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 that observable modes are in the Bunch-Davies (BD) vacuum at the onset of the slow roll. LQC analysis shows that modes with physical wavelength \lambda > r_{\rm LQC} at the bounce experience curvature as they evolve in the Planck regime. Therefore they have excitations over the BD vacuum. While the use of non-BD states has been considered before, in LQC these states are not postulated, or obtained by tweaking the potential, but arrived at from the Planck scale dynamics. Therefore, quantum gravity effects could manifest themselves in the longest wavelength modes in the CMB. Interestingly, recent CMB observations have revealed anomalies at large angular scales, e.g. suppression of power in temperature anisotropy spectrum at \ell<30. As suggested in PLANCK 2015 XVI paper, although these anomalies have been observed only at a ~2-3 \sigma level, they could be “the visible traces of fundamental physical processes occurring in the early Universe”.


Brajesh Gupt (picture taken as a postdoc at Penn State) is a postdoctoral researcher at the Institute for Gravitation and the Cosmos at the Pennsylvania State University.

The question then is whether modes with \lambda > r_{\rm LQC} at the bounce are in the observable range of CMB. The answer depends on the number of e-folds in the pre-inflationary dynamics.  We use the details of quantum geometry to introduce a new principle to constrain this phase of dynamics. Recall that, because `dark energy’ dominates the late time dynamics, there are cosmological horizons. For observations it suffices to restrict oneself to the interior of the 2-sphere obtained by the intersection of the cosmological horizon with the CMB surface. Given a viable inflationary model –e.g. with the Starobinsky potential– one can trace the evolution of this 2-sphere back in time. At the onset of inflation, the radius is astonishingly small, about 107 Planck lengths; by contrast, the proton radius is ~1020 Planck lengths! In LQC, we can evolve the ball back in time all the way to the bounce. Our principle asks that the radius be the minimum allowed by the smallest non-zero eigenvalue of the area operator, called the area gap. This fixes the pre-inflationary history of the cosmological background; for the Strobinsky potential, for example, there are 17 pre-inflationary e-folds. To specify the initial conditions for cosmological perturbations at the bounce, we use a quantum generalization of Penrose’s Weyl curvature hypothesis (discussed in an accompanying CQG+ article).

With these initial conditions, the existing LQC framework leads to specific predictions.  For the temperature-temperature (TT) correlation function, there is excellent agreement with standard inflation for \ell>30 but there is a power suppression at large angular scales, \ell<30. Consequently, the LQC power spectrum provides a better fit to the data than the standard inflation. Our analysis also predicts specific power suppression in the E-mode polarization spectrum, which will be tested in the upcoming data release by PLANCK mission. Furthermore, predictions involving E-mode polarization are distinct from those of non-primordial mechanisms such as the integrated Sachs-Wolf effect. Thus LQC, together with our principles to fix initial conditions, opens a new window to confront quantum gravity with observations. Finally, there is also a feedback from observations to the fundamental theory. By leaving the area gap as a free parameter, we find the value that would provide a best fit to the observed TT spectrum at all angular scales.  The commonly used value in LQC comes from the black hole entropy calculations. Interestingly, it lies well within the 68% confidence level of the PLANCK data, providing an independent probe into the quantum nature of geometry.

At long last, quantum gravity is ready to leave the pristine perch of mathematical physics and dive into cosmological phenomenology.

[1] Ade, P.A.R., Aghanim, N., Akrami, Y., Aluri, P.K., Arnaud, M., Ashdown, M. and Aumont, J. et al. Planck 2015 results-XVI. Isotropy and statistics of the CMB. Astronomy & Astrophysics. 594, A16 (2016).

Read the full article in Classical and Quantum Gravity:
Quantum gravity in the sky: interplay between fundamental theory and observations
Ashtekar and Gupt 2016. Class. Quantum Grav 34014002

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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.

Continue reading

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

by Parampreet Singh.

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 Troebs in the lab

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.

How does tilt-to-length coupling come about? Continue reading

Setting space on fire

by Yasaman K. Yazdi and Niayesh Afshordi.

Niayesh Afshordi and Yasaman Yazdi discover that firewalls have consequences

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

Written by Madhavan Varadarajan


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 Harvard Physics PhD, 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". Coughlin dances on the Harvard ballroom dance team and enjoys the chaos of teaching 3rd and 4th graders in an after-school math and science program at a local elementary school.

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.

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