arXiv subjects

O. Elgaroy

Publications and source records attributed to O. Elgaroy.

13 recordsLinked to original sources

arXiv2006

Cosmological constraints on f(R) gravity theories within the Palatini approach

We investigate f(R) theories of gravity within the Palatini approach and show how one can determine the expansion history, H(a), for an arbitrary choice of f(R). As an example, we consider cosmological constraints on such theories arising from the supernova type Ia, large scale structure formation and cosmic microwave background observations. We find that best fit to the data is a non-null leading order correction to the Einstein gravity, but the current data exhibits no significant preference over the concordance LCDM model. Our results show that the often considered 1/R models are not compatible with the data. The results demonstrate that the background expansion alone can act as a good discriminator between modified gravity models when multiple data sets are used.

astro-ph↗

arXiv2005

Loitering universe models in light of the CMB

Spatially flat loitering universe models have recently been shown to arise in the context of brane world scenarios. Such models allow more time for structure formation to take place at high redshifts, easing, e.g., the tension between the observed and predicted evolution of the quasar population with redshift. While having the desirable effect of boosting the growth of structures, we show that in such models the position of the first peak in the power spectrum of the cosmic microwave background anisotropies severely constrains the amount of loitering at high redshifts.

astro-ph↗

arXiv2004

Geometrical Constraints on Dark Energy

We explore the recently introduced statefinder parameters. After reviewing their basic properties, we calculate the statefinder parameters for a variety of cosmological models, and investigate their usefulness as a means of theoretical classification of dark energy models. We then go on to consider their use in obtaining constraints on dark energy from present and future supernovae type Ia data sets. Provided that the statefinder parameters can be extracted unambiguously from the data, they give a good visual impression of where the correct model should lie. However, it is non-trivial to extract the statefinders from the data in a model-independent way, and one of our results indicates that an expansion of the dark energy density to second order in the redshift is inadequate. Hence, while a useful theoretical and visual tool, applying the statefinders to observations is not straightforward.

astro-ph↗

arXiv2004

Meissner Effect and Vortex Dynamics in Quark Stars -- A Model for Soft Gamma-Ray Repeaters

We present a new model for soft gamma-ray repeaters based on a quark star born with temperatures above the critical value (T_c) for the onset of the colour-flavor locked superconductivity. The quark star then quickly cools below T_c, expelling a fraction of the surface magnetic field via the Meissner effect. We show that if a small fraction (\leq 10%) of the surface magnetic field (10^{14} - 10^{15} {\rm G}) is expelled, it quickly decays via magnetic reconnection and heats up the quark star surface to temperatures > 10^9 {\rm K}. Created (e^{+},e^{-}) pairs annihilate into gamma rays emitted in a giant burst (the first burst in our model), with a luminosity of \sim 10^{45} {\rm ergs} {\rm s}^{-1}. Subsequent bursts result from the restructuring of the surface magnetic field following the formation and relaxation of a vortex lattice which confines the internal magnetic field. During this phase, energy is sporadically released as a consequence of magnetic reconnection events in the entangled surface magnetic field as it evolves into a smooth, more stable, configuration. The star eventually enters a quiescent phase in which energy is continuously supplied by vortex annihilation at the surface. As the star spins down, the outermost vortex lines will be pushed to the surface where they annihilate and release their confined magnetic field. We show that the corresponding luminosity is L_v \sim 10^{36} {\rm ergs} {\rm s}^{-1} for a typical soft gamma-ray repeater spinning with a period of 8 {\rm s} and a surface magnetic field not exceeding 10^{15} {\rm G}. Our model can be applied to any situation where a T>T_{\rm c} quark star is generated. We discuss the connection between anomalous X-ray pulsars and soft gamma-ray repeaters in the context of our model.

astro-ph↗

arXiv2003

The Integrated Sachs-Wolfe effect as a probe of non-standard cosmological evolution

The Integrated Sachs-Wolfe effect is studied in non-standard cosmologies. By considering flat universes with a non-fluctuating dark energy component, it is shown how the quadrupole power can be suppressed by atypical evolution of the scale factor. For example, a brief period of non-standard evolution at a high redshift can suppress the quadrupole significantly. The effect on the overall normalization of the CMB power spectrum is also discussed. Non-standard cosmologies can affect the overall normalization significantly and enhance the primordial fluctuations. The possibility of constraining such non-standard models with CMB and independent measures of $σ_8$, is considered.

astro-ph↗

arXiv2002

A new upper limit on the total neutrino mass from the 2dF Galaxy Redshift Survey

We constrain f_nu = Omega_nu / Omega_m, the fractional contribution of neutrinos to the total mass density in the Universe, by comparing the power spectrum of fluctuations derived from the 2dF Galaxy Redshift Survey with power spectra for models with four components: baryons, cold dark matter, massive neutrinos and a cosmological constant. Adding constraints from independent cosmological probes we find f_nu < 0.13 (at 95% confidence) for a prior of 0.1< Omega_m <0.5, and assuming the scalar spectral index n=1. This translates to an upper limit on the total neutrino mass and m_nu,tot < 1.8 eV for "concordance" values of Omega_m and the Hubble constant. Very similar results are obtained with a prior on Omega_m from Type Ia supernovae surveys, and with marginalization over n.

