Quantum gravity in the sky?

by Abhay Ashtekar and Brajesh Gupt.


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

ashtekar_oxford

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”.

gupt_pennstate

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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So long, and thanks for all the manuscripts

Adam Day

Adam Day is the Publisher of Classical and Quantum Gravity

Years ago, I sat, somewhat nervously, in a small, dimly lit room in an old office block. I’d applied for a dream job and I was expecting to learn the outcome of that application. A senior member of staff tactfully began the meeting with some friendly small-talk that did absolutely nothing to calm my nerves.

I’d heard of CQG – even before I’d seen the job advert. Reputed for its high standards of peer-review, it also held the distinction of being the first physics journal on the web. Clearly, this was a journal for brilliant pioneers and innovators (submit here) and I wanted to be part of that. Furthermore, I’d enjoyed studying relativity as an undergraduate and had hoped to become a gravitational-wave researcher, so the science of CQG was already close to my heart.  I can’t even remember the colour of the walls in that old office, but I can still hear the words “I’d like to offer you the job” very clearly.

Looking back Continue reading

Pulsed Gravitational Waves

timothyjwalton

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.


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

Highlights of 2016 now free to read 

By Clifford Will.


Clifford Will

Clifford Will is the Editor-in-Chief of Classical and Quantum Gravity, Distinguished Professor of Physics at the University of Florida, Chercheur Associé at the Institut d’Astrophysique de Paris, and James McDonnell Professor of Space Sciences Emeritus at Washington University in St. Louis.

I am delighted to present the CQG Highlights of 2016 which are now free to read.  This prestigious annual collection is selected by the editorial board and includes notable papers on gravitational waves, black holes, general relativity, cosmology, quantum gravity and more.

As well as being free to read on the web, each paper is promoted by the journal in a number of campaigns.  Watch for the CQG Highlights brochure at your next conference.

CQG Highlights remains one of CQG’s most popular promotions.  Don’t miss your chance to be included in CQG Highlights of 2017 by publishing your next great paper in CQG.

 

 

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

Happy new year!

By Clifford Will.


Clifford Will

Clifford Will is the Editor-in-Chief of Classical and Quantum Gravity, Distinguished Professor of Physics at the University of Florida, Chercheur Associé at the Institut d’Astrophysique de Paris, and James McDonnell Professor of Space Sciences Emeritus at Washington University in St. Louis.

What a year for gravitational physics!  In February, the LIGO and Virgo Collaborations (LVC) announced the first detection of gravitational waves.  The MICROSCOPE satellite test of the equivalence principle took to the skies in April and, in June, LISA Pathfinder surpassed all expectations in demonstrating the key technologies required to detect gravitational waves in space.  As if all that wasn’t enough, the LVC announced a second detection of a binary black hole merger later that month.  By September, NASA revealed that it would rejoin ESA in funding the LISA mission with a view to launching a 3-armed space interferometer by 2030.  Could we have wished for more?

CQG launched a focus issue on the topic of gravitational waves in 2016 edited by Peter Shawhan and Deirdre Shoemaker.  You can submit your next great paper on gravitational waves to the issue which is currently open to submissions and will be promoted in a number of channels throughout 2017.  All submissions will be subject to CQG’s usual high standard of peer review.

To keep track of the latest CQG publications and news in 2017, you can follow the CQG+ blog or follow the journal on social media (Twitter, Facebook).

I want to express my appreciation to all CQG authors, referees and readers who supported the journal in 2016.  I particularly wish to thank the journal’s Editorial Board Members and Advisory Panel Members who assist in directing the strategy of the journal and who oversee CQG’s peer review.  I also welcome new Board and Panel members to CQG. I look forward to working with all of you in the coming year.

With the LIGO detectors’ second observation run underway, I am certain that we have more to look forward to in 2017. Continue reading