News and Views: Challenges of Relativistic Astrophysics

I discuss some of the most outstanding challenges in relativistic astrophysics in the subjects of compact objects (black holes and neutron stars), dark sector (dark matter and dark energy), plasma astrophysics (origin of jets, cosmic rays, and magnetic fields), and the primordial universe (physics at the beginning of the Universe). In these four subjects, I discuss 12 of the most important challenges. These challenges give us insight into new physics that can only be studied in the large scale universe. The near-future possibilities, in observations and theory, for addressing these challenges are also discussed.     

Tracing activity in distant supermassive black holes with X-rays

Supermassive black holes at the centres of galaxies have long been thought to be the engines of quasars, which emit more energy than any other sources in the Universe. In the local Universe, dormant supermassive black holes have been detected through the motions of stars and gas near the galactic centres. In the distant Universe, high energy X-ray observations are now revealing the accretion of matter onto supermassive black holes, even when the black holes are highly obscured by gas and dust. Great advances are being made in obtaining a cosmic census of supermassive black holes. The duration, times, and mass inflow rates to these black holes are being traced via multiwavelength follow-up observations with ground-based telescopes and a time history of the accretion is thereby being reconstructed.     

Chemical Evolution of the Universe and its Consequences for Gravitational-Wave Astrophysics

Gravitational waves (GW) emitted by merging black holes (BH) and neutron stars are now routinely detected. Those are the afterlives of massive stars that formed all across the Universe—at different cosmic times and with different metallicities. Birth metallicity plays an important role in the evolution of massive stars. Consequently, the population properties of mergers are sensitive to the metallicity dependent cosmic star formation history (f_SFR(Z,z)). In particular, within the isolated formation scenarios (the focus of this paper), a strong low metallicity preference of the formation of BH mergers is found. The origin of this dependence and its consequences are discussed. Most importantly, uncertainty in the f_SFR(Z,z) (substantial even at low redshifts) cannot be ignored in the models. This poses a challenge for the interpretation of the observed GW source population properties. Possible improvements and the role of future GW detectors are considered. Recent efforts to determine f_SFR(Z,z) and the factors that dominate its uncertainty are summarized. Many of those factors stem from the uncertain properties of faint and distant galaxies. The fact that they leave imprint on the redshift-dependent properties of mergers makes GW a promising (and complementary to electromagnetic observations) tool to study galaxy chemical evolution.     

Scientific discovery with the James Webb Space Telescope

For the past 400 years, astronomers have sought to observe and interpret the Universe by building more powerful telescopes. These incredible instruments extend the capabilities of one of our most important senses, sight, towards new limits such as increased sensitivity and resolution, new dimensions such as exploration of wavelengths across the full electromagnetic spectrum, new information content such as analysis through spectroscopy, and new cadences such as rapid time-series views of the variable sky. The results from these investments, from small to large telescopes on the ground and in space, have completely transformed our understanding of the Universe; including the discovery that Earth is not the centre of the Universe, that the Milky Way is one among many galaxies in the Universe, that relic cosmic background radiation fills all space in the early Universe, that that the expansion rate of the Universe is accelerating, that exoplanets are common around stars, that gravitational waves exist, and much more. For modern astronomical research, the next wave of breakthroughs in fields ranging over planetary, stellar, galactic, and extragalactic science motivate a general-purpose observatory that is optimised at near- and mid-infrared wavelengths, and that has much greater sensitivity, resolution, and spectroscopic multiplexing than all previous telescopes. This scientific vision, from measuring the composition of rocky worlds in the nearby Milky Way galaxy to finding the first sources of light in the Universe to other topics at the forefront of modern astrophysics, motivates the state-of-the-art James Webb Space Telescope (Webb). In this review paper, I summarise the design and technical capabilities of Webb and the scientific opportunities that it enables.     

Non-equilibrium in cosmology

All the non-trivial features of the Universe we see around us, such as particles, stars, galaxies, and clusters of galaxies, are the result of non-equilibrium processes in the cosmic evolution. These lectures aim to provide some general background in cosmology and to examine specific, and notable, examples of departures from thermal equilibrium. They are organized as follows: 1) Overview of the thermal history of the Universe after the Big Bang: the relevant time-scales and the mechanism of particle decoupling from the themal bath; 2) Explicit examples of cosmic relics: nucleosynthesis, photons and the cosmic microwave background, neutrinos, and cold dark matter; 3) Baryogenesis: the generation of the baryon asymmetry of the Universe; 4) The formation of cosmic structures (galaxies, clusters of galaxies): from the Vlasov equation to the renormalization group.     

