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LIGO Confirms 1989 Hebrew University Prediction About Neutron Star Mergers Producing Gamma Ray Bursts


Prof. Tsvi Piran at Hebrew University's Racah Institute of Physics led a team that published an accurate prediction in 'Nature' that met with skepticism for years

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Two years ago, the LIGO gravitational wave detector stunned the world with the discovery of a merger of two black holes. This past August, LIGO did it again: with the help of a second detector called VIRGO, it discovered a new source of gravitational radiation. Seconds later, NASA’s Fermi satellite detected a gamma-ray burst from the same direction. Several hours later, a telescope in Chile identified the source at a Galaxy located 120 million light years away. While this is an enormous distance for us, on a cosmological scale it is relatively close. 

Since these initial discoveries, most of the telescopes in the world, including the Hubble Space Telescope, have observed this galactic event. The results, which have been kept secret until now (despite a partial leak), are reported today in several scientific papers published in the prestigious journals Physical Review LettersNature, Science and the Astrophysical Journal.

These observations confirm a longstanding prediction made almost thirty years ago by a team headed by Prof. Tsvi Piran at the Hebrew University of Jerusalem. Piran is the Schwartzman Chair for Theoretical Physics at the Hebrew University's Racah Institute of Physics. The prediction, published in Nature in 1989 ("Nucleosynthesis, neutrino bursts and γ-rays from coalescing neutron stars"), suggests that when two neutron star merger they emit, in addition to gravitational waves, a burst of gamma-rays. They also synthesize and eject to outer space rare heavy elements, like gold plutonium and uranium. The merged neutron stars form a black hole in this process.

Neutron stars are rare types of stars that are produced in supernova explosions when a regular star dies. Unlike regular matter that is composed from 50% neutron and 50% protons, neutron stars are made just from neutrons. Due to their strange composition, they are extremely dense: a teaspoon of neutron star matter weights about 100 million tons, and a neutron star of 10 km (smaller than the width of Jerusalem) weights about a million times the mass of Earth. 

The first neutron star was discovered in 1967 by Antony Hewish, who received the 1974 Nobel Prize in Physics. Later a binary pair of neutron stars rotating around each other was discovered by Hulse and Taylor, who were awarded the 1993 Nobel Prize in Physics. (And on October 3, LIGO’s three leading champions were awarded the 2017 Nobel Prize in Physics: Barry Barish and Kip Thorne of Caltech and Rainer Weiss of MIT.)

Shortly after the discovery of a binary neutron star pair in 1975, researchers realized that that such a pair would emit gravitational radiation and eventually merge. The question that Piran and colleagues asked in 1989 was: in addition to the gravitational radiation, what else will be emitted as a result of this merger? They suggested that the merger will produce a burst of gamma-rays — which have the smallest wavelengths and the most energy of any other wave in the electromagnetic spectrum — and at the same time will synthesize and eject into outer space freshly synthesized heavy elements like gold, plutonium and uranium. The ultimate result will be a black hole. This prediction, which Piran and colleagues published in Nature, was met with skepticism and initially ignored. However, Piran continued to work on it, and indirect evidence in its favor mounted over the years. These last observations confirm it without any doubt.

“I am exhilarated by this confirmation of a prediction we made nearly thirty years ago,” said Prof. Tsvi Piran following today’s announcement confirming his prediction. “I also remember how difficult it was to convince the scientific community of our idea: at the time it was against the standard model that was published even in freshman textbooks on astronomy. When we made this prediction in 1989, we did not expect it to be confirmed within our lifetimes. But with continued curiosity and the development of new technologies, we are able learn ever deeper truths about the nature of our Universe."

LIGO’s observations confirmed that the event involved a binary neutron star merger, and the formation of a black hole. The Fermi satellite detected the predicted gamma-rays, and the optical observation confirmed the nucleosynthesis of heavy elements. All of this is published today in multiple research papers, with Piran’s participation in several papers published in the journals Nature, Science and The Astrophysical Journal. These observations solve several puzzles that have bothered astronomers over the years, and open new ways to understand the nature of our Universe.

