Gravity Originates from Variable Energy Density of Quantum Vacuum

<p><strong>The future of theoretical physics is unclear. Two large areas that fall under the umbrella of theoretical physics are cosmology and quantum gravity. Modern cosmology is relatively a much younger field than quantum gravity, and both of these fields require further developments of general relativity. In this thesis we do not hope to resolve the problems facing modern cosmology or theories of quantum gravity. Rather, we will conduct original research into aspects of general relativity that may be used in the future to aid the development and testing of theories of cosmology and quantum gravity.</strong></p><p>It is our view that the largest problem facing astrophysics and cosmology stem from the existence of the dark sector of the Universe. The implication here being that more than ninety percent of the energy density of the Universe is “missing in action” and seemingly consists of dark energy and dark matter. Furthermore, it is apparent that there exist conceptual flaws in our understanding of observational concepts such as expansion versus motion and observer biases. To this end, we investigate the standard spacetime metric used in cosmology, the Friedmann–Lemaˆıtre-Roberston–Walker (FLRW) metric in a peculiar coordinate system — the Painlev´e–Gullstrand coordinates. In this coordinate system (slicing), space is no longer expanding, rather, the galaxies are receding from each other. We hope this will aid in the understanding of expansion, motion, curvature, and observer bias with future work. We further investigate the possibility of black holes in cosmology being directly coupled to the accelerated expansion of the Universe — in other words, black holes as a source for dark energy. However, we show that this is highly implausible.</p><p>Relatively recently it has been postulated that the near black hole horizon limit may be a regime where quantum gravity effects become relevant i.e., quantum gravity may not be restricted to near the Planck scale. We investigate a curious model of black and white holes that shows how one may transition into the other over a finite period of time. This is research conducted in the near horizon limit of the Schwarzschild black hole. We introduce a time dependent function into the usual Schwarzschild black hole spacetime (leaving this new spacetime not a simple coordinate transformed version of the original). This function allows the black hole to transition into a white hole. Importantly, the action for this transition can be shown to be zero, meaning it can be added to the Feynman path integral at no cost.</p><p>Finally, we move to investigating the black hole memory effect. During the last decade, there has been an interesting connection made between the Bondi– Metzner–Sachs (BMS) group — an infinite dimensional group of symmetries found at null infinity — and the gravitational memory effect. In particular, it was shown that the passage of a gravitational wave that alters a Schwarzschild black hole is seen as a supertranslation of the spacetime at null infinity. We extend these calculations to the Kerr and Kerr–Newman black holes. Hence, showing that there may be a way to verify the abstract mathematical ideas predicated on the BMS group by detection of the memory effect in future observations.</p><p>It is our hope that when future gravitational wave detectors such as the laser-interferometer-space-antenna (LISA) are launched, research conducted in this thesis may shed light on how the memory may relate to black holes in their asymptotic & near horizon limits to aid our understanding of the nature of quantum gravity.</p>

Bridging the gap between the approximately ten solar mass 'stellar mass' black holes and the 'supermassive' black holes of millions to billions of solar masses are the elusive 'intermediate-mass' black holes. Their discovery is key to understanding whether supermassive black holes can grow from stellar-mass black holes or whether a more exotic process accelerated their growth soon after the Big Bang. Currently, tentative evidence suggests that the progenitors of supermassive black holes were formed as ∼10(4)-10(5) M(⊙) black holes via the direct collapse of gas. Ongoing searches for intermediate-mass black holes at galaxy centres will help shed light on this formation mechanism.

Thermodynamic stability and holographic heat engine efficiency of a Kerr–Newmann-NUT-Kiselev-AdS black hole in Rastall gravity

The aim of the article is to obtain an intuitive understanding of the recently explored deep connections between thermal physics, quantum field theory and general relativity. A special case in which a detector moves with constant acceleration through a quantum vacuum is examined to clarify the fact that such a detector becomes thermally excited, with a temperature proportional to its acceleration. An elementary physical explanation of this fundamental result is provided. The uniformly accelerated observer finds his space-time manifold bounded by an event horizon and so realizes a ‘model’ black hole. Real black holes also have thermal properties when quantum effects are taken into account; these are described and the correspondences with the accelerated case are pointed out. In particular, an elementary account is given of the thermal Hawking radiation emitted by the black holes formed by collapsed stars.

