Search results for "Universe"
showing 10 items of 2171 documents
Majorons: a simultaneous solution to the large and small scale dark matter problems
1984
Abstract It is shown that the existence of majorons, which enable a heavy neutrino, 500 eV ≲ mνH ≲ 25 keV to decay into a light neutrino mνL ≲ 8 eV and a majoron, with lifetime 104 yr ≲ τνH ≲ 108 yr can solve both the large and small scale dark matter problems. For a primordial “Zeldovich” spectrum of fluctuations the limits are m v H ≲ 550 eV and τ v H > 107 to 108 yr (the ranges mνH ≲ eV and τνH ≳ 108 yr are allowed by the model but galaxy formation becomes problematic). The large scale dark matter problem is how to achieve the critical density as implied by inflation, the small scale problems deal with the halos of galaxies and galaxy formation and perturbation growth. The heavy neutrino…
Bouncing Cosmologies in Palatini $f(R)$ Gravity
2009
7 pages, 4 figures.-- PACS nrs.: 04.50.Kd; 98.80.-k; 98.80.Qc.-- ArXiv pre-print available at: http://arxiv.org/abs/0907.0318
Little Rip, ΛCDM and singular dark energy cosmology from Born–Infeld-f(R) gravity
2014
Abstract We study late-time cosmic accelerating dynamics from Born–Infeld- f ( R ) gravity in a simplified conformal approach. We find that a variety of cosmic effects such as Little Rip, ΛCDM universe and dark energy cosmology with finite-time future singularities may occur. Unlike the convenient Born–Infeld gravity where in the absence of matter only de Sitter expansion may emerge, apparently any FRW cosmology maybe reconstructed from this conformal version of the Born–Infeld- f ( R ) theory. Despite the fact that the explicit form of f ( R ) is fixed by the conformal ansatz, the relation between the two metrics in this approach may be changed so as to bring out any desired FRW cosmology.
Accurate modeling of weak lensing with the stochastic gravitational lensing method
2011
We revise and extend the stochastic gravitational lensing method (the sGL method) first introduced by Kainulainen and Marra [Phys. Rev. D 80, 123020 (2009)]. Here we include a realistic halo-mass function and density profiles to model the distribution of mass between and within galaxies, galaxy groups, and galaxy clusters. We also introduce a modeling of the filamentary large-scale structures and a method to embed halos into these structures. We show that the sGL method naturally reproduces the weak lensing results for the Millennium simulation. The strength of the sGL method is that a numerical code based on it can compute the lensing probability distribution function (PDF) for a given inh…
New High-Precision Measurement of the Reaction Rate of the 18O(p, α)15N Reaction via THM
2008
The 18O(p,alpha)15N reaction rate has been extracted by means of the Trojan-Horse method. For the first time the contribution of the 20-keV peak has been directly evaluated, giving a value about 35% larger than previously estimated. The present approach has allowed to improve the accuracy of a factor 8.5, as it is based on the measured strength instead of educated guesses or spectroscopic measurements. The contribution of the 90-keV resonance has been determined as well, which turned out to be of negligible importance to astrophysics.
Dark Matter Bound States from Three-Body Recombination
2020
The small-scale structure problems of the universe can be solved by self-interacting dark matter that becomes strongly interacting at low energies. A particularly predictive model is resonant short-range self-interactions, with a dark-matter mass of about 19 GeV and a large S-wave scattering length of about 17 fm. Such a model makes definite predictions for the few-body physics of weakly bound clusters of the dark-matter particles. We calculate the production of two-body bound clusters by three-body recombination in the early universe under the assumption that the dark matter particles are identical bosons, which is the most favorable case for forming larger clusters. The fraction of dark m…
A full picture of large lepton number asymmetries of the Universe
2017
A large lepton number asymmetry of (0.1−1) at present Universe might not only be allowed but also necessary for consistency among cosmological data. We show that, if a sizeable lepton number asymmetry were produced before the electroweak phase transition, the requirement for not producing too much baryon number asymmetry through sphalerons processes, forces the high scale lepton number asymmetry to be larger than about 03. Therefore a mild entropy release causing (10-100) suppression of pre-existing particle density should take place, when the background temperature of the Universe is around T = (10−2-102) GeV for a large but experimentally consistent asymmetry to be present today. We also …
Probing photophobic axion and relaxion dark matter
2021
We investigate the interplay between early universe cosmology and dark matter direct detection, considering axion models with naturally suppressed couplings to photons. In the context of the cosmological relaxation of the electroweak scale, we focus on a scenario of relaxion dark matter, in which the relaxion field constitutes all the observed dark matter relic density and its allowed mass range is fixed to a few keV by construction. In particular, we show that a relaxion particle with mass mϕ=3.0 keV which couples to electrons with gϕ,e=6.8×10−14 is consistent with the XENON1T excess, while accounting for the observed dark matter and satisfying astro/cosmo probes. This scenario uses the el…
Gravitational wave probes of axionlike particles
2020
We have recently shown that axions and axion-like particles (ALPs) may emit an observable stochastic gravitational wave (GW) background when they begin to oscillate in the early universe. In this note, we identify the regions of ALP parameter space which may be probed by future GW detectors, including ground- and space-based interferometers and pulsar timing arrays. Interestingly, these experiments have the ability to probe axions from the bottom up, i.e. in the very weakly coupled regime which is otherwise unconstrained. Furthermore, we discuss the effects of finite dark photon mass and kinetic mixing on the mechanism, as well as the (in)sensitivity to couplings of the axion to Standard Mo…
Lepton number asymmetries and the lower bound on the reheating temperature
2017
We show that the reheating temperature of a matter-domination era in the early universe can be pushed down to the neutrino decoupling temperature at around $2 \ {\rm MeV}$ if the reheating takes place through non-hadronic decays of the dominant matter and neutrino-antineutrino asymmetries are still large enough, $|L| \gtrsim \mathcal{O}(10^{-2})$ (depending on the neutrino flavor) at the end of reheating.