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Tag Archives: black hole

On June 15, 2016, scientists at LIGO announced they had detected a second pair of black holes merging, at a distance of 1.4 billion light years, releasing the energy equivalent of the mass of the Sun. The consequent ripples in space-time shook the twin detectors at LIGO on December 26, 2015.

This brings the number of confirmed detections by LIGO to two within just four months, giving scientists optimism that more events will follow, enabling quantitative predictions about how frequently these high-energy events occur across space and time.

Read more in this NYT article, including a short video.


An article in the New York Times by Dennis Overbye gives the latest chapter in Stephen Hawking’s saga concerning whether the properties of matter that has fallen into a black hole are lost forever, or whether there is a way out.

Forty years ago, Hawking showed theoretically that black holes were not ‘eternal prisons’ but could leak radiation. There ensued a long-running debate about whether this radiation retained any information or attributes of the original matter. If it does not, this violates a tenet of modern physics, that it is always possible, in theory, to reverse time. This became known as the ‘information paradox’ and was the subject of a famous bet between Hawking and Caltech professor John Preskill. (Hawking conceded defeat 10 years ago, admitting that advances in string theory, had left no room in the universe for information loss.)

In a paper published to be published this week in Physical Review Letters, Hawking and his colleagues Andrew Strominger (Harvard) and Malcolm Perry (Cambridge) announce they have found a clue pointing the way out of black holes. They new results undermine John Wheeler’s famous notion that black holes have “no hair” — that they are shorn of the essential properties of the things they have consumed.

Looked at from the right vantage point — from a far distance in time, technically known as “null infinity” — black holes might not be not be bald at all. A tell-tale pattern of light rays bordering the event horizon contains information about what has passed through. This has been dubbed in their paper a “soft hair” theory.

For a more complete description of this story, see the full NYT article.

In a subsequent article, Dennis Overbye answers questions on black holes submitted by his readers.


An update for Section 13.3f (pp. 352-355) in Pasachoff & Filippenko, The Cosmos, 4th ed:
In what many think is the most important step in astronomy since Galileo first turned a telescope on the heavens in 1609, ripples in space-time known as gravitational waves were detected on September 14, 2015, and reported to the world at large on February 11, 2016. Scientists gathered all over in auditoriums at the National Press Club in Washington, at Caltech, at MIT, in Moscow, and in many other places for the epochal event. “Ladies and gentlemen, we have detected gravitational waves,” said the executive director of the LIGO Laboratory. “We did it!”

The observational existence of gravitational radiation had been established by the changing period of the Hulse-Taylor “double pulsar” at a rate that matched the energy loss expected from the emission of gravitational waves (Section 13.3f, pp. 352-354), but the gravitational waves themselves – distortions of space-time – had not been detected directly until the Advanced LIGO (Laser Interferometer Gravitational Wave Detector) picked up a signal in its engineering run on September 14, 2015.  The signal, which could be heard as an upwards “chirp” in the audio range, resulted from two black holes 1.3 billion light years away, each containing about 30 solar masses, spiraling rapidly into each other and merging to become a single black hole with almost the same total mass but with 3 solar masses of material converted into energy in the form of gravitational waves.

Advanced LIGO has about 3 times the sensitivity of the earlier LIGO, and this event was within that advance of sensitivity. A further gain of 3x is expected with Advanced LIGO in the future.

Scientific papers were published February 11, 2016, in the Physical Review Letters and in the Astrophysical Journal Letters.

A few days later, I [JMP] attended a two-hour evening session at Caltech’s main auditorium to hear the principals of the LIGO project speaking, and I had the feeling that it was just like being at a meeting of the Lincei Academy in Florence in 1610 to hear Galileo speak about his new discoveries with the [optical] telescope.

There are many links to discussions and animations available. Here are some of them:

Popular and academic press:
New York Times coverage (includes movie); also NYT Opinion piece and an NYT Editorial a few days later, justifying scientific inquiry; a congressman’s letter, and sound bites from scientists.
Nature coverage (with explanatory graphics)
Science magazine in depth (requires subscription to access full text)
New Yorker article (with an account of LIGO’s inception and development)
Physics Today comparison of gravitational waves and sound waves
The Wall Street Journal and Michio Kaku’s desciption.