Stellar-remnant black holes (BH) in dense stellar clusters comprise a natural\nsetup to trigger general-relativistic (GR) inspiral and merger of binary black\nholes (BBH), detectable by the LISA and the LIGO-Virgo, through dynamical\nencounters inside such environments. In this work, the intricacies of such\ndynamical interactions are probed utilizing realistic, self-consistent,\npost-Newtonian, direct N-body evolutionary models of young massive and open\nstellar clusters. Particularly, the configurations of the compact subsystems,\nthat drive the in-cluster GR BBH coalescences, are tracked on the fly. Such an\napproach reveals that the GR coalescences within the open clusters take place\nprimarily via chaotic interactions involving triple BH systems. Although less\nfrequently, such mergers are found to happen also in higher-order subsystems\nsuch as quadruples and in subsystems involving non-BH members; the mergers can\nthemselves be BH---non-BH, which events would leave electromagnetic signatures.\nClose, fly-by encounters inside the clusters can also make BBHs and other types\nof double-compact binaries temporarily post-Newtonian; such binaries would\npotentially contribute to the GW background for the LISA and the PTA. These\ncalculations, furthermore, suggest that open clusters are potential hosts for\nnot only detached BH---main-sequence binaries, as recently identified in the\nglobular cluster NGC 3201, but also a wide variety of other types of\nremnant---non-remnant binaries, which are assembled via dynamical interactions\ninside the clusters and which have the prospects of being discovered in\nradial-velocity surveys.\n

This thesis consists of three separate studies of roles that black holes might play in our universe. In the first part we formulate a statistical method for inferring the cosmological parameters of our universe from LIGO/VIRGO measurements of the gravitational waves produced by coalescing black-hole/neutron-star binaries. This method is based on the cosmological distance-redshift relation, with "luminosity distances" determined directly, and redshifts indirectly, from the gravitational waveforms. Using the current estimates of binary coalescence rates and projected "advanced" LIGO noise spectra, we conclude that by our method the Hubble constant should be measurable to within an error of a few percent. The errors for the mean density of the universe and the cosmological constant will depend strongly on the size of the universe, varying from about 10% for a "small" universe up to and beyond 100% for a "large" universe. We further study the effects of random gravitational lensing and find that it may strongly impair the determination of the cosmological constant. In the second part of this thesis we disprove a conjecture that black holes cannot form in an early, inflationary era of our universe, because of a quantum-field-theory induced instability of the black-hole horizon. This instability was supposed to arise from the difference in temperatures of any black-hole horizon and the inflationary cosmological horizon; it was thought that this temperature difference would make every quantum state that is regular at the cosmological horizon be singular at the black-hole horizon. We disprove this conjecture by explicitly constructing a quantum vacuum state that is everywhere regular for a massless scalar field. We further show that this quantum state has all the nice thermal properties that one has come to expect of "good" vacuum states, both at the black-hole horizon and at the cosmological horizon. In the third part of the thesis we study the evolution and implications of a hypothetical primordial black hole that might have found its way into the center of the Sun or any other solar-type star. As a foundation for our analysis, we generalize the mixing-length theory of convection to an optically thick, spherically symmetric accretion flow (and find in passing that the radial stretching of the inflowing fluid elements leads to a modification of the standard Schwarzschild criterion for convection). When the accretion is that of solar matter onto the primordial hole, the rotation of the Sun causes centrifugal hangup of the inflow near the hole, resulting in an "accretion torus" which produces an enhanced outflow of heat. We find, however, that the turbulent viscosity, which accompanies the convective transport of this heat, extracts angular momentum from the inflowing gas, thereby buffering the torus into a lower luminosity than one might have expected. As a result, the solar surface will not be influenced noticeably by the torus's luminosity until at most three days before the Sun is finally devoured by the black hole. As a simple consequence, accretion onto a black hole inside the Sun cannot be an answer to the solar neutrino puzzle.

