arXiv subjects

Oystein Elgaroy

Publications and source records attributed to Oystein Elgaroy.

At least 19 recordsLinked to original sources

arXiv2012

A physical interpretation of Hubble's law and the cosmological redshift from the perspective of a static observer

We derive explicit and exact expressions for the physical velocity of a free particle comoving with the Hubble flow as measured by a static observer, and for the frequency shift of light emitted by a comoving source and received, again, by a static observer. The expressions make it clear that an interpretation of the redshift as a kind of Doppler effect only makes sense when the distance between the observer and the source vanishes exactly.

gr-qc↗

arXiv2011

Reactor sterile neutrinos, dark energy and the age of the universe

There are indications that the neutrino oscillation data from reactor experiments and the LSND and MiniBooNE experiments show a preference for two sterile neutrino species, both with masses in the eV region. We show that this result has a significant impact on some important cosmological parameters. Specifically, we use a combination of CMB, LSS and SN1A data and show that the existence of two light, sterile neutrinos would rule out the cosmological constant as dark energy at 95% confidence level, and lower the expansion age of the universe to 12.58 \pm 0.26 Gyr.

astro-ph.CO↗

arXiv2008

Neutrino Mass, Dark Energy, and the Linear Growth Factor

We study the degeneracies between neutrino mass and dark energy as they manifest themselves in cosmological observations. In contradiction to a popular formula in the literature, the suppression of the matter power spectrum caused by massive neutrinos is not just a function of the ratio of neutrino to total mass densities f_nu=Omega_nu/Omega_m, but also each of the densities independently. We also present a fitting formula for the logarithmic growth factor of perturbations in a flat universe, f(z, k;f_nu,w,Omega_DE)= (1-A(k)*Omega_DE*f_nu+B(k)*f_nu^2-C(k)*f_nu^3)*Omega_m(z)^alpha, where alpha depends on the dark energy equation of state parameter w. We then discuss cosmological probes where the f factor directly appears: peculiar velocities, redshift distortion and the Intergrated Sachs-Wolfe effect. We also modify the approximation of Eisenstein & Hu (1999) for the power spectrum of fluctuations in the presence of massive neutrinos and provide a revised code (http://www.star.ucl.ac.uk/~lahav/nu_matter_power.f)

astro-ph↗

arXiv2008

Perihelion precession in spherically symmetric Kaluza-Klein theories

We consider the perihelion precession and bending of light in a class of Kaluza-Klein models and show that the "electric redshift" model, proposed in Zhang (2006) to explain the redshift of Quasars, does not agree with observations. As Zhang's model only considers the Jordan frame, we also compute the perihelion precession as seen in the Einstein frame and show that, to lowest order, the perihelion precession can only be consistent with observation in the Einstein frame. However, when we consider the corresponding "electric redshift" in the Einstein frame, we find that the redshift is significantly lower than for the Jordan frame and is therefore very unlikely to explain the redshift of quasars.

astro-ph↗

arXiv2008

Detection of transplanckian effects in the cosmic microwave background

Quantum gravity effects are expected to modify the primordial density fluctuations produced during inflation and leave their imprint on the cosmic microwave background observed today. We present a new analysis discussing whether these effects are detectable, considering both currently available data and simulated results from an optimal CMB experiment. We find that the WMAP (Wilkinson Microwave Anisotropy Probe) data show no evidence for the particular signature considered in this work but give an upper bound on the parameters of the model. However, a hypothetical experiment shows that with proper data, the trans-Planckian effects should be detectable through alternate sampling methods. This fuzzy conclusion is a result of the nature of the oscillations, since they give rise to a likelihood hypersurface riddled with local maxima. A simple Bayesian analysis shows no significant evidence for the simulated data to prefer a trans-Planckian model. Conventional Markov chain Monte Carlo (MCMC) methods are not suitable for exploring this complicated landscape, but alternative methods are required to solve the problem. This, however, requires extremely high-precision data.

