Gravitational Microlensing by Ellis Wormhole: Second Order Effects

Gravitational lensing is the effect of light bending in a gravitational field. It can be used as a possible observational method to detect or exclude the existence of wormholes. In this work, we extend the work by Abe on gravitational microlensing by Ellis wormhole by including the second order deflection term. Using the lens equation and definition of Einstein radius, we find the angular locations of the physical image inside and outside Einstein ring. The work contains a comparative analysis of light curves between the Schwarzschild black hole and the Ellis wormhole that can be used to distinguish such objects though such distinctions are too minute to be observable even in the near future. We also tabulate the optical depth and event rate for lensing by bulge and Large Magellanic Cloud (LMC) stars.     

Constraining fast radio burst progenitors with gravitational lensing

Fast Radio Bursts(FRBs)are new transient radio sources discovered recently.Because of the angular resolution restriction in radio surveys,no optical counter part has been identified yet so it is hard to determine the progenitor of FRBs.In this paper we propose to use radio lensing survey to constrain FRB progenitors.We show that,different types of progenitors lead to different probabilities for a FRB to be gravitationally lensed by dark matter halos in foreground galaxies,since different type progenitors result in different redshift distributions of FRBs.For example,the redshift distribution of FRBs arising from double stars shifts toward lower redshift than of the FRBs arising from single stars,because double stars and single stars have different evolution timescales.With detailed calculations,we predict that the FRB sample size for producing one lensing event varies significantly for different FRB progenitor models.We argue that this fact can be used to distinguish different FRB models and also discuss the practical possibility of using lensing observation in radio surveys to constrain FRB progenitors.     

Fermion-Fermion Stars

The fermion-fermion stars, i.e., the dark matter self-gravitating systems made from two kinds of fermions with different masses, are considered. We review the stability of the systems, present a comparison between the maxima of gravitational redshift for fermion stars, compact stars, Bondi stars, bonson stars and fermion-fermion stars, and then investigate rotation curves of fermion-fermion stars (two-component concentric spheres) which might be polytropic dark matter halos of galaxies. Results show that the fermion-fermion stars would give rotation curves with flat part at large radii. This presents a striking contrast with the rotation curve of a single component fermion star which has no flat parts.     

Strong field gravitational lensing by a stringy charged black hole

In this paper, we study gravitational lensing of magnetically charged black hole of string theory as a strong field approximation for the supermassive black hole at the center of NGC4486B. We evaluate light deflection angle numerically, from which we obtain magnifications, Einstein rings and observables for the relativistic images. Finally, we explore time delay between relativistic images when they are on the same as well as opposite side of the lens. It is concluded that charge parameter plays a prominent role in the strong gravitational lensing.     

A minimal model for two-component FIMP dark matter:A basic search

In the multi-component configurations of dark matter phenomenology,we propose a minimal twocomponent configuration which is an extension of the Standard Model with only three new fields;one scalar and one fermion interact with the thermal soup through Higgs portal,mediated by the other scalar in such a way that the stabilities of dark matter candidates are made simultaneously by an explicit Z2 symmetry.Against the most common freeze-out framework,we look for dark matter particle signatures in the freeze-in scenario by evaluating the relic density and detection signals.A simple distinguishing feature of the model is the lack of dark matter conversion,so the dark matter components act individually and the model can be adapted entirely to both singlet scalar and singlet fermionic models,separately.We find dark matter self-interaction as the most promising approach to probe such feeble models.Although the scalar component satisfies this constraint,the fermionic one refuses it even in the resonant region.     

Microlensing of Kepler stars as a method of detecting primordial black hole dark matter

If the dark matter consists of primordial black holes (PBHs), we show that gravitational lensing of stars being monitored by NASA's Kepler search for extrasolar planets can cause significant numbers of detectable microlensing events. A search through the roughly 150,000 light curves would result in large numbers of detectable events for PBHs in the mass range 5×10(-10) M(⊙) to 10(-4) M(⊙). Nondetection of these events would close almost 2 orders of magnitude of the mass window for PBH dark matter. The microlensing rate is higher than previously noticed due to a combination of the exceptional photometric precision of the Kepler mission and the increase in cross section due to the large angular sizes of the relatively nearby Kepler field stars. We also present a new formalism for calculating optical depth and microlensing rates in the presence of large finite-source effects.     

