Celebrating quantum gravity: the moon’s craters and conceptual revolutions

by Seth K. Asante, Bianca Dittrich, and Hal M. Haggard 

Fifty years ago this December the astronauts of the Apollo 8 mission were the first humans to ever see the far side of the moon. As they passed behind the moon they lost radio contact with mission control in Houston. They were completely isolated. Only recently have cockpit recordings of their reactions become public [1]. At first they couldn’t see the moon at all, but then the command module pilot James A. Lovell Jr. exclaims “Hey, I got the moon!”. William A. Anders, the lunar module pilot, asks excitedly “Is it below us?” and Lovell begins “Yes, and it’s—” when Anders interrupts him having spotted it. Deeply enthused the astronauts have dropped their technical patter and systems checks, which make up the main fabric of the recordings. Anders marvels “I have trouble telling the bumps from the holes.” In his excitement Anders completely loses his technical jargon. He can’t even recall the word ‘crater’. He is reacting to the moon. It is easy to feel his enthusiasm at this hidden wonder.


Hal Haggard, Seth Asante, and Bianca Dittrich form a triangle area, the main variable in their new study of discrete gravity [5]. If you squint the image is even a bit like The Dark Side of the Moon’s cover art. The picture is taken in front of artwork by Elizabeth McIntosh hanging in the main atrium of the Perimeter Institute.

Quantum gravity is a deep puzzle of modern physics. Like the far side of the moon, much of the full theory is still hidden from view. But, it seems to me that we too seldom celebrate the great accomplishments that thinking about this puzzle has yielded. Two grand anniversaries both connected to gravity are to be celebrated this year. It’s a perfect moment to feel again the excitement that these discoveries represent and to connect to the enthusiasm and sense of exploration that quantum gravity can inspire. Continue reading

The Sound of Exotic Astrophysical “Instruments”

by Sebastian Völkel and Kostas Kokkotas

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.

Continue reading

Undergraduate research and publications

by Nelson Christensen

The participation of undergraduates in scientific research is important for a number of reasons. First and foremost, undergraduates can make significant contributions to the science. In addition, research by undergraduates is now recognised to be an extremely important part of the educational process for these students. LIGO and Virgo have provided wonderful opportunities for undergraduates to experience the joys of physics  research. With guidance, students across the undergraduate physics spectrum can find a project suited to their level of expertise and their interests.


Professor Nelson Christensen, who has conducted research and published with numerous undergraduates over the years.

Over the years at Carleton College I have had the thrill of seeing many students make real and significant contributions to LIGO and Virgo’s research efforts. When the students take their success from the classroom to research their joy for physics really springs out. But it should be noted that research is not a sure success for all undergraduate physics majors. I have seen “A” students who could never make the connection to the independent and original work required with a research project; that’s okay, research is not for everyone. On the other hand, I have worked with students who earned B’s and C’s in their physics classes, yet exploded with the opportunity of research; the applied nature of the physics motivated them, and consequently, often encouraged them to become better students in the classroom as well. Continue reading

COSMOLOGICAL CONS(tant → erved charge)

The road to black hole thermodynamics with Λ

by  Dmitry Chernyavsky and Kamal Hajian

What are volume and pressure in black hole thermodynamics? That is the question!

Chernyavsky Balloon

What do the gas in a balloon and a black hole have in common? For a regular CQG reader the answer should be obvious; both can be described within the framework of thermodynamics. However we know that the gas in balloon is characterised by volume and pressure, as well as other  thermodynamic quantities. So, a natural question arises about analogues of the volume and pressure for a black hole.
Answering this question, black hole physicists have noticed that if the universe is filled with a non-zero cosmological constant Λ, this mysterious entity can be absorbed in the energy-momentum tensor of matter, and its contribution resembles a perfect fluid with a pressure proportional to Λ. Continuing with this analogy, one can also introduce a ‘thermodynamic volume’ for a black hole. For instance, the appropriate volume which satisfies the first law of thermodynamics for the Schwarzschild black hole is equal to the volume of a ball with the same radius, but in flat space! Using the notions of the black hole pressure P and volume V, it is standard to vary the cosmological constant generalising the first law of black hole thermodynamics by V δP.

Chernyavsky authors

Dmitry Chernyavsky and Kamal Hajian Sevan lake in Armenia where we started to think about the cosmological conserved charge instead of cosmological constant.

Continue reading

How to reach infinity?

Bypassing stability conditions and curing logarithmic singularities

By Jörg Frauendiener and Jörg Hennig

Assume you want to model a general relativistic spacetime. Due to the annoying limitations of conventional computers, like finite memory and processing speed, it is tempting to focus on a finite portion of the spacetime. Then, without waiting endlessly, one can obtain an approximate description of this portion. One just has to choose a suitable numerical method and solve the field equations for the metric at some set of grid-points. While this approach is standard, it introduces unpleasant problems. Firstly, the set of equations needs to be complemented with boundary conditions at the outer edges of this finite portion, in order to obtain a complete mathematical problem. This, however, is quite unphysical as usually no information about the actual behaviour at such an artificial boundary is available. Consequently, spurious gravitational radiation enters the numerical domain. Secondly, if one is interested in accurately describing gravitational waves, one should recall that these are only well-defined at infinity. Hence it is desirable to extend the simulation up to infinity.


Jörg Frauendiener and Jörg Hennig trapped at infinity.

Continue reading

Can You See Asymptotic Symmetries?

