# Quantum gravity in the sky?

by Abhay Ashtekar and Brajesh Gupt.

Quantum gravity effects in the very early Universe can leave observable imprints.

Abhay Ashtekar (picture taken as a postdoc at Oxford University) is the Eberly Professor of Physics and the Director of the Institute for Gravitation and the Cosmos at the Pennsylvania State University.

The inflationary paradigm traces the genesis of the large-scale structure of the cosmos to astonishingly early times. However, at the onset of inflation spacetime curvature is only about 10-14 times the Planck curvature where quantum gravity effects dominate. Therefore, it is natural to ask if the earlier, pre-inflationary phase of dynamics would change observable predictions of standard inflation. The answer is often assumed to be in the negative. Our CQG paper shows that this conclusion is premature. Specifically, in Loop Quantum Cosmology (LQC) there is an unforeseen interplay between the ultraviolet effects that tame the big bang singularity, and dynamics of infrared modes of cosmological perturbations. As a result, imprints of the quantum spacetime geometry in the Planck regime can manifest themselves at the largest angular scales in the CMB.

In LQC, quantum geometry effects dominate in the Planck regime, replacing the big bang by a quantum bounce, where scalar curvature reaches its finite and universal upper bound. Therefore the radius of curvature has a non-zero lower bound, $r_{\rm LQC}$. Over the last 7 years, techniques have been developed to describe dynamics of the cosmological perturbations on this quantum background geometry, thereby facing the trans-Planckian issues squarely. Standard inflation assumes Continue reading

# Pulsed Gravitational Waves

Timothy J. Walton occupies some quantum state between a physicist and a mathematician, having obtained his PhD from the physics department at Lancaster University in 2008 but now masquerading as a lecturer in mathematics at the University of Bolton.

by Timothy J. Walton.

Applying techniques from classical electrodynamics to generate new gravitational wave perturbations

I must begin with a confession: I don’t view myself as a gravitational physicist. Despite my PhD at Lancaster University involving a formulation of relativistic elasticity and an awful lot of differential geometry, my research thus far has been within the realm of classical and quantum electrodynamics. But it was precisely within that domain, along one particular avenue of investigation, where the first seeds of an idea were sown. Following my earlier work on a class of exact finite energy, spatially compact solutions to the vacuum source-free Maxwell equations – pulsed electromagnetic waves – describing single cycle pulses of laser light [1], together with Shin Goto at Kyoto University in Japan and my former PhD supervisor Robin Tucker at Lancaster University, a new question arose: “do pulsed gravitational waves exist?’’

As I recall, this question was posed and began to take root during one of the regular meetings I have with Robin. Within my institution, I am fortunate enough Continue reading

# Quantum mechanics meets CMB physics

by Massimo Giovannini.

Since 1991 Massimo Giovannini has extensively researched, taught and written on high-energy physics, gravitation and cosmology. He wrote over 180 papers and various review articles. He is the author of a book entitled “A primer on the physics of Cosmic Microwave Background” published in 2008.

Which is the origin of the temperature and polarization anisotropies of the Cosmic Microwave Background? Classical or quantum? The temperature and the polarization anisotropies of the Cosmic Microwave Background (CMB) are customarily explained in terms of large-scale curvature inhomogeneities. Are curvature perturbations originally classical or are they inherently quantum mechanical, as speculated many years ago by Sakharov?

In the conventional view these questions are quickly dismissed since the quantum origin of large-scale curvature fluctuations is, according to some, an indisputable fact of nature. This is true if and when Continue reading

# Pushing post-Newtonian theory even further!

by Tanguy Marchand, Luc Blanchet and Guillame Faye.

With the spectacular discoveries by the LIGO/VIRGO collaboration of gravitational waves from the coalescence of black-hole binaries, we foresee the possibility of extremely accurate measurements of the so-called post-Newtonian (PN) coefficients that describe the gravitational waveform of these systems in the inspiral phase prior to the final coalescence. The PN coefficients are especially important because they probe the non-linear structure of general relativity (GR) and provide thus very constraining tests of this theory. In turn, they permit accurate measurements of the physical parameters of the binary, essentially the mass of the compact objects and their moment of rotation or spin.

# Want to crush a singularity? First make it strong and then …

by Parampreet Singh.

Parampreet Singh with a young student who often asks him the most difficult and so far unanswerable questions on the resolution of singularities. Dr Parampreet Singh is Associate Professor at Department of Physics and Astronomy at Louisiana State University.

Einstein’s theory of classical general relativity breaks down when spacetime curvature
becomes extremely large near the singularities. To answer the fundamental questions
about the origin of our Universe or what happens at the central singularity of the black holes thus lies beyond the validity of Einstein’s theory. Our research deals with discovering the framework which guarantees resolution of singularities.

It has been long expected that quantum gravitational effects tame the classical singularities leading to insights on the above questions. A final theory of quantum gravity is not yet there but the underlying techniques can be used to understand whether quantum gravitational effects resolve cosmological and black hole singularities. Our goal is Continue reading

# Tilting laser beams in LISA

by Michael Tröbs.

Michael Tröbs in the lab. Michael Tröbs is an experimental physicist at Max Planck Institute for Gravitational Physics (AEI). The LISA optical bench test bed was built in collaboration with Airbus DS and University of Glasgow. At AEI Michael is responsible for the project.

A testbed to experimentally investigate tilt-to-length coupling for LISA, a gravitational-wave detector in space.

