Einstein and Beyond: A Critical Perspective on General Relativity

An alternative approach to Einstein's theory of General Relativity (GR) is reviewed, which is motivated by a range of serious theoretical issues inflicting the theory, such as the cosmological constant problem, presence of non-Machian solutions, problems related with the energy-stress tensor $T^{ik}$ and unphysical solutions. The new approach emanates from a critical analysis of these problems, providing a novel insight that the matter fields, together with the ensuing gravitational field, are already present inherently in the spacetime without taking recourse to $T^{ik}$. Supported by numerous evidences, the new insight revolutionizes our views on the representation of the source of gravitation and establishes the spacetime itself as the source, which becomes crucial for understanding the unresolved issues in a unified manner. This leads to a new paradigm in GR by establishing equation $R^{ik}=0$ as the field equation of gravitation plus inertia in the very presence of matter.Comment: An invited review accepted for publication by `Universe' in its Special Issue "100 Years of Chronogeometrodynamics: the Status of the Einstein's Theory of Gravitation in Its Centennial Year

Editorial introduction to the special issue “The Renaissance of Einstein’s Theory of Gravitation”

[No abstract available

Large-Scale surveys for continuous gravitational waves: from data preparation to multi-stage hierarchical follow-ups

The gravitational wave event GW150914 was the first direct detection of gravitational waves roughly 100 years after their prediction by Albert Einstein. The detection was a breakthrough, opening another channel to observe the Universe. Since then over 90 detections of merging compact objects have been made, most of them coalescences of binary black holes of different masses. There have been two black hole-neutron star, and two binary neutron-star mergers. Another breakthrough was the first binary neutron-star merger, GW170817, associated with a slew of electromagnetic observations, including a gamma-ray burst 1.7s after the merger. Compact binary coalescence events are cataclysmic events in which multiple solar masses are emitted in gravitational waves in ~seconds. Still, their gravitational wave detection requires sophisticated measuring devices: kilometer-scale laser interferometers. Another not yet detected form of gravitational radiation are continuous gravitational waves from e.g., but not limited to, fast-spinning neutron stars nonaxisymmetric relatively to their rotational axis. The gravitational wave amplitude on Earth is orders of magnitude weaker than the compact binary coalescence events, but, in the case of the nonaxisymmetric neutron star, is emitted as long as the neutron star is spinning and sustaining the deformation, which may be months to years. The gravitational wave is mostly emitted at twice the rotational frequency, with a possible frequency evolution (spin-down) due to the energy emitted by gravitational waves, as well as other braking mechanisms. This nearly monochromatic continuous wave is received by observers on Earth Doppler modulated by Earth's orbit and spin. Although the waveform is seemingly simple, the detection problem for signals from unknown sources is very challenging. The all-sky search for unknown neutron stars in our galaxy detailed in this work used the volunteer distributed computing project Einstein@Home and the ATLAS supercomputer for several months, taking tens of thousands of total CPU-time years to complete. In this work I describe the full-scale data analysis procedure, including data preparation, search set-up optimization and post-processing of search results, whose design and implementation is the core of my doctoral research work. I also present a number of observational results that demonstrate the real-world application of the methodologies that I designed.Das Gravitationswellenereignis GW150914 war der erste direkte Nachweis von Gravitationswellen rund 100 Jahre nach deren Vorhersage durch Albert Einstein. Die Entdeckung war ein Durchbruch und eröffnete einen weiteren Kanal zur Beobachtung des Universums. Seitdem wurden über 90 weitere verschmelzende kompakte Objekte entdeckt, die meisten binäre schwarze Löcher unterschiedlicher Masse, aber auch zweimal verschmelzende Schwarze Löcher mit Neutronensternen und zwei Verschmelzungen von binären Neutronensternen. Ein weiterer Durchbruch war die Beobachtung der ersten Verschmelzung zweier Neutronensterne, GW170817, die mit einer Reihe von elektromagnetischen Beobachtungen einherging, darunter ein Gammastrahlenausbruch 1.7s nach der Verschmelzung. Bei der Verschmelzung kompakter Objekte handelt es sich um kataklysmische Ereignisse, bei denen innerhalb von ~Sekunden mehrere Sonnenmassen in Form von Gravitationswellen ausgestoßen werden. Ihr Nachweis erfordert jedoch hochentwickelte Messgeräte: Laserinterferometer im Kilometermaßstab. Eine weitere, noch nicht nachgewiesene Form der Gravitationsstrahlung sind kontinuierliche Gravitationswellen, die z.B., aber nicht nur, von schnell rotierenden Neutronensternen ausgehen, die relativ zu ihrer Rotationsachse nicht achsensymmetrisch sind. Die Amplitude der kontinuierlichen Gravitationswellen auf der Erde ist um Größenordnungen schwächer als die der verschmelzenden kompakten Objekte, wird aber im Fall des nicht achsensymmetrischen Neutronensterns so lange abgestrahlt, wie der Neutronenstern rotiert und die Deformation aufrechterhält, was Monate bis Jahre sein können. Die Gravitationswelle wird meist mit der doppelten Rotationsfrequenz ausgestrahlt, wobei eine Frequenzentwicklung (Spin-down) aufgrund der von Gravitationswellen ausgesandten Energie, sowie anderer Bremsmechanismen möglich ist. Diese nahezu monochromatische, kontinuierliche Welle wird von einem Beobachter auf der Erde Doppler-moduliert durch die Erdumlaufbahn und die Erddrehung empfangen. Obwohl die Wellenform scheinbar einfach ist, ist das Problem des Nachweises von Signalen aus unbekannten Quellen eine große Herausforderung. Die in dieser Arbeit beschriebene Suche nach unbekannten Neutronensternen in unserer Galaxie über den kompletten Himmel verwendete über mehrere Monate hinweg das Volunteer-Computing-Projekt Einstein@Home und den ATLAS-Supercomputer und benötigte insgesamt Zehntausende von Jahren an Rechenzeit. In dieser Arbeit beschreibe ich das vollständige Datenanalyseverfahren einschließlich der Datenvorbereitung, der Optimierung der Suchparameter und der Nachbearbeitung der Suchergebnisse, dessen Entwurf und Implementierung das Kernstück meiner Doktorarbeit darstellt. Außerdem stelle ich eine Reihe von Beobachtungsergebnissen vor, welche die praktische Anwendung der von mir entwickelten Methoden demonstrieren

