Keywords

In this paper, we consider the impact of the Lorentz invariance violation (LIV) on the γ − γ opacity of the universe to very high energy (VHE) gamma-rays, compared to the effect of local underdensities (voids) of the extragalactic background light, and on the Compton scattering process. Both subluminal and superluminal modifications of the photon dispersion relation are considered. In the subluminal case, LIV effects may result in a significant reduction of the γ − γ opacity for photons with energies ≳10 TeV. However, the effect is not expected to be sufficient to explain the apparent spectral hardening of several observed VHE γ-ray sources in the energy range from 100 GeV to a few TeV, even when including effects of plausible inhomogeneities in the cosmic structure. Superluminal modifications of the photon dispersion relation lead to a further enhancement of the EBL γγ opacity. We consider, for the first time, the influence of LIV on the Compton scattering process. We find that this effect becomes relevant only for photons at ultra-high energies, E ≳ 1 PeV. In the case of a superluminal modification of the photon dispersion relation, both the kinematic recoil effect and the Klein–Nishina suppression of the cross section are reduced. However, we argue that the effect is unlikely to be of astrophysical significance.

Galaxy formation depends critically on the physical state of gas in the circumgalactic medium (CGM) and its interface with the intergalactic medium (IGM), determined by the complex interplay between inflow from the IGM and outflows from supernovae and/or AGN feedback. The average Lyα absorption profile around galactic halos represents a powerful tool to probe their gaseous environments. We compare predictions from Illustris and Nyx hydrodynamical simulations with the observed absorption around foreground quasars, damped Lyα systems, and Lyman-break galaxies. We show how large-scale BOSS and small-scale quasar pair measurements can be combined to precisely constrain the absorption profile over three decades in transverse distance $20\,\mathrm{kpc}\lesssim b\lesssim 20\,\mathrm{Mpc}$. Far from galaxies, $\gtrsim 2\,\mathrm{Mpc}$, the simulations converge to the same profile and provide a reasonable match to the observations. This asymptotic agreement arises because the ΛCDM model successfully describes the ambient IGM and represents a critical advantage of studying the mean absorption profile. However, significant differences between the simulations, and between simulations and observations, are present on scales $20\,\,\mathrm{kpc}\lesssim b\lesssim 2\,\mathrm{Mpc}$, illustrating the challenges of accurately modeling and resolving galaxy formation physics. It is noteworthy that these differences are observed as far out as $\sim 2\,\mathrm{Mpc}$, indicating that the "sphere of influence" of galaxies could extend to approximately ∼7 times the halo virial radius. Current observations are very precise on these scales and can thus strongly discriminate between different galaxy formation models. We demonstrate that the Lyα absorption profile is primarily sensitive to the underlying temperature–density relationship of diffuse gas around galaxies, and argue that it thus provides a fundamental test of galaxy formation models.

There is substantial and growing observational evidence from the normalized luminosity density in the near-infrared that the local universe is underdense on scales of several hundred megaparsecs. We test whether our parameterization of the observational data of such a "void" is compatible with the latest supernovae type Ia data and with constraints from line-of-sight peculiar-velocity motions of galaxy clusters with respect to the cosmic microwave background rest-frame, known as the linear kinematic Sunyaev–Zel'dovich (kSZ) effect. Our study is based on the large local void (LLV) radial profile observed by Keenan, Barger, and Cowie (KBC) and a theoretical void description based on the Lemaître–Tolman–Bondi model with a nonzero cosmological constant (ΛLTB). We find consistency with the measured luminosity distance–redshift relation on radial scales relevant to the KBC LLV through a comparison with 217 low-redshift supernovae type Ia over the redshift range $0.0233\lt z\lt 0.15$. We assess the implications of the KBC LLV in light of the tension between "local" and "cosmic" measurements of the Hubble constant, H₀. We find that when the existence of the KBC LLV is fully accounted for, this tension is reduced from $3.4\sigma$ to $2.75\sigma$. We find that previous linear kSZ constraints, as well as new ones from the South Pole Telescope and the Atacama Cosmology Telescope, are fully compatible with the existence of the KBC LLV.

For Type Ia supernovae (SNe Ia) observed through a nonuniform interstellar medium (ISM) in its host galaxy, we investigate whether the nonuniformity can cause observable time variations in dust extinction and in gas absorption due to the expansion of the SN photosphere with time. We show that, owing to the steep spectral index of the ISM density power spectrum, sizable density fluctuation amplitudes at the length scale of typical ISM structures ($\gtrsim 10\,\mathrm{pc}$) will translate to much smaller fluctuations on the scales of an SN photosphere. Therefore, the typical amplitude of time variation due to a nonuniform ISM, of absorption equivalent widths, and of extinction, would be small. As a result, we conclude that nonuniform ISM density should not impact cosmology measurements based on SNe Ia. We apply our predictions based on the ISM density power-law power spectrum to the observations of two highly reddened SNe Ia, SN 2012cu and SN 2014J.