Societies and organizations:
American Physical Society Viewpoint
The Kavli Foundation Scientific spotlight
Caltech (press releasesillustrations, movies and animations)
NSF press release
AAPT resources on gravitational waves
STFC (UK) press release
Cornell Chronicle and media statement
University of Texas Rio Grande Valley press release
ESA congratulations
CSIRO (Australia) news release

LIGO websites:
LIGO labs (Observatories: Livingston | Hanford); Advanced LIGO; LIGO Scientific Collaboration; LIGO Partner Experiments and Collaborations






From a press release from the Chandra X-ray Center and NASA’s Marshall Space Flight Center, September 23, 2015:

Three orbiting X-ray space telescopes have detected an increased rate of X-ray flares from the usually quiet giant black hole at the center of our Milky Way galaxy after new long-term monitoring. Scientists are trying to learn whether this is normal behavior that was unnoticed due to limited monitoring, or these flares are triggered by the recent close passage of a mysterious, dusty object.

Credit: NASA/CXC/MPE/G.Ponti et al; Illustration: NASA/CXC/M.Weiss

Credit: NASA/CXC/MPE/G.Ponti et al; Illustration: NASA/CXC/M.Weiss

By combining information from long monitoring campaigns by NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton, with observations by the Swift satellite, astronomers were able to carefully trace the activity of the Milky Way’s supermassive black hole over the last 15 years. The supermassive black hole, Sagittarius A*, weighs in at slightly more than 4 million times the mass of the Sun. X-rays are produced by hot gas flowing toward the black hole.

The new study reveals that Sagittarius A* (Sgr A* for short) has been producing one bright X-ray flare about every ten days. However, within the past year, there has been a ten-fold increase in the rate of bright flares from Sgr A*, at about one every day. This increase happened soon after the close approach to Sgr A* by a mysterious object called G2.

Originally, astronomers thought G2 was an extended cloud of gas and dust. However, after passing close to Sgr A* in late 2013, its appearance did not change much, apart from being slightly stretched by the gravity of the black hole. This led to new theories that G2 was not simply a gas cloud, but instead a star swathed in an extended dusty cocoon.

While the timing of G2’s passage with the surge in X-rays from Sgr A* is intriguing astronomers see other black holes that seem to behave like Sgr A*. Therefore, it’s possible this increased chatter from Sgr A* may be a common trait among black holes and unrelated to G2. For example, the increased X-ray activity could be due to a change in the strength of winds from nearby massive stars that are feeding material to the black hole.

If the G2 explanation is correct, the spike in bright X-ray flares would be the first sign of excess material falling onto the black hole because of the cloud’s close passage. Some gas would likely have been stripped off the cloud, and captured by the gravity of Sgr A*. It then could have started interacting with hot material flowing towards the black hole, funneling more gas toward the black hole that could later be consumed by Sgr A*.

Links: Full Chandra press release; detailed image description; MNRAS paper by G. Ponti et al.

From a press release of the European Space Agency:

Over the past week, ESA’s Integral satellite has been observing an exceptional outburst of high-energy light produced by a black hole that is devouring material from its stellar companion.


Credit and copyright: ESA/ATG medialab

X-rays and gamma rays point to some of the most extreme phenomena in the Universe, such as stellar explosions, powerful outbursts and black holes feasting on their surroundings. In contrast to the peaceful view of the night sky we see with our eyes, the high-energy sky is a dynamic light show, from flickering sources that change their brightness dramatically in a few minutes to others that vary on timescales spanning years or even decades.

On 15 June 2015, a long-time acquaintance of X-ray and gamma ray astronomers made its comeback to the cosmic stage: V404 Cygni, a system comprising a black hole and a star orbiting one another. It is located in our Milky Way galaxy, almost 8000 light-years away in the constellation Cygnus, the Swan. In this type of binary system, material flows from the star towards the black hole and gathers in a disc, where it is heated up, shining brightly at optical, ultraviolet and X-ray wavelengths before spiralling into the black hole.

The V404 Cygni black hole system has not been this bright and active since 1989, when it was observed with the Japanese X-ray satellite Ginga and high-energy instruments on board the Mir space station.

Link: the full ESA press release.

From a press release of the Max Planck Institute for Radio Astronomy, April 21, 2015:

Joint observations by the Atacama Pathfinder Experiment (APEX) telescope in Chile and Antarctica’s largest astronomical telescope come closer to making detailed images of the supermassive black hole in the centre of the Milky Way, up to its very edge, the “event horizon”. These successful observations were conducted within the framework of the Event Horizon Telescope – a virtual telescope as big as planet Earth.

