Ramin G. Daghigh

Bio: Ramin G. Daghigh is an academic researcher from Metropolitan State University of Denver. The author has contributed to research in topics: Quasinormal mode & Black hole. The author has an hindex of 9, co-authored 35 publications receiving 245 citations. Previous affiliations of Ramin G. Daghigh include Queen's University Belfast & University of Winnipeg.

A detailed analytic study of the asymptotic quasinormal modes of Schwarzschild?anti de Sitter black holes

Abstract: We analyze analytically the asymptotic regions of the quasinormal mode frequency spectra with infinitely large overtone numbers for D-dimensional Schwarzschild black holes in anti de Sitter spacetimes. In this limit, we confirm the analytic results obtained previously in the literature using different methods. In addition, we show that in certain spacetime dimensions these techniques imply the existence of other regions of the asymptotic quasinormal mode frequency spectrum which have not previously appeared in the literature. For large black holes, some of these modes have a damping rate of 1.2TH, where TH is the Hawking temperature. This is less than the damping rate of the lowest overtone quasinormal mode calculated by other authors. It is not completely clear whether these modes actually exist or are an artifact of an unknown flaw in the analytic techniques being used. We discuss the possibility of the existence of these modes and explore some of the consequences. We also examine the possible connection between the asymptotic quasinormal modes of Schwarzschild–anti de Sitter black holes and the quantum level spacing of their horizon area spectrum.

Highly Damped Quasinormal Modes of Generic Single Horizon Black Holes

Abstract: We calculate analytically the highly damped quasinormal mode spectra of generic single-horizon black holes using the rigorous WKB techniques of Andersson and Howls (2004 Class. Quantum Grav. 21 1623). We thereby provide a firm foundation for previous analysis, and point out some of their possible limitations. The numerical coefficient in the real part of the highly damped frequency is generically determined by the behaviour of coupling of the perturbation to the gravitational field near the origin, as expressed in tortoise coordinates. This fact makes it difficult to understand how the famous ln(3) could be related to the quantum gravitational microstates near the horizon.

High temperature matter and gamma ray spectra from microscopic black holes

Abstract: The relativistic viscous fluid equations describing the outflow of high temperature matter created via Hawking radiation from microscopic black holes are solved numerically for a realistic equation of state. We focus on black holes with initial temperatures greater than 100 GeV and lifetimes less than 6 days. The spectra of direct photons and photons from ${\ensuremath{\pi}}^{0}$ decay are calculated for energies greater than 1 GeV. We calculate the diffuse gamma ray spectrum from black holes distributed in our galactic halo. However, the most promising route for their observation is to search for point sources emitting gamma rays of ever-increasing energy.

The highly damped quasinormal modes of d-dimensional Reissner-Nordström black holes in the small charge limit

Abstract: We analyse in detail the highly damped quasinormal modes of d-dimensional Reissner-Nordstrbm black holes with small charge, paying particular attention to the large but finite damping limit in which the Schwarzschild results should be valid. In the infinite damping limit we confirm, using different methods, the results obtained previously in the literature for higher dimensional Reissner-Nordstrom black holes. Using a combination of analytic and numerical techniques, we also calculate the transition of the real part of the quasinormal mode frequency from the Reissner-Nordstrom value for very large damping to the Schwarzschild value of In(3)T bh for intermediate damping. The real frequency does not interpolate smoothly between the two values. Instead there is a critical value of the damping at which the topology of the Stokes/anti-Stokes lines changes, and the real part of the quasinormal mode frequency dips to zero.

The mystery of the asymptotic quasinormal modes of Gauss-Bonnet black holes with small Gauss-Bonnet coupling

Abstract: We analyse the quasinormal modes of D-dimensional Schwarzschild black holes with the Gauss–Bonnet correction in the large damping limit and show that standard analytic techniques cannot be applied in a straightforward manner to the case of infinite damping. However, by using a combination of analytic and numeric techniques, which can only be applied to vector perturbations, we are able to calculate the quasinormal mode frequencies of vector perturbations for a range where the damping is large but finite. We show that in this 'intermediate' damping region, the famous ln(3) appears in the real part of the quasinormal mode frequency. In our calculations, the Gauss–Bonnet coupling, α, is taken to be much smaller than the parameter μ, which is related to the black hole mass. Even though our calculations are done for the simplest case of five spacetime dimensions, it is shown that the results hold in every dimension greater than four by substituting an approximate expression for the metric in the calculations, which is valid in the intermediate damping region for small α.

