Archivi tag: gravitational waves

Gravitational Wave Astrophysics

The detection and analysis of the gravitational radiation emitted by diverse astrophysical and cosmological sources promises to open a completely new window to the exploration of the Universe. The developments towards Gravitational Wave Astronomy are currently following different paths. On one hand, second-generation ground-based interferometric detectors [LIGO (USA), VIRGO (Italy-EGO), and KAGRA (Japan)] in the high-frequency band (1 − 104 Hz) are going to start operations during this decade, and are expected to provide the first detections. There are also plans for a third-generation observatory, the Einstein Telescope, an European project currently funded by the ASPERA network. These detectors will observe general relativistic phenomena such as the coalescence and merger of stellar compact binaries containing neutron stars and black holes. They are also sensitive to gravitational emissions from core-collapse supernovae and to early-universe backgrounds. On the other hand, the design of space-based detectors that will cover the low-frequency band (10−5 − 1 Hz) has a long history, and in particular the European project eLISA, which aims at becoming an European Space Agency Large mission, will search for supermassive black hole binary coalescence, inspirals of stellar compact objects into supermassive black holes, thousands of galactic binaries, and stochastic backgrounds of gravitational waves. Finally, we have the Pulsar Timing Arrays (PTAs), which look at variations in the arrival times of radio emissions from millisecond pulsars due to the passage of gravitational waves. There are three PTAs [NANOGrav (USA), EPTA (Europe), and PPTA (Australia)] operating radio-telescopes that monitor sets of millisecond pulsars to detect the presence of gravitational waves in the very low frequency band (10−9 − 10−7 Hz). These arrays, organized in the International PTA (IPTA) consortium, are expected to make the first detections on a timescale of decades, and are currently providing unique constraints on gravitational waves from inspirals of supermassive black holes, cosmic strings, and other sources. There is therefore a strong motivation to study the astrophysical mechanisms that create the different gravitational-wave sources (to understand their distribution and event rates) and also the physical connection between these sources and other astrophysical and cosmological processes. For instance, observations by large telescopes for electromagnetic transient signals together with gravitational-wave observations will provide rich information for high-energy phenomena such as gamma-ray bursts. Moreover, the detection of supermassive black hole mergers can help us understand the mechanisms of galaxy formation, explore the structure of black holes and test General Relativity and other theories of gravity. Taking into account the technological developments in gravitational-wave detectors, it is an excellent time to organize a workshop to discuss the state of the art of the main astrophysics associated with the sources of gravitational waves and the hidden universe that can be unveiled through these observations. At the same time, we will aim to produce a reference book on the topic, going beyond normal proceedings, which could be an useful entrance to the field and provide a wide panorama of the astrophysics of gravitational waves. The book will be edited by the organizers, and contributed by participants, especially invited speakers. Following tradition, the SOC will grant the Sant Cugat Forum of Astrophysics Award to Young Scientists.

All of these issues, as well as the logistics of the workshop in the framework of the Sant Cugat Forum of Astophysics are detailed in this web.

The Strong Gravity Regime of Black Holes and Neutron Stars

The 558th WE-Heraeus-Seminar on The Strong Gravity Regime of Black holes and Neutron stars is kindly funded by the Wilhelm und Else Heraeus Foundation. It will be held from March 31st to April 4th, 2014 at the Physikzentrum of the German Physical Society in Bad Honnef near Bonn and Cologne, Germany.

The main theme of this seminar is the observation and theoretical description of systems where gravity is strong and non-linear, in particular systems containing black holes and neutron stars which are ideal gravitational laboratories. To cover the complete complexity of this field of research, experts and graduate students from the observational and theoretical community are invited to bring together their expertise.

As a rough guideline, we have the following categories:

  • Strong-field gravity in GR and its alternatives
  • Black holes as strong field probes
  • Neutron stars as strong field probes
  • Gravitational wave observations and merger events

More info: The Strong Gravity Regime of Black Holes and Neutron Stars

Using the Universe as ‘tool’ to detect gravitons

La gravità è l’unica tra le quattro forze fondamentali per la quale gli scienziati non hanno ancora rivelato la sua unità fondamentale. Infatti, secondo il modello standard delle particelle elementari si ritiene che l’interazione gravitazionale venga trasmessa attraverso il gravitone, allo stesso modo con cui l’interazione elettromagnetica viene trasmessa dai fotoni. Oggi, nonostante esistano delle basi teoriche a favore dell’esistenza dei gravitoni rimane, però, il problema di rivelarli, almeno sulla Terra dove le possibilità sono estremamente basse se non quasi nulle.

