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Post Info TOPIC: Gravity Waves
Blobrana


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RE: Gravity Waves
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Title: The effect of metallicity on the detection prospects for gravitational waves
Authors: K. Belczynski, M. Dominik, T. Bulik, R. O'Shaughnessy, C.L. Fryer, D.E. Holz

By combining advances in observational astrophysics with recent progress in stellar evolution, we show that there will be a remarkably high number of black holes with compact object (neutron star or black hole) companions in the local Universe. Data from the Sloan Digital Sky Survey (300,000 galaxies) indicates that recent star formation (within the last 1 billion years) is bimodal: half the stars form from gas with high amounts of metals (solar metallicity), and the other half form with small contribution of elements heavier than Helium (20% solar). Theoretical studies of mass loss derive significantly higher stellar-origin black hole masses (30-80 Msun) than previously estimated for sub-solar compositions. We combine these findings to estimate the probability for detection of gravitational waves arising from the inspiral of double compact objects. Our results show that a low metallicity environment significantly boosts the formation of double compact object binaries with at least one black hole. In particular, we find the gravitational-wave detection rate is increased by a factor of 20 if the metallicity is decreased from solar (as in all previous estimates) to a 50-50 mixture of solar and 10% solar metallicity. The current sensitivity of the largest instruments to double neutron-star binaries (VIRGO: 9 Mpc; LIGO: 18) is not high enough to ensure a first detection. However, our results indicate that if a future instrument increased the reach to 50-100Mpc, a detection of gravitational waves would be expected within the first year of observation. It was previously thought that binary neutron stars were the most likely source, but our results indicate that black-hole binaries are 25-times more likely. We are therefore truly on the cusp of seeing gravitational waves, and the first source ever to be seen is likely to be a black hole binary.

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Blobrana


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Title: Precise Measurement of Gravity Variations During a Total Solar Eclipse
Authors: Qian-shen Wang, Xin-She Yang, Chuan-zhen Wu, Hong-gang Guo, Hong-chen Liu, Chang-chai Hua

The variations of gravity were measured with a high precision LaCoste-Romberg D gravimeter during a total solar eclipse to investigate the effect of solar eclipse on the gravitational field. The observed anomaly (7.0 ±2.7) x 10^{-8} m/s˛  during the eclipse implies that there may be a shielding property of gravitation.

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Blobrana


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Radio telescopes vie with laser detectors to hunt for signs of massive cosmic collisions.

Aided by the Universe's best celestial clocks, radio astronomers are embarking on a search for the almost-imperceptible stretching of the fabric of space by gravitational waves - predicted by Einstein's theory of general relativity but not yet detected directly. The approach is competing with more elaborate and expensive approaches to gravitational wave detection.
Since the late 1970s, astronomers have known that gravitational waves affect the arrival time of radio-wave bursts that emanate with clockwork regularity from pulsars, the spinning neutron stars left over from exploded supernovae. Now, the idea has moved from theory to application with the recent discoveries of many millisecond pulsars, which emit radio-wave bursts every thousandth of a second or so, more rapidly and more reliably than 'normal' pulsars.
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Blobrana


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Gravity
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New theory on the force of gravity.

Hij vermoedt dat Albert Einstein zich in zijn graf zal omdraaien, maar volgens de Amsterdamse theoretisch fysicus Erik Verlinde is de zwaartekracht niet wat we denken. De laatste maanden werkte hij aan een theorie waarin de aantrekking tussen twee massa's vanzelf ontstaat door informatieverschillen in de ruimte tussen de massa's en die daarbuiten.
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Blobrana


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Title: On the Origin of Gravity and the Laws of Newton
Authors: Erik P. Verlinde

Starting from first principles and general assumptions Newton's law of gravitation is shown to arise naturally and unavoidably in a theory in which space is emergent through a holographic scenario. Gravity is explained as an entropic force caused by changes in the information associated with the positions of material bodies. A relativistic generalisation of the presented arguments directly leads to the Einstein equations. When space is emergent even Newton's law of inertia needs to be explained. The equivalence principle leads us to conclude that it is actually this law of inertia whose origin is entropic.

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Blobrana


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What On Earth Is Wrong With Gravity? [BBC 2008 - Horizon]



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Blobrana


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Title: A quantum trampoline for ultra-cold atoms
Authors: Martin Robert De Saint Vincent (LCFIO), Jean-Philippe Brantut (LCFIO), Christian J. Bordé (LPL, SYRTE), Alain Aspect (LCFIO), Thomas Bourdel (LCFIO), Philippe Bouyer (LCFIO)

We have observed the interferometric suspension of a free-falling Bose-Einstein condensate periodically submitted to multiple-order diffraction by a vertical 1D standing wave. The various diffracted matter waves recombine coherently, resulting in high contrast interference in the number of atoms detected at constant height. For long suspension times, multiple-wave interference is revealed through a sharpening of the fringes. We use this scheme to measure the acceleration of gravity.

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Blobrana


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Listening for Gravity Waves, Silence Becomes Meaningful
Gravity waves spread through space and time like ripples on a pond, warping the fabric of the universe as they pass. The largest waves emanate from the most cataclysmic events in the universe: stellar explosions, mergers of black holes, and the violent first moments of cosmological history. Or so the venerable theory of general relativity goes - although many predictions of Albert Einstein's theory of gravity have been proved, only indirect evidence for gravity waves has been found.

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Blobrana


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Sensitive search fails to find ripples in space, but boosts hopes for future hunts.
The hunt for gravitational waves may not have found the elusive ripples in space-time predicted by Albert Einstein, but the latest results from the most sensitive survey to date are providing clear insight into the origins and fabric of the Universe.

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Blobrana


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Quantum theory of gravity
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Old methods lead to a new approach to finding a quantum theory of gravity
Quantum mechanics and Einstein's theory of general relativity are both extremely accurate theories of how the universe works, but all attempts to combine the two into a unified theory have ended in failure. When physicists try to calculate the properties of a quantum theory of gravity, they find quantities that become infinite -- infinities that are so bad they can't be removed by mathematical gambits that work in other areas of physics.
Now, Zvi Bern, John Carrasco, and Henrik Johanssen at UCLA, Lance Dixon at the Stanford Linear Accelerator Centre, and Radu Roiban at Pennsylvania State University have found a way to carry out a new set of gravity calculations with the help of an older theory that has been known since the 1980s to be finite.
Their new results are reported in Physical Review Letters and highlighted in a commentary by Hermann Nicolai at the Max Planck Institute for Gravitational Physics in Potsdam, Germany, in Physics.

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