Gravitational phase transition of fermionic matter in a general-relativistic framework

The Thomas–Fermi model at finite temperature is extended to describe a system of self-gravitating weakly interacting massive fermions in a general-relativistic framework. The existence and properties of the gravitational phase transition in this model are investigated numerically. It is shown that when a nondegenerate gas of weakly interacting massive fermions is cooled below some critical temperature, a condensed phase emerges, consisting of quasidegenerate fermion stars. For fermion masses of 10 to 25 keV, these fermion stars may very well provide an alternative explanation for the supermassive compact dark objects that are observed at galactic centers. Received: 23 April 1999 / Revised version: 24 June 1999 / Published online: 28 September 1999     

Exact removal of the center-of-mass spurious states from level densities

We give recursive formulae for the exact removal of the contribution of the center-of-mass spurious states from the fixed-spin and parity nuclear level density found in shell-model calculations, provided the total level density for restricted configurations is known. The method is valid for a large class of problems using a harmonic oscillator basis. Using our earlier methods based on statistical spectroscopy that utilize the centroids and widths for a restricted class of fixed-spin configurations, such as Nvariant Planck's over 2piomega excitations, one can calculate very accurately level densities free of spurious states. The approach is applicable to other fermion and boson systems trapped by an oscillator potential.     

Gravitational Lensing by Charged Black Holes

We formulate the lensing effects of a spherically symmetric electrically charged black hole using thin lens equations. The charged black hole leads to three images and could lead to three Einstein rings provided the parameters such as the mass, charge and the distances satisfy certain constraints. We have computed the exact positions of images and magnification properties for a super-massive black hole with electric charge.     

Weak lensing,dark matter and dark energy

Weak gravitational lensing is rapidly becoming one of the principal probes of dark matter and dark energy in the universe. In this brief review we outline how weak lensing helps determine the structure of dark matter halos, measure the expansion rate of the universe, and distinguish between modified gravity and dark energy explanations for the acceleration of the universe. We also discuss requirements on the control of systematic errors so that the systematics do not appreciably degrade the power of weak lensing as a cosmological probe.     

Quasar microlensing

Quasar microlensing deals with the effect of compact objects along the line of sight on the apparent brightness of the background quasars. Due to the relative motion between quasar, lenses and observer, the microlensing magnification changes with time which results in uncorrelated brightness variations in the various images of multiple quasar systems. The amplitudes of the signal can be more than a magnitude with time scales of weeks to months to years. The effect is due to the “granular” nature of the gravitational microlenses—stars or other compact objects in the stellar mass range. Quasar microlensing allows to study the quasar accretion disk with a resolution of tens of microarcseconds, hence quasar microlensing can be used to explore an astrophysical field that is hardly accessible by any other means. Quasar microlensing can also be used to study the lensing objects in a statistical sense, their nature (compact or smoothly distributed, normal stars or dark matter) as well as transverse velocities. Quasar microlensing light curves are now being obtained from monitoring programs across the electromagnetic spectrum from the radio through the infrared and optical range to the X-ray regime. Recently, spectroscopic microlensing was successfully applied, it provides quantitative comparisons with quasar/accretion disk models. There are now more than a handful of systems with several-year long light curves and significant microlensing signal, lending to detailed analysis. This review summarizes the current state of the art of quasar microlensing and shows that at this point in time, observational monitoring programs and complementary intense simulations provide a scenario where some of the early promises of quasar microlensing can be quantitatively applied. It has been shown, e.g., that smaller sources display more violent microlensing variability, first quantitative comparison with accretion disk models has been achieved, and quasar microlensing has been used to determine the fraction of dark matter in a lensing galaxy for the first time. This is the quantitative beginning. The future of quasar microlensing is bright.     

Natural multiparticle entanglement in a Fermi gas

We investigate multipartite entanglement in a noninteracting fermion gas, as a function of fermion separation, starting from the many particle fermion density matrix. We prove that all multiparticle entanglement can be built only out of two-fermion entanglement. Although from the Pauli exclusion principle we would always expect entanglement to decrease with fermion distance, we surprisingly find the opposite effect for certain fermion configurations. The von Neumann entropy is found to be proportional to the volume for a large number of particles even when they are arbitrarily close to each other. We will illustrate our results using different configurations of two, three, and four fermions at zero temperature although all our results can be applied to any temperature and any number of particles.     

Lensing by exotic objects

Many exotic astronomical objects have been introduced. Usually the objects have masses, therefore they may act as gravitational lenses. We briefly discuss gravitational lensing with cosmic strings. As is well-known, dark matter is one of the most important components of the Universe. Recent computer simulations indicate that dark matter may form clumps. We review gravitational lensing (including microlensing) for the clumps.     

