Radiation astronomy/Gravitationals

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The images show LIGO and Livingston, Louisiana, measurement of gravitational waves. Credit: B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration).{{free media}}
This photo shows the Livingston LIGO detector. Credit: Caltech/MIT/LIGO Laboratory.{{free media}}
This gravitational wave spectrum includes sources and detectors. Credit: NASA Goddard Space Flight Center.{{free media}}

Gravitational radiation appears to be cylindrical waves of radiation produced by relativistic, undulatory gravitational fields in Euclidean space.[1]

Interaction Mediator Relative Magnitude Behavior Range
Strong interaction gluon 1038 1 10−15 m
Electromagnetic interaction photon 1036 1/r2 universal
Weak interaction W and Z bosons 1025 1/r5 to 1/r7 10−16 m
Gravitational interaction photon or graviton ? 10 1/r2 universal

As the gravitational interaction is 10-36 that of the electromagnetic interaction to produce gravitational radiation requires a massive oscillator.

At right are the results from the first gravitational radiation detection. The images show the radiation signals received by the Laser Interferometer Gravitational Observatory (LIGO) instruments at Hanford, Washington (left) and Livingston, Louisiana (right) and comparisons of these signals to the signals expected due to a black hole merger event.

The wavelength of the gravitational waves is given by for example: 3 x 108 m‧s-1/400 Hz = 750,000 m, which is way longer than radio waves but expected for such a weak oscillator. 35 Hz corresponds to 8,600,000 m.

LIGO operates two detectors located 3000 km (1800 miles) apart: One in eastern Washington near Hanford, and the other near Livingston, Louisiana. The photo on the left shows the Livingston detector.

"According to general relativity, a pair of black holes orbiting around each other lose energy through the emission of gravitational waves, causing them to gradually approach each other over billions of years, and then much more quickly in the final minutes. During the final fraction of a second, the two black holes collide at nearly half the speed of light and form a single, more massive black hole, converting a portion of the combined black holes' mass to energy, according to Einstein's formula E=mc2. This energy is emitted as a final strong burst of gravitational waves. These are the gravitational waves that LIGO observed."[2]

"LIGO’s twin interferometers bounce laser beams between mirrors at the opposite ends of 4-kilometre-long vacuum pipes that are set perpendicularly to each other. A gravitational wave passing through will alter the length of one of the arms, causing the laser beams to shift slightly out of sync."[3]

Later detection confirmed the fusion of two massive stellar-sized objects, a binary neutron star merger.[4]

"According to Einstein's field equations, photon matter subject to quadruple oscillations is a source of gravitational waves."[5]

"In this work, we present a solution to the first stage of a new two-stage global treatment of the vacuum binary black hole problem [1, 2]. The approach, based upon characteristic evolution, has been carried out in the regime of Schwarzschild perturbations where advanced and retarded solutions of the linearized problem can be rigorously identified [3]. Computational experiments are necessary to study the applicability of the approach to the nonlinear regime. From a time-reversed viewpoint, this first stage is equivalent to the determination of the outgoing radiation emitted from the fission of a white hole in the absence of ingoing radiation. This provides the physically correct “retarded” waveform for a white hole fission, were such events to occur in the universe. Although there is no standard astrophysical mechanism for producing white holes from a nonsingular matter distribution, white holes of primordial or quantum gravitational origin cannot be ruled out."[6]

"This fission problem has a simpler formulation as a characteristic initial value problem than the black hole merger problem. The boundary of the (conformally compactified) exterior spacetime contains two null hypersurfaces where boundary conditions must be satisfied: past null infinity I−, where the incoming radiation must vanish, and the white hole event horizon H−, which must describe a white hole, which is initially in equilibrium with no ingoing radiation and then distorts and ultimately fissions into two white holes with the emission of outgoing gravitational waves."[6]

An almost identical signal could originate from a comparable much more massive neutron star fission.

"This is an exciting time to study gravitation, astrophysics and cosmology. Through challenging cosmic microwave background (CMB) and supernovae observations cosmology has been turned on its head. Gravitational radiation astronomy should be the next contributor to this revolution in astrophysics and cosmology."[7]

Pulsars[edit]

As the pulsar picks up speed through accretion, it becomes distorted from a perfect sphere due to subtle changes in the crust, depicted here by an equatorial bulge. Credit: Dana Berry/NASA Goddard Space Flight Center.{{free media}}
A bizarre stellar pair consists of the most massive neutron star confirmed so far, orbited by a white dwarf star. Credit: ESO/L. Calçada.{{free media}}

