A Successful Eccentric

by Blake Moore

Humankind has been obsessed with circles for a long time. It comes as no surprise then that the modeling of gravitational waves had focused until recently on those emitted by black holes or neutron stars in circular orbits around each other. But in the case of gravitational wave modeling, there is good reason for this obsession. Gravitational waves remove energy and angular momentum from a binary, forcing the eccentricity to decay and the orbit to circularize rapidly. Since the 1960s, the expectation has then been that the gravitational waves that ground-based detectors would observe would correspond to circular binaries.

Travis Robson (right), Blake Moore (center), and David Anderson (left) are members of the eXtreme Gravity Institute at Montana State University. Here they are at nearby Yellowstone National Park.

But as with most things in physics, Nature adores the complex if one looks closely enough. Several astrophysical studies have recently shown that binaries may form with moderate eccentricities at orbital separations at which they would be emitting gravitational waves that ground-based detectors could observe very soon. These binaries would form near a supermassive black hole or in globular clusters, where three- or many-body interactions may source eccentricity. And if they are detected, they could shed light on their true population and formation scenario.

Gravitational waves measure colour of black holes

By Enrico Barausse, Richard Pires Brito, Vitor Cardoso, Irina Dvorkin, and Paolo Pani

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Black holes predicted by Einstein are, well, black. In classical physics, nothing can escape their event horizon, not light, not matter, and neither gravitational waves.

There is a deep reason for this absolute blackness. If the event horizon were not a perfectly absorbing surface, but rather a partially reflective one, spinning black holes would become unstable and would shed most of their rotational energy into gravitational waves. This process is known as superradiant instability [1], and is tightly linked to the presence of an ergoregion, a region of spacetime just outside the event horizon, where modes of negative energy are allowed to exist. Negative energy-modes can form in the ergoregion of normal humdrum black holes, but are eventually doomed to fall in the event horizon.

Enrico Barausse

Richard Pires Brito

Professor Vitor Cardoso

Irina Dvorkin

Paolo Pani

If (what look like) black objects had a surface, such modes would be partially reflected by it, and they would bounce back and forth between the horizon and the boundary of the ergoregion (which they cannot cross, since negative energy modes cannot travel to infinity). Each time they reach the ergoregion boundary, they come out as positive energy-modes, thus inside the ergoregion they would keep growing in amplitude (i.e. their energy would keep decreasing and becoming more negative) eventually producing an instability. Indeed, these ‘bounces’ produce ‘echoes’ in the gravitational wave signal [2] from the remnant black hole forming from binary mergers, and there are claims [3] (albeit controversial [4]) that they may have been seen in the LIGO data.

In this paper we do not look at the black holes that form from binary mergers, but rather at isolated ones. These black holes can have a wide range of masses (from stellar masses for stellar-origin black holes up to millions or billions solar masses for supermassive black holes) and a variety of spins (on which we have some knowledge thanks to electromagnetic observations). Normally, isolated black holes do not emit gravitational waves, but if their event horizon had some reflectivity (that is, if these objects were not totally black), they would turn into black-hole bombs due to superradiance, and they would shed almost all their angular momentum in gravitational waves. These gravitational wave signals would be too weak to be detected singularly, but because there are in general many more black holes in isolation than in binaries, they can produce a very large stochastic background. Indeed, this background would be orders of magnitude larger than the current upper bounds from LIGO/Virgo. Similar results also apply to supermassive black holes, in the yet-unexplored LISA band.

So in conclusion, the existing stochastic background constraints from LIGO and Virgo show that black holes are very black, although some shades of grey may still be allowed. Indeed, while 100% reflection from the horizon is ruled out, smaller reflection coefficients may still be possible depending on the spin of the object [5].

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References:
[1] W. H. Press and S. A. Teukolsky, “Floating Orbits, Superradiant Scattering and the Black-hole Bomb“. Nature. 238 (5361): 211-212 (1972);
Brito, Cardoso, Pani; “Superradiance“, Springer (2015)
[2] Cardoso, Franzin, Pani, “Is the gravitational-wave ringdown a probe of the event horizon?“, Phys. Rev. Lett. 116, 171101 (2016)
[3] Abedi & Afshordi, “Echoes from the Abyss: Tentative evidence for Planck-scale structure at black hole horizons“, Phys. Rev. D 96, 082004 (2017)
[4] Ashton+ https://arxiv.org/abs/1612.05625; Abedi, Dykaar, Afshordi, https://arxiv.org/abs/1701.03485 and https://arxiv.org/abs/1803.08565;
Westerweck+, “Low significance of evidence for black hole echoes in gravitational wave data“, Phys. Rev. D 97, 124037 (2018)
[5] Maggio, Pani, Ferrari “Exotic Compact Objects and How to Quench their Ergoregion Instability“, Phys. Rev. D 96, 104047 (2017); Maggio, Cardoso, Dolan, Pani, http://arxiv.org/abs/arXiv:1807.08840

