Many cosmological models predict the existence of a stochastic Gravitational-Wave (GW) background produced just after the universe was born. As gravitational waves do not interact with matter, their detection would give us a unique and pristine probe to study the very first instants of the Universe: when it was 50 orders of magnitude younger than its age at the epoch of the photon decoupling. Such a detection would be as important as the discovery of the Cosmological Microwave Background (CMB). CMB studies tell us what the universe looked like when it became optically thin (~300,000 years after the Big Bang). They help us to establish the standard ΛCDM model of cosmology and to understand the important role of inflation. Continue reading
How does matter couple in theories involving several metrics? We unveil the possibility for a new effective metric.
While the theory of general relativity will mark its 100 year anniversary next fall, the realization that the expansion of our universe may currently be accelerating has opened up the door for a series of investigations to understand the behavior of gravity at large distances – as large as the current observable Universe or about 1010 light years. Among the different possible modifications of gravity explored in the past decade, theories of gravity which involve several metrics have played a crucial role. The idea that gravity could be the outcome of several interacting metrics is of course not a new concept and such theories have been explored for more than 70 years, but their consistent realization has only been derived very recently in the past few years, and we are finally reaching a stage where we can understand more precisely how matter couples to gravity in such theories. Continue reading
As features editor of Physics World magazine, my search for stories to share with our readers takes me far and wide – from nuclear reactors to the quietest lab in the world. But sometimes I need look no further than the very office in which I work. That’s because I share my workplace with the staff behind nearly 70 journals published by IOP Publishing. So it was that one lunchtime earlier this year, I got chatting to Adam Day, publisher of Classical and Quantum Gravity (CQG).
The next generation gravitational wave interferometers, known as Advanced LIGO, located in Hanford, WA and Livingston, LA have been installed and are in the process of achieving a sensitivity required for the first direct detection of a gravitational wave. The goal of their design is to measure a gravitational strain as small as 4×10–24/√Hz, requiring a length resolution of approximately 10–19 rms within a 100 Hz bandwidth. This high sensitivity demands multiple optical cavities to enhance the response Continue reading
Black holes have a potential technological application that is frequently overlooked: they allow you to look at the back of your own head. This could be useful for checking that your tie is properly tucked into your shirt collar, or – perhaps more relevant for physicists – that your pony tail is straight. This technology relies on the fact that there exist circular photon orbits in all members of the Kerr-Newman-de Sitter family of spacetimes for which the parameters (mass, charge and cosmological constant) correspond to a black hole.
The question arises as to whether this characteristic feature of electro-vac Continue reading
Gauge-gravity duality allows us to calculate properties of certain quantum field theories (QFT) from classical general relativity. One famous piece of this conjecture, due to Ryu and Takayanagi, relates the entanglement entropy in a QFT region to the area of a surface in the gravitational theory. In addition to being a clue about quantum gravity, this proposal is one of the few tools which allow us to calculate entanglement entropy analytically. Since the entanglement entropy is of increasing interest for field theory and condensed matter applications, it is important to check if the conjecture is true.
One important property of the entropy is strong subadditivity (SSA). This quantum inequality says that the sum of the entropies in two regions is always greater than the sum of the entropies of their union and intersection. My article uses proof Continue reading
Cosmic strings have been a source of fascination in cosmology since Tom Kibble first proposed their existence 40 years ago. Like an imperfection in a solidifying crystal, a cosmic string is a thread of energy that might have formed in the early universe during a symmetry breaking phase transition. Twenty years ago Ruth Gregory pointed out that a black hole could have a cosmic string as a single “hair”. Turning this idea around, in this article we have proposed that a Continue reading
The quest to detect gravitational waves directly has seen great advances over the past five decades, with the earlier resonant “bar” detectors being surpassed in sensitivity by large laser interferometers in the last decade. The first generation of interferometric detectors proved the viability of the approach, progressively improving sensing and control techniques and running up against the fundamental limitations of their designs. Along the way, many searches for gravitational wave signals were carried out and published, but none achieved the milestone of detecting a clear gravitational-wave signal.
All of that is about to change. The lessons learned from the first full-scale interferometric detectors fed into the design of advanced detectors which are now being constructed and commissioned and will soon begin collecting data. Higher laser power, Continue reading
Can the supertranslation symmetry of radiating spacetimes affect angular momentum loss?
That was the question on my mind when I went to a workshop at Berkeley, California last winter. I knew that the supertranslations were a global aspect of the gravitational memory effect, which produces a net displacement between particles after passage of a gravitational wave. What I didn’t know, and learned from David Garfinkle at Berkeley, was that there was an electromagnetic analog of radiation memory, which produces a momentum kick on test charges after passage of a wave. Surprisingly, this result has apparently gone unnoticed in Continue reading
Quantum gravity alone is not the only major theoretical open problem in fundamental physics: gravity, quantum theory and thermodynamics form a triple, whose full interconnections we have definitely not yet understood. As soon as quantum effects appear in a curved spacetime, thermal aspects appear to be unavoidable. Combining thermodynamics and (full) gravity might turn out to be even more crucial than understanding the quantum aspects of the gravitational field alone. In recent years, it has become increasingly clear that entanglement entropy is a central ingredient for the synthesis we are seeking. Continue reading