The planned space-based gravitational-wave detector LISA will consist of three satellites in a triangle with million kilometer long laser arms. This constellation will orbit the Sun, following the Earth. LISA is expected to be laser shot-noise limited in its most sensitive frequency band (in the Millihertz range). The second largest contribution to the noise budget is the coupling from laser beam tilt to the interferometric length measurement, which we will call tilt-to-length (TTL) coupling in the following.

# Setting space on fire

by Yasaman K. Yazdi and Niayesh Afshordi.

Niayesh Afshordi and Yasaman Yazdi discover that firewalls have consequences. Yasaman K. Yazdi is a PhD candidate at the University of Waterloo and the Perimeter Institute for Theoretical Physics. Niayesh Afshordi is an associate professor at the University of Waterloo and the Perimeter Institute for Theoretical Physics.

Thought experiments highlight the edge of our understanding of our theories.  Sometimes, however, we can get so caught up in heated debates about the solution to a thought experiment, that we may forget that we are talking about physical objects, and that an actual experiment or observation may give the answer.  In this Insight we discuss a proposed solution to the black hole information puzzle, and a possible observational signal that might confirm it.

The black hole information puzzle and a potential solution

The black hole information loss problem is a decades old problem that highlights the tensions between some of the pillars of modern theoretical physics. It has evolved from being Continue reading

# Propagation in the absence of classical spacetime

The author’s research group busy at work. Madhavan Varadarajan is a Professor at the Raman Research Institute in Bangalore, India.

At the Planck scale of 10−33cm, where the very notion of classical spacetime ceases to exist due to large quantum fluctuations of spacetime geometry, can meaning be given to the notion of “causality”? We are interested in this question in the context of Loop Quantum Gravity (LQG).

The basic quantum states of LQG are labelled by graphs. Each such state describes discrete one dimensional excitations of spatial geometry along the edges of its graph label. These ‘graphical’ states provide the Continue reading

# OK, so what happens now?

Written by Michael Coughlin

# The future of gravitational-wave astronomy after the first detection

Michael Coughlin is currently a post-doctoral fellow at Harvard University with Prof. Christopher Stubbs. In September 2016, he successfully defended his physics PhD at Harvard, titled “Gravitational-wave astronomy in the LSST era”. He began researching gravitational waves with LIGO over eight years ago as a college freshman at Carleton College in Northfield, MN and it was very exciting for him to be part of LIGO’s historical confirmation in February 2016. At Harvard, he added the Large Synoptic Survey Telescope (LSST), Pan-STARRS, and ATLAS to his research areas, including designing and building a prototype calibration system, which he nicknamed “CaBumP”.

Since LIGO announced the detection of gravitational waves from binary black hole mergers in its first observing run [1-2], the most common question I have received is “What was it like to be part of such a historic scientific discovery?” The second most common question has been: “So what happens now?” The answer is a lot of stuff! Here I’ll focus on three main goals:

1. Using LIGO to detect other sources of gravitational-waves
2. Improving the gravitational-wave detectors in order to probe farther into the cosmos
3. Electromagnetic follow-up of gravitational-wave events with telescopes to get a more complete picture

What else does nature have in store for us?

The detection of gravitational waves from binary black hole mergers has been incredibly exciting, and we look forward to the detection of more such systems. Of course, there are many other sources (pulsars, supernovae, binary neutron stars, etc.) that we hope to detect as well. As a member of the group in LIGO searching for a stochastic background of gravitational waves, I am particularly interested in the processes that could create such a signal. This includes backgrounds from compact binary coalescences, pulsars, magnetars, or core-collapse supernovae. A cosmological background (such as from inflation!) could be generated by various physical processes in the early universe. In particular, with the recent discovery of binary black-hole mergers, there is a really good chance of observing a stochastic gravitational-wave background from these systems [3].

There are other sources that are likely to produce long-lived transients, including emission from rotational instabilities in proto-neutron stars and black-hole accretion disk instabilities. There is ongoing significant effort to improve Continue reading

# “There’s no way it’s real”

Written by Samantha Usman, who is currently pursuing an MPhil at Cardiff University, UK under the supervision of Prof. Stephen Fairhurst. She graduated in May 2016 with a BS in Mathematics and Physics at Syracuse University. While at Syracuse, Usman worked with Prof. Duncan Brown on improving LIGO’s sensitivity to gravitational waves from binary star systems. In her spare time, Usman trains in Brazilian jiu jitsu and Muay Thai kickboxing and enjoys walks with her Australian Shepherd, Marble.

# The discovery of gravitational waves from an undergraduate’s perspective

Author Samantha Usman training for competition in Brazilian jiu jitsu.

The first time I learned LIGO might have detected a gravitational wave, I was listening in on a conference call on September 16, 2015. Two days earlier, ripples in the fabric of space from massive black holes crashing into each other at half the speed of light had passed through the Earth. The LIGO detectors picked up these faint changes in the length of space, but they pick up all sorts of extra noise that you’d never expect; how could we be sure this was really a gravitational wave?

On September 16th, I was an undergraduate starting my senior year at Syracuse University. I’d been doing LIGO research with my advisor, Prof. Duncan Brown, for almost two and a half years. Since LIGO had yet to start an observing run, my research had been focused on testing improvements to the codes that we use to search for gravitational waves. I’d been told in those two and a half years that it would take a few years to get our detectors to design sensitivity and not to expect a detection until I was well into graduate school.

So when I sat in my boss’ office listening to a colleague in Germany say he thought we’d really seen something, I rolled my eyes and muttered, “There’s no way it’s real.” I was convinced people were Continue reading