arXiv subjects

Nils A. Nilsson

Publications and source records attributed to Nils A. Nilsson.

26 records · Page 2Linked to original sources

arXiv2022

Tests of Standard Cosmology in Horava Gravity and Bayesian Evidence for a Closed Universe, and the Hubble Tension

We consider some background tests of standard cosmology in the context of Horava gravity with different scaling dimensions for space and time, which has been proposed as a renormalizable, higher-derivative, Lorentz-violating quantum gravity model without ghost problems. We obtain the "very strong" and "strong" Bayesian evidence for our two cosmology models A and B, respectively, depending on the choice of parametrization based on Horava gravity, against the standard, spatially-flat, LCDM cosmology model based on general relativity. An MCMC analysis with observational data, including BAO, shows (a) preference of a closed universe with the curvature density parameter Omega_k=-0.005+- 0.0007, -0.004+0.003-0.001 and (b) reduction of the Hubble tension with the Hubble constant H_0=71.4+1.2-0.9, 69.5+1.6-0.9 km s^{-1} Mpc^{-1} for the models A, B. We comment on some possible further improvements for the "cosmic-tension problem" by considering more complete early universe physics, based on the Lorentz-violating standard model with anisotropic space-time scaling, consistently with Horava gravity, as well as the observational data which are properly adopted for the closed universe.

hep-th↗

arXiv2022

SME Gravity in the Early Universe

In this talk, we give a short overview of recent work on cosmological solutions within the SME gravitational sector under the assumption of explicit spacetime-symmetry breaking. We show that for the special case of timelike diffeomorphism breaking, the resulting Friedmann equations can be written as standard FLRW cosmology with added dynamical dark energy, and we discuss primordial gravitational waves in the context of this model.

gr-qc↗

arXiv2021

3+1 Formulation of the Standard-Model Extension Gravity Sector

We present a 3+1 formulation of the effective field theory framework called the Standard-Model Extension in the gravitational sector. The explicit local Lorentz and diffeomorphism symmetry breaking assumption is adopted and we perform a Dirac-Hamiltonian analysis. We show that the structure of the dynamics presents significant differences from General Relativity and other modified gravity models. We explore Hamilton's equations for some special choices of the coefficients. Our main application is cosmology and we present the modified Friedmann equations for this case. The results show some intriguing modifications to standard cosmology. In addition, we compare our results to existing frameworks and models and we comment on the potential impact to other areas of gravitational theory and phenomenology.

gr-qc↗

arXiv2019

Preferred-frame Effects, the $H_0$ Tension, and Probes of Hořava-Lifshitz Gravity

We discuss implications on the $H_0$ tension due to preferred-frame effects in the context of Hořava-Lifshitz gravity. By using a combination of low-redshift data (Sne1a, elliptical and lenticular galaxies, GRB's, and quasars) we study the $H_0$ tension and its appearance as a preferred-frame effect, as well as present new constraints on the model parameter $λ$. Moreover, from the structure of the Friedmann equations, we argue that up to $38\%$ of the Hubble tension can be explained by Lorentz-violating effects in a Hořava-Lifshitz scenario.

gr-qc↗

arXiv2019

A 3+1 Decomposition of the Minimal Standard-Model Extension Gravitational Sector

The 3+1 (ADM) formulation of General Relativity is used in, for example, canonical quantum gravity and numerical relativity. Here we present a 3+1 decomposition of the minimal Standard-Model Extension gravity Lagrangian. By choosing the leaves of foliation to lie along a timelike vector field we write the theory in a form which will allow for comparison and matching to other gravity models.

gr-qc↗

arXiv2018

Hořava-Lifshitz cosmology in light of new data

We present new observational constraints on Lorentz violating Hořava-Lifshitz cosmological scenarios using an updated cosmological data set from Cosmic Microwave Background (Planck CMB), expansion rates of elliptical and lenticular galaxies, JLA compilation (Joint Light-Curve Analysis) data for Type Ia supernovae (SneIa), Baryon Acoustic Oscillations (BAO) and priors on the Hubble parameter with an alternative parametrisation of the equations. Unlike in other approaches we consider the curvature parameter $Ω_k$ as a free parameter in the analysis we considered the parameters $Ω_k$ and $ΔN_ν$ as completely free, which helped to place new, updated bounds on several of the theory parameters. Remarkably, the detailed balance scenario exhibits positive spatial curvature to more than 3$σ$, whereas for further theory generalizations we found evidence for positive spatial curvature at 1$σ$. This could create circumstantial evidence from observations and could be used to single out distinct formulations and scenarios.

gr-qc↗

arXiv2018

Cosmology in a toy model of Lorentz breaking gravity

We explore cosmological solutions to Lorentz breaking gravity using the gravitational sector of the Standard Model Extension (SME). By using a simple toy model for Lorentz violation and under the assumption that the so-called $\mathfrak{T}$-tensor is covariantly constant, we show that the gravity sector SME influences basic cosmology. If the approach used in this toy model is valid these results should point the way for more sofisticated studies.

gr-qc↗

arXiv2017

Energy Scale of Lorentz Violation in Rainbow Gravity

We modify the standard relativistic dispersion relation in a way which breaks Lorentz symmetry - the effect is predicted in a high-energy regime of some modern theories of quantum gravity. We show that it is possible to realise this scenario within the framework of Rainbow Gravity which introduces two new energy-dependent functions $f_1(E)$ and $f_2(E)$ into the dispersion relation. Additionally, we assume that the gravitational constant $G$ and the cosmological constant $Λ$ also depend on energy $E$ and introduce the scaling function $h(E)$ in order to express this dependence. For cosmological applications we specify the functions $f_1$ and $f_2$ in order to fit massless particles which allows us to derive modified cosmological equations. Finally, by using Hubble+SNIa+BAO(BOSS+Lyman $α$)+CMB data, we constrain the energy scale $E_{LV}$ to be at least of the order of $10^{16}$ GeV at $1σ$ which is the GUT scale or even higher $10^{17}$ GeV at $3σ$. Our claim is that this energy can be interpreted as the decoupling scale of massless particles from spacetime Lorentz violating effects.

astro-ph.CO↗