For example, the conventional way of measuring gravitational forces, by bouncing light off a set of mirrors to measure tiny shifts in their separation, would be impossible in the case of gravitons. According to physicist Freeman Dyson, the sensitivity required to detect such a miniscule distance change caused by a graviton requires the mirrors to be so massive and heavy that they’d collapse and form a black hole. Because of this, some have claimed that measuring a single graviton is hopeless. But what if you used the largest entity you know of, in this case the Universe, to search for the telltale effects of gravitons. That is what two physicists are proposing. In the paper, “Using cosmology to establish the quantization of gravity”, published in Physical Review D (Feb. 20, 2014), Lawrence Krauss, a cosmologist at Arizona State University, and Frank Wilczek, a Nobel-prize winning physicist with MIT and ASU, have proposed that measuring minute changes in the cosmic background radiation of the Universe could be a pathway of detecting the telltale effects of gravitons.

Krauss and Wilczek suggest that the existence of gravitons, and the quantum nature of gravity, could be proved through some yet-to-be-detected feature of the early Universe.

This may provide, if Freeman Dyson is correct about the fact that terrestrial detectors cannot detect gravitons, the only direct empirical verification of the existence of gravitons”, Krauss said. “Moreover, what we find most remarkable is that the Universe acts like a detector that is precisely the type that is impossible or impractical to build on Earth”. It is generally believed that in the first fraction of a second after the Big Bang, the Universe underwent rapid and dramatic growth during a period called “inflation.” If gravitons exist, they would be generated as “quantum fluctuations” during inflation. Ultimately, these would evolve, as the Universe expanded, into classically observable gravitational waves, which stretch space-time along one direction while contracting it along the other direction. This would affect how electromagnetic radiation in the cosmic microwave background (CMB) radiation left behind by the Big Bang is produced, causing it to become polarized. Researchers analyzing results from the European Space Agency’s Planck satellite are searching for this “imprint” of inflation in the polarization of the CMB.

Krauss said his and Wilczek’s paper combines what already is known with some new wrinkles.

While the realization that gravitational waves are produced by inflation is not new, and the fact that we can calculate their intensity and that this background might be measured in future polarization measurements of the microwave background is not new, an explicit argument that such a measurement will provide, in principle, an unambiguous and direct confirmation that the gravitational field is quantized is new”, he said. “Indeed, it is perhaps the only empirical verification of this very important assumption that we might get in the foreseeable future”. Using a standard analytical tool called dimensional analysis, Wilczek and Krauss show how the generation of gravitational waves during inflation is proportional to the square of Planck’s constant, a numerical factor that only arises in quantum theory. That means that the gravitational process that results in the production of these waves is an inherently quantum-mechanical phenomenon. This implies that finding the fingerprint of gravitational waves in the polarization of CMB will provide evidence that gravitons exist, and it is just a matter of time (and instrument sensitivity) to finding their imprint. “I’m delighted that dimensional analysis, a simple but profound technique whose virtues I preach to students, supplies clear, clean insight into a subject notorious for its difficulty and obscurity”, said Wilczek. “It is quite possible that the next generation of experiments, in the coming decade or maybe even the Planck satellite, may see this background”, Krauss added.

ASU: Researchers propose a new way to detect the elusive graviton

arXiv: Using Cosmology to Establish the Quantization of Gravity

How to ‘listen to’ the birth of black holes?

La partecipazione dell’Australia alla potenziale scoperta delle onde gravitazionali, e quindi alla capacità di “ascoltare” la nascita di un buco nero, riceverà oggi una accelerata. Questo è il giorno in cui i fisici di tutto il continenti australiano si incontreranno all’Australian International Gravitational Research Centre presso Gingin, quasi 100 Km da Perth. L’obiettivo del meeting è quello di lanciare una missione a livello nazionale che abbia lo scopo di espandere la partecipazione dell’Australia ai progetti americani ed europei unendosi così alla ricerca delle elusive perturbazioni dello spaziotempo.