Hopefully yes: Measure their Berry phases.

By Blagoje Oblak

Some years ago, at a dinner party, I met a fellow physicist who asked me what I was working on. I told him I was studying asymptotic symmetries — symmetries of space-time seen by observers located far away from all sources of the gravitational field. Remarkably, I said, these symmetries often have a beautiful infinite-dimensional structure and may provide new insights in our understanding of gravity. Somewhat sceptical, he replied: `Well surely this must be in some toy model — some extra dimensions, or postulated particles and fields… There’s no way this is directly relevant to our actual, real world!’ While I could understand his perspective, I also felt a little hurt by his cynicism towards theoretical science, so I was happy to retort: No, asymptotic symmetries do not require anything beyond what has been firmly established by experiment; just take pure general relativity, and their magic reveals itself.

Blagoje Oblak performing a gravitational experiment in the Mediterranean. Photo credit: Geoffrey Mullier.

Continue reading

Constructing Spacetimes

By Steven Carlip and Samuel Loomis

Imagine you are given a bucket of points and asked to assemble them into a spacetime. What kind of “glue” would you need?

In causal set theory, the only added ingredient is the set of causal relations, the knowledge of which points are to the past and future of which. In particular, suppose your points were taken at random from a real spacetime, at some typical length scale ℓ. Then on scales large compared to ℓ, the causal diamonds – the sets formed by intersecting the past of one point with the future of another – determine the topology; the causal relations determine the metric up to a scale factor; and the remaining scale factor is just a local volume, which can be obtained by counting points. As the slogan of Rafael Sorkin, the founder of the field, goes, “Order + Number = Geometry.”

Carlip photo

Samuel Loomis and Steven Carlip with their causal set.

Continue reading

Is Gravity Parity Violating?

I-Love-Q Probes of Modied Gravity

By Toral Gupta, Barun Majumder, Kent Yagi, and  Nicolás Yunes

Although General Relativity has passed all tests carried out so far with flying colors, probes of the extreme gravity regime, where the gravitational interaction is simultaneously strong, non-linear and highly dynamical, have only recently began. This is timely because attempts to reconcile general relativity with quantum mechanics, be it in the form of string theory or loop quantum gravity, and attempts to explain cosmological observations, be it in the early or late universe, may require modifications to Einstein’s general theory. New electromagnetic telescopes, like the Neutron Star Interior Composition Explorer, and gravitational wave detectors, like advanced LIGO and Virgo, can now provide the first detailed observations of the extreme gravity regime. These new telescopes herald the era of extreme experimental relativity, allowing for new stringent constraints of deviations from Einstein’s theory, or perhaps, if we are lucky, pointing to signals of departures.

Continue reading

Bouncing a cosmic brew

From quantum gravity to early universe cosmology using group field theory condensates

By Marco de Cesare, Daniele Oriti, Andreas Pithis, and Mairi Sakellariadou 

“If you can look into the seeds of spacetime,
And say which grain will grow and which will not,
Speak then to me.”
– adapted quote from William Shakespeare’s, Macbeth

When we try to describe the earliest stages of the expansion of our Universe, the current picture of spacetime and its geometry as given by Einstein’s theory of General Relativity (GR) breaks down due to the extreme physical conditions faced at the Big Bang. More specifically, theorems by Hawking and Penrose imply that the cosmos emerged from a spacetime singularity. The existence of a cosmological singularity represents a main obstacle in obtaining a complete and consistent picture of cosmic evolution. However, there are reasons to believe that quantum gravitational effects taking place at the smallest scale could lead to a resolution of such singularities. This would have a huge impact for our understanding of gravity at a microscopic level, and for Cosmology of the very early Universe.

Continue reading

A Kind Of Magic

The road from Dunsink to the exceptional symmetries of M-theory

By Leron Borsten and Alessio Marrani 

Our journey starts in the fall of 1843 at the Dunsink Observatory[1], presiding from its hill-top vantage over the westerly reaches of Dublin City, seat to the then Astronomer Royal Sir William Rowan Hamilton. In the preceding months Hamilton had become preoccupied by the observation that multiplication by a complex phase induces a rotation in the Argand plane, revealing an intimate link between two-dimensional Euclidean geometry and the complex numbers ℂ. Fascinated by this unification of geometry and algebra, Hamilton set about the task of constructing a new number system that would do for three dimensions what the complexes did for two. After a series of trying failures, on October 16th 1843, while walking from the Dunsink Observatory to a meeting of the Royal Irish Academy on Dawson Street, Hamilton surmounted his apparent impasse in a moment of inspired clarity: rotations in three dimensions require a four-dimensional algebra with one real and three imaginary units satisfying the fundamental relations i= j= k= ijk = -1. The quaternions ℍ were thus born. Taken in that instant of epiphany, Hamilton etched his now famous equations onto the underside of Broome bridge, a cave painting illuminated not by campfire, but mathematical insight and imagination.  Like all great mathematical expressions, once seen they hang elegant and timeless, eternal patterns in the fixed stars merely chanced upon by our ancestral explorers.


Leron Borsten (left) and Alessio Marrani (right) stood before Hamilton’s fundamental relations, Broome bridge Dublin. Leron is currently a Schrödinger Fellow in the School of Theoretical Physics, Dublin Institute for Advanced Studies. Alessio is currently a Senior Grantee at the Enrico Fermi Research Centre, Roma.

Continue reading