Some features of the Cosmos in Loop Quantum Gravity

By Parampreet Singh, Louisiana State University, USA


CQG Editorial Board member, Param Singh, Guest Edited the Applications of loop quantum gravity to cosmology focus issue in 2016 and 2017

A successful union of Einstein’s general relativity and quantum theory is one of the most fundamental problems of theoretical physics. Though a final theory of quantum gravity is not yet available, its lessons and techniques can already be used to understand  quantization of various spacetimes. Of these, cosmological spacetimes are of special interest. They provide a simpler yet a non-trivial and a highly rich setting to explore detailed implications of quantum gravitational theories. Various conceptual and technical difficulties encountered in understanding quantum dynamics of spacetime in  quantum gravity can be bypassed in such a setting. Further, valuable lessons can be learned for the quantization of more general spacetimes.

In the last decade, progress in loop quantum gravity has provided avenues which allow us to reliably answer various interesting questions about the quantum dynamics of spacetime in the cosmological setting. Quantum gravitational dynamics of cosmological spacetimes obtained using techniques of  loop quantum gravity leads to a novel picture where singularities of Einstein’s theory of general relativity are overcome and a new window opens to test loop quantum gravity effects through astronomical observations.

The scope of the Focus Issue: Applications of loop quantum gravity to cosmology, published last year in CQG, is to provide a snapshot of some of the rigorous and novel results on this research frontier in the cosmological setting.

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Advancing the Search for Gravitational Waves with Next-Generation Citizen Science

by Michael Zevin.


Michael Zevin is a third-year doctoral student in astrophysics at Northwestern University. He is a member of the LIGO Scientific Collaboration, and in addition to citizen science and LIGO detector characterization his research focuses on utilizing gravitational-wave detections to learn about binary stellar evolution and the environments in which compact binaries form.

With the first observations of gravitational waves and the discovery of binary black hole systems, LIGO has unveiled a new domain of the universe to explore. Though the recent signals persisted in LIGO’s sensitive band for a second or less, these last words of the binary that were spewed into the cosmos provided an unprecedented test of general relativity and insight into the progenitor stars that subsequently formed into the colliding black holes. However, the hunt is far from over. With LIGO’s second observing run underway, we can look forward to many more gravitational-wave signals, and as is true with any new mechanism for studying the cosmos, we can also expect to find the unexpected.

The extreme sensitivity required to make such detections was acquired through decades of developing methods and machinery to isolate the sensitive components of LIGO from non-gravitational-wave disturbances. Nonetheless, as a noise-dominated experiment, LIGO is still susceptible to Continue reading

Can astrophysical black holes have “hair”?

by Carlos Herdeiro and Eugen Radu, Guest Editors of Focus Issue: Hairy Black Holes.


Carlos A. R. Herdeiro (left) got his PhD from Cambridge University (U.K.) in 2002. He is currently an assistant professor at Aveiro University, Portugal, and an FCT principal researcher. He is also the founder and coordinator of the Gravitation group at Aveiro University ( Eugen Radu (right) got his PhD from Freiburg University (Germany) in 2002. He is currently an FCT principal researcher at Aveiro University (Portugal).

One of the most recognizable statements about black holes is that they have “no-hair”. Close inspection, however, shows that this is a belief rather than a mathematically proven theorem. Moreover, decades of research on this topic have shown that, depending on what one precisely means, this statement may be simply wrong. That is, as solutions of Einstein’s equations, in a generic context, black holes are not necessarily “bald”. Then, less ambitious, but perhaps more relevant questions are: “Can astrophysical black holes have hair?” and “Can we test the existence of black hole hair with present and future astrophysical observations?”.

This CQG focus issue brings together a set of papers describing models in which black holes do have “hair”, as well as observational efforts that have the potential to assess if this is (or not) the case for astrophysical black hole candidates. This collection of research papers is by no means a faithful and complete description of all possible alternatives to the Kerr paradigm in the literature. Rather, the selected papers focus on Continue reading

Focus issue on gravitational waves, now open for submissions

Peter Shawhan and Deirdre Shoemaker invite you to publish your next paper on gravitational waves in CQG’s new open focus issue on the topic.

Peter Shawhan

Peter Shawhan, University of Maryland

Sometimes things come together in unexpected, happy ways. At the CQG editorial board meeting in London last July, we discussed ideas for new focus issues and there was a consensus that the time was right to organize one on the general theme of gravitational waves. We could claim amazing prescience, but honestly we had no idea that Continue reading

Black-hole laboratories in the era of gravitational-wave astronomy

Paolo Pani and Helvi Witek

Helvi and Paolo visiting Toronto during the International Conference on Black Holes at the Fields Institute last year.
Helvi is Research Fellow in the School of Mathematical Sciences at the University of Nottingham. Paolo is Assistant Professor at Sapienza University of Rome and Research Scientist at the Instituto Superior Técnico in Lisbon.