Gravitational waves are ripples in the curvature of spacetime. They are thought to mark the beginning of time at the Big Bang and the end of time as black holes are born. They are generated by extreme cosmic events such as colliding stars and supernova explosions. Theory predicts that they carry vast amounts of energy at the speed of light. While their power can exceed the power of all the stars in the Universe, their effects are miniscule and difficult to detect. Centre Director, The University of Western Australia’s Winthrop Professor David Blair, said 1000 physicists around the world are currently involved in the search which is focused on the commissioning of three enormous supersensitive detectors that will start operating within the next few years in the USA and Europe, with another under construction in Japan. “The expected step in sensitivity will extend their reach tenfold and increase the number of expected signals 1000-fold“, he said. Professor Peter Veitch, Chair of the Australian Consortium for Gravitational Astronomy, said: “The new advanced detectors change the whole game. For the first time we have firm predictions: both the strength and the number of signals. No longer are we hoping for rare and unknown events. We will be monitoring a significant volume of the Universe and for the first time we can be confident that we will ‘listen’ to the coalescence of binary neutron star systems and the formation of black holes. Once these detectors reach full sensitivity we should hear signals almost once a week“. Data from the detectors will be used in conjunction with optical telescopes that will search the sky for visible signs of the catastrophic events signaled by the gravitational waves. Australia is contributing two telescopes to the search: the Zadko telescope at Gingin and the Skymapper telescope at Coonabarrabran in New South Wales. The data from the detectors will be distributed to data analysis teams in many countries. The Australian data analysis team has developed special techniques for digging signals out of the unavoidable noise in the detectors, plus special techniques that use graphics processing units for detecting signals the instant they occur (instead of traditional techniques which can take minutes or hours to identify signals). This fast detection method is especially important if optical telescopes are going to be able to locate distant explosions the moment they occur.

One of the most exciting sources is expected to be the coalescence of pairs of neutron stars to form a black hole, giving out a burst of gamma rays and a flash of light that astronomers call a kilonova.

In this project the Pawsey Centre supercomputers will be equipped with ‘search pipelines’ developed at ANU, Melbourne and UWA. These are massive computer codes designed to separate signals from the noise. Each pipeline is optimised for a specific type of signal, such as the chirps expected as neutron stars spiral together and black holes form. Using these codes, Australian students will be able to play a major role in the first discovery of gravitational waves. The project will be launched at the Gravity Discovery Centre (GDC) by the Chair of GDC Fred Deshon, the Chair of the Gravitational Wave Observatory Development Committee Jens Balkau and the Chair of the Australian Consortium for Gravitational Astronomy Peter Veitch. The GDC which also includes the Gingin Observatory shares the Gingin site with the Australian International Gravitational Research Centre and provides public education on the big questions of the Universe.

UWA: Australian scientists to ‘listen’ to the formation of black holes

27° Texas Symposium on Relativistic Astrophysics

The 27th Texas Symposium on Relativistic Astrophysics will be held in downtown Dallas December 8 – 13, 2013. It is organized by the Department of Physics at The University of Texas at Dallas (UTD) and is chaired by Wolfgang Rindler and Mustapha Ishak. The Symposium will include both invited and contributed talks and posters. This will be a special and historically meaningful Jubilee meeting, marking the 50th anniversary, almost to the day, of the very first of these Texas Symposia, held in Dallas in December 1963. We are excited to welcome hundreds of international astrophysicists back to Dallas fifty years later, both to celebrate the past 50 years of Texas Symposia and relativistic astrophysics and to kick off the next 50 years of remarkable discoveries.

