GRG Editor’s Choice: Extreme Gravity Tests with Gravitational Waves from Compact Binary Coalescences

Wednesday, April 4, 2018

The first part of our review article Extreme gravity tests with gravitational waves from compact binary coalescences was published today in General Relativity and Gravitation and selected as an Editor’s Choice.

This review is part of the Topical Collection Testing the Kerr spacetime with gravitational-wave and electromagnetic observations (Guest Editor: Emanuele Berti).

New PRL: Spontaneous Scalarization for Black Holes and Neutron Stars in Scalar-Gauss-Bonnet Gravity

Friday, March 30, 2018

In a new paper published today in Physical Review Letters we identify a class of modified theories of gravity where gravity is mediated by a scalar field coupled to quadratic terms in the curvature (the so-called “Gauss-Bonnet invariant”) that could be particularly interesting in the context of testing general relativity with gravitational wave detectors. These theories exhibit the phenomenon called “spontaneous scalarization” for both black holes and compact stars: for certain classes of solutions the scalar field can “grow” and modify the structure of these compact objects, with possible observational signatures. These theories formally admit all of the stationary solutions of general relativity, but the general relativistic solutions are not dynamically preferred if certain conditions are satisfied. Remarkably, black holes exhibit scalarization if their mass lies within one of many narrow bands. Two related papers (one by Antoniou et al., the other by Doneva and Yazadjiev) appeared in the same issue of Physical Review Letters.

82nd Mississippi Academy of Sciences Meeting

Wednesday, February 28, 2018

Some of our graduate students took part in the 82nd Annual Mississippi Academy of Sciences Meeting on February 22-23 at the University of Southern Mississippi. Shrobana Ghosh and Sunethra Dayavansha shared the first prize for the best talk. BB Pilgrim won the second prize for talks in the Physics and Engineering division. Ashoka Karunarathne won the third prize for a poster at the Mississippi INBRE (IDeA Network of Biomedical Research) graduate scholars symposium, held at the same meeting. Congratulations!

New NASA Award

Wednesday, February 7, 2018

Our NASA Astrophysics Theory Program (ATP) Grant Proposal 17-ATP17-0047 (Exploring Extreme Gravity with LISA: Developing a Science Case for Tests of General Relativity) (PI: Emanuele Berti, Co-PI: Nicolás Yunes) was recommended for funding. The total award amount is $815,554 over 3 years. This is the proposal summary:

This proposal is focused on developing the experimental-relativity science case for the Laser Interferometer Space Antenna (LISA). We propose to explore tests of General Relativity with LISA gravitational wave data using different astrophysical population models and astrophysical sources to forecast what will be possible in the era of space-based detectors. We will create and develop tools to carry out consistency checks of Einstein’s theory and to search for modified gravity anomalies with LISA data. We will explore how the strength of these tests varies with population models and with astrophysical sources, mapping out the theory space that will be constrainable with LISA. We will also explore the strength of combining LISA data with ground-based gravitational wave observations to carry out tests of Einstein’s theory with multi-wavelength observations.

The proposed work is of direct relevance to NASA’s strategic mission to better understand the universe through observation, and to NASA’s mission of discovery and knowledge. The region of the universe where gravity is very strong and dynamically changing (the extreme gravity universe) is one of the last unturned stones. This is in part because extreme gravity objects, like black holes, are difficult to resolve due to their size and distance from Earth. NASA’s investment in space-borne gravitational wave astrophysics as a partner to ESA is aimed at resolving such objects and, for the first time, exploring the extreme gravity universe in detail. The focus of this proposal is to aid in this endeavor by developing the understanding needed to extract the most information about theoretical physics and modified gravity constraints from LISA data.

New PRL: Light-Ring Stability for Ultracompact Objects

Monday, December 18, 2017

Five of the groundbreaking gravitational wave detections by the LIGO/Virgo collaboration have been interpreted as black hole collisions forming a more massive black hole. It is hard to demonstrate conclusively that these objects are indeed black holes, and there is a lively debate on the intriguing possibility that other, more exotic alternatives could explain the observations. An article by Pedro Cunha, Emanuele Berti and Carlos Herdeiro, published today in Physical Review Letters, provides a generic no-go theorem for these exotic alternatives.

The remnant black hole born from black hole collisions vibrates with a characteristic signature - a “sound” similar to a ringing bell. This special signature, called ringdown, is related to the existence of special orbits called “light rings”: around a black hole, gravity bends light so much that light rays can circumnavigate the black hole (so if you were close enough to a black hole, you could see the back of your head). Some exotic alternatives to black holes can also have light rings, and therefore they can ring down just like black holes. It has been proposed that these “black hole mimickers” could have produced the events observed by LIGO/Virgo.

This Letter shows that if the compact object is not a black hole, it must necessarily have a second light ring. This second light ring differs from the first in one crucial way: it traps radiation. The trapped radiation piles up and destabilizes the exotic compact object, making it unlikely to exist in Nature. The implication is that these exotic objects are generically unstable, and therefore that the LIGO-Virgo detections are really evidence for black holes.

The figure below is a schematic illustration of an ultracompact object with two light rings: the unstable light ring produces gravitational radiation that can potentially imitate the signal from a black hole, but the second (stable) light ring traps radiation and destabilizes the object.