Archivi tag: general relativity

100 Years to General Relativity: From Theory to Experiment and Back

The 32nd winter school in theoretical physics is titled “100 years of General Relativity: From theory to experiment and back“. Speakers include: Arkani-Hamed, Bekenstein, Damour, Gibbons, Gross, Maldacena, Mukhanov, Nicolai, Peebles, Polchinski, Thorne, Van Raamsdonk*. It is intended especially for graduate students. Continua a leggere 100 Years to General Relativity: From Theory to Experiment and Back


27° Texas Symposium on Relativistic Astrophysics

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. Continua a leggere 27° Texas Symposium on Relativistic Astrophysics

Extreme-Astrophysics in an Ever-Changing Universe

Since ancient times, astronomers’ attention has been drawn to changes in the sky. Today we know that most phenomena observed in “time-domain” astronomy are related to extreme astrophysical events or processes. Whether it is the explosion of stars in supernovae or the observations of flare stars, pulsars, gamma-ray bursts, blazars or active galactic nuclei, time-domain astronomy stretches across the whole electromagnetic spectrum and beyond. With increasing technical capabilities, the 21st century will see corresponding new instruments being developed or coming online, revolutionising our view of the ever-changing Universe. Continua a leggere Extreme-Astrophysics in an Ever-Changing Universe

The X-ray Universe

The XMM-Newton Science Operations Centre is organising a major astrophysical symposium from Monday 16th to Thursday 19th of June 2014 in Dublin, Ireland. The symposium is the fourth international meeting in the series “The X-ray Universe”. The intention is to gather a general collection of research in high energy astrophysics. The symposium will provide a showcase for results, discoveries and expectations from current and future X-ray missions. Continua a leggere The X-ray Universe

99 years of Black Holes: from Astronomy to Quantum Gravity

This meeting shall bring together the Black Hole community (from Quantum to supermassive) as well as the Gravitational Wave Community. The main topics of the conference will cluster around Black Holes, Gravitational Waves and the Future of General Relativity and Quantum Physics.
Continua a leggere 99 years of Black Holes: from Astronomy to Quantum Gravity

The ‘liquid’ nature of spacetime in quantum gravity models

An illustration of the liquid spacetime concept. Credit: Jason Ralston/Flickr
Se lo spaziotempo fosse un liquido, avrebbe una viscosità bassissima, come i “superfluidi“. Un lavoro che ha visto collaborare la Scuola Internazionale Superiore di Studi Avanzati (SISSA) di Trieste con l’Università Ludwig Maximilian di Monaco ha mostrato come dovrebbero comportarsi gli “atomi” che compongono il fluido dello spaziotempo, secondo alcuni modelli di gravità quantistica. Le considerazioni proposte in questo lavoro impongono vincoli molto stretti al verificarsi di effetti legati a questa eventuale natura “fluida” dello spaziotempo, mostrando che è possibile discriminare tra i modelli di gravità quantistica finora sviluppati al fine di superare la Relatività Generale.

Continua a leggere The ‘liquid’ nature of spacetime in quantum gravity models

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

Trying to fix the black hole information paradox

Black HoleI fisici hanno discusso le affermazioni che sono state avanzate di recente da Stephen Hawking in merito ai buchi neri (post). Ormai sono decenni che si sta cercando di svelare il mistero che avvolge questi affascinanti “mostri del cielo” la cui estrema forza di attrazione gravitazionale è così intensa che nemmeno la luce riesce a sfuggire. Oggi il professor Chris Adami, della Michigan State University, ha deciso di buttarsi, per così dire, nella mischia per capirne di più e tentare di risolvere l’enigma.

The debate about the behavior of black holes, which has been ongoing since 1975, was reignited when Hawking posted a blog on Jan. 22, 2014, stating that event horizons, the invisible boundaries of black holes, do not exist. Hawking, considered to be the foremost expert on black holes, has over the years revised his theory and continues to work on understanding these cosmic puzzles. One of the many perplexities is a decades-old debate about what happens to information, matter or energy and their characteristics at the atomic and subatomic level, in black holes. “In 1975, Hawking discovered that black holes aren’t all black. They actually radiate a featureless glow, now called Hawking radiation”, Adami said. “In his original theory, Hawking stated that the radiation slowly consumes the black hole and it eventually evaporates and disappears, concluding that information and anything that enters the black hole would be irretrievably lost”.

But this theory created a fundamental problem, dubbed the information paradox. Now Adami believes he’s solved it.

According to the laws of quantum physics, information can’t disappear”, Adami said. “A loss of information would imply that the Universe itself would suddenly become unpredictable every time the black hole swallows a particle. That is just inconceivable. No law of physics that we know allows this to happen”. So if the black hole sucks in information with its intense gravitational pull, then later disappears entirely, information and all, how can the laws of quantum physics be preserved?