The Symposium will cover the following topics:


  • Cosmic acceleration/dark energy
  • Cosmic microwave background
  • Early universe (Inflation, Cyclic Model, CCC cosmology …)
  • Galaxy formation and reionization
  • Inhomogeneous cosmologies, averaging, and backreaction
  • Large-scale surveys
  • Quantum gravity/cosmology and string cosmology
  • Weak gravitational lensing
  • Experimental/observational cosmology – other topics
  • Theoretical cosmology – other topics
Compact objects and galactic/cluster scales
  • Black holes, mergers, and accretion discs
  • Galaxy evolution and supermassive black holes
  • Imaging black holes
  • Microlensing and exoplanets
  • Neutron stars, pulsars, magnetars, and white dwarfs
  • Nuclear Equation of State for Compact Objects
  • Singularities
  • Strong gravitational lensing
  • Supermassive black hole binaries
  • Tidal disruption of stars by supermassive black holes
  • Compact object observations – other topics
  • Compact object theory – other topics
High-energy astrophysics and astroparticle physics
  • Active galactic nuclei and jets
  • Cosmological implications of the Higgs and the LHC
  • Dark matter astrophysics
  • Dark matter experiments and data
  • Gamma-ray bursts, SNe connection, and sources
  • High-energy cosmic rays (VHE, UHE, mechanisms, etc.)
  • High-energy gamma-rays
  • Nuclear Astrophysics
  • Supernovae and their remnants
  • High-energy astrophysics/astroparticle physics – other topics
Testing general relativity and modified gravity
  • Alternative theories of gravity
  • Strong-field tests of general relativity
  • Testing general relativity at cosmological scales
  • Testing general relativity – other topics
  • Modified gravity – other topics
Gravitational waves
  • Electromagnetic counterparts of gravitational wave sources
  • Ongoing and planned gravitational wave experiments
  • Gravitational wave theory and simulations
  • Results and progress from gravitational wave searches
  • Supernovae and Gravitational Wave Emission
  • Gravitational waves – other topics
Numerical relativity
  • Computer algebra and symbolic programming
  • Locating black hole horizons
  • Numerical simulations
  • Relativistic magnetohydrodynamics
  • Numerical relativity – other topics
Other ongoing and future experiments and surveys
  • ACT, AMS, BOSS, CFHT, Chandra, DES, Euclid, Fermi, HETDEX, HSC, JWST,
  • (to be completed after abstract submissions)
And also:
History of relativistic astrophysics
History of the Texas Symposium and interface with other anniversaries
The Kerr solution – 50 years later

New hints on the primordial gravitational waves in the CMB

Grazie ad una serie di osservazioni realizzate mediante il telescopio del Polo Sud in Antartide e l’osservatorio spaziale Herschel, gli astronomi sono stati in grado di rivelare per la prima volta un segnale molto debole nella radiazione cosmica di fondo che potrebbe fornire informazioni di vitale importanza sui primi momenti della creazione dell’Universo. Le misure di questo segnale elusivo sono state realizzate studiando il modo con cui la luce viene deflessa nel suo viaggio cosmico prima di arrivare sulla Terra, passando attraverso gli ammassi di galassie e la distribuzione della materia scura. La scoperta permette di fornire nuovi indizi su come rivelare le onde gravitazionali che si sono originate durante l’epoca dell’inflazione cosmica, un risultato cruciale anticipato dalla missione Planck (post).

The relic radiation from the Big Bang, the Cosmic Microwave Background, or CMB, was imprinted on the sky when the Universe was just 380 000 years old. Today, some 13.8 billion years later, we see it as a sky filled with radio waves at a temperature of just 2.7 degrees above absolute zero. Tiny variations in this temperature, around a few tens of millionths of a degree, reveal density fluctuations in the early Universe corresponding to the seeds of galaxies and stars we see today. The most detailed all-sky map of temperature variations in the background was revealed by Planck in March (post). But the CMB also contains a wealth of other information. A small fraction of the light is polarised, like the light we can see using polarised glasses. This polarised light has two distinct patterns: E-modes and B-modes. E-modes were first found in 2002 with a ground-based telescope. B-modes, however, are potentially much more exciting to cosmologists, although much harder to detect. They can arise in two ways. The first involves adding a twist to the light as it crosses the Universe and is deflected by galaxies and dark matter, a phenomenon known as gravitational lensing. The second has its roots buried deep in the mechanics of a very rapid phase of enormous expansion of the Universe, which cosmologists believe happened just a tiny fraction of a second after the Big Bang, namely the ‘inflation’.

The new study has combined data from the South Pole Telescope and Herschel to make the first detection of B-mode polarisation in the CMB due to gravitational lensing.

