The Black Hole information problem: past, present, and future

Donald Marolf
We give a brief overview of the black hole information problem emphasizing fundamental issues and recent proposals for its resolution. The focus is on broad perspective and providing a guide to current literature rather than presenting full details. We concentrate on resolutions restoring naive unitarity…
Read more at https://arxiv.org/pdf/1703.02143.pdf

GW151226: Observation of Gravitational Waves from a 22 Solar-mass Binary Black Hole Coalescence

Figure 1. (Adapted from figure 1 of our publication). The gravitational wave event GW151226 as observed by the twin Advanced LIGO instruments: LIGO Hanford (left) and LIGO Livingston (right). The images show the data recorded by the detectors during the last second before merger as the signal varies as a function of time (in seconds) and frequency (in Hertz or the number of wave cycles per second). To be certain that a real gravitational wave has been observed, we compare the data from the detectors against a pre-defined set of models for merging binaries. This allows us to find gravitational wave signals which are buried deep in the noise from the instruments and nearly impossible to find by eye. The animation shows the detector data with and without removing the best-matching model gravitational-wave signal, making it much easier to identify. The signal can be seen sweeping up in frequency as the two black holes spiral together. This signal is much more difficult to spot by eye than the first detection GW150914!

Figure 1. (Adapted from figure 1 of our publication). The gravitational wave event GW151226 as observed by the twin Advanced LIGO instruments: LIGO Hanford (left) and LIGO Livingston (right). The images show the data recorded by the detectors during the last second before merger as the signal varies as a function of time (in seconds) and frequency (in Hertz or the number of wave cycles per second). To be certain that a real gravitational wave has been observed, we compare the data from the detectors against a pre-defined set of models for merging binaries. This allows us to find gravitational wave signals which are buried deep in the noise from the instruments and nearly impossible to find by eye. The animation shows the detector data with and without removing the best-matching model gravitational-wave signal, making it much easier to identify. The signal can be seen sweeping up in frequency as the two black holes spiral together. This signal is much more difficult to spot by eye than the first detection GW150914!

A few months after the first detection of gravitational waves from the black hole merger event GW150914, the Laser Interferometer Gravitational-Wave Observatory (LIGO) has made another observation of gravitational waves from the collision and merger of a pair of black holes. This signal, called GW151226, arrived at the LIGO detectors on 26 December 2015 at 03:38:53 UTC.

The signal, which came from a distance of around 1.4 billion light-years, was an example of a compact binary coalescence, when two extremely dense objects merge. Binary systems like this are one of many sources of gravitational waves for which the LIGO detectors are searching. Gravitational waves are ripples in space-time itself and carry energy away from such a binary system, causing the two objects to spiral towards each other as they orbit. This inspiral brings the objects closer and closer together until they merge. The gravitational waves produced by the binary stretch and squash space-time as they spread out through the universe. It is this stretching and squashing that can be detected by observatories like Advanced LIGO, and used to reveal information about the sources which created the gravitational waves.

GW151226 is the second definitive observation of a merging binary black hole system detected by the LIGO Scientific Collaboration and Virgo Collaboration. Together with GW150914, this event marks the beginning of gravitational-wave astronomy as a revolutionary new means to explore the frontiers of our Universe….

Read more at: http://www.ligo.org/science/Publication-GW151226/index.php#sthash.GM0EB4ib.dpuf>

Soft Hair on Black Holes

Stephen W. Hawking, Malcolm J. Perry, and Andrew Strominger
It has recently been shown that BMS supertranslation symmetries imply an infinite number of conservation laws for all gravitational theories in asymptotically Minkowskian spacetimes. These laws require black holes to carry a large amount of soft (i.e. zero-energy) supertranslation hair. The presence of a Maxwell field similarly implies soft electric hair. This paper gives an explicit description of soft hair in terms of soft gravitons or photons on the black hole horizon, and shows that complete information about their quantum state is stored on a holographic plate at the future boundary of the horizon. Charge conservation is used to give an infinite number of exact relations between the evaporation products of black holes which have different soft hair but are otherwise identical. It is further argued that soft hair which is spatially localized to much less than a Planck length cannot be excited in a physically realizable process, giving an effective number of soft degrees of freedom proportional to the horizon area in Planck units.
Read more at https://arxiv.org/pdf/1601.00921v1.pdf

Read also: “Gary T. Horowitz, Black Holes Have Soft Quantum Hair

Testing General Relativity in a Black Hole’s Shadow

Deviations in the shadow of our Galaxy’s supermassive black hole could reveal violations of general relativity.
PhysRevLett.116

The Event Horizon Telescope (EHT), a planet-wide network of radio telescopes, is compiling the first direct image of the giant black hole in the center of our Galaxy. The trapping of light by the hole will produce a dark shadow surrounded by a bright circular ring. A new analysis shows that the EHT could potentially detect small deviations in the shadow size, which are predicted by alternative theories of gravity.

Like most galaxies, our Milky Way has a supermassive black hole, called Sagittarius A* (Sgr A*). Observations of stars orbiting Sgr A* have provided estimates of its mass (about 4 million solar masses) and its distance from us (roughly 27,000 light years). The radius of the black hole, defined by its event horizon, is just 17 times that of the Sun. To image this compact object, astronomers formed the EHT project, which performs interferometry on data from several radio telescopes from around the globe. The group expects to have the first snapshot of Sgr A* in the next few years.

According to general relativity, the warping of space around Sgr A* creates a shadow with an apparent radius of exactly 50 microarcseconds. By contrast, many alternative gravity theories predict a larger or smaller shadow. Tim Johannsen from the Perimeter Institute for Theoretical Physics, Canada, and colleagues analyzed a previously constructed simulation of EHT data for Sgr A*. They first showed that the EHT will dramatically reduce the uncertainties in the mass and distance measurements. They also describe in detail how the EHT will test general relativity by measuring certain spacetime deviation parameters that have nonzero values in alternative gravity theories.

This research is published in Physical Review Letters.

–Michael Schirber – http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.116.031101