astro-ph↗

arXiv2008

Cosmological implications of the KATRIN experiment

The upcoming Karlsruhe Tritium Neutrino (KATRIN) experiment will put unprecedented constraints on the absolute mass of the electron neutrino, $\mnue$. In this paper we investigate how this information on $\mnue$ will affect our constraints on cosmological parameters. We consider two scenarios; one where $\mnue=0$ (i.e., no detection by KATRIN), and one where $\mnue=0.3$eV. We find that the constraints on $\mnue$ from KATRIN will affect estimates of some important cosmological parameters significantly. For example, the significance of $n_s<1$ and the inferred value of $Ω_Λ$ depend on the results from the KATRIN experiment.

astro-ph↗

arXiv2007

Using the cluster mass function from weak lensing to constrain neutrino masses

We discuss the variation of cosmological upper bounds on $M_ν$, the sum of the neutrino masses, with the choice of data sets included in the analysis, pointing out a few oddities not easily seen when all data sets are combined. For example, the effect of applying different priors varies significantly depending on whether we use the power spectrum from the 2dFGRS or SDSS galaxy survey. A conservative neutrino mass limit of $M_ν< 1.43$eV (95%C.L.) is obtained by combining the WMAP 3 year data with the cluster mass function measured by weak gravitational lensing. This limit has the virtue of not making any assumptions about the bias of luminous matter with respect to the dark matter, and is in this sense (and this sense only) bias-free.

astro-ph↗

arXiv2007

On using the CMB shift parameter in tests of models of dark energy

The so-called shift parameter is related to the position of the first acoustic peak in the power spectrum anisotropies of the cosmic microwave background (CMB) anisotropies. It is an often used quantity in simple tests of dark energy models. However, the shift parameter is not directly measurable from the cosmic microwave background, and its value is usually derived from the data assuming a spatially flat cosmology with dark matter and a cosmological constant. Our aim in this paper is to evaluate the effectiveness of the shift parameter as a constraint on dark energy models, and the potential pitfalls in using it as a test of non-standard models of dark energy. By comparing to full CMB fits, we show that combining the shift parameter with the position of the first acoustic peak in the CMB power spectrum improves the accuracy of the test considerably.

astro-ph↗

arXiv2006

Revised WMAP constraints on neutrino masses and other extensions of the minimal $Λ$CDM model

Recently, two issues concerning the three-year WMAP likelihood code were pointed out. On large angular scales ($l \lesssim 30$), a sub-optimal likelihood approximation resulted in a small power excess. On small angular scales ($l \gtrsim 300$), over-subtraction of unresolved point sources produced a small power deficit. For a minimal six-parameter cosmological model, these two effects conspired to decrease the value of $n_s$ by $\sim 0.7 σ$. In this paper, we study the change in preferred parameter ranges for more extensive cosmological models, including running of $n_s$, massive neutrinos, curvature, and the equation of state for dark energy. We also include large-scale structure and supernova data in our analysis. We find that the parameter ranges for $α_s$, $Ω_k$ and $w$ are not much altered by the modified analysis. For massive neutrinos the upper limit on the sum of the neutrino masses decreases from $M_ν< 1.90$eV to $M_ν< 1.57$eV when using the modified WMAP code and WMAP data only. We also find that the shift of $n_s$ to higher values is quite robust to these extensions of the minimal cosmological model.

astro-ph↗

arXiv2006

Upper limits on neutrino masses from cosmology

Upper limits on neutrino masses from cosmology have been reported recently to reach the impressive sub-eV level, which is competitive with future terrestrial neutrino experiments. In this brief overview of the latest limits from cosmology I point out some of the caveats that should be borne in mind when interpreting the significance of these limits.

hep-ph↗

arXiv2006

Is space expanding in the Friedmann universe models?

The interpretation of the expanding universe as an expansion of space has recently been challenged. From the geodesic equation in Friedmann universe models and the empty Milne model, we argue that a Newtonian or special relativistic analysis is not applicable on large scales, and the general relativistic interpretation in terms of expanding space has the advantage of being globally consistent. We also show that the cosmic redshift, interpreted as an expansion effect, containts both the Doppler effect and the gravitational frequency shift.

astro-ph↗

arXiv2006

Sub-eV upper limits on neutrino masses from cosmology

Upper limits on neutrino masses from cosmology have been reported recently to reach the impressive sub-eV level, which is competitive with future terrestrial neutrino experiments. In this brief review of the latest limits from cosmology we point out some of the caveats that should be borne in mind when interpreting the significance of these limits.