This measurement was made possible by a clever and unique combination of ground-based observations from the South Pole Telescope, which measured the light from the Big Bang, with space-based observations from Herschel, which is sensitive to the galaxies that trace the dark matter which caused the gravitational lensing,” says Joaquin Vieira, of the California Institute of Technology and the University of Illinois at Urbana-Champaign, who led the Herschel survey used in the study. By using Herschel’s observations, the scientists mapped the gravitational lensing material along the line of sight, and then searched for correlations between that pattern and the polarised light coming from the CMB, as measured by the South Pole Telescope. “It’s an important checkpoint that we’re able to detect this small lensing B-mode signal and it bodes well for our ability to ultimately measure an even more elusive type of B-mode created during the inflationary Big Bang”, adds Duncan Hanson of McGill University, Montreal, Canada. Scientists believe that during inflation, violent collisions between clumps of matter and between matter and radiation, should have created a sea of gravitational waves. Today, those waves would be imprinted in a primordial B-mode component of the CMB.

Finding such a signal would yield crucial information about the very early Universe, well before the time when the CMB itself was generated, and would provide confirmation of the inflation scenario.

In 2014, new results will be released from ESA’s Planck, and the most eagerly anticipated is whether primordial B-modes have been detected. In the meantime, Herschel has helped to point the way.


arXiv: Detection of B-mode Polarization in the Cosmic Microwave Background with Data from the South Pole Telescope

arXiv: A CMB lensing mass map and its correlation with the cosmic infrared background

A new approach to exploring quantum gravity

I fisici Lawrence Krauss e Frank Wilczek, rispettivamente dell’Arizona State University e dell’Australian National University, hanno pubblicato un lavoro in cui essi propongono una nuovo approccio sulla possibilità di quantizzare la forza di gravità misurando la polarizzazione della radiazione cosmica di fondo. Secondo gli scienziati, questo metodo potrebbe portare ad una connessione tra la radiazione cosmica di fondo e le onde gravitazionali causate dall’inflazione cosmica durante le epoche primordiali della storia cosmica.

Physicists, as most are aware, have been stymied in their efforts to discover a way to unify quantum mechanics and gravity, most scientists in the field believe there is likely a gravity particle they call it a graviton, that carries the force known as gravity. No one of course has ever seen one, or been able to prove it exists. This is because, they say, of how weak it is compared to the other forces, such as electromagnetism, to be able to see it, some have suggested, would require a device so massive that it would collapse in on itself into a black hole. For this reason, some researchers have suggested that we will never be able to see it.

In their paper, Krauss and Wilczek suggest that it might not be necessary to see it, because there might be a way to infer its existence by measuring the CMB.

Their idea is that in the early Universe, just after the Big Bang, as inflation was occurring, gravitational waves should have been created which in turn would have caused photons present in the CMB to scatter in a certain pattern. Finding that pattern would mean finding evidence of a particle that was carrying the gravitational force, the graviton. And if evidence for the existence of a graviton could be found, then physicists would finally have their “universal theory”. They add that they believe that dimensional analysis could provide a link between those early gravitational waves and Planck’s constant, which is of course used in quantum mechanics. There are a couple of issues with the new theory, the first is that technology does not yet exist to measure the CMB in a way that would allow scientists to detect those early gravitational waves and another is proving that any polarization found in the CMB can indeed be attributable to gravitational waves and not some other mechanism, force or process.

arXiv: Using Cosmology to Establish the Quantization of Gravity

Stellar dynamics and growth of massive black holes

The study of galactic nuclei has advanced rapidly during the past few years.  Observations carried out with space-borne telescopes, such as the Hubble Space Telescope, or from the ground, using adaptive optics, have allowed us to study the kinematics of stars and gas in regions reaching down to sub-parsec scales for external galaxies, and to the milliparsec range for the Milky Way. An outstanding conclusion is that dark compact objects, very probably massive black holes (MBH), with masses ranging between a million and a thousand million solar masses, occupy the centres of most galaxies for which such observations can be made.