Spacetimes representing black holes or central objects embedded in cosmological backgrounds provide explicit examples of time-dependent (apparent) horizons for the study of Hawking radiation and black hole thermodynamics in fully dynamical situations, allow one to explore the spatial variation of the gravitational coupling in scalar-tensor gravity, and help clarifying the long-standing issue of cosmological expansion versus local dynamics 1. Further, alternative gravity may be needed to explain the current acceleration of the universe without dark energy and theories designed for this purpose contain a time-dependent effective cosmological constant so that their black holes are asymptotically Friedmann-Lemâitre-Robertson-Walker (FLRW). Finally, they help studying the accretion of dark energy by black holes. A handful of spherical metrics describing inhomogeneities in FLRW backgrounds are known in General Relativity (GR) and in alternative gravity, but most are not understood. Relevant questions are: are there spacetime singularities, and are they hidden by apparent horizons (AHs)? Where are these AHs, do they expand, and how? Does the black hole mass (if a black hole exists) change? What are the matter sources and the conformal diagrams?

The anticipated discovery of a pulsar in orbit with a black hole is expected to provide a unique laboratory for black hole physics and gravity. In this context, the next generation of radio telescopes, like the Five-hundred-meter Aperture Spherical radio Telescope (FAST) and the Square Kilometre Array (SKA), with their unprecedented sensitivity, will play a key role. In this paper, we investigate the capability of future radio telescopes to probe the space–time of a black hole and test gravity theories by timing a pulsar orbiting a stellar-mass black hole (SBH). Based on mock data simulations, we show that a few years of timing observations of a sufficiently compact pulsar–SBH (PSR–SBH) system with future radio telescopes would allow precise measurements of the black hole mass and spin. A measurement precision of 1 per cent can be expected for the spin. Measuring the quadrupole moment of the black hole, needed to test general relativity's (GR's) no-hair theorem, requires extreme system configurations with compact orbits and a large SBH mass. Additionally, we show that a PSR–SBH system can lead to greatly improved constraints on alternative gravity theories even if they predict black holes (practically) identical to GR's. This is demonstrated for a specific class of scalar–tensor theories. Finally, we investigate the requirements for searching for PSR–SBH systems. It is shown that the high sensitivity of the next generation of radio telescopes is key for discovering compact PSR–SBH systems, as it will allow for sufficiently short survey integration times.

Following a brief review of the “black hole dark energy radiation” and “gravitized vacuum” references, a novel theory of how gravity might affect the quantum vacuum is proposed. This overarching theory proposes that the gravitational field of a sufficiently concentrated collection of matter and/or energy upregulates the virtual particle activity of the adjacent quantum vacuum, thus its energy density and lensing capacity. In contrast to general relativity, the particle and wave duality of quantum physics is necessary for understanding quantum vacuum gravitational effects. Very recent supporting and pending observational studies are discussed, including the ingenious and extremely sensitive vacuum scale to be deployed for the Archimedes Experiment. Support or falsification of this proposal may be imminent.

Exact solutions describing rotating black holes can offer important tests for alternative theories of gravity, motivated by the dark energy and dark matter problems. We present an analytic rotating black hole solution for a class of vector-tensor theories of modified gravity, valid for arbitrary values of the rotation parameter. The new configuration is characterised by parametrically large deviations from the Kerr-Newman geometry, controlled by non-minimal couplings between vectors and gravity. It has an oblate horizon in Boyer-Lindquist coordinates, and it can rotate more rapidly and have a larger ergosphere than black holes in General Relativity (GR) with the same asymptotic properties. We analytically investigate the features of the innermost stable circular orbits for massive objects on the equatorial plane, and show that stable orbits lie further away from the black hole horizon with respect to rotating black holes in GR. We also comment on possible applications of our findings for the extraction of rotational energy from the black hole.