"Gravitational radiation, ripples in the fabric of space predicted by Albert Einstein, may serve as a cosmic traffic enforcer, protecting reckless pulsars from spinning too fast and blowing apart, according to a report published in the July 3 issue of Nature. Pulsars, the fastest spinning stars in the Universe, are the core remains of exploded stars, containing the mass of our Sun compressed into a sphere about 10 miles across. Some pulsars gain speed by pulling in gas from a neighboring star, reaching spin rates of nearly one revolution per millisecond, or almost 20 percent light speed. These "millisecond" pulsars would fly apart if they gained much more speed."[8]

"Using NASA's Rossi X-ray Timing Explorer, scientists have found a limit to how fast a pulsar spins and speculate that the cause is gravitational radiation: The faster a pulsar spins, the more gravitational radiation it might release, as its exquisite spherical shape becomes slightly deformed. This may restrain the pulsar's rotation and save it from obliteration."[8]

"Nature has set a speed limit for pulsar spins. Just like cars speeding on a highway, the fastest-spinning pulsars could technically go twice as fast, but something stops them before they break apart. It may be gravitational radiation that prevents pulsars from destroying themselves."[9]

"Gravitational waves, analogous to waves upon an ocean, are ripples in four-dimensional spacetime. These exotic waves, predicted by Einstein's theory of relativity, are produced by massive objects in motion."[8]

"Created in a star explosion, a pulsar is born spinning, perhaps 30 times per second, and slows down over millions of years. Yet if the dense pulsar, with its strong gravitational potential, is in a binary system, it can pull in material from its companion star. This influx can spin up the pulsar to the millisecond range, rotating hundreds of times per second."[8]

"In some pulsars, the accumulating material on the surface occasionally is consumed in a massive thermonuclear explosion, emitting a burst of X-ray light lasting only a few seconds. In this fury lies a brief opportunity to measure the spin of otherwise faint pulsars. Scientists report in Nature that a type of flickering found in these X-ray bursts, called "burst oscillations," serves as a direct measure of the pulsar's spin rate. Studying the burst oscillations from 11 pulsars, they found none spinning faster than 619 times per second."[8]

"The Rossi Explorer is capable of detecting pulsars spinning as fast as 4,000 times per second. Pulsar break-up is predicted to occur at 1,000 to 3,000 revolutions per second. Yet scientists have found none that fast. From statistical analysis of 11 pulsars, they concluded that the maximum speed seen in nature must be below 760 revolutions per second."[8]

"This observation supports the theory of a feedback mechanism involving gravitational radiation limiting pulsar speeds. As the pulsar picks up speed through accretion, any slight distortion in the star's dense, half-mile-thick crust of crystalline metal will allow the pulsar to radiate gravitational waves. (Envision a spinning, oblong rugby ball in water, which would cause more ripples than a spinning, spherical basketball.) An equilibrium rotation rate is eventually reached where the angular momentum shed by emitting gravitational radiation matches the angular momentum being added to the pulsar by its companion star."[10]

"Accreting millisecond pulsars could eventually be studied in greater detail in an entirely new way, through the direct detection of their gravitational radiation. LIGO, the Laser Interferometer Gravitational-Wave Observatory now in operation in Hanford, Washington, and in Livingston, Louisiana, will eventually be tunable to the frequency at which millisecond pulsars are expected to emit gravitational waves."[10]

"The waves are subtle, altering spacetime and the distance between objects as far apart as the Earth and the Moon by much less than the width of an atom. As such, gravitational radiation has not been directly detected yet. We hope to change that soon."[11]

For the image second down on the right: "This artist’s impression shows the exotic double object that consists of a tiny, but very heavy neutron star that spins 25 times each second (right), orbited every two and a half hours by a white dwarf star (left). The neutron star is a pulsar named PSR J0348+0432 that is giving off radio waves that can be picked up on Earth by radio telescopes. Although this unusual pair is very interesting in its own right it is also a unique laboratory for testing the limits of physical theories."[12]

Black holes[edit]

Frame is from a simulation of the merger of two black holes and the resulting emission of gravitational radiation (colored fields). Credit: NASA/Ames Research Center/Christopher E. Henze.{{free media}}
Numerical simulations are of the gravitational waves emitted by the inspiral and merger of two black holes. Credit: NASA/Ames Research Center/C. Henze.{{free media}}

"According to Einstein, whenever massive objects interact, they produce gravitational waves — distortions in the very fabric of space and time — that ripple outward across the universe at the speed of light. While astronomers have found indirect evidence of these disturbances, the waves have so far eluded direct detection. Ground-based observatories designed to find them are on the verge of achieving greater sensitivities, and many scientists think that this discovery is just a few years away."[13]

"Catching gravitational waves from some of the strongest sources — colliding black holes with millions of times the sun's mass — will take a little longer. These waves undulate so slowly that they won't be detectable by ground-based facilities. Instead, scientists will need much larger space-based instruments, such as the proposed Laser Interferometer Space Antenna, which was endorsed as a high-priority future project by the astronomical community."[13]