We have discovered that there exists a deep link between the central MBH and its host galaxy. Claims of detection of “intermediate-mass” black holes (IMBHs, with masses between 100 and 10,000 solar masses) raise the possibility that these correlations extend to much smaller systems, but the strongest -if not totally conclusive- observational evidences for the existence of IMBHs are ultra-luminous X-ray sources.  The origins of these IMBH are still shrouded in mystery, and many aspects of their interplay with the surrounding stellar cluster remain to be elucidated. The particularly important modes of interaction between stars and the MBH in galactic nuclei are, firstly, that stars can produce gases which are accreted onto the MBH through normal stellar evolution, collisions or disruptions of stars by the strong central tidal field.  Secondly, compact stars can be swallowed whole if they gradually inspiral due to the emission of GWs. The latter process, is known as an “Extreme Mass Ratio Inspiral”. Indeed, in the past few years, several galaxies have exhibited X-ray/UV flares consistent with the tidal disruption of a star. Tidal disruptions trigger phases of bright accretion that may reveal the presence of a MBH in an otherwise quiescent, possibly very distant, galaxy, with important implications for extra-galactic UV/X-ray astronomy. Partial disruptions, for instance stripping the envelope of a giant star, may also have tell-tale observational consequences.  Such outbursts may have been detected already and space missions such as SWIFT, the Advanced Telescope for High Energy Astrophysics or the Energetic X-ray Imaging Survey Telescope (EXIST) and the extended ROentgen Survey with an Imaging Telescope Array (eRosita) should yield a rich harvest of data.

The workshop will bring experts from rather different fields: stellar dynamicists, observers from optical and X/UV-ray, astrometry, data analysts, general relativity and numerical modeling of different regimes (from kiloparsec distances to parsecs down to the horizon of the massive black hole). The workshop is born from the need for a place where the distinct communities involved in X-ray astronomy, gravitation wave including electromagnetic counterparts might gather. While these communities – theoretical and observational astrophysics, general relativity, cosmology and data analysis – have made significant collaborative progress over recent years, we believe that it is indispensable to future advancement that they draw closer, and that they speak a common idiom.

Massive Black Holes: Birth, Growth and Impact

During the past decade, massive black holes have become central objects of study in areas of astrophysics that were traditionally not connected. Along with traditional studies of black holes as high energy astrophysical sources, massive black holes have become pivotal to the understanding of galaxy formation and evolution. Similarly, massive black hole binaries have become the main targets of the future generation of gravitational wave experiments, motivating new research on the orbital decay and merging of black holes.  Finally, studies of our own Galactic Center have also undergone tremendous progress and are expected be able to probe general relativistic effects induced by the central supermassive black hole. With this conference, we will bring together experts from the diverse groups involved in the study of massive black holes, producing a novel summary of the status of knowledge and fostering a productive interaction between various research communities that normally operate separately.

Themes that we will focus on will include

(1) Formation mechanisms of massive black hole seeds, confronting weaknesses and strengths of different models and placing them in the context of cosmic structure formation.

(2) Co-evolution of galaxies and massive black holes, in particular the role of black hole feedback on galaxy formation.

(3) Evolution of massive black hole binaries, from the Newtonian to the relativistic regime, including predictions for gravitational wave experiments.

(4) Modeling of accretion discs, especially the latest generation of three-dimensional numerical simulations, addressing the state-of-the art in the field and discussing how to transfer the acquired knowledge to sub-grid models of black hole accretion during galaxy formation.

We expect the conference will generate the most up-to-date synthesis of our current knowledge on massive black holes.

Neutron Stars: Nuclear Physics, Gravitational Waves and Astronomy

A basic cornerstone of modern physics is the quest to describe quantitatively the properties of nuclear matter. Neutron stars are unique beacons in this journey, as their interiors expose matter to extreme regimes of density, temperature and energy, not accessible to terrestrial experiments. Moreover, the intense gravitational fields in these astrophysical compact objects, particularly in binaries, could give rise to potentially detectable signals in the next generation of gravitational wave detectors. The astronomical observation of compact objects thus provides a unique insight into the properties of nuclear matter in extreme regimes. Better and more reliable theoretical tools and a more thorough modeling are required to interpret observations. Finally, one needs to connect present and future observation to the underlying microphysics associated to the strong interaction.

This international workshop aims at bringing together a number of historically disjoint research communities: nuclear physicists, astrophysicists and general relativists. Taking advantage of a multi-disciplinary environment, we plan to identify key issues in compact star physics and to develop strategies to make the most of the new generation of astronomical observatories, gravitational wave detectors and nuclear experiments.