Archivi tag: general relativity

Hot Topics in General Relativity and Gravitation

This international conference will be held as part of the framework of the Recontres du Vietnam. Our aims are to discuss and review recent developments on:

  • Astrophysics of compact objects
  • Gravitational waves (experimental and theoretical)
  • Experimental gravity and tests of General Relativity
  • Alternative gravity theories and Cosmology related issues

The conference will consist of plenary sessions for indepth oral presentations (review talks and talks on specific specialised topics), parrallel and poster sessions (contributions sollicitated or selected from abstract submission). Special emphasis is being placed on active participation by young researchers and post-docs. A common plenary session with the conference Cosmology in the Planck Era is planned.

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Karl Schwarzschild Meeting On Gravitation

The first Karl Schwarzschild Meeting (KSM) on Gravitational Physics will be held in Frankfurt am Main, Germany on 22-26 July, 2013. The foundational spirit of the meeting can be summarized as: “by acknowledging the past we open a route to the future”. The five-day meeting will bring together both working specialists in the field of black hole physics and rising young researchers to foster new conversations and collaborations.

G191-B2B, a stellar test on a constant of nature

Un gruppo di fisici della University of New South Wales (UNSW) hanno studiato una nana bianca distante dove la gravità diventa oltre 30.000 volte maggiore rispetto alla superficie terrestre per verificare una teoria controversa sulla variabilità, o meno, di una delle costanti della natura.

Julian Berengut and his international team used the Hubble Space Telescope to measure the strength of the electromagnetic force, known as alpha, on a white dwarf star.

Berengut, of the UNSW School of Physics, said the team’s previous research on light from distant quasars suggests that alpha, known as the fine-structure constant, may vary across the Universe.

This idea that the laws of physics are different in different places in the cosmos is a huge claim, and needs to be backed up with solid evidence”, he says. “A white dwarf star was chosen for our study because it has been predicted that exotic, scalar energy fields could significant alter alpha in places where gravity is very strong. Scalar fields are forms of energy that often appear in theories of physics that seek to combine the Standard Model of particle physics with Einstein’s general theory of relativity. By measuring the value of alpha near the white dwarf and comparing it with its value here and now in the laboratory we can indirectly probe whether these alpha-changing scalar fields actually exist”. White dwarfs are very dense stars near the ends of their lives. The researchers studied the light absorbed by nickel and iron ions in the atmosphere of a white dwarf called G191-B2B. The ions are kept above the surface by the star’s strong radiation, despite the pull of its extremely strong gravitational field. “This absorption spectrum allows us to determine the value of alpha with high accuracy. We found that any difference between the value of alpha in the strong gravitational field of the white dwarf and its value on Earth must be smaller than one part in ten thousand”, Berengut says. “This means any scalar fields present in the star’s atmosphere must only weakly affect the electromagnetic force”. Berengut said that more precise measurements of the iron and nickel ions on earth are needed to complement the high-precision astronomical data. “Then we should be able to measure any change in alpha down to one part per million. That would help determine whether alpha is a true constant of Nature, or not”.

UNSW: White dwarf star throws light on constant of Nature

Physical Review Letters: Fundamental Constant Doesn’t Budge in High Gravity

arXiv: Limits on variations of the fine-structure constant with gravitational potential from white-dwarf spectra

Simulating a stellar-mass black hole

Grazie ad uno studio guidato da alcuni astronomi della Johns Hopkins University e della Rochester Institute of Technology, è stato possibile trovare nuovi indizi che confermano una ipotesi a lungo dibattuta su come i buchi neri di massa stellare producono radiazione di alta energia.

We’re accurately representing the real object and calculating the light an astronomer would actually see”, says Scott Noble, associate research scientist in RIT’s Center for Computational Relativity and Gravitation. “This is a first-of-a-kind calculation where we actually carry out all the pieces together. We start with the equations we expect the system to follow, and we solve those full equations on a supercomputer. That gives us the data with which we can then make the predictions of the X-ray spectrum”. Lead researcher Jeremy Schnittman, an astrophysicist at NASA’s Goddard Space Flight Center, says the study looks at one of the most extreme physical environments in the Universe: “Our work traces the complex motions, particle interactions and turbulent magnetic fields in billion-degree gas on the threshold of a black hole”.

By analyzing a supercomputer simulation of gas flowing into a black hole, the team finds they can reproduce a range of important X-ray features long observed in active black holes.

