Astrophysics

Starting with the de Broglie--Proca Lagrangian for a massive vector field, we calculate the number density of particles resulting from gravitational particle production (GPP) during inflation, with detailed consideration to the evolution of the number density through the reheating. We find plausible scenarios for the production of dark-photon dark matter of mass in a wide range, as low as a micro-electron volt to 10 14 GeV. Gravitational particle production does not depend on any coupling of the dark photon to standard-model particles.

We study perturbations of a complex scalar field during reheating with no self-interaction in the regime μ?�H , when the scalar field has a fast oscillatory behaviour (close to a pressure-less fluid). We focus on the precise determination of the instability scale and find it differs from that associated with a real scalar field. We further look at the probability that unstable fluctuations form Primordial Black Holes (PBHs) obtaining a significant production of tiny PBHs which quickly evaporate and may subsequently leave a population of Planck-mass relics. We finally impose restrictions on the duration and energy scale of the fast oscillations period by considering that such relics constitute, at most, the totality of dark matter in the Universe.

The problem under consideration is to determine the change of the Cosmic Microwave Background (CMB) spectral shape due to the thermal Sunyaev-Zeldovich effect. We numerically model the spectral intensity of the CMB radiation Comptonized by the hot intergalactic Maxwellian plasma. To this aim, a relativistic Monte Carlo code with photon weights is developed. The code enables us to construct the Comptonized CMB spectrum in a wide energy range. The results are compared with known analytical solutions and previous numerical simulations. We also calculate the angular distributions of the intensity of radiation emerging from the cloud, which show that the spectral shape of the tSZ effect is not universal for different directions of escaping photons. The numerical method can be applied to simulate the processes of Comptonization for different optical depths, temperatures, initial spectra of photon sources and their spatial distributions, the obtained results may have implications on investigating the profiles of galaxy clusters.

We propose a new algorithm for computing the luminosity distance in the flat universe with a cosmological constant based on Shchigolev's homotopy perturbation method, where the optimization idea is applied to prevent the arbitrariness of initial value choice in Shchigolev's homotopy. Compared with the some existing numerical methods, the result of numerical simulation shows that our algorithm is a very promising and powerful technique for computing the luminosity distance, which has obvious advantages in computational accuracy,computing efficiency and robustness for a given {\Omega_m}.

A range of cosmological observations demonstrate an accelerated expansion of the Universe, and the most likely explanation of this phenomenon is a cosmological constant. Given the importance of understanding the underlying physics, it is relevant to investigate alternative models. This article uses numerical simulations to test the consistency of one such alternative model. Specifically, this model has no cosmological constant, instead the dark matter particles have an extra force proportional to velocity squared, somewhat reminiscent of the magnetic force in electrodynamics. The constant strength of the force is the only free parameter. Since bottom-up structure formation creates cosmological structures whose internal velocity dispersions increase in time, this model may mimic the temporal evolution of the effect from a cosmological constant. It is shown that models with force linearly proportional to internal velocites, or models proportional to velocity to power three or more cannot mimic the accelerated expansion induced by a cosmological constant. However, models proportional to velocity squared are still consistent with the temporal evolution of a Universe with a cosmological model.

Fast radio bursts (FRBs) are astrophysical transients of still debated origin. So far several hundred events have been detected, mostly at extragalactic distances, and this number is expected to grow significantly over the next years. The radio signals from the burst experience dispersion as they travel through the free electrons along the line-of-sight characterised by the dispersion measure (DM) of the radio pulse. In addition, each photon also experiences a gravitational Shapiro time delay while travelling through the potentials generated by the large-scale structure. If the equivalence principle (EP) holds, the Shapiro delay is the same for photons of all frequencies. In case the EP is broken, one would expect an additional dispersion to occur which could be either positive or negative for individual sources. Here we suggest to use angular statistics of the DM fluctuations to put constraints on the EP parametrized by the post-Newtonian parameter γ . Previous studies suffer from the problem that the gravitational potential responsible for the delay diverges in a cosmological setting, which our approach avoids. We carry out a forecast for a population of FRBs observable within the next years and show that any significant detection of the DM angular power spectrum will place constraints on the EP that are by a few orders of magnitude more stringent than current limits.

