes parameter correlation. Unfortunately these tests cannot be expected to be definitive. However, we do show that if this $TQ$-correlation is observed to be sufficiently large over an appropriately chosen angular range, then the hypothesis can be rejected at a high confidence level. We quantify these statements and optimize the statistics we have constructed to apply to the anticipated polarization data. We find that we can construct a statistic that has a 25 per cent chance of excluding the hypothesis that we live in a rare realization of LCDM at the 99.9 per cent confidence level.

astro-ph.CO↗

arXiv2013

Effect of Our Galaxy's Motion on Weak Lensing Measurements of Shear and Convergence

In this work we investigate the effect on weak-lensing shear and convergence measurements due to distortions from the Lorentz boost induced by our Galaxy's motion. While no ellipticity is induced in an image from the Lorentz boost to first order in beta = v/c, the image is magnified. This affects the inferred convergence at a 10 per cent level, and is most notable for low multipoles in the convergence power spectrum C κκ and for surveys with large sky coverage like LSST and DES. Experiments which image only small fractions of the sky and convergence power spectrum determinations at l > 5 can safely neglect the boost effect to first order in beta.

astro-ph.CO↗

arXiv2011

Improved constraints on cosmological parameters from SNIa data

We present a new method based on a Bayesian hierarchical model to extract constraints on cosmological parameters from SNIa data obtained with the SALT-II lightcurve fitter. We demonstrate with simulated data sets that our method delivers tighter statistical constraints on the cosmological parameters over 90% of the time, that it reduces statistical bias typically by a factor ~ 2-3 and that it has better coverage properties than the usual chi-squared approach. As a further benefit, a full posterior probability distribution for the dispersion of the intrinsic magnitude of SNe is obtained. We apply this method to recent SNIa data, and by combining them with CMB and BAO data we obtain Omega_m=0.28 +/- 0.02, Omega_Lambda=0.73 +/- 0.01 (assuming w=-1) and Omega_m=0.28 +/- 0.01, w=-0.90 +/- 0.05 (assuming flatness; statistical uncertainties only). We constrain the intrinsic dispersion of the B-band magnitude of the SNIa population, obtaining sigma_mu^int = 0.13 +/- 0.01 [mag]. Applications to systematic uncertainties will be discussed in a forthcoming paper.

astro-ph.CO↗

arXiv2010

Should we doubt the cosmological constant?

While Bayesian model selection is a useful tool to discriminate between competing cosmological models, it only gives a relative rather than an absolute measure of how good a model is. Bayesian doubt introduces an unknown benchmark model against which the known models are compared, thereby obtaining an absolute measure of model performance in a Bayesian framework. We apply this new methodology to the problem of the dark energy equation of state, comparing an absolute upper bound on the Bayesian evidence for a presently unknown dark energy model against a collection of known models including a flat LambdaCDM scenario. We find a strong absolute upper bound to the Bayes factor B between the unknown model and LambdaCDM, giving B < 3. The posterior probability for doubt is found to be less than 6% (with a 1% prior doubt) while the probability for LambdaCDM rises from an initial 25% to just over 50% in light of the data. We conclude that LambdaCDM remains a sufficient phenomenological description of currently available observations and that there is little statistical room for model improvement.

astro-ph.CO↗

arXiv2010

Vector field models of modified gravity and the dark sector

We present a comprehensive investigation of cosmological constraints on the class of vector field formulations of modified gravity called Generalized Einstein-Aether models. Using linear perturbation theory we generate cosmic microwave background and large-scale structure spectra for general parameters of the theory, and then constrain them in various ways. We investigate two parameter regimes: a dark-matter candidate where the vector field sources structure formation, and a dark-energy candidate where it causes late-time acceleration. We find that the dark matter candidate does not fit the data, and identify five physical problems that can restrict this and other theories of dark matter. The dark energy candidate does fit the data, and we constrain its fundamental parameters; most notably we find that the theory's kinetic index parameter $n_{\mathrm{ae}}$ can differ significantly from its $Λ$CDM value.

astro-ph.CO↗

arXiv2007

On the growth of structure in theories with a dynamical preferred frame

We study the cosmological stability of a class of theories with a dynamical preferred frame. For a range of actions, we find cosmological solutions which are compatible with observations of the recent history of the Universe: a matter dominated era followed by accelerated expansion. We then study the evolution of linear perturbations on these backgrounds and find conditions on the parameters of the theory which allow for the growth of structure sourced by the new degrees of freedom.