The unexpectedly hard very-high-energy (VHE; E > 100 GeV) γ-ray spectra of a few distant blazars have been interpreted as evidence of a reduction of the γγ opacity of the universe due to the interaction of VHE γ-rays with the extragalactic background light (EBL) compared to the expectation from current knowledge of the density and cosmological evolution of the EBL. One of the suggested solutions to this problem involves the inhomogeneity of the EBL. In this paper, we study the effects of such inhomogeneity on the energy density of the EBL (which then also becomes anisotropic) and the resulting γγ opacity. Specifically, we investigate the effects of cosmic voids along the line of sight to a distant blazar. We find that the effect of such voids on the γγ opacity, for any realistic void size, is only of the order of ≲1% and much smaller than expected from a simple linear scaling of the γγ opacity with the line-of-sight galaxy underdensity due to a cosmic void.

The study of cosmology, galaxy formation, and exoplanets has now advanced to a stage where a cosmic inventory of terrestrial planets (TPs) may be attempted. By coupling semianalytic models of galaxy formation to a recipe that relates the occurrence of planets to the mass and metallicity of their host stars, we trace the population of TPs around both solar-mass (FGK type) and lower-mass (M dwarf) stars throughout all of cosmic history. We find that the mean age of TPs in the local universe is $7\pm 1\,\mathrm{Gyr}$ for FGK hosts and $8\pm 1\,\mathrm{Gyr}$ for M dwarfs. We estimate that hot Jupiters have depleted the population of TPs around FGK stars by no more than $\approx 10 \%$, and that only $\approx 10 \%$ of the TPs at the current epoch are orbiting stars in a metallicity range for which such planets have yet to be confirmed. The typical TP in the local universe is located in a spheroid-dominated galaxy with a total stellar mass comparable to that of the Milky Way. When looking at the inventory of planets throughout the whole observable universe, we argue for a total of $\approx 1\times {10}^{19}$ and $\approx 5\times {10}^{20}$ TPs around FGK and M stars, respectively. Due to light travel time effects, the TPs on our past light cone exhibit a mean age of just 1.7 ± 0.2 Gyr. These results are discussed in the context of cosmic habitability, the Copernican principle, and searches for extraterrestrial intelligence at cosmological distances.

We investigate two dark energy cosmological models (i.e., the ΛCDM and ϕCDM models) with massive neutrinos assuming two different neutrino mass hierarchies in both the spatially flat and non-flat scenarios, where in the ϕCDM model the scalar field possesses an inverse power-law potential, V(ϕ) ∝ ϕ^−α (α > 0). Cosmic microwave background data from Planck 2015, baryon acoustic oscillation data from 6dFGS, SDSS-MGS, BOSS-LOWZ and BOSS CMASS-DR11, the joint light-curve analysis compilation of SNe Ia apparent magnitude observations, and the Hubble Space Telescope H₀ prior, are jointly employed to constrain the model parameters. We first determine constraints assuming three species of degenerate massive neutrinos. In the spatially flat (non-flat) ΛCDM model, the sum of neutrino masses is bounded as Σm_ν < 0.165(0.299) eV at 95% confidence level (CL). Correspondingly, in the flat (non-flat) ϕCDM model, we find Σm_ν < 0.164(0.301) eV at 95% CL. The inclusion of spatial curvature as a free parameter results in a significant broadening of confidence regions for Σm_ν and other parameters. In the scenario where the total neutrino mass is dominated by the heaviest neutrino mass eigenstate, we obtain similar conclusions to those obtained in the degenerate neutrino mass scenario. In addition, the results show that the bounds on Σm_ν based on two different neutrino mass hierarchies have insignificant differences in the spatially flat case for both the ΛCDM and ϕCDM models; however, the corresponding differences are larger in the non-flat case.

We present the interesting coincidence of cosmology and astrophysics that points toward a dimensionless age of the Universe H₀t₀ that is close to one. Despite cosmic deceleration for 9 Gyr and acceleration since then, we find H₀t₀ = 0.96 ± 0.01 for the ΛCDM model that fits SN Ia data from Pan-STARRS, CMB power spectra, and baryon acoustic oscillations. Similarly, astrophysical measures of stellar ages and the Hubble constant derived from redshifts and distances point to H₀t ∼ 1.0 ± 0.1. The wide range of possible values for H₀t₀ realized during cosmic evolution means that we live at what appears to be a special time. This "synchronicity problem" is not precisely the same as the usual coincidence problem, because there are combinations of Ω_M and Ω_Λ for which the usual coincidence problem holds but for which H₀t₀ is not close to 1.

At redshifts beyond $z\sim 1$, measuring the black hole (BH) galaxy relations proves to be a difficult task. The bright light of the active galactic nuclei aggravates the deconvolution of BH and galaxy properties. However, high-redshift data on these relations are vital to understand the ways in which galaxies and BHs co-evolve and the ways in which they do not. In this work we use BH and stellar mass densities (BHMDs and SMDs) to constrain the possible co-evolution of BHs with their host galaxies since $z\sim 5$. The BHMDs are calculated from quasar luminosity functions using the Soltan argument, while we use integrals over stellar mass functions or the star-formation rate density to obtain values for the SMD. We find that both quantities grow in lock-step below redshifts of $z\sim 3$ with a non-evolving BHMD to SMD ratio. A fit to the data assuming a power-law relation between the BHMD and the SMD yields exponents around unity (1.0–1.5). Up to $z\sim 5$ the BHMD to SMD ratio does not show a strong evolution given the larger uncertainty in the completeness of high-redshift data sets. Our results, always applying the same analysis technique, seem to be consistent across all adopted data sets.