The solution, Adami says, is that the information is contained in the stimulated emission of radiation, which must accompany the Hawking radiation, the glow that makes a black hole not so black. Stimulated emission makes the black hole glow in the information that it swallowed.

Stimulated emission is the physical process behind LASERS (Light Amplification by Stimulated Emission of Radiation). Basically, it works like a copy machine: you throw something into the machine, and two identical somethings come out. If you throw information at a black hole, just before it is swallowed, the black hole first makes a copy that is left outside. This copying mechanism was discovered by Albert Einstein in 1917, and without it, physics cannot be consistent”, Adami said. Do others agree with Adami’s theory that stimulated emission is the missing piece that solves the information paradox? According to Paul Davies, cosmologist, astrobiologist and theoretical physicist at Arizona State University, “In my view Chris Adami has correctly identified the solution to the so-called black hole information paradox. Ironically, it has been hiding in plain sight for years. Hawking’s famous black hole radiation is an example of so-called spontaneous emission of radiation, but it is only part of the story. There must also be the possibility of stimulated emission, the process that puts the S in LASER”. With so many researchers trying to fix Hawking’s theory, why did it take so long if it was hiding in plain sight? “While a few people did realize that the stimulated emission effect was missing in Hawking’s calculation, they could not resolve the paradox without a deep understanding of quantum communication theory”, Adami said. Quantum communication theory was designed to understand how information interacts with quantum systems, and Adami was one of the pioneers of quantum information theory back in the ’90s. Trying to solve this information paradox has kept Adami awake many nights as demonstrated by his thick notebooks filled with 10 years of mathematical calculations. So where does this leave us, according to Adami? “Stephen Hawking’s wonderful theory is now complete in my opinion. The hole in the black hole theory is plugged, and I can now sleep at night”, he said. Adami may now sleep well at night, but his theory is sure to keep other physicists up trying to confirm whether he has actually solved the mystery.

Michigan State University: Plugging the hole in Hawking’s black hole theory
arXiv: Classical information transmission capacity of quantum black holes


MSU Professor Chris Adami believes he has found the solution to a long-standing problem with Stephen Hawking’s black hole theory in a groundbreaking new study recently published in the journal Classical and Quantum Gravity. Shot by G.L. Kohuth 

Alan Guth commenta i risultati di BICEP2

E’ ancora vivo il fermento che ha generato in questi giorni l’annuncio dei ricercatori dell’Harvard CMB Group sull’esperimento BICEP2 in merito alla rivelazione di un segnale presente nella radiazione cosmica di fondo associato al passaggio di onde gravitazionali primordiali, una forte evidenza indiretta dell’inflazione cosmica (post). Il modello inflazionistico fu inizialmente proposto negli anni ’80 da Alan Guth, oggi Victor F. Weisskopf Professor of Physics presso il MIT, che commenta qui di seguito il significato scientifico dei dati ottenuti da BICEP2.

Q: Can you explain the theory of cosmic inflation that you first put forth in 1980?

A: I usually describe inflation as a theory of the “bang” of the Big Bang: It describes the propulsion mechanism that drove the universe into the period of tremendous expansion that we call the Big Bang. In its original form, the Big Bang theory never was a theory of the bang. It said nothing about what banged, why it banged, or what happened before it banged. The original Big Bang theory was really a theory of the aftermath of the bang. The universe was already hot and dense, and already expanding at a fantastic rate. The theory described how the universe was cooled by the expansion, and how the expansion was slowed by the attractive force of gravity. Inflation proposes that the expansion of the universe was driven by a repulsive form of gravity. According to Newton, gravity is a purely attractive force, but this changed with Einstein and the discovery of general relativity. General relativity describes gravity as a distortion of spacetime, and allows for the possibility of repulsive gravity. Modern particle theories strongly suggest that at very high energies, there should exist forms of matter that create repulsive gravity. Inflation, in turn, proposes that at least a very small patch of the early universe was filled with this repulsive-gravity material. The initial patch could have been incredibly small, perhaps as small as 10-24 centimeter, about 100 billion times smaller than a single proton. The small patch would then start to exponentially expand under the influence of the repulsive gravity, doubling in size approximately every 10-37 second. To successfully describe our visible universe, the region would need to undergo at least 80 doublings, increasing its size to about 1 centimeter. It could have undergone significantly more doublings, but at least this number is needed. During the period of exponential expansion, any ordinary material would thin out, with the density diminishing to almost nothing. The behavior in this case, however, is very different: The repulsive-gravity material actually maintains a constant density as it expands, no matter how much it expands! While this appears to be a blatant violation of the principle of the conservation of energy, it is actually perfectly consistent. This loophole hinges on a peculiar feature of gravity: The energy of a gravitational field is negative. As the patch expands at constant density, more and more energy, in the form of matter, is created. But at the same time, more and more negative energy appears in the form of the gravitational field that is filling the region. The total energy remains constant, as it must, and therefore remains very small. It is possible that the total energy of the entire universe is exactly zero, with the positive energy of matter completely canceled by the negative energy of gravity. I often say that the universe is the ultimate free lunch, since it actually requires no energy to produce a universe. At some point the inflation ends because the repulsive-gravity material becomes metastable. The repulsive-gravity material decays into ordinary particles, producing a very hot soup of particles that form the starting point of the conventional Big Bang. At this point the repulsive gravity turns off, but the region continues to expand in a coasting pattern for billions of years to come. Thus, inflation is a prequel to the era that cosmologists call the Big Bang, although it of course occurred after the origin of the universe, which is often also called the Big Bang.