"In the turbulent environment near the merging black holes, the magnetic field intensifies as it becomes twisted and compressed. [...] The most interesting outcome of the magnetic simulation is the development of a funnel-like structure — a cleared-out zone that extends up out of the accretion disk near the merged black hole. The most important aspect of the study is the brightness of the merger's flash. The team finds that the magnetic model produces beamed emission that is some 10,000 times brighter than those seen in previous studies, which took the simplifying step of ignoring plasma effects in the merging disks."[13]

In the image on the left: "Numerical simulations of the gravitational waves emitted by the inspiral and merger of two black holes. The colored contours around each black hole represent the amplitude of the gravitational radiation; the blue lines represent the orbits of the black holes and the green arrows represent their spins."[14]

References[edit]

  1. A. Einstein and N. Rosen (January 1937). "On gravitational waves". Journal of the Franklin Institute 223 (1): 43-54. doi:10.1016/S0016-0032(37)90583-0. http://www.sciencedirect.com/science/article/pii/S0016003237905830?via%3Dihub. Retrieved 2018-1-3. 
  2. Ivy F. Kupec (11 February 2016). Gravitational waves detected 100 years after Einstein's prediction. 2415 Eisenhower Avenue, Alexandria, Virginia 22314, USA: National Science Foundation. p. 1. Retrieved 3 January 2018.
  3. Davide Castelvecchi & Alexandra Witze (11 February 2016). "Einstein's gravitational waves found at last LIGO 'hears' space-time ripples produced by black-hole collision". Nature. doi:10.1038/nature.2016.19361. http://www.nature.com/news/einstein-s-gravitational-waves-found-at-last-1.19361. Retrieved 2018-1-3. 
  4. B. P. Abbott, the LIGO Scientific Collaboration & the Virgo Collaboration (16 October 2017). "GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral". Physical Review Letters 119 (16). doi:10.1103/PhysRevLett.119.161101. 
  5. Constantin Sandu and Dan Brasoveanu (2007). Sonic Electromagnetic Gravitational Spacecraft, Part - Principles, In: AIAA SPACE 2007 Conference & Exposition. AIAA 2007-6203. American Institute of Aeronautics and Astronautics. Retrieved 10 January 2018.
  6. 6.0 6.1 Roberto Gómez, Sascha Husa, Luis Lehner, and Jeffrey Winicour (15 September 2002). "Gravitational waves from a fissioning white hole". Physical Review D 66 (6): 1-9. doi:10.1103/PhysRevD.66.064019. https://arxiv.org/pdf/gr-qc/0205038. Retrieved 2018-1-10. 
  7. Nelson Christensen, Renate Meyer and Adam Libson (1 December 2003). "A Metropolis–Hastings routine for estimating parameters from compact binary inspiral events with laser interferometric gravitational radiation data". Classical and Quantum Gravity 21 (1): 317-330. doi:10.1088/0264-9381/21/1/023. http://people.carleton.edu/~nchriste/CQG03.pdf. Retrieved 2018-1-19. 
  8. 8.0 8.1 8.2 8.3 8.4 8.5 Lynn Jenner (2 July 2003). EINSTEIN'S GRAVITATIONAL WAVES MAY SET SPEED LIMIT FOR PULSAR SPIN. Greenbelt, MD USA: Goddard Space Flight Center. Retrieved 4 April 2018.
  9. Deepto Chakrabarty (2 July 2003). EINSTEIN'S GRAVITATIONAL WAVES MAY SET SPEED LIMIT FOR PULSAR SPIN. Greenbelt, MD USA: Goddard Space Flight Center. Retrieved 4 April 2018.
  10. 10.0 10.1 Lars Bildsten (2 July 2003). EINSTEIN'S GRAVITATIONAL WAVES MAY SET SPEED LIMIT FOR PULSAR SPIN. Greenbelt, MD USA: Goddard Space Flight Center. Retrieved 4 April 2018.
  11. Barry Barish (2 July 2003). EINSTEIN'S GRAVITATIONAL WAVES MAY SET SPEED LIMIT FOR PULSAR SPIN. Greenbelt, MD USA: Goddard Space Flight Center. Retrieved 4 April 2018.
  12. L. Calçada (25 April 2013). Artist’s impression of the pulsar PSR J0348+0432 and its white dwarf companion. European Southern Observatory. Retrieved 4 April 2018.
  13. 13.0 13.1 13.2 Christopher E. Henze (27 September 2012). Simulations Uncover 'Flashy' Secrets of Merging Black Holes. Greenbelt, MD USA: Goddard Space Flight Center. Retrieved 4 April 2018.
  14. Emanuele Berti (11 February 2016). "Viewpoint: The First Sounds of Merging Black Holes". Physics 9: 17. https://physics.aps.org/articles/v9/17. Retrieved 2018-4-4.