Wormhole supported by dark energy admitting conformal motion

In this article, we study the possibility of sustaining static and spherically symmetric traversable wormhole geometries admitting conformal motion in Einstein gravity, which presents a more systematic approach to search a relation between matter and geometry. In wormhole physics, the presence of exotic matter is a fundamental ingredient and we show that this exotic source can be dark energy type which support the existence of wormhole spacetimes. In this work we model a wormhole supported by dark energy which admits conformal motion. We also discuss the possibility of the detection of wormholes in the outer regions of galactic halos by means of gravitational lensing. Studies of the total gravitational energy for the exotic matter inside a static wormhole configuration are also performed.     

Dark matter and baryogenesis in the Fermi-bounce curvaton mechanism

We elaborate on a toy model of matter bounce, in which the matter content is constituted by two fermion species endowed with four fermion interaction terms. We describe the curvaton mechanism that is thus generated, and then argue that one of the two fermionic species may realize baryogenesis, while the other(lighter) one is compatible with constraints on extra hot dark matter particles.     

Fermion Particle Production in a Periodic Potential

In this paper we study fermion particle production in the early universe. The present work is motivated to restudy the fermion particle production from the basics and compare the results in the literature through another method developed by one of the present author. One of the authors (SB) has developed a method, known as complex trajectory WKB method, to study particle production in curved as well as flat spacetime. In the present work we have tried to compare the CWKB method with that of other works, current in the literature. In this work we have obtained the particle production amplitude starting from the basics and test our results through both analytical and numerical calculations. For fermion particle production, we first do analytical calculations with a toy example to calculate the production amplitude and verify the same doing fourth order Runge-Kutta calculation. As most problems relevant to early universe are not amenable to analytical calculations, we then take up to study the particle production in periodic potential, generally used in inflationary cosmology. We recheck two recent approaches and obtain almost identical results as that obtained by Greene and Kofman. We also verify the result through CWKB method. Boson particle production has been discussed elsewhere, we discuss it briefly in connection with CWKB. In the present work we generalize the CWKB results of boson production to fermion production. Our works will enable one to understand the various phenomena in early universe related to particle production. Using CWKB we calculate the occupation number and some other results for fermion particle production. The present work will help us clarify the variant results of fermion production current in the literature.     

Searching for decaying axionlike dark matter from clusters of galaxies

We constrain the lifetime of radiatively decaying dark matter in clusters of galaxies inspired by generic Kaluza-Klein axions, which have been invoked as a possible explanation for the solar coronal x-ray emission. These particles can be produced inside stars and remain confined by the gravitational potential of clusters. By analyzing x-ray observations of merging clusters, where gravitational lensing observations have identified massive, baryon poor structures, we derive the first cosmological lifetime constraint on this kind of particles of tau > or = 10(23) sec.     

Pixel lensing

Pixel lensing is gravitational microlensing of unresolved stars. The main target explored up to now has been the nearby galaxy of Andromeda, M31. The scientific issues of interest are the search for dark matter in form of compact halo objects, the study of the characteristics of the luminous lens and source populations and the possibility of detecting extra-solar (and extra-galactic) planets. In the present work we intend to give an updated overview of the observational status in this field.     

Cryogenics and Einstein Telescope

The dominant noises which limit the present sensitivity of the gravitational wave detectors are the thermal noise of the suspended mirrors and the shot noise. For the third generation of gravitational wave detectors as the Einstein Telescope (ET), the reduction of the shot noise implies to increase the power stored in the detector at 1 MW level and, at the same time, to compensate the huge optic distortion due to induced thermal lensing. At low temperature it is possible to reduce both these effects. However, lowering the temperature of the test masses without injecting vibration noise from the cooling system is a technological challenge. We review here the thermal noise impact on the ultimate ET sensitivity limit and we discuss possible cryogenic configurations to cool the mirror.     

Modeling of a Wide Class of Static Stars Within the Framework of a Unified Approach

A class of static spherically symmetrical models of relativistic stars with a preset energy density distribution is examined. An analytical expression for an approximate solution of the Einstein equations is derived by the method of successive approximations in terms of a small parameter determining the star compactness. Equilibrium star configurations are constructed for a family of models and their stability and basic physical characteristics are investigated. The results of modeling of three different astrophysical objects (a neutron star, a white dwarf (Sirius B), and a star of the main sequence (Sun)) have been compared with the available observational data. This allows us to conclude that the suggested model approach is applicable to a wide class of stars.