Q: What is the new result announced this week, and how does it provide critical support for your theory?

A: The stretching effect caused by the fantastic expansion of inflation tends to smooth things out — which is great for cosmology, because an ordinary explosion would presumably have left the universe very splotchy and irregular. The early universe, as we can see from the afterglow of the cosmic microwave background (CMB) radiation, was incredibly uniform, with a mass density that was constant to about one part in 100,000. The tiny nonuniformities that did exist were then amplified by gravity: In places where the mass density was slightly higher than average, a stronger-than-average gravitational field was created, which pulled in still more matter, creating a yet stronger gravitational field. But to have structure form at all, there needed to be small nonuniformities at the end of inflation. In inflationary models, these nonuniformities — which later produce stars, galaxies, and all the structure of the universe — are attributed to quantum theory. Quantum field theory implies that, on very short distance scales, everything is in a state of constant agitation. If we observed empty space with a hypothetical, and powerful, magnifying glass, we would see the electric and magnetic fields undergoing wild oscillations, with even electrons and positrons popping out of the vacuum and then rapidly disappearing. The effect of inflation, with its fantastic expansion, is to stretch these quantum fluctuations to macroscopic proportions. The temperature nonuniformities in the cosmic microwave background were first measured in 1992 by the COBE satellite, and have since been measured with greater and greater precision by a long and spectacular series of ground-based, balloon-based, and satellite experiments. They have agreed very well with the predictions of inflation. These results, however, have not generally been seen as proof of inflation, in part because it is not clear that inflation is the only possible way that these fluctuations could have been produced. The stretching effect of inflation, however, also acts on the geometry of space itself, which according to general relativity is flexible. Space can be compressed, stretched, or even twisted. The geometry of space also fluctuates on small scales, due to the physics of quantum theory, and inflation also stretches these fluctuations, producing gravity waves in the early universe. The new result, by John Kovac and the BICEP2 collaboration, is a measurement of these gravity waves, at a very high level of confidence. They do not see the gravity waves directly, but instead they have constructed a very detailed map of the polarization of the CMB in a patch of the sky. They have observed a swirling pattern in the polarization (called “B modes”) that can be created only by gravity waves in the early universe, or by the gravitational lensing effect of matter in the late universe. But the primordial gravity waves can be separated, because they tend to be on larger angular scales, so the BICEP2 team has decisively isolated their contribution. This is the first time that even a hint of these primordial gravity waves has been detected, and it is also the first time that any quantum properties of gravity have been directly observed.

Q: How would you describe the significance of these new findings, and your reaction to them?

A: The significance of these new findings is enormous. First of all, they help tremendously in confirming the picture of inflation. As far as we know, there is nothing other than inflation that can produce these gravity waves. Second, it tells us a lot about the details of inflation that we did not already know. In particular, it determines the energy density of the universe at the time of inflation, which is something that previously had a wide range of possibilities. By determining the energy density of the universe at the time of inflation, the new result also tells us a lot about which detailed versions of inflation are still viable, and which are no longer viable. The current result is not by itself conclusive, but it points in the direction of the very simplest inflationary models that can be constructed. Finally, and perhaps most importantly, the new result is not the final story, but is more like the opening of a new window. Now that these B modes have been found, the BICEP2 collaboration and many other groups will continue to study them. They provide a new tool to study the behavior of the early universe, including the process of inflation. When I (and others) started working on the effect of quantum fluctuations in the early 1980s, I never thought that anybody would ever be able to measure these effects. To me it was really just a game, to see if my colleagues and I could agree on what the fluctuations would theoretically look like. So I am just astounded by the progress that astronomers have made in measuring these minute effects, and particularly by the new result of the BICEP2 team. Like all experimental results, we should wait for it to be confirmed by other groups before taking it as truth, but the group seems to have been very careful, and the result is very clean, so I think it is very likely that it will hold up.

Courtesy MIT: 3 Questions: Alan Guth on new insights into the ‘Big Bang’