Search results for "COSMOLOGY"
showing 10 items of 2905 documents
Boson stars in Palatini $f(\mathcal{R})$ gravity
2021
We explore equilibrium solutions of spherically symmetric boson stars in the Palatini formulation of $f(\mathcal{R})$ gravity. We account for the modifications introduced in the gravitational sector by using a recently established correspondence between modified gravity with scalar matter and general relativity with modified scalar matter. We focus on the quadratic theory $f(\mathcal{R})=R+\xi R^2$ and compare its solutions with those found in general relativity, exploring both positive and negative values of the coupling parameter $\xi$. As matter source, a complex, massive scalar field with and without self-interaction terms is considered. Our results show that the existence curves of bos…
GW190521 as a merger of Proca stars: a potential new vector boson of $8.7 \times 10^{-13}$ eV
2020
Advanced LIGO-Virgo reported a short gravitational-wave signal (GW190521) interpreted as a quasi-circular merger of black holes, one populating the pair-instability supernova gap, forming a remnant black hole of $M_f\sim 142 M_\odot$ at a luminosity distance of $d_L \sim 5.3$ Gpc. With barely visible pre-merger emission, however, GW190521 merits further investigation of the pre-merger dynamics and even of the very nature of the colliding objects. We show that GW190521 is consistent with numerically simulated signals from head-on collisions of two (equal mass and spin) horizonless vector boson stars (aka Proca stars), forming a final black hole with $M_f = 231^{+13}_{-17}\,M_\odot$, located …
Inflation meets neutrinos
2019
Constraints on inflationary models typically assume only the standard models of cosmology and particle physics. By extending the neutrino sector to include a new interaction with a light scalar mediator ($m_{\phi}\sim$MeV), it is possible to relax these constraints, in particular via opening up regions of the parameter space of the spectral index $n_s$. These new interactions can be probed at IceCube via interactions of astrophysical neutrinos with the Cosmic Neutrino Background for nearly all of the relevant parameter space.
Diffuse axion-like particle searches
2017
We propose a new method to search for axion-like particles (ALPs) based on the gamma-rays produced concomitant with high-energy astrophysical neutrinos. The existence of high-energy neutrinos implies production of gamma-rays in the same sources. Photons can convert into ALPs in the sources' magnetic fields, and will travel as ALPs through extragalactic space. Back-conversion in the Milky Way's magnetic field leads to a diffuse anisotropic high-energy photon flux that existing and upcoming gamma-ray detectors, like HAWC, CTA, and LHAASO can detect. This method probes unexplored ALP parameter space, with LHAASO being realistically sensitive to couplings above $10^{-11}\, \rm{GeV^{-1}}$ and ma…
Stability and physical properties of spherical excited scalar boson stars
2023
We study the time evolution of spherical, excited -- with $n$ radial nodes -- scalar boson stars in General Relativity minimally coupled to a complex massive scalar field with quartic self-interactions. We report that these stars, with up to $n=10$, can be made dynamically stable, up to timescales of $t\sim\frac{10^{4}}{c\mu}$, where $\mu$ is the inverse Compton wavelength of the scalar particle, for sufficiently large values of the self-interactions coupling constant $\lambda$, which depend on $n$. We observe that the compactness of these solutions is rather insensitive to $n$, for large $\lambda$ and fixed frequency. Generically, along the branches where stability was studied, these excit…
Jet launching from merging magnetized binary neutron stars with realistic equations of state
2021
We perform general relativistic, magnetohydrodynamic (GRMHD) simulations of binary neutron stars in quasi-circular orbit that merge and undergo delayed or prompt collapse to a black hole (BH). The stars are irrotational and modeled using an SLy or an H4 nuclear equation of state. To assess the impact of the initial magnetic field configuration on jet launching, we endow the stars with a purely poloidal magnetic field that is initially unimportant dynamically and is either confined to the stellar interior or extends from the interior into the exterior as in typical pulsars. Consistent with our previous results, we find that only the BH + disk remnants originating from binaries that form hype…
Reply to comment on "Searching for Topological Defect Dark Matter via Nongravitational Signatures"
2015
In the comment of Avelino, Sousa and Lobo [arXiv:1506.06028], it is argued, by comparing the kinetic energy of a topological defect with the overall energy of a pulsar, that the origin of the pulsar glitch phenomenon due to the passage of networks of topological defects through pulsars is faced with serious difficulties. Here, we point out that topological defects may trigger pulsar glitches within traditional scenarios, such as vortex unpinning. If the energy transfer from a topological defect exceeds the activation energy for a single pinned vortex, this may lead to an avalanche of unpinning of vortices and consequently a pulsar glitch, and therefore the source of angular momentum and ene…
On the dynamics of tilted black hole-torus systems
2016
We present results from three-dimensional, numerical relativity simulations of a {\it tilted} black hole-thick accretion disc system. The simulations are analysed using tracer particles in the disc which are advected with the flow. Such tracers, which we employ in these new simulations for the first time, provide a powerful means to analyse in detail the complex dynamics of tilted black hole-torus systems. We show how its use helps to gain insight in the overall dynamics of the system, discussing the origin of the observed black hole precession and the development of a global non-axisymmetric $m=1$ mode in the disc. Our three-dimensional simulations show the presence of quasi-periodic oscil…
Correlated optical/X-ray variability in the high-mass X-ray binary SAX J2103.5+4545
2009
SAX J2103.5+4545 is the Be/X-ray binary with the shortest orbital period. It shows extended bright and faint X-ray states that last for a few hundred days. The main objective of this work is to investigate the relationship between the X-ray and optical variability and to characterise the spectral and timing properties of the bright and faint states. We have found a correlation between the spectral and temporal parameters that fit the energy and power spectra. Softer energy spectra correspond to softer power spectra. That is to say, when the energy spectrum is soft the power at high frequencies is suppressed. We also present the results of our monitoring of the Halpha line of the optical cou…
Numerically solving the relativistic Grad–Shafranov equation in Kerr spacetimes: numerical techniques
2018
The study of the electrodynamics of static, axisymmetric and force-free Kerr magnetospheres relies vastly on solutions of the so called relativistic Grad-Shafranov equation (GSE). Different numerical approaches to the solution of the GSE have been introduced in the literature, but none of them has been fully assessed from the numerical point of view in terms of efficiency and quality of the solutions found. We present a generalization of these algorithms and give detailed background on the algorithmic implementation. We assess the numerical stability of the implemented algorithms and quantify the convergence of the presented methodology for the most established setups (split-monopole, parab…