Quasinormal modes of black holes and black branes

Abstract: Quasinormal modes are eigenmodes of dissipative systems. Perturbations of classical gravitational backgrounds involving black holes or branes naturally lead to quasinormal modes. The analysis and classification of the quasinormal spectra require solving non-Hermitian eigenvalue problems for the associated linear differential equations. Within the recently developed gauge-gravity duality, these modes serve as an important tool for determining the near-equilibrium properties of strongly coupled quantum field theories, in particular their transport coefficients, such as viscosity, conductivity and diffusion constants. In astrophysics, the detection of quasinormal modes in gravitational wave experiments would allow precise measurements of the mass and spin of black holes as well as new tests of general relativity. This review is meant as an introduction to the subject, with a focus on the recent developments in the field.

New cosmological constraints on primordial black holes

Abstract: We update the constraints on the fraction of the Universe going into primordial black holes in the mass range ${10}^{9}--{10}^{17}\text{ }\text{ }\mathrm{g}$ associated with the effects of their evaporations on big bang nucleosynthesis and the extragalactic photon background. We include for the first time all the effects of quark and gluon emission by black holes on these constraints and account for the latest observational developments. We then discuss the other constraints in this mass range and show that these are weaker than the nucleosynthesis and photon background limits, apart from a small range ${10}^{13}--{10}^{14}\text{ }\text{ }\mathrm{g}$, where the damping of cosmic microwave background anisotropies dominates. Finally we review the gravitational and astrophysical effects of nonevaporating primordial black holes, updating constraints over the broader mass range $1--{10}^{50}\text{ }\text{ }\mathrm{g}$.

Quasinormal modes of black holes: From astrophysics to string theory

Abstract: Perturbations of black holes, initially considered in the context of possible observations of astrophysical effects, have been studied for the past 10 years in string theory, brane-world models, and quantum gravity. Through the famous gauge/gravity duality, proper oscillations of perturbed black holes, called quasinormal modes, allow for the description of the hydrodynamic regime in the dual finite temperature field theory at strong coupling, which can be used to predict the behavior of quark-gluon plasmas in the nonperturbative regime. On the other hand, the brane-world scenarios assume the existence of extra dimensions in nature, so that multidimensional black holes can be formed in a laboratory experiment. All this stimulated active research in the field of perturbations of higher-dimensional black holes and branes during recent years. In this review recent achievements on various aspects of black hole perturbations are discussed such as decoupling of variables in the perturbation equations, quasinormal modes (with special emphasis on various numerical and analytical methods of calculations), late-time tails, gravitational stability, anti--de Sitter/conformal field theory interpretation of quasinormal modes, and holographic superconductors. We also touch on state-of-the-art observational possibilities for detecting quasinormal modes of black holes.

Constraints on Primordial Black Holes

Abstract: We update the constraints on the fraction of the Universe going into primordial black holes (PBHs) over the mass range $10^{-5}$--$10^{50}$ g. Those smaller than $\sim 10^{15}$ g would have evaporated by now due to Hawking radiation, so their abundance at formation is constrained by the effects of evaporated particles on big bang nucleosynthesis, the cosmic microwave background (CMB), the Galactic and extragalactic $\gamma$-ray and cosmic ray backgrounds and the possible generation of stable Planck mass relics. PBHs larger than $\sim 10^{15}$ g are subject to a variety of constraints associated with gravitational lensing, dynamical effects, influence on large-scale structure, accretion and gravitational waves. We discuss the constraints on both the initial collapse fraction and the current fraction of the cold dark matter in PBHs at each mass scale but stress that many of the constraints are associated with observational or theoretical uncertainties and some are no longer applicable. We also consider indirect constraints associated with the amplitude of the primordial density fluctuations, such as second-order tensor perturbations and $\mu$-distortions arising from the effect of acoustic reheating on the CMB, but these only apply if PBHs are created from the high-$\sigma$ peaks of nearly Gaussian fluctuations. Finally we discuss how the constraints are modified if the PBHs have an extended mass function, this being relevant if PBHs provide some combination of the dark matter, the LIGO/Virgo coalescences and the seeds for cosmic structure.