Constant-rate inflation, including ultra-slow-roll as a special case, has been widely applied to the formation of primordial black holes with significant deviation from the standard slow-roll conditions at both the growing and decaying phases of the power spectrum. We derive analytic solutions for the curvature perturbations with respect to the late-time scaling dimensions (conformal weights) constrained by the dilatation symmetry of the de Sitter background and show that continuous momentum scaling generically occurs at the transition across different conformal dimensionalities. The temporal excitation of subleading states (with the next-to-lowest conformal weights) is recorded as the "steepest growth" of the power spectrum in the transition from slow-roll to constant-rate phases.

We generate constrained realizations (CRs) of the density and peculiar velocity fields within 200 h ?? Mpc from the final release of the Two-Micron All-Sky Redshift Survey (2MRS) ??the densest all-sky redshift survey to date. The CRs are generated by combining a Wiener filter estimator in spherical Fourier-Bessel space with random realizations of log-normally distributed density fields and Poisson-sampled galaxy positions. The algorithm is tested and calibrated on a set of semi-analytic mock catalogs mimicking the environment of the Local Group (LG), to rigorously account for the statistical and systematic errors of the reconstruction method. By comparing our peculiar velocity CRs with the observed velocities from the Cosmicflows-3 catalog, we constrain the normalized linear growth rate to f ? lin 8 =0.363±0.070 , which is consistent with the latest Planck results as well as other direct probes. Simultaneously, we estimate a bulk flow contribution from sources beyond the 2MRS reconstruction volume of B ext =164±68km s ?? towards l=311± 24 ??, b=0± 23 ??. The total reconstructed velocity field at the position of the LG, smoothed with a 1 h ?? Mpc Gaussian, is 552±71km s ?? towards l=274.3± 7.9 ??, b=33.9± 8.1 ??, in good agreement with the observed CMB dipole. The total reconstructed bulk flow within different radii is compatible with other measurements. Within a 50 h ?? Mpc Gaussian window we find a bulk flow of 229±45km s ?? towards l=295± 11 ??, b=5± 11 ??. The code used to generate the CRs and obtain these results, dubbed CORAS, is made publicly available.

Dark Matter (DM) can be trapped by the gravitational field of any star, since collisions with nuclei in dense environments can slow down the DM particle below the escape velocity ( v esc ) at the surface of the star. If captured, the DM particles can self annihilate, and, therefore, provide a new source of energy for the star. We investigate this phenomenon for capture of DM particles with mass ( m X ) heavier than 100 GeV by the first generation of stars (Pop III stars), by using the recently developed multiscatter capture formalism. Pop III stars are particularly good DM captors, since they form in DM rich environments, at the center of ∼ 10 6 M ⊙ DM minihalos, at redshifts z ∼15 . Assuming a DM-proton scattering cross section ( σ) at the deepest current exclusion limits provided by the XENON1T experiment, we find that captured DM annihilations at the core of Pop III stars can lead, via the Eddington limit, to upperbounds in stellar masses that can be as low as a few M ⊙ if the ambient DM density ( ρ X ) at the location of the Pop III star is sufficiently high. Conversely, when Pop III stars are identified, one can use their observed mass ( M ⋆ ) to place bounds on ρ X σ . Using adiabatic contraction to estimate the ambient DM density in the environment surrounding Pop III stars, we place projected upper limits on σ , for M ⋆ in the 100−1000 M ⊙ range, and find bounds that are competitive with, or deeper than, those provided by the most sensitive current direct detection experiments. Most intriguingly, we find that each of the Pop III stars considered could be used to probe below the "neutrino floor," and identify the corresponding necessary ambient DM density.

We show that the nonperturbative decay of ultralight scalars into Abelian gauge bosons, recently proposed as a possible solution to the Hubble tension, produces a stochastic background of gravitational waves which is constrained by the cosmic microwave background. We simulate the full nonlinear dynamics of resonant dark photon production and the associated gravitational wave production, finding the signals to exceed constraints for the entire parameter space we consider. Our findings suggest that gravitational wave production from the decay of early dark energy may provide a unique probe of these models.