A significant fraction of the stellar population in the cusp around central\nblack holes of galaxies consists of compact remnants of evolved stars, such as\nwhite dwarfs, neutron stars and stellar mass black holes. We estimate the rate\nof capture of compact objects by massive central black holes, assuming most\nspiral galaxies have a central black hole of modest mass ($\\sim 10^6 \\msun$),\nand a cuspy spheroid. It is likely that the total capture rate is dominated by\nnucleated spirals. We estimate the flux of gravitational wave radiation from\nsuch coalescences, and the estimated detectable source count for proposed\nspace--based gravitational wave observatories such as LISA. About one event per\nyear should be detectable within $1 \\, {\\rm Gpc}$, given very conservative\nestimates of the black hole masses and central galactic densities. We expect\n$10^2$--$10^3$ detectable sources at lower frequencies ($10^{-4}$ Hz) ``en\nroute'' to capture. If stellar mass black holes are ubiquitous, the signal may\nbe dominated by stellar mass black holes coalescing with massive black holes.\nThe rate of white dwarf--white dwarf mergers in the cores of nucleated spirals\nis estimated at $\\sim10^{-6}$ per year per galaxy.\n

Challenges in X-ray binary timing: current and future

Modern models predict that galaxies do not evolve in isolation, but exist in haloes of dark matter and gas, embedded in the large-scale cosmic web. To build a self-consistent model for how galaxies form and evolve, it is vital to understand how galaxies regulate their formation through interaction with the circumgalactic medium that surrounds them, and at present the nature and impact of these interactions is poorly-constrained by both theory and observations. This thesis presents new insights into how galaxies affect their surroundings through feedback processes, the impact of the properties of the CGM on galaxy evolution, and how the properties of galaxy-CGM ecosystems are fundamentally connected to the assembly histories of their dark matter haloes. I begin by examining the origin of scatter in the relationship between the gas fraction and mass of dark matter haloes hosting present-day ~L* central galaxies in the EAGLE simulations. The scatter is uncorrelated with the accretion rate of the central galaxy's black hole (BH), but correlates strongly and negatively with the BH's mass, implicating differences in the expulsion of gas by active galactic nucleus (AGN) feedback, throughout the assembly of the halo, as the main cause of scatter. Haloes whose central galaxies host undermassive BHs also tend to retain a higher gas fraction, and exhibit elevated star formation rates (SFRs). Diversity in the mass of central BHs stems primarily from diversity in the dark matter halo binding energy, as these quantities are strongly and positively correlated at fixed halo mass, such that ~L* galaxies hosted by haloes that are more (less) tightly-bound develop central BHs that are more (less) massive than is typical for their halo mass. Variations in the halo gas fraction at fixed halo mass are reflected in both the soft X-ray luminosity and thermal Sunyaev-Zel'dovich flux, suggesting that the prediction of a strong coupling between the properties of galaxies and their halo gas fractions can be tested with measurements of these diagnostics for galaxies with diverse SFRs but similar halo masses. I then examine the connection between the properties of the CGM and the quenching and morphological evolution of central galaxies in both the EAGLE and IllustrisTNG simulations. The simulations yield very different median CGM mass fractions as a function of halo mass, with low-mass haloes being significantly more gas-rich in IllustrisTNG than in EAGLE. Nonetheless, in both cases the scatter in the CGM mass fraction at fixed halo mass is strongly correlated with the specific star formation rate and the kinematic morphology of central galaxies. This feedback elevates the CGM cooling time, preventing gas from accreting onto the galaxy to fuel star formation, and thus establishing a preference for quenched, spheroidal galaxies to be hosted by haloes with low CGM mass fractions for their mass. In both simulations, the CGM mass fraction correlates negatively with the host halo's intrinsic concentration, and hence with its binding energy and formation redshift, primarily because early halo formation fosters the rapid early growth of the central black hole (BH). This leads to a lower CGM mass fraction at fixed halo mass in EAGLE because the BH reaches high accretion rates sooner, whilst in IllustrisTNG it occurs because the central BH reaches the mass threshold at which AGN feedback is assumed to switch from thermal to kinetic injection earlier. Despite these differences, there is consensus from these state-of-the-art simulations that the expulsion of efficiently-cooling gas from the CGM is a crucial step in the quenching and morphological evolution of central galaxies. The above results suggest a connection between the assembly histories of dark matter haloes and the properties of their galaxy-CGM ecosystems. I clearly demonstrate this connection by performing a controlled and systematic experiment in which I adjust the assembly history of a single EAGLE halo hosting a moderately star-forming Milky Way-like galaxy by ``genetically modifying its initial conditions, keeping all other variables fixed. Shifting the halo assembly history to earlier times increases the integrated feedback injected by the AGN, ejecting a greater fraction of the CGM baryons and leading to the quenching and morphological transformation of the central galaxy. These effects can only have originated from differences in the assembly history of the halo, providing compelling evidence for this novel picture of the self-regulation of galaxy formation.