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Read the full article in Classical and Quantum Gravity:
The stochastic gravitational-wave background in the absence of horizons
Enrico Barausse et al 2018 Class. Quantum Grav. 35 20LT01

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Fast Self-forced Inspirals

by Niels Warburton and Maarten van de Meent

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LISA will fly. Since being given the green light by the European Space Agency a year ago, the scientific consortium around the Laser Interferometer Space Antenna (LISA) has been reorganising as it gears up to meet the challenge of building and operating a gravitational wave detector in space. This process has led to a renewed focus on the waveform templates that will be needed to extract the signals and estimate source parameters.

One of the key sources for LISA are extreme mass-ratio inspirals (EMRIs). In these binaries a stellar mass compact object (such as a black hole or neutron star) spirals into a massive black hole. Emitting hundreds of thousands of gravitational wave cycles in the millihertz band, LISA will detect individual EMRIs for months or even years. The low instantaneous signal-to-noise-ratio of the gravitational waves necessitates accurate waveform templates that can be used with matched filtering techniques to extract the signal from the detectors data stream. Coherently matching a signal over months or even years requires going beyond leading-order, flux-based black hole perturbation models and calculating the so-called ‘self-force’ that drives the inspiral [1]. Roughly, one can think of this self-force as arising from the smaller orbiting body interacting with its own perturbation to the metric of the massive black hole. To this end the recent “LISA Data Analysis Work Packages” document defined a number of source-modelling challenges that must be overcome before LISA flies [2]. One of these requires the community to:

“Design and implement a framework for incorporating self-force-based numerical calculations, as they become available, into a flexible semi-analytical Kludge model that enables fast production of waveform templates”

Our work [3], “Fast Self-forced Inspirals”, is a response to this challenge.

The Sound of Exotic Astrophysical “Instruments”

by Sebastian Völkel and Kostas Kokkotas

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Could you distinguish the sound of a wormhole from an ultra compact star or black hole?

Such an exotic, though quite fundamental question, could be asked to any physicist after the groundbreaking and Nobel Prize winning discoveries of gravitational waves from merging black holes and neutron stars. Gravitational waves provide mankind with a novel sense, the ability to hear the universe. This analogy, between sound waves and gravitational waves, will bring to the minds of many physicists Mark Kac’s famous  question: “Can One hear the Shape of a Drum?” [1], and not just to the drummers amongst us. The possibility of this analogy is one of the ways in which gravitational waves are very distinct from the usual tool of astronomy, light.

To answer the question for our exotic instruments, we will rephrase it in a more technical form. In the simplest version one can describe linear perturbations of spherically symmetric and non-rotating models of wormholes and ultra compact stars. It is well known that the perturbation equations for these cases can simplify to the study of the one-dimensional wave equation with an effective potential. The solutions, which are usually given as a set of modes, represent the characteristic sound of the object. The so-called quasi-normal mode (QNM) spectrum is the starting point for our discussion.

FIG. 1. Sebastian Völkel (right) is a PhD student in the Theoretical Astrophysics group of Professor Kostas Kokkotas at the University of Tübingen, located in the south of Germany. Among his research interests is the study of compact objects along with the associated gravitational wave emissions. More information about his research can be found here.
Professor Kostas Kokkotas (left) is leading the group of Theoretical Astrophysics at the University of Tübingen. The focus of his research is on the dynamics of compact objects (neutron stars & black-holes) as sources of gravitational waves in general relativity and in alternative theories of gravity. More information about the group can be found here.
Photo by Severin Frank.

The gravitational-wave story of a neutron-star merger

by Jocelyn Read, California State University Fullerton

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With several binary black hole mergers observed in the past two years, astronomers and relativists have become familiar with their general features: a quick chirp signal lasting seconds or less, a familiar inspiral-merger-ringdown pattern of waves, and a dark event in a distant galaxy, billions of light-years away.

GW170817 is a little bit different.

We’ve already seen systems like its presumed antecedent in our galaxy, where pulsars with neutron-star companions precisely map out their hours-long orbits with radio blips. We can imagine, then, the last 80 million or so years of GW170817’s source. Two neutron stars, in a galaxy only 40 Mpc away, driven through a slow but steady inspiral by gravitational radiation. For us distant observers, things become more interesting when the increasing orbital frequency sends the emitted gravitational waves into the sensitive range of our ground-based detectors.