hep-ph↗

arXiv2006

Bayesian analysis of Friedmannless cosmologies

Assuming only a homogeneous and isotropic universe and using both the 'Gold' Supernova Type Ia sample of Riess et al. and the results from the Supernova Legacy Survey, we calculate the Bayesian evidence of a range of different parameterizations of the deceleration parameter. We consider both spatially flat and curved models. Our results show that although there is strong evidence in the data for an accelerating universe, there is little evidence that the deceleration parameter varies with redshift.

astro-ph↗

arXiv2005

Neutrino Masses from Cosmological Probes

There is a renewed interest in constraining the sum of the masses of the three neutrino flavours by using cosmological measurements. Solar, atmospheric, and reactor neutrino experiments have confirmed neutrino oscillations, implying that neutrinos have non-zero mass, but without pinning down their absolute masses. While it is established that the effect of light neutrinos on the evolution of cosmic structure is small, the upper limits derived from large-scale structure could help significantly to constrain the absolute scale of the neutrino masses. It is also important to know the sum of neutrino masses as it is degenerate with the values of other cosmological parameters, e.g. the amplitude of fluctuations and the primordial spectral index. A summary of cosmological neutrino mass limits is given. Current results from cosmology set an upper limit on the sum of the neutrino masses of ~1 eV, somewhat depending on the data sets used in the analyses and assumed priors on cosmological parameters. It is important to emphasize that the total neutrino mass (`hot dark matter') is derived assuming that the other components in the universe are baryons, cold dark matter and dark energy. We assess the impact of neutrino masses on the matter power spectrum, the cosmic microwave background, peculiar velocities and gravitational lensing. We also discuss future methods to improve the mass upper limits by an order of magnitude.

hep-ph↗

arXiv2004

Combined constraints on Cardassian models from supernovae, CMB and large-scale structure observations

We confront Cardassian models with recent observational data. These models can be viewed either as purely phenomenological modifications of the Friedmann equation, or as arising from cosmic fluids with non-standard properties. In the first case, we find that the models are consistent with the data for a wide range of parameters but no significant preference over the Lambda-CDM model is found. In the latter case we find that the fact that the sound speed is non-zero in these models makes them inconsistent with the galaxy power spectrum from the Sloan Digital Sky Survey.

astro-ph↗

arXiv2004

Weighing Neutrinos with Large-Scale Structure

While it is established that the effect of neutrinos on the evolution of cosmic structure is small, the upper limits derived from large-scale structure could help significantly to constrain the absolute scale of the neutrino masses. Current results from cosmology set an upper limit on the sum of the neutrino masses of ~ 1 eV, somewhat depending on the data sets used in the analyses and assumed priors on cosmological parameters. In this review we discuss the effects of neutrinos on large-scale structure which make these limits obtainable. We show the impact of neutrino masses on the matter power spectrum, the cosmic microwave background and the clustering amplitude. A summary of derived cosmological neutrino mass upper limits is given, and we discuss future methods which will improve the mass upper limits by an order of magnitude.

astro-ph↗

arXiv2004

Cosmic acceleration and extra dimensions: constraints on modifications of the Friedmann equation

An alternative to dark energy as an explanation for the present phase of accelerated expansion of the Universe is that the Friedmann equation is modified, e.g. by extra dimensional gravity, on large scales. We explore a natural parametrization of a general modified Friedmann equation, and find that the present supernova type Ia and cosmic microwave background data prefer a correction of the form 1/H over a cosmological constant. We also explore the constraints that can be expected in the future, and find that there are good prospects for distinguishing this model from the standard cosmological constant to very high significance if one combines supernova data with a precise measurement of the matter density.

astro-ph↗

arXiv2003

Observational constraints on particle production during inflation

Resonant particle production, along with many other physical processes which change the effective equation of state (EOS) during inflation, introduces a step-like feature in the primordial power spectrum. We calculate observational constraints on resonant particle production, parameterised in form of an effective step height, N_eff and location in k-space, k_break. Combining data from the cosmic microwave background and the 2dF Galaxy Redshift Survey yields strong constraints in some regions of parameter space, although the range in k-space which can be probed is restricted to k ~ 0.001 - 0.1 h Mpc^-1. We also discuss the implications of our findings for general models which change the effective EOS during inflation.

astro-ph↗