We consider some observational consequences of replacing all black holes (BHs) with a class of non-singular solutions that mimic BHs but with dark energy (DE) interiors; GEneric Objects of DE (GEODEs). We focus on the BH mass function and chirp-mass redshift distribution of mergers visible to gravitational-wave observatories. We incorporate the GEODE blueshift into an initially Salpeter stellar remnant distribution, and model the binary population by evolving synthesized binary remnant distributions, published before LIGO’s first measurements. We find that a GEODE produced between 20 ≲ z ≲ 40, and observed at z ∼ 7, will have its initial mass amplified by ∼20–140×. This can relieve tension between accretion-only growth models and the inferred masses of BHs in quasars at z ≳ 6. Moreover, we find that merger rates of GEODE binaries increase by a factor of ∼2× relative to classical BHs. The resulting GEODE mass function is consistent with the most recent LIGO constraints at <0.5σ. In contrast, a Salpeter stellar distribution that evolves into classical remnants is in tension at ≳2σ. This agreement occurs without low-metallicity regions, abnormally massive progenitor stars, novel formation channels, or primordial object formation at extreme rates. In particular, we find that solar metallicity progenitors, which produce 1.1–1.8M ⊙ remnants, overlap with many LIGO observations when evolved as GEODEs.

Recent baryon acoustic oscillation (BAO) measurements by the Dark Energy Spectroscopic Instrument (DESI) provide evidence that dark energy (DE) evolves with time, as parameterized by a w 0 w a equation of state. Cosmologically coupled black holes (BHs) provide a DE source that naturally evolves with time, because BH production tracks cosmic star-formation. Using DESI BAO measurements and priors informed by Big Bang Nucleosynthesis, we measure the fraction of baryonic density converted into BHs, assuming that all DE is sourced by BH production. We find that the best-fit DE density tracks each DESI best-fit w 0 w a model within 1σ, except at redshifts z ≲ 0.2, highlighting limitations of the w 0 w a parameterization. Cosmologically coupled BHs produce H 0 = (69.94 ± 0.81) km s-1 Mpc-1, with the same χ 2 as ΛCDM, and with two fewer parameters than w 0 w a . This value reduces tension with SH0ES to 2.7σ and is in excellent agreement with recent measurements from the Chicago-Carnegie Hubble Program. Because cosmologically coupled BH production depletes the baryon density established by primordial nucleosynthesis, these BHs provide a physical explanation for the “missing baryon problem” and the anomalously low sum of neutrino masses preferred by DESI.The global evolution of DE is an orthogonal probe of cosmological coupling, complementing constraints on BH mass-growth from elliptical galaxies, stellar binaries, globular clusters, the LIGO-Virgo-KAGRA merging population, and X-ray binaries.A DE density that correlates with cosmic star-formation: 1) is a natural outcome of cosmological coupling in BH populations; 2) eases tension between early and late-time cosmological probes; and 3) produces time-evolution toward a late-time ΛCDM cosmology different from Cosmic Microwave Background projections.