by Astrid Eichhorn, Sebastian Mizera and Sumati Surya
Silence is a rare commodity in our everyday, technology-driven lives; from the outright blare of car horns in India, to the quieter, but persistent sounds of mobile phones in Germany, constantly alerting us to the incoming messages, to the 24 hour news cycle on Canadian TV. Finding a few quiet moments, unhindered by the constant onslaught of information requires a considerable effort. Indeed, it often feels that noise, not silence is the generic state of our world.
Sebastian Mizera is a graduate student at Perimeter Institute working on various aspects of quantum field theories. He enjoys quiet evenings reading a book.
Sumati Surya is on the faculty of the Raman Research Institute, in Bangalore, India and is currently an Emmy-Noether visiting fellow at the Perimeter Institute, Canada. Although she loves the quiet she also loves the sounds created by her two very active and engaging daughters.
Astrid Eichhorn is currently an Emmy-Noether junior research group leader at the University of Heidelberg, where she and her group explore the fundamental quantum structure of spacetime and matter. When she isn’t exploring asymptotic silence in quantum gravity, she likes to look for silence in the colder more remote regions of our planet.
by Ivan Agullo, Abhay Ashtekar and Brajesh Gupt
Can observations determine the quantum state of the very early Universe?
Can we hope to know even in principle what the universe was like in the beginning? This ancient metaphysical question has acquired new dimensions through recent advances in cosmology on both observational and theoretical fronts. To the past of the surface of last scattering, the universe is optically opaque. Yet, theoretical advances inform us that dynamics of the universe during earlier epochs leaves specific imprints on the cosmic microwave background (CMB). Therefore, we can hope to deduce what the state of the universe was during those epochs. In particular, success of the inflationary scenario suggests that the universe is well described by a spatially flat Friedmann, Lemaître, Robertson, Walker (FLRW) space-time, all the way back to the onset of the slow roll phase. This is an astonishingly early time when space-time curvature was some times that on the horizon of a solar mass black hole and matter density was only 11 orders of magnitude smaller than the Planck scale.
Clockwise from top left: Brajesh Gupt (Pennsylvania State University), Abhay Ashtekar (Pennsylvania State University) and Ivan Agullo (Louisiana State University)
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 9th – 14th 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.
At the beginning of next week Jennifer Sanders and I will be representing the CQG editorial team at the Loops’ 17 conference at the University of Warsaw in Poland.
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!
by J. Fernando Barbero G., Benito A. Juárez-Aubry, Juan Margalef-Bentabol and
Eduardo J. S. Villaseñor.
Boundaries are ubiquitous in physics, if anything because most material objects tend to have one… In the context of gravity they play a number of interesting roles: from the definition of conserved quantities in asymptotically flat spacetimes to holography (no lasers here, sorry) or the modeling of black holes. A natural question in the context of the canonical quantization of gravitational theories is how to obtain their Hamiltonian description – in particular the constraints – in the presence of boundaries.
Eduardo and Fernando trying to get inspiration for the new group logo
“Juan, can you hear us?”
“Yes, the connection seems to be working better these days.”
“Hi Benito. Good to see you! Can you also see us?”
“Sorry for the delay, it is early in the morning here. Yeah, I can see you perfectly. Hi, Eduardo and Fernando! Hi, Juan!”
“So, as I told you in my last email, we have to write this CQG+ Insight piece about our traces paper. You know, it should be informative, informal and infused with deep physical insights, so, any suggestions? — Yes, Fernando.”
Stanley Deser is emeritus Ancell Professor of Physics at Brandeis University and a Senior Research Associate at Caltech
Prior to Supergravity’s (SUGRA’s) inception, the ideas in the air came from two new, quite different realms. One realm was supersymmetry (SUSY); the other arose from the emerging difficulties in achieving consistent interactions between gravity and higher (s > 1) spin gauge fields.
Indeed, the Western discoverers of SUSY, Julius Wess and Bruno Zumino , would frequently visit Boston from NYU to spread the SUSY gospel, which did get even our blasé attention after a while, especially since the simplest SUSY multiplet pattern (s; s + 1/2) linking adjoining Fermi-Bose fields had no obvious reason to stop at the s = 0 and s = 1/2 models that had been studied thus far.
by Joseph Samuel.
Cricketing nations have a very good idea what a boundary is, it’s good for a cool four runs, without the bother of running! Corners are tense moments in a football (soccer to some) match when a well struck ball can curve into the goal. The crease is what a batsman lunges for when the wicket keeper ….. wait! this is not a sports column, but CQG+! Let’s back up and explain what our paper really is about.
In a path integral approach to quantum gravity, one has to divide up spacetime into pieces and focus on the action within each piece. In the elementary case of particle mechanics, this “skeletonisation” converts the action expressed as a Riemann integral into a discrete sum. A desirable property of the action is that it should be additive when we glue the pieces back together. This is achieved only when one properly takes into account the boundaries of the pieces. The boundaries can be spacelike, timelike or null. Much work has focused on the first two cases. The Einstein–Hilbert Action principle for spacetime regions with null boundaries has only recently attracted attention (look up the Arxiv for papers by E. Poisson et al and Parattu et al; references would not be consistent with the chatty, informal style of CQG+). These papers deal with the appropriate boundary terms that appear in all boundary signatures.
Daniela Saadeh – UCL Astrophysics Group
CQG is proud to sponsor the IOP Gravitational Physics Group (GPG) thesis prize. This year the prize was awarded to Daniela Saadeh, who we have interviewed below. Congratulations Daniela!
Can you tell us a little bit about the work in your thesis?
A fundamental assumption of the standard model of cosmology is that the large-scale Universe is isotropic – i.e. that its properties are independent of direction. Historically, this concept stemmed from the Copernican Principle, the philosophical statement that we do not occupy a ‘special’ place in the Universe. In physical terms, this idea is converted into the assumption that all positions and directions in the Universe are equivalent, so that no observer is ‘privileged’.
However, assumptions must be tested, especially foundational ones. General relativity – our standard theory of gravity – allows for many ways in which spacetime could be anisotropic: directional symmetry is not fundamentally required. If the Universe were indeed to be anisotropic, we would actually need to carefully revise our understanding (for instance, calculations about its history or content). Making this health check is very important! Continue reading