We are proud to present the completed Focus Issue on “Black holes and Fundamental Fieldsone year after its first contribution has been published online.

This issue appears serendipitously at the same time as LIGO’s historic detection of gravitational waves which, simultaneously, provided us with the first direct observational evidence for the existence of black holes (BHs). We wish to take this opportunity to congratulate the LIGO/VIRGO Scientific Collaboration and everyone involved on  their breakthrough discovery!

The true excitement around this discovery arises from the fact that it marks the beginning of the long-sought-for era of gravitational-wave astronomy. As Kip Thorne recently put it, “Recording a gravitational wave […] has never been a big motivation for LIGO, the motivation has always been to open a new window to the Universe”. The outstanding observation of a BH binary coalescence — and the expectation of Continue reading

Black-hole superradiance and the hunt for dark matter

Paulo Pani

Paolo Pani is a Marie Curie Fellow at Sapienza University of Rome and FCT Researcher at Instituto Superior Tecnico in Lisbon. His research interests include black holes, foundations of General Relativity and relativistic astrophysics. He is co-author of the book “Superradiance” (Springer-Verlag), now in press.

Little is known about dark matter, despite the numerous searches for its constituents. Fortunately, everything falls in the same way, so possible imprints of dark matter can be found in gravitational fields. In particular, if ultralight bosons exist in nature, they would make spinning black holes unstable. How does such instability evolve in realistic scenarios? And what can it teach us about the existence of dark matter?

In our recent CQG paper, we take the first step to address these questions by studying how a light scalar field grows near Continue reading

Focus issue: Astrophysics and general relativity of dense stellar systems

Clifford Will and Pau Amaro-Seoane

Guest Editors: Clifford Will and Pau Amaro-Seoane

We invite you to read the latest CQG focus issue on “Astrophysics and General Relativity of dense stellar systems“, which is available to read now.

Dense stellar systems such as galactic nuclei and stellar clusters are unique laboratories, not only for astrophysics, but also for general relativity. The complexity of these systems is such that, in spite of a huge theoretical, observational and numerical effort there are still a large number of open key questions. This focus issue on the “Astrophysics and General Relativity of Dense Stellar Systems” brings together an array of invited articles on important aspects of these questions.

We hope that you will enjoy reading the articles in the focus issue; all of which are free for a period of time following publication.

Focus issue: Relativistic effects in cosmology

Kazuya Koyama

Kazuya Koyama is a Reader in Cosmology in the Institute of Cosmology and Gravitation, at the University of Portsmouth,

I am very pleased to bring you this focus issue summarising the recent developments in computing relativistic effects in cosmology.

The issue consists of two parts, with the first part examining general relativistic formulations of the observed over-density of galaxies.

The second part discusses relativistic effects on the formation of large scale structures.

The authors pioneered the development of our understanding of general relativistic effects in cosmological observations and we hope that this focus issue will be a basis for further advancement of the field. Continue reading

New focus issue: Advanced interferometric gravitational wave detectors

Peter Shawhan and Marie-Anne Bizouard

Peter Shawhan is an Associate Professor of Physics at the University of Maryland, USA. His primary research area is in the analysis of data from gravitational wave detectors and connections with astrophysical events.
Marie-Anne Bizouard is a research fellow at CNRS, Laboratoire de l’Accélérateur Linéaire, Orsay, France. She is an experimental physicist working on gravitational wave searches with ground based interferometric detectors.

The quest to detect gravitational waves directly has seen great advances over the past five decades, with the earlier resonant “bar” detectors being surpassed in sensitivity by large laser interferometers in the last decade.  The first generation of interferometric detectors proved the viability of the approach, progressively improving sensing and control techniques and running up against the fundamental limitations of their designs.  Along the way, many searches for gravitational wave signals were carried out and published, but none achieved the milestone of detecting a clear gravitational-wave signal.

All of that is about to change.  The lessons learned from the first full-scale interferometric detectors fed into the design of advanced detectors which are now being constructed and commissioned and will soon begin collecting data.  Higher laser power, Continue reading

New focus issue: Entanglement and quantum gravity

Eugenio Bianchi and Carlo Rovelli

Eugenio Bianchi (left) is an assistant professor at Pennsylvania State University and Carlo Rovelli (right) is a professor at Aix-Marseille University at the Centre de Physique Theorique de Luminy

Quantum gravity alone is not the only major theoretical open problem in fundamental physics: gravity, quantum theory and thermodynamics form a triple, whose full interconnections we have definitely not yet understood. As soon as quantum effects appear in a curved spacetime, thermal aspects appear to be unavoidable. Combining thermodynamics and (full) gravity might turn out to be even more crucial than understanding the quantum aspects of the gravitational field alone. In recent years, it has become increasingly clear that entanglement entropy is a central ingredient for the synthesis we are seeking. Continue reading