Event Horizon Telescope

Credit: © Dan Marrone/University of Arizona

The South Pole Telescope (SPT) and APEX joined together in a ‘Very Long Baseline Interferometry’ experiment for the first time in January 2015. The two telescopes together observed two sources — the black hole at the centre of the Milky Way galaxy, Sagittarius A*, and the black hole at the centre of the nearby galaxy Centaurus A — and combined their signals to synthesize a telescope 7,000 kilometres across. With this success, the SPT joins the Event Horizon Telescope array, which connects APEX, the Large Millimeter Telescope in Mexico, the Submillimeter Telescope in Arizona, the Combined Array for Research in Millimeter-wave Astronomy in California, the Submillimeter Array and James Clerk Maxwell Telescope in Hawaii, and the Institute for Radio Astronomy Millimetrique (IRAM) telescopes in Spain and France.

Even though the Milky Way’s black hole is 4 million times more massive than the Sun, it is tiny to the eyes of astronomers. Smaller than Mercury’s orbit around the Sun, yet almost 26,000 light years away, studying its event horizon in detail is equivalent to standing in New York and reading the date on a cent in Germany.

With its unprecedented resolution, more than 1,000 times better than the Hubble Telescope, the EHT will see swirling gas on its final plunge over the event horizon, never to regain contact with the rest of the universe. If the theory of general relativity is correct, the black hole itself will be invisible because not even light can escape its immense gravity. However, it might still be seen as a silhouette against the background.

Links: MPIfR press release; EHT home.

A recent article in Nature describes how a supermassive black hole’s spin has been measured via the gravitational lens of a foreground galaxy, that fortuitously lies along the same line-of-sight. The new measurements have enabled astronomers to find that a supermassive black hole powering a distant quasar has grown through coherent, rather than random, episodes of mass accretion (see Section 17.3, p. 458-459). The following text is a digest of Guido Risalti’s summary in Nature‘s ‘News and views’ section, March 13, 2014.

Supermassive black holes are simple systems. They are characterized by just two quantities, their mass and their spin. Whereas the total amount of accretion and any mergers that a supermassive black hole undergoes are encoded in its mass, how this mass was assembled is encoded in its spin. A few ordered accretion events or mergers of large black holes produce high spins, and short, random accretion processes produce low spins. Measuring these spins is therefore a major goal of extragalactic astronomy: the spins of supermassive black holes hold a key to understanding the evolution of their host galaxies.

But how can we measure the spins? According to Einstein’s general theory of relativity, a black hole’s gravitational field twists space-time around it. Such twisting depends on the black hole’s spin, so measuring the twisting allows the spin to be estimated. The signature of space-time distortion is imprinted on the emission of radiation from regions close to the black hole’s event horizon – the surface beyond which no radiation can escape. The best way to perform such a measurement is to observe X-rays reflected by the disk.

In their study, R. C. Reis and colleagues break new ground by obtaining a spin measurement of a quasar at a distance of more than 6 billion light years from Earth, from a time when the Universe was about half its current age. This remarkable result was possible owing to the exceptional nature of the observed source – a quadruply imaged, gravitationally lensed quasar.


The light from the distant quasar is both magnified and split into four different images by the gravitational field of a foreground elliptical galaxy (the lens) that, by chance, is on the line of sight of the quasar. For this reason, the authors could analyse four ‘copies’ of the X-ray spectrum of the quasar, each with an intensity significantly magnified by the lens. The resulting X-ray spectra have a quality that matches the best that has been obtained for nearby sources, and allowed a robust measurement of the black hole’s spin. As it turns out, the spin is large (close to the highest possible value that theory predicts), suggesting that the black hole acquired its mass through coherent phases of mass accretion.

Links: The Nature article (behind paywall); a widefield view of the lens via U. Michigan press release.

Stephen Hawking, very famous for his ideas about black holes seeming to emit radiation (dubbed ‘Hawking radiation’) and also well known for his ability to function as a theoretical astrophysicist in the face of tremendous personal disability, has made some recent statements in a new paper that seem to challenge the nature of black holes’ event horizons (see Section 14.3, p. 363).