We’ve predicted and come to the same evidence that the observers have”, Noble says. “This is very encouraging because it says we actually understand what’s going on. If we made all the correct steps and we saw a totally different answer, we’d have to rethink what our model is”. Gas falling toward a black hole initially orbits around it and then accumulates into a flattened disk. The gas stored in this disk gradually spirals inward and becomes compressed and heated as it nears the center. Ultimately reaching temperatures up to 20 million degrees Fahrenheit (12 million C), some 2,000 times hotter than the Sun’s surface, the gas shines brightly in low-energy, or soft, X-rays. For more than 40 years, however, observations show that black holes also produce considerable amounts of “hard” X-rays, light with energy 10 to hundreds of times greater than soft X-rays. This higher-energy light implies the presence of correspondingly hotter gas, with temperatures reaching billions of degrees.

The new study bridges the gap between theory and observation, demonstrating that both hard and soft X-rays inevitably arise from gas spiraling toward a black hole.

Working with Noble and Julian Krolik, a professor at Johns Hopkins, Schnittman developed a process for modeling the inner region of a black hole’s accretion disk, tracking the emission and movement of X-rays, and comparing the results to observations of real black holes. Noble developed a computer simulation solving all of the equations governing the complex motion of inflowing gas and its associated magnetic fields near an accreting black hole. The rising temperature, density and speed of the infalling gas dramatically amplify magnetic fields threading through the disk, which then exert additional influence on the gas. The result is a turbulent froth orbiting the black hole at speeds approaching the speed of light. The calculations simultaneously tracked the fluid, electrical and magnetic properties of the gas while also taking into account Einstein’s theory of relativity. Running on the Ranger supercomputer at the Texas Advanced Computing Center located at the University of Texas in Austin, Noble’s simulation used 960 of Ranger’s nearly 63,000 central processing units and took 27 days to complete. Over the years, improved X-ray observations provided mounting evidence that hard X-rays originated in a hot, tenuous corona above the disk, a structure analogous to the hot corona that surrounds the Sun. “Astronomers also expected that the disk supported strong magnetic fields and hoped that these fields might bubble up out of it, creating the corona”, Noble says. “But no one knew for sure if this really happened and, if it did, whether the X-rays produced would match what we observe”. Using the data generated by Noble’s simulation, Schnittman and Krolik developed tools to track how X-rays were emitted, absorbed and scattered throughout both the accretion disk and the corona region.

Combined, they demonstrate for the first time a direct connection between magnetic turbulence in the disk, the formation of a billion-degree corona, and the production of hard X-rays around an actively “feeding” black hole.

In the corona, electrons and other particles move at appreciable fractions of the speed of light. When a low-energy X-ray from the disk travels through this region, it may collide with one of the fast-moving particles. The impact greatly increases the X-ray’s energy through a process known as inverse Compton scattering. “Black holes are truly exotic, with extraordinarily high temperatures, incredibly rapid motions and gravity exhibiting the full weirdness of general relativity”, Krolik says. “But our calculations show we can understand a lot about them using only standard physics principles”. The study was based on a non-rotating black hole. The researchers are extending the results to spinning black holes, where rotation pulls the inner edge of the disk further inward and conditions become even more extreme. They also plan a detailed comparison of their results to the wealth of X-ray observations now archived by NASA and other institutions. Black holes are the densest objects known. Stellar-mass black holes form when massive stars run out of fuel and collapse, crushing up to 20 times the Sun’s mass into compact objects less than 75 miles (120 kilometers) wide.

The following video animation of supercomputer data takes you to the inner zone of the accretion disk of a stellar-mass black hole. Gas heated to 20 million degrees Fahrenheit as it spirals toward the black hole glows in low-energy, or soft, X-rays. Just before the gas plunges to the center, its orbital motion is approaching the speed of light. X-rays up to hundreds of times more powerful (“harder”) than those in the disk arise from the corona, a region of tenuous and much hotter gas around the disk. Coronal temperatures reach billions of degrees. Credit: NASA’s Goddard Space Flight Center

RIT: NASA-led study explains decades of black hole observations

arXiv: X-ray Spectra from MHD Simulations of Accreting Black Holes

STARS 2013 e SMFNS 2013

The events are the second and third in a series of meetings gathering scientists working on astroparticle physics, cosmology, gravitation, nuclear physics, and related fields. As in previous years, the meeting sessions will consist of invited and contributed talks and will cover recent developments in the following topics:
STARS2013 – New phenomena and new states of matter in the Universe, general relativity, gravitation, cosmology, heavy ion collisions and the formation of the quark-gluon plasma, white dwarfs, neutron stars and pulsars, black holes, gamma-ray emission in the Universe, high energy cosmic rays, gravitational waves, dark energy and dark matter, strange matter and strange stars, antimatter in the Universe, and topics related to these.
SMFNS2013 – Strong magnetic fields in the Universe, strong magnetic fields in compact stars and in galaxies, ultra-strong magnetic fields in neutron star mergers, quark stars and magnetars, strong magnetic fields and the cosmic microwave background, and topics related to these.