Event to Commemorate 3 Decades of IGC
On September 14, 2021, IGC held an event to commemorate Professor Abhay Ashtekar's tenure as the founding Director of the institute which he has led with great vision for almost three decades. During this period, the institute has grown enormously and has had great impact on generations of graduate students, postdoctoral fellows and faculty. Several speakers at the event gave tribute to Abhay's leadership in making IGC an international leader in several areas in Fundamental Physics, Theoretical and Observational Cosmology, and Multi-messenger Astrophysics. The meeting culminated with a short talk by Abhay on his recollections of how the institute was founded, transformations it has undergone and its impact on science and the role played by staff in maintaining a great atmosphere.
First detection of light echoes from behind a black hole
Light cannot escape from a black hole, but for the first time ever, researchers have observed light from behind a black hole — a scenario that was predicted by Einstein’s theory of General Relativity but never confirmed, until now. In a paper published July 28 in Nature, a team including Niel Brandt reports recordings of X-ray emissions from the far side of a black hole.
Abhay Ashtekar featured in the Oral History Program
Abhay Ashtekar was interviewed by the American Institute of Physics under their Oral History Program which aims to “offer unique insights into the lives, works, and personalities of modern scientists.” The transcript of this 2 hour long interview is now available at the Niels Bohr Library & Archives.
Six Lectures at the United Center for Gravitational Waves Summer School; August 2021
Since the first detection of Gravitational Waves six years ago, the field has literally exploded in multiple directions that include multi-wavelength astronomy and astrophysics, approximation methods in general relativity, numerical relativity, application of Machine Learning to waveform model building, forefront cosmological issues such as the Hubble tension, and nuclear physics issues related to the equation of state of neutron stars and nuclear processes at extreme temperature. Therefore Gravitational Wave Science has emerged as one of the most exciting research areas that now attracts young researchers in large numbers. At the same time this very explosion of the field makes it difficult for young researchers to grasp, even in broad terms, the conceptual and mathematical foundation of the theory of Gravitational Waves since the investigations that built these foundations are rarely discussed in the specific areas these researchers work in everyday. The purpose of these six lectures by Abhay Ashtekar is to fill this gap. The notion of `radiation’ requires global and rather subtle constructions. Because of these subtleties, there was considerable confusion even about the physical reality of gravitational waves in full general relativity for several decades! This confusion was dispelled, thanks to a beautiful interplay between physics and geometry. Every theoretical researcher in the field should be aware of how difficulties associated with coordinate invariance are overcome and fully gauge invariant quantities representing physical observables are extracted. This awareness would provide a broad perspective that can guide their own research. Furthermore, as discussed in the last two lectures, foundational issues can also have concrete applications in addressing `practical issues’.
Observation of gravitational waves from two neutron star-black hole mergers
The LIGO-Virgo-KAGRA Scientific Collaboration, has announced the discovery of two neutron star-black hole mergers in the data from the third observing run, separated by 10 days on 5 and 15 January 2021. The IGC LIGO group played a crucial role in this new discovery: Both of these mergers were detected as part of real-time gravitational wave processing conducted by members of the IGC LIGO group. LIGO and Virgo detectors have previously observed the merger of dozens of binary black holes and two binary neutron stars. Neutron star-black hole binaries were believed to exist but this is the first time ever astronomers have witnessed such a phenomena. In each case, the neutron star was likely swallowed whole by its black-hole partner without emitting any electromagnetic radiation. The system observed on January 5 had companion masses of 1.5 solar mass for the neutron star and 5.6 solar mass for the black hole, while the one observed on January 15 had masses 1.9 solar mass for the neutron star and 8.7 solar mass for the black hole. Both the systems came from roughly a distance of 300 Mpc. The details of the announcement can be found in the Penn State News Article.
Book: The Origin of Spacetime Physics
As Abhay Ashtekar explains in his Foreword to the monograph “Origin of Spacetime Physics” our understanding of space, time and the universe underwent a revolution during the first three decades of the 20th century. This Monograph is a collection of 13 seminal papers by Lorentz, Poincaré, Einstein, de Sitter, Friedmann and Lemaîre. The Foreward puts these original writings in historical context. Its use of contemporary language and concepts makes the rather complicated evolution of ideas accessible to younger researchers. It is quite astonishing to see how close Lorentz and Poincarè came to developing special relativity, but did not take the bold leap of abolishing Newton’s absolute simultaneity. Similarly, younger researchers would find the delay in accepting the Friedmann-Lemaître, matter filled, dynamical universes very surprising. These historical developments shed much light on how scientific revolutions actually happen. They do not follow a simple trajectory as presented in modern discussions! The monograph was published by Minkowski Press and is available on Amazon.