Space-borne gravitational wave observatories

An overview of our current knowledge of black seed formation models following their growth history over cosmic time is presented. Both light seed formation channels remnants of the first stars and the more massive direct collapse seed formation scenarios are outlined. In particular, the focus is on the implications of these various scenarios and what these initial conditions imply for the highest redshift black holes, the local black hole population, the highest mass black holes at each epoch and the low mass end of the black hole mass function all of which are currently observed. The goal is to present a broad and comprehensive picture of the current status; the open questions and challenges faced by black hole growth models in matching current observational data and the prospects for future observations that will help discriminate between competing models.     

Gravitational collapse of dark energy field configurations and supermassive black hole formation

Dark energy is the dominant component of the total energy density of our Universe. The primary interaction of dark energy with the rest of the Universe is gravitational. It is therefore important to understand the gravitational dynamics of dark energy. Since dark energy is a low-energy phenomenon from the perspective of particle physics and field theory, a fundamental approach based on fields in curved space should be sufficient to understand the current dynamics of dark energy. Here, we take a field theory approach to dark energy. We discuss the evolution equations for a generic dark energy field in curved space-time and then discuss the gravitational collapse for dark energy field configurations. We describe the 3 + 1 BSSN formalism to study the gravitational collapse of fields for any general potential for the fields and apply this formalism to models of dark energy motivated by particle physics considerations. We solve the resulting equations for the time evolution of field configurations and the dynamics of space-time. Our results show that gravitational collapse of dark energy field configurations occurs and must be considered in any complete picture of our Universe. We also demonstrate the black hole formation as a result of the gravitational collapse of the dark energy field configurations. The black holes produced by the collapse of dark energy fields are in the supermassive black hole category with the masses of these black holes being comparable to the masses of black holes at the centers of galaxies.     

The gravitational wave symphony of the Universe

The new millennium will see the upcoming of several ground-based interferometric gravitational wave antennas. Within the next decade a space-based antenna may also begin to observe the distant Universe. These gravitational wave detectors will together operate as a network taking data continuously for several years, watching the transient and continuous phenomena occurring in the deep cores of astronomical objects and dense environs of the early Universe where gravity was extremely strong and highly nonlinear. The network will listen to the waves from rapidly spinning non-axisymmetric neutron stars, normal modes of black holes, binary black hole inspiral and merger, phase transitions in the early Universe, quantum fluctuations resulting in a characteristic background in the early Universe. The gravitational wave antennas will open a new window to observe the dark Universe unreachable via other channels of astronomical observations.     

Some highlights of the first four years of the Fermi Gamma-ray Space Telescope

The Fermi Gamma-ray Space Telescope, formerly called GLAST, measures the cosmic gamma-ray flux in the energy range 8 keV to 〉 300 GeV. In addition to breakthrough capabilities in energy coverage and localization, the very large field of view enables observations of 20~ of the sky at any instant, and the entire sky on a timescale of a few hours. With its launch in 2008, Fermi opens a new and important window on a wide variety of phenomena, including pulsars, black holes and active galactic nuclei, gamma-ray bursts, supernova remnants and the origins of cosmic rays, and searches for hypothetical new phenomena such as particle dark matter annihilations. A brief overview and selected science highlights from the first four years are provided.     

Primordial black hole as a source of the boost factor

Primordial black holes (PBHs) accumulate weakly interacting massive particles (WIMPs) around them and form ultracompact minihalos (UCMHs), if the WIMP is a dominant component of the dark matter (DM). In this Letter, we discuss that the UCMHs seeded by the PBHs with sub-earth mass enhance the WIMP annihilation in the present Universe and can successfully explain the positron and/or electron excess in cosmic ray observed by PAMELA/Fermi experiments. The signal is very similar to that from a decaying dark matter, which can explain the PAMELA and/or Fermi anomaly without conflict with any constraints as long as the decay mode is proper. In this scenario, the boost factor can be as large as 10⁵. In addition, we discuss testability of our scenario by gamma-ray point source and gravitational-wave experiments.     