Credit and copyright: Alain Riazuelo

Marek Demianski, a cosmologist at Warsaw University and often a visiting professor at Williams College, evaluated the controversy as follows (February 5, 2014):”Yes, I read this short paper of Hawking. It is really nothing new; when one takes into account quantum effects a black hole is not black any more. Unfortunately we do not have yet a quantum theory of gravity so all these considerations are only hypothetical. From our practical point of view, quantum effects do not influence properties of astrophysical black holes.”

It looks like this theoretical debate is set to continue.

Links: accessible New Scientist article and overview of the Firewall Paradox; New Republic overview; Nature news article; Preprint of Stephen Hawking’s paper.

The European Research Council (ERC) has awarded 14 million euros (around $19 million) to a team of European astrophysicists to construct the first accurate image of a black hole. The team will test the predictions of current theories of gravity, including Einstein’s general theory of relativity. The funding is provided in the form of a synergy grant, the largest and most competitive type of grant of the ERC. This is the first time an astrophysics proposal has been awarded such a grant.

The team, led by investigators at the University of Nijmegen, the Max Planck Institute for Radio Astronomy, and Goethe University in Frankfurt, hopes to measure the shadow cast by the event horizon of the black hole in the center of the Milky Way, find new radio pulsars near this black hole, and combine these measurements with advanced computer simulations of the behavior of light and matter around black holes as predicted by theories of gravity. They will combine several telescopes around the globe to peer into the heart of our own galaxy, which hosts a mysterious radio source called Sagittarius A* which is considered to be the central supermassive black hole. (See p. 383 and Section 15.5, p. 391.)


Credit and © M. Moscibrodzka & H. Falcke, Radboud-Universität Nijmegen

Black holes are notoriously elusive with a gravitational field so large that even light cannot escape their grip. The team plans to make an image of the event horizon – the border around a black hole which light can enter, but not leave.  The scientists want to peer into the heart of our own galaxy, which hosts a mysterious radio source called Sagittarius A*. The object is known to have a mass of around 4 million times the mass of the Sun and is considered to be the central supermassive black hole of the Milky Way.

As gaseous matter is attracted towards the event horizon by the black hole’s gravitational attraction, strong radio emission is produced before the gas disappears. The event horizon should then cast a dark shadow on that bright emission. Given the huge distance to the center of the Milky Way, the shadow is equivalent to the size of an apple on the Moon seen from Earth. By combining high-frequency radio telescopes around the world, in a technique called very long baseline interferometry (VLBI), even such a tiny feature is, in principle, detectable.

In addition, the group wants to use the same radio telescopes to find and measure pulsars around the very same black hole. Pulsars are rapidly spinning neutron stars, which can be used as highly accurate natural clocks in space. While radio pulsars are found throughout the Milky Way, surprisingly none had been found in the center of the Milky Way until very recently.

A recent article in the New York Times highlights the ongoing debate among theoretical cosmologists about what happens when you enter a black hole, the so-called “Firewall Paradox”. At stake are some of the basic tenets of modern science, in particular Einstein’s general theory of relativity, the theory of gravity, on which our understanding of the Universe is based.

The traditional view holds that an astronaut falling into a black hole would not be physically aware of crossing the point of no return, known as the event horizon. (Of course, he or she will inevitably be crushed by the monstrous gravitational forces…)

In 1974, British cosmologist Stephen Hawking theorized, using general relativity and quantum theory (the laws which govern behaviors on the smallest, subatomic scales) that black holes, could leak particles and radiation back into space. This in itself generated a 30-year debate, and a famous wager with Caltech physicist John Preskill, on whether such escaping particles would carry some quantum information with them or not. In 2004, Hawking conceded that such information could survive.

A group of researchers at the University of California, Santa Barbara studying how information escapes a black hole’s clutches have presented the “Firewall Paradox”: that having information flowing out of a black hole is incompatible with having an otherwise smooth space-time at its boundary, i.e. the traditional event horizon. Instead there would be a discontinuity in the vacuum that would manifest itself as energetic particles — a literal “firewall” — lurking just inside the black hole.

If the firewall argument proves to be correct, one of three ideas that lie at the heart of modern physics, must be wrong. Either information can be lost in a black hole after all; Einstein’s principle of equivalence is wrong; or quantum field theory, which describes how elementary particles and forces interact, is wrong and needs fixing.

To find out more about possible solutions to this problem, read Dennis Overbye’s full New York Times article here.