Detecting the gravito-magnetic field of the dark halo of the Milky Way - the LaDaHaD mission concept

We propose to locate transponders and atomic clocks in at least three of the Lagrange points of the Sun-Earth pair, with the aim of exploiting the time of flight asymme- try between electromagnetic signals travelling in opposite directions along polygonal loops having the Lagrange points at their vertices. The asymmetry is due to the pres- ence of a gravito-magnetic field partly caused by the angular momentum of the Sun, partly originating from the angular momentum of the galactic dark halo in which the Milky Way is embedded. We list also various opportunities which could be associated with the main objective of this Lagrange Dark Halo Detector (LaDaHaD)

PHEMTO: the polarimetric high energy modular telescope observatory

Based upon dual focusing techniques, the Polarimetric High-Energy Modular Telescope Observatory (PHEMTO) is designed to have performance several orders of magnitude better than the present hard X-ray instruments, in the 1–600 keV energy range. This, together with its angular resolution of around one arcsecond, and its sensitive polarimetry measurement capability, will give PHEMTO the improvements in scientific performance needed for a mission in the 2050 era in order to study AGN, galactic black holes, neutrons stars, and supernovae. In addition, its high performance will enable the study of the non-thermal processes in galaxy clusters with an unprecedented accuracy

Compact Stars in the QCD Phase Diagram

The book edition of the Universe Special Issue “Compact Stars in the QCD Phase Diagram” is devoted to the overarching aspects shared between heavy-ion collisions and compact star astrophysics in investigating the hadron-to-quark matter phase transition in the equation of state of strongly interacting matter in different regions of the phase diagram of QCD. It comprises 22 review and research articles that, together, will serve as a useful guide in educating both young and senior scientists in this emerging field that represents an intersection of the communities of strongly interacting matter theory, heavy-ion collision physics and compact star astrophysics

The Prediction of Anyons: Its History and Wider Implications

Prediction of ``anyons'', often attributed exclusively to Wilczek, came first from Leinaas & Myrheim in 1977, and independently from Goldin, Menikoff, & Sharp in 1980-81. In 2020, experimentalists successfully created anyonic excitations. This paper discusses why the possibility of quantum particles in two-dimensional space with intermediate exchange statistics eluded physicists for so long after bosons and fermions were understood. The history suggests ideas for the preparation of future researchers. I conclude by addressing failures to attribute scientific achievements accurately. Such practices disproportionately hurt women and minorities in physics, and are harmful to science.Comment: Based on a presentation in the 50th Anniversary Special Session of the 34th International Colloquium on Group Theoretical Methods in Physics, Strasbourg, France, July 202

Past, Present and Future of a Habitable Earth

This perspective of this book views Earth's various layers as a whole system, and tries to understand how to achieve harmony and sustainable development between human society and nature, with the theme of " habitability of the Earth." This book is one effort at providing an overview of some of the recent exciting advances Chinese geoscientists have made. It is the concerted team effort of a group of researchers from diverse backgrounds to generalize their vision for Earth science in the next 10 years. The book is intended for scholars, administrators of the Science and Technology policy department, and science research funding agencies. This is an open access book

GrailQuest: hunting for atoms of space and time hidden in the wrinkle of Space-Time

GrailQuest (Gamma Ray Astronomy International Laboratory for QUantum Exploration of Space-Time) is a mission concept based on a constellation (hundreds/thousands) of nano/micro/small-satellites in low (or near) Earth orbits. Each satellite hosts a non-collimated array of scintillator crystals coupled with Silicon Drift Detectors with broad energy band coverage (keV-MeV range) and excellent temporal resolution (≤ 100 nanoseconds) each with effective area ∼100cm2. This simple and robust design allows for mass-production of the satellites of the fleet. This revolutionary approach implies a huge reduction of costs, flexibility in the segmented launching strategy, and an incremental long-term plan to increase the number of detectors and their performance; this will result in a living observatory for next-generation, space-based astronomical facilities. GrailQuest is conceived as an all-sky monitor for fast localisation of high signal-to-noise ratio transients in the X-/gamma-ray band, e.g. the elusive electromagnetic counterparts of gravitational wave events. Robust temporal triangulation techniques will allow unprecedented localisation capabilities, in the keV-MeV band, of a few arcseconds or below, depending on the temporal structure of the transient event. The ambitious ultimate goal of this mission is to perform the first experiment, in quantum gravity, to directly probe space-time structure down to the minuscule Planck scale, by constraining or measuring a first-order dispersion relation for light in vacuo. This is obtained by detecting delays between photons of different energies in the prompt emission of Gamma-Ray Bursts