Different approaches to the study of the gravitational radiation emitted by astrophysical sources

Stars and black holes are sources of gravitational radiation in many phases of their life, and the signals they emit exhibit features that are characteristic of the generating process. Emitted since the beginning of star formation, these signals also contribute to create a stochastic background of gravitational waves. We shall show how the spectral properties of this background can be estimated in terms of the energy spectrum of each single event and of the star formation rate history, which is now deducible from astronomical observations. We shall further discuss the process of scattering of masses by stars and black holes, showing that, unlike black holes, stars emit signals that carry a clear signature of the nature of the source.     

Letter: A Model of Dark Energy for the Accelerating Universe

Recent observations of large scale structure of the Universe, especially that of Type Ia supernovae, indicate that the Universe is flat and is accelerating, and that the dominant energy density in the Universe is the cosmic dark energy. We propose a model in which the cosmic effective Yang-Mills condensate familiar in particle physics plays the role of the dark energy that causes the acceleration of the Universe. Since the quantum effective Yang-Mills field in certain states has the equation of state p _y = – _y , when employed as the cosmic matter source, it naturally results in an accelerating expansion of the Universe. With the matter components ( _m 1/3) being added into the model, the composition of YM condensate and matter components can give rise to the desired equation of state w –2/3 for the Universe.     

Neutrino masses, dark matter and the mysterious early quasars

The lightest right-handed or sterile neutrino, that is embedded in a renormalizable seesaw-like extension of the standard model, with a mass and a tiny mixing θ∼10^−6.5 to one of the left-handed active neutrinos, is an attractive quasi-stable dark matter particle candidate. This sterile neutrino is produced in the early universe with the dark matter abundance required by WMAP. It is quasi-stable, decaying in about 10¹⁹ years into two neutrinos and an antineutrino, and it may be observed directly through its subdominant radiative decay into an active neutrino and a photon in about 10²¹ years. In contrast to the galaxies, that are known to form hierarchically, the supermassive black holes are formed anti-hierarchically, i.e. the most massive quasars first, and the least massive active galactic nuclei last. Here we argue that the anti-hierarchical formation of the supermassive black holes may be due to the possibility that both, the quasars and active galactic nuclei, may originate from supermassive degenerate neutrino balls that are swallowed up by stellar-mass black holes, produced by supernova explosions of massive stars at the centers of the neutrino balls.     

Cosmological Black Holes

In this paper we propose a model for the formation of the cosmological voids. We show that cosmological voids can form directly after the collapse of extremely large wavelength perturbations into low-density black holes or cosmological black holes (CBH). Consequently the voids are formed by the comoving expansion of the matter that surrounds the collapsed perturbation. It follows that the universe evolves, in first approximation, according to the Einstein-Straus cosmological model. We discuss finally the possibility to detect the presence of these black holes through their weak and strong lensing effects and their influence on the cosmic background radiation.     

Gravitational wave formation from the collapse of dark energy field configurations

Dark energy is the dominant component of the energy density in the Universe. In a previous paper, we have shown that the collapse of dark energy fields leads to the formation of supermassive black holes with masses comparable to the masses of black holes at the centers of galaxies. Thus, it becomes a pressing issue to investigate the other physical consequences of the collapse of dark energy fields. Given that the primary interactions of dark energy fields with the rest of the Universe are gravitational, it is particularly interesting to investigate the gravitational wave signals emitted during the collapse of dark energy fields. This is the focus of the current work described in this paper. We describe and use the 3+1 BSSN formalism to follow the evolution of the dark energy fields coupled with gravity and to extract the gravitational wave signals. Finally, we describe the results of our numerical computations and the gravitational wave signals produced by the collapse of dark energy fields.     

Can Superconducting Cosmic Strings Piercing Seed Black Holes Generate Supermassive Black Holes in the Early Universe?