astro-ph↗

arXiv2002

Features in the primordial power spectrum: constraints from the CMB and the limitation of the 2dF and SDSS redshift surveys to detect them

We allow a more general (step-function) form of the primordial power spectrum than the usual featureless power-law Harrison--Zeldovich (with spectral index n=1) power spectrum, and fit it to the latest Cosmic Microwave Background data sets. Although the best-fitting initial power spectrum can differ significantly from the power-law shape, and contains a bump at scales k ~ 0.003 h/Mpc, we find Omega_m to be approximately 0.24, consistent with previous analyses that assume power-law initial fluctuations. We also explore the feasibility of the 2dF and SDSS galaxy redshifts surveys to observe these features, and we find that even if features exist in the primoridal power spectrum, they are washed out by the window functions of the redshift surveys on scales k< 0.03 h/Mpc.

astro-ph↗

arXiv2001

Characteristic length scale and energy of a vortex line in a dilute, superfluid Fermi gas

We calculate the characteristic length scale of a vortex line in a dilute, superfluid gas of fermionic atoms, and find that it is in general smaller, and has a weaker density dependence than the BCS coherence length. Taking this into account, we find the energy of a vortex line to be larger than has previously been estimated. As a consequence, the critical frequency for formation of vortices in trapped fermion gases can be significantly larger than earlier calculations have suggested.

cond-mat.stat-mech↗

arXiv2001

Superfluid vortices in neutron stars

A microscopic, quantum mechanical model for neutron vortices in the crust of a neutron star is presented. After a brief introduction to the Bogoliubov- de Gennes equations, which form the basis for our calculations, we present results for density distributions, vortex core sizes and vortex energies, both for an isolated neutron vortex and for the case when the vortex core overlaps with a cylindrical nucleus. Earlier results on the vortex core size are confirmed, indicating a much less dramatic variation of the vortex core size with density than predicted by the BCS formula.

astro-ph↗

arXiv1998

Microscopic structure of a vortex line in superfluid neutron star matter

The microscopic structure of an isolated vortex line in superfluid neutron star matter is studied by solving the Bogoliubov-de Gennes equations. Our calculation, which is the starting point for a microscopic calculation of pinning forces in neutron stars, shows that the size of the vortex core varies differently with density, and is in general smaller than assumed in some earlier calculations of vortex pinning in neutron star crusts. The implications of this result are discussed

nucl-th↗

arXiv1997

Properties of Pairing Correlations in Infinite Nuclear Matter

We discuss various properties of singlet S pairing in neutron and nuclear matter in terms of the bare nuclon-nucleon interaction. Relations to the NN phase shifts are discussed, as well as various properties of the BCS wave function and the coherence length for fermion superfluids in infinite matter.

nucl-th↗

arXiv1996

Model--space approach to $^{1}S_{0}$ neutron and proton pairing with the Bonn meson--exchange potentials

In this work we calculate neutron and proton energy gaps in neutron star matter, using the Bonn meson--exchange interactions and a model--space approach to the gap equation. This approach allows a consistent calculation of energy gaps and single particle energies with the model--space Brueckner--Hartree--Fock (MBHF) method, without double counting of two--particle correlations. Neutron energy gaps are calculated at zero and finite temperature. Proton energy gaps are calculated at beta equilibrium, and it is shown that the inclusion of muons has a significant effect. The results are compared with those of other works, and the implications for neutron star physics are briefly discussed.

nucl-th↗

arXiv1995

Muons and emissivities of neutrinos in neutron star cores

In this work we consider the role of muons in various URCA processes relevant for neutrino emissions in the core region of neutron stars. The calculations are done for $β$--stable nuclear matter with and without muons. We find muons to appear at densities $ρ= 0.15$ fm$^{-3}$, slightly around the saturation density for nuclear matter $ρ_0 =0.16$ fm$^{-3}$. The direct URCA processes for nucleons are forbidden for densities below $ρ= 0.5$ fm$^{-3}$, however the modified URCA processes with muons $(n+N\rightarrow p+N +μ+\overlineν_μ, p+N+μ\rightarrow n+N+ν_μ$), where $N$ is a nucleon, result in neutrino emissivities comparable to those from $(n+N\rightarrow p+N +e +\overlineν_e, p+N+e \rightarrow n+N+ν_e$). This opens up for further possibilities to explain the rapid cooling of neutrons stars. Superconducting protons reduce however these emissivities at densities below $0.4$ fm$^{-3}$.

nucl-th↗