The Hebrew University of Jerusalem is Israel’s leading university and premier research institution. Founded in 1918 by innovative thinkers including Albert Einstein, the Hebrew University is a pluralistic institution that advances science and knowledge for the benefit of humankind. For more information, please visit

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 LIGO Confirms 1989 Hebrew University Prediction About Neutron Star Mergers Producing Gamma Ray Bursts
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Both push and pull drive our Galaxy’s race through space


Discovery of the “Dipole Repeller” confirms that both attraction and repulsion are at play in our extragalactic neighborhood

Although we can’t feel it, we’re in constant motion: the earth spins on its axis at about 1,600 km/h; it orbits around the sun at about 100,000 km/h; the sun orbits our Milky Way galaxy at about 850,000 km/h; and the Milky Way galaxy and its companion galaxy Andromeda are moving with respect to the expanding universe at roughly 2 million km/h (630 km per second). But what is propelling the Milky Way’s race through space?

Until now, scientists assumed that a dense region of the universe is pulling us toward it, in the same way that gravity made Newton’s apple fall to earth. The initial “prime suspect” was called the Great Attractor, a region of a half dozen rich clusters of galaxies 150 million lightyears from the Milky Way. Soon after, attention was drawn to an area of more than two dozen rich clusters, called the Shapley Concentration, which sits 600 million lightyears beyond the Great Attractor.

Now researchers led by Prof. Yehuda Hoffman at the Hebrew University of Jerusalem report that our galaxy is not only being pulled, but also pushed. In a new study in the forthcoming issue of Nature Astronomy, they describe a previously unknown, very large region in our extragalactic neighborhood. Largely devoid of galaxies, this void exerts a repelling force on our Local Group of galaxies.

“By 3-d mapping the flow of galaxies through space, we found that our Milky Way galaxy is speeding away from a large, previously unidentified region of low density. Because it repels rather than attracts, we call this region the Dipole Repeller,” said Prof. Yehuda Hoffman. “In addition to being pulled towards the known Shapley Concentration, we are also being pushed away from the newly discovered Dipole Repeller. Thus it has become apparent that push and pull are of comparable importance at our location.”

PHOTOS and VIDEO are available at Use of these materials is permitted on condition of respecting the publication embargo and including the proper credit information.

The presence of such a low density region has been suggested previously, but confirming the absence of galaxies by observation has proved challenging. But in this new study, Hoffman, at the Hebrew university’s Racah Institutes of Physics, working with colleagues in the USA and France, tried a different approach.

Using powerful telescopes, among them the Hubble Space Telescope, they constructed a 3-dimensional map of the galaxy flow field. Flows are direct responses to the distribution of matter, away from regions that are relatively empty and toward regions of mass concentration; the large scale structure of the universe is encoded in the flow field of galaxies. They studied the peculiar velocities – those in excess of the Universe’s rate of expansion – of galaxies around the Milky Way, combining different datasets of peculiar velocities with a rigorous statistical analysis of their properties. They thereby inferred the underlying mass distribution that consists of dark matter and luminous galaxies — over-dense regions that attract and under-dense ones that repel.

By identifying the Dipole Repeller, the researchers were able to reconcile both the direction of the Milky Way’s motion and its magnitude. They expect that future ultra-sensitive surveys at optical, near-infrared and radio wavelengths will directly identify the few galaxies expected to lie in this void, and directly confirm the void associated with the Dipole Repeller.

Hoffman’s collaborators include Daniel Pomarède, Institut de Recherche sur les Lois Fondamentales de l’Univers, CEA, Université Paris-Saclay, Gif-sur-Yvette, France; R. Brent Tully, Institute for Astronomy (IFA), University of Hawaii, USA; and Hélène M. Courtois, IPN Lyon, University of Lyon, France.

CITATION: The dipole repeller. Yehuda Hoffman, Daniel Pomarède, R. Brent Tully and Hélène M. Courtois. Nature Astronomy, Advance Online Publication January 30, 2017. doi: 10.1038/s41550-016-0036

SUPPORT: The researchers thank the Israel Science Foundation (1013/12), the Institut Universitaire de France, the US National Science Foundation, Space Telescope Science Institute (for Observations with Hubble Space Telescope), the Jet Propulsion Lab (for observations with Spitzer Space Telescope) and NASA (for analysis of data from the Wide-field Infrared Survey Explorer).

Both push and pull drive our Galaxy’s race through space
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