astro-ph↗

arXiv2005

On the large-angle anomalies of the microwave sky

[Abridged] We apply the multipole vector framework to full-sky maps derived from the first year WMAP data. We significantly extend our earlier work showing that the two lowest cosmologically interesting multipoles, l=2 and 3, are not statistically isotropic. These results are compared to the findings obtained using related methods. In particular, the planes of the quadrupole and the octopole are unexpectedly aligned. Moreover, the combined quadrupole plus octopole is surprisingly aligned with the geometry and direction of motion of the solar system: the plane they define is perpendicular to the ecliptic plane and to the plane defined by the dipole direction, and the ecliptic plane carefully separates stronger from weaker extrema, running within a couple of degrees of the null-contour between a maximum and a minimum over more than 120deg of the sky. Even given the alignment of the quadrupole and octopole with each other, we find that their alignment with the ecliptic is unlikely at >98% C.L., and argue that it is in fact unlikely at >99.9% C.L. We explore the role of foregrounds showing that the known Galactic foregrounds are unlikely to lead to these correlations. Multipole vectors, like individual a_lm, are very sensitive to sky cuts, and we demonstrate that analyses using cut skies induce relatively large errors, thus weakening the observed correlations but preserving their consistency with the full-sky results. Finally we apply our tests to COBE cut-sky maps and briefly extend the analysis to higher multipoles. If the correlations we observe are indeed a signal of non-cosmic origin, then the lack of low-l power will very likely be exacerbated, with important consequences for our understanding of cosmology on large scales.

astro-ph↗

arXiv2004

Multipole Vectors--a new representation of the CMB sky and evidence for statistical anisotropy or non-Gaussianity at 2<=l<=8

We propose a novel representation of cosmic microwave anisotropy maps, where each multipole order l is represented by l unit vectors pointing in directions on the sky and an overall magnitude. These "multipole vectors and scalars" transform as vectors under rotations. Like the usual spherical harmonics, multipole vectors form an irreducible representation of the proper rotation group SO(3). However, they are related to the familiar spherical harmonic coefficients, alm, in a nonlinear way, and are therefore sensitive to different aspects of the CMB anisotropy. Nevertheless, it is straightforward to determine the multipole vectors for a given CMB map and we present an algorithm to compute them. Using the WMAP full-sky maps, we perform several tests of the hypothesis that the CMB anisotropy is statistically isotropic and Gaussian random. We find that the result from comparing the oriented area of planes defined by these vectors between multipole pairs 2<=l1!=l2<=8 is inconsistent with the isotropic Gaussian hypothesis at the 99.4% level for the ILC map and at 98.9% level for the cleaned map of Tegmark et al. A particular correlation is suggested between the l=3 and l=8 multipoles, as well as several other pairs. This effect is entirely different from the now familiar planarity and alignment of the quadrupole and octupole: while the aforementioned is fairly unlikely, the multipole vectors indicate correlations not expected in Gaussian random skies that make them unusually likely. The result persists after accounting for pixel noise and after assuming a residual 10% dust contamination in the cleaned WMAP map. While the definitive analysis of these results will require more work, we hope that multipole vectors will become a valuable tool for various cosmological tests, in particular those of cosmic isotropy.

astro-ph↗

arXiv1995

A Warm-Plus-Hot Dark Matter Universe

We investigate a new hybrid-model universe containing two types of dark matter, one ``warm'' and the other ``hot''. The hot component is an ordinary light neutrino with mass $\sim 25h^2$~eV while the warm component is a sterile neutrino with mass $\sim 700h^2$~eV. The two types of dark matter arise entirely within the neutrino sector and do not require separate physical origins. We calculate the linear transfer functions for a representative sample of warm-plus-hot models. The transfer functions, and results from several observational tests of structure formation, are compared with those for the cold-plus-hot models that have been studied extensively in the literature. On the basis of these tests, we conclude that warm-plus-hot dark matter is essentially indistinguishable from cold-plus-hot dark matter, and therefore provides a viable scenario for large scale structure. We demonstrate that a neutrino mass matrix can be constructed which provides the requisite dark matter constituents, while remaining consistent with all cosmological bounds.

astro-ph↗

arXiv1993

Neutrino Lasing in the Sun

Applying the phenomenon of neutrino lasing in the solar interior, we show how the rate for the generic neutrino decay process `ν-> fermion + boson', can in principal be enhanced by many orders of magnitude over its normal decay rate. Such a large enhancement could be of import to neutrino-decay models invoked in response to the apparent deficit of electron neutrinos observed from the sun. The significance of this result to such models depends on the specific form of the neutrino decay, and the particle model within which it is embedded.

hep-ph↗

arXiv1993

Mixed Dark Matter from Neutrino Lasing

We investigate the production of dark matter with non-standard momentum distributions arising from the decay of a relativistic massive neutrino into a lighter fermion and boson H$\rightarrow$F$+$B. We develop the Boltzmann equations for this process, paying particular attention to spin polarisation effects. Such decays can, under certain circumstances, lead to a process of runaway stimulated decays or `neutrino-lasing' resulting, for suitable boson mass, in the production of mixed-dark-matter. We discuss a number of variations on the model; we show how the yield of cold particles can be increased by a simple modification to our original proposal. We discuss the singlet-majoron model as a possible framework for these decays.

astro-ph↗

arXiv1993

Neutrino-Lasing in The Early Universe

Recently, Madsen has argued that relativistic decays of massive neutrinos into lighter fermions and bosons may lead, via thermalization, to the formation of a Bose condensate. If correct, this could generate mixed hot and cold dark matter, with important consequences for structure formation. From a detailed study of such decays, we arrive at substantially different conclusions; for a wide range of masses and decay times, we find that stimulated emission of bosons dominates the decay. This phenomenon can best be described as a neutrino laser, pumped by the QCD phase transition. We discuss the implications for structure formation and the dark-matter problem.

hep-ph↗