Listening to the Universe with New Gravitational Wave Observatories
B. Sathyaprakash is a lead author on the cover story of the May 2021 issue of Nature Physics Reviews on the future of gravitational-wave astronomy. As Abhay Ashtekar explained, “Gravitational-wave observations of binary black-hole and neutron-star mergers by LIGO and Virgo have opened a completely new window on the Universe. The gravitational-wave spectrum, extending from attohertz to kilohertz frequencies, provides a fertile ground for exploring many fundamental questions in physics and astronomy.” Pulsar timing arrays probe the nanohertz to microhertz frequency band to detect gravitational-wave remnants from past mergers of supermassive black holes. The space-based Laser Interferometer Space Antenna will target gravitational-wave sources from microhertz up to hundreds of millihertz and trace the evolution of black holes from the early Universe through the peak of the star formation era. Einstein Telescope and Cosmic Explorer, two future ground-based observatories now under development for the 2030s, are pursuing new technologies to achieve a tenfold increase in sensitivity to study compact object evolution to the beginning of the star formation era. As Sathyaprakash put it, “Gravitational-wave observations provide a new tool for observing the Universe, and future observatories are guaranteed to make discoveries that could transform our understanding of many of the current problems in physics and Astronomy.”
Abhay Ashtekar delivered a Colloquium to the Consortium of Brazilian Universities and Research Institutes drawn from all Brazilian Provinces on April 7th 2021: 'Black Holes, Big Bang and Gravitational Waves: Examples of Paradigm Shifts in Science'
Big Bang, Black Holes, Big Bang and Gravitational Waves now appear as compelling –even obvious– consequences of general relativity. Therefore it may seem surprising that none of these ideas were readily accepted. Not only was there considerable debate, but in fact leading figures were often arguing on what turned out to be the `wrong side’ of history. These developments provide excellent lessons for younger researchers on how science unfolds. Paradigm shifts in science occur when younger researchers have the courage not to accept ideas merely because they are mainstream; patience to systematically develop novel avenues they deeply believe in; and maturity to accept that a variety of factors –not all logical or even science related– can drive or slow down scientific progress.
Abhay Ashtekar gave the Inaugural Interview in the series “On the Shoulders of Giants” to Nawat@nuscidev.or
An Arabian initiative to promote forefront science in the regional Universities and Research Institutes, on the theme Einstein’s Cosmos and the Quantum on March 4th 2021
The IGC ApJL “Dark sirens to resolve the Hubble-Lemaître tension” was highlighted in AAS Nova.
Besides tests of GR, Gravitational Wave (GW) observations of compact binary mergers also play a unique role for cosmology: They facilitate the measurement of the Hubble constant through a luminosity distance estimate in combination with an independent, electromagnetic redshift measurement from the GW source’s host galaxy. In the light of the tension between the electromagnetic measurements of the Hubble constant from the early and late universe, the GW-assisted method has gained even more importance. While other studies have investigated methods to resolve the Hubble-Lemaître tension with binary neutron stars, IGC researchers Ssohrab Borhanian, Arnab Dhani, Anuradha Gupta, K.G. Arun and B.S. Sathyaprakash demonstrated that there is an observable population of golden dark sirens that could facilitate this resolution in the next five years without direct electromagnetic counterparts or statistical methods. Instead, improved waveform models using higher spherical harmonic modes and planned upgrades to GW detectors will compound together to localize these golden events with high-precision—in distance and on the sky—to uniquely determine their host galaxies in the local universe and consequently measure the Hubble constant to the required precision.”
Professor Stephanie Wissel’s Antarctic balloon mission was chosen for NASA Pioneers program
Professor Wissel and her team in the IGC will search for the highest energy neutrinos using the long duration balloon experiment, PUEO, flown in Antarctica. PUEO is the first and only balloon payload to date selected for the new NASA Pioneers program. As Dr. Thomas H. Zurbuchen, associate administrator of NASA’s Science Mission Directorate of the new program, put it, “These concept studies bring innovative, out-of-the-box thinking to the problem of how to do high-impact astrophysics experiments on a small budget.”
Eberly College of Science 2021 Frontiers of Science Lecture Series entitled “Sustainability: How Science Can Help Achieve the UN’s 2030 Sustainable Development Goals” The series will consist of 6 public lectures, held on consecutive Saturdays via Zoom.