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In this paper, we investigate the optical properties of a charged black hole with scalar hair (CSH) within the context of four-dimensional Einstein-Maxwell-Dilaton gravity. To achieve this, we consider three distinct toy models of thin accretion disks. The presence of dilaton coupling allows us to express both the solutions of CSH and the Reissner-Nordstr\"om (RN) black hole in terms of their mass ($M$) and charge ($Q$). Our findings reveal differences in the effective potentials $V_{eff}$, photon sphere radii $r_{ph}$, and innermost stable circular orbit $r_{isco}$ between the CSH and RN black hole cases, which become increasingly pronounced as the charge parameter $Q$ increases. However, no noticeable distinctions are observed concerning the critical impact parameter $b_{ph}$. When the ratio of the photon ring band and the lensed ring band exceeds 0.1, it may suggest the presence of a charged black hole with scalar hair. Furthermore, our results underscore the significant influence of the charge parameter $Q$ on the brightness distributions of the direct, lensed ring, and photon ring for three standard emission functions. These findings emphasize the potential for distinguishing between CSH and RN black holes through an analysis of direct intensity and peak brightness in specific accretion disk models., Comment: 13 pages, 13figures

Dark matter, if represented by a $\mathbb{Z}_2$-symmetric scalar field, can manifest as both particles and condensates. In this paper, we study the evolution of an oscillating homogeneous condensate of a $\mathbb{Z}_2$-symmetric scalar field in a thermal plasma in an FLRW universe. We focus on the perturbative regime where the oscillation amplitude is sufficiently small so that parametric resonance is inefficient. This perturbative regime necessarily comprises the late stage of the condensate decay and determines its fate. The coupled coarse-grained equations of motion for the condensate, radiation, and spacetime are derived from first principles using nonequilibrium quantum field theory. We obtain analytical expressions for the relevant microscopic quantities that enter the equations of motion and solve the latter numerically. We find that there is always a nonvanishing relic abundance for a condensate with a $\mathbb{Z}_2$ symmetry that is not spontaneously broken. This is because its decay rate decreases faster than the Hubble parameter at late times due to either the amplitude dependence or the temperature dependence in the condensate decay rate. Consequently, accounting for the condensate contribution to the overall dark matter relic density is essential for $\mathbb{Z}_2$ scalar singlet dark matter., Comment: 56 pages, 17 figures; v2: refs updated, minor changes, matches the version published in JCAP

We study the optical appearance of a de Sitter-Schwarzschild black hole and its distinguishability from a Schwarzschild black hole. By exploring various accretion models and emission profiles, we investigate the impact of different parameters on the observed shadows and intensity profiles. Our analysis reveals that the outer edge of the shadow, corresponding to the apparent radius of the photon sphere, remains consistent regardless of the spherical accretion details or the size of the black hole. However, subtle differences in the overall brightness and intensity distribution can arise between these two black holes, especially for emission models with sharp peaks near the event horizon. We find that the de Sitter-Schwarzschild black hole tends to exhibit a slightly darker appearance in certain scenarios, while in others, it can appear slightly brighter than the Schwarzschild black hole. These distinctions become more prominent as the radial emission decreases more rapidly. Nevertheless, the size of the shadow alone is not sufficient to differentiate the potential differences in the optical appearance between the de Sitter-Schwarzschild black hole and the Schwarzschild black hole. Instead, distinctions may be observed in the overall brightness of the image., Comment: v1:25 pages, 14 figures; v2:published version

We investigate a new type of Schwarzschild wormhole, which relies on a 3-dimensional spacetime defect with degenerate metrics. This particular wormhole is a solution of the vacuum Einstein equations. We also study the generalized Schwarzschild-type defect wormhole and discuss the Null Energy Condition. In particular, we investigate the geodesics and geodesic congruences of the generalized Schwarzschild-type defect wormhole. Additionally, we explore the optical appearance of these wormholes, shedding light on their observable features., Comment: 21 pages, 5 figures. v2:typos fixed

We study the dissipation of oscillating scalar backgrounds in a spatially flat Friedmann-Lema\^{i}tre-Robertson-Walker universe using non-equilibrium quantum field theory. To be concrete, a $Z_2$-symmetric two-scalar model with quartic interactions is used. For quasi-harmonic oscillations, we adopt the multi-scale analysis to obtain analytical approximate expressions for the evolution of the scalar background in terms of the retarded self-energy and retarded proper four-vertex function. Different from the case in flat spacetime, we find that in an expanding universe the condensate decay in this model can be complete only if the imaginary part of the retarded self-energy is not negligibly small. The microphysical interpretation of the imaginary parts of the retarded self-energy and retarded proper four-vertex function in terms of particle production is also discussed., Comment: 32 pages + appendix, 18 figures; v2: minor typos corrected; v3: clarifying comments added; v4: time-dependence of the microscopic quantities discussed, refs updated, to appear in JHEP; v5: matches the published version