Dr. Jocelyn Read explains gravitational waves to undergraduate students Isabella Molina and Erick Leon.

I wanted to take this opportunity to give a sense of scale, so consider this a tour of some interesting way-points along the signal’s path through that sensitive range of frequencies. Many thanks to my colleagues in the LIGO and Virgo collaborations who’ve helped lay out these markers over the last weeks – and of course, any remaining errors are my own.

Tails of gravitational waves and mechanics of compact binaries

by Luc Blanchet and Alexandre Le Tiec

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The first law of binary black hole mechanics can be extended to include non local tail effects.

Ever since Kepler’s discovery of the laws of planetary motion, the “two-body problem” has always played a central role in gravitational physics. In Einstein’s general theory of relativity, the simplest and most “universal,” purely gravitational, two-body problem is that of a binary system of black holes. The inspiral and merger of two compact objects (i.e. bodies whose radius is comparable to their mass, in “geometric” units where G = c = 1) produces copious amounts of gravitational radiation, as was recently discovered by LIGO’s multiple detections of gravitational waves from black hole binary systems.

Alexandre Le Tiec (left) celebrates the detection of gravitational waves and Luc Blanchet (right) thinks about gravitational waves in Quy Nhon, Vietnam

In general relativity, the inspiral and onset of the merger of two compact objects is indeed universal, as it does not depend on the nature of the bodies, be they black holes or neutron stars, or possibly more exotic objects like boson stars or even naked singularities. However, the gravitational waves generated during the post-merger phase depend on the internal structure of the compact objects, and in the case of neutron stars should reveal many details about the scenario for the formation of the final black hole after merger, and the equation of state of nuclear matter deep inside the neutron stars

Surfing a wave to Stockholm

by Clifford M. Will, CQG Editor-in-Chief

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What a week for gravitational physics!   

First came the September 27th announcement of another detection of gravitational waves, this time by the three-detector network that included Virgo along with the two LIGO observatories. The source of the gravitational waves was another fairly massive black hole binary merger, with black holes of 30 and 26 solar masses. Once again, about 3 solar masses were converted to energy in a fraction of a second, leaving behind a 53 solar mass black hole spinning at about 70 percent of the maximum allowed. With  Virgo included in the detection, the localization of the source on the sky was improved dramatically over earlier detections by LIGO alone, dropping to a small blob on the sky measuring 60 square degrees, from the large, 1000 square degree banana-shaped regions of earlier detections.

For the first time, a test of gravitational-wave polarizations was carried out.  Because the arms of the two LIGO instruments are roughly parallel, they have very weak sensitivity to different polarization modes of the waves.  But with Virgo’s very different orientation, it was possible to show that the data favor the two spin-2 modes of general relativity over pure spin-0 or pure spin-1 modes.

But then, six days later came the announcement of the Nobel Prize in Physics, awarding one half of the prize to Rainer Weiss of MIT and the other half shared between Kip Thorne and Barry Barish of Caltech, for decisive contributions to the detection of gravitational radiation. CQG congratulates the winners!

Amaldi12 – The Edoardo Amaldi Conference on Gravitational Waves

It’s been a busy few weeks for CQG – we’ve been to the Era of Gravitational Wave Astronomy conference in Paris, hosted the annual Editorial Board meeting in London, attended the Loops17 conference in Warsaw and now it’s time to fly off to California for Amaldi12.

Amaldi12, named after Edoardo Amaldi, will be held at the Hilton Hotel in Pasadena, CA from 9^th – 14^th July. The conference will explore the science around gravitational waves and their detection, particularly in light of the confirmed detections by LIGO-Virgo and new advances with the LISA mission.

I will be at the conference Monday through Friday with a table top booth at the event, located near the international ballroom in the hotel. I’m really interested in hearing your thoughts about the journal, so please do stop by say hello and have a chat.

The Era of Gravitational-Wave Astronomy

At the beginning of next week I will be attending the Era of Gravitational Wave Astronomy conference (or TEGRAW 2017, for short) at the Institut D’Astrophysique in Paris, France.

The conference aims to highlight the most recent developments in both theoretical works (such as the two-body problem, effective theories, numerical relativity, and tests of gravity theories) and experimental works (such as future detectors, both on ground and in space).

IOP Publishing/ CQG will have a small table top booth at the event so feel free to stop by if you fancy having a chat. I’ll only be there Monday through Wednesday (unfortunately missing the social event) but am looking forward to meeting you.

I hope to see you in Paris!

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

by Michael Zevin.

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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