The discovery of a large number of supermassive black holes (SMBH) at redshifts , when the Universe was only 900 million years old, raises the question of how such massive compact objects could form in a cosmologically short time interval. Each of the standard scenarios proposed, involving rapid accretion of seed black holes or black hole mergers, faces severe theoretical difficulties in explaining the short‐time formation of supermassive objects. In this work we propose an alternative scenario for the formation of SMBH in the early Universe, in which energy transfer from superconducting cosmic strings piercing small seed black holes is the main physical process leading to rapid mass increase. As a toy model, the accretion rate of a seed black hole pierced by two antipodal strings carrying constant current is considered. Using an effective action approach, which phenomenologically incorporates a large class of superconducting string models, we estimate the minimum current required to form SMBH with masses of order by . This corresponds to the mass of the central black hole powering the quasar ULAS J112001.48+064124.3 and is taken as a test case scenario for early‐epoch SMBH formation. For GUT scale strings, the required fractional increase in the string energy density, due to the presence of the current, is of order 10⁻⁷, so that their existence remains consistent with current observational bounds on the string tension. In addition, we consider an “exotic” scenario, in which an SMBH is generated when a small seed black hole is pierced by a higher‐dimensional string, predicted by string theory. We find that both topological defect strings and fundamental strings are able to carry currents large enough to generate early‐epoch SMBH via our proposed mechanism.     

The history of star formation in the universe

Our primary means of studying how galaxies form and evolve over cosmic time is through measurements of the rate at which massive stars are born in galaxies per unit comoving volume of the universe. Only recently have the most distant, most massively star-forming galaxies in the universe even been discovered, and these discoveries are challenging the standard cosmological scenario in which galaxies form hierarchically, with low-mass objects collapsing first and then merging to form larger and larger systems over cosmic time. We now know that these incredibly luminous galaxies, which have all but vanished in the local universe, are cocooned in dust, hiding them from optical view. They can only be detected at long wavelengths where the dust reradiates the starlight that it has absorbed. In this review, I trace the spectacular progress that has been made over the last decade towards understanding the cosmic history of star formation, with a particular emphasis on the important role of dusty sources in that history.     

A common origin of neutralino stars and supermassive black holes

To account for the microlensing events observed in the Galactic halo, Gurevich, Zybin, and Sirota have proposed a model of gravitationally bound, noncompact objects with masses of ～(0.01–1)M _⊙. These objects are formed in the expanding Universe from adiabatic density perturbations and consist of weakly interacting particles of dark matter, for example, neutralinos. They assumed the perturbation spectrum on some small scale to have a distinct peak. We show that the existence of this peak would inevitably give rise to a large number of primordial black holes (PBHs) with masses of ～10⁵ M _⊙ at the radiation-dominated evolutionary stage of the Universe. Constraints on the coefficient of nonlinear contraction and on the compactness parameter of noncompact objects were derived from constraints on the PBH number density. We show that noncompact objects can serve as gravitational lenses only at a large PBH formation threshold, δ_c > 0.5, or if noncompact objects are formed from entropic density perturbations.     

Hydrodynamic and hydromagnetic stability of black holes with radiative transfer

Subrahmanyan Chandrasekhar (Chandra) was just eight years old when the first astrophysical jet was discovered in M87. Since then, jets have been uncovered with a wide variety of sources including accretion disks orbiting stellar and massive black holes, neutron stars, isolated pulsars, γ-ray bursts, protostars and planetary nebulae. This talk will be primarily concerned with collimated hydromagnetic outflows associated with spinning, massive black holes in active galactic nuclei. Jets exhibit physical processes central to three of the major research themes in Chandrasekhar’s research career – radiative transfer, magnetohydrodynamics and black holes. Relativistic jets can be thought of as ‘exhausts’ from both the hole and its orbiting accretion disk, carrying away the energy liberated by the rotating spacetime and the accreting gas that is not radiated. However, no aspect of jet formation, propagation and radiation can be regarded as understood in detail. The combination of new γ-ray, radio and optical observations together with impressive advances in numerical simulation make this a good time to settle some long-standing debates.     

Will black holes eventually engulf the Universe?

The Babichev–Dokuchaev–Eroshenko model for the accretion of dark energy onto black holes has been extended to deal with black holes with non-static metrics. The possibility that for an asymptotic observer a black hole with large mass will rapidly increase and eventually engulf the Universe at a finite time in the future has been studied by using reasonable values for astronomical parameters. It is concluded that such a phenomenon is forbidden for all black holes in quintessential cosmological models.