Pulsars May Reveal Gravitational Ripples from Supermassive Black Holes
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April 13, 2012

Pulsars May Reveal Gravitational Ripples from Supermassive Black Holes




In 2010, an international team of scientists discovered a promising way to fine-tune pulsars into the best precision time-pieces in the Universe and provide astronomers with a new tool to study the powerful gravitational forces that shaped the universe.

Gravitational waves can be described as ripples in the spacetime fabric, just like those created in a lake when someone casts a stone. Electromagnetic waves are created when we have accelerating charges. Gravitational waves are in complete analogy created when we have accelerating masses. However, their amplitude is very small, something that can be better understood by the fact that spacetime needs rather immense amounts of energy in order to be disturbed (curved). 

Gravitational waves are detectable only from rather extreme dynamical phenomena such as the coalescing of binary black holes or relativistic stars, core collapses during a supernova explosion creating neutron stars or pulsars (such as the Crab Nebula pulsar above), or the Big Bang itself. Whenever such an event takes place, it transmits information about the whole dynamical process through the emission of gravitational waves.

Pulsars rank at or near the top of freaky phenomena found in our Universe. In the early 1930s, California Institute of Technology astrophysicist, Fred Zwicky, an immigrant from Bulgaria, focused his attention on a question that had long troubled astronomers: the appearance of random, unexplained points of light.

It occurred to Zwicky that if a star collapsed to the sort of density found in the core of atoms, the result would be an unimaginably compacted core: atoms would be crushed together with their electrons squeezed into the nucleus, forming neutrons and a neutron star, with a core so dense that a single spoonful would weigh 200 billion pounds.

But there's more, Zwicky  concluded: with the collapse of the star there would be  huge amounts of leftover energy that would result in a massive explosion,  the biggest in the known universe that we called today supernovas.

Most neutron stars house incredibly large magnetic fields. If they are spinning rapidly they make fabulous clocks, cosmic radio beacons we call pulsars. Pulsars can keep time to an accuracy better that one microsecond per year. Some pulsars generate more than 1000 pulses per second, which means, as Lawrence Krauss wrote in The Physics of Star Trek, that an object with the mass of the Sun packed into an object 10 to 20 kilometers across is rotating over 1000 times per second, or more that half the speed of light!

The extremely stable rotation of these 'cosmic clocks' has enabled astronomers to discover the first planets orbiting other stars and provided stringent tests for theories of the Universe.
However, until now, slight irregularities in their spin have puzzled scientists and significantly reduced their usefulness as precision tools.

Astronomers have observed that pulsar spin rates slow very gradually over time. The team, led by the University of Manchester's Professor Andrew Lyne, used decades-worth of observations to determine that pulsars actually exhibit two different rates of spin change, not one as previously thought, and switch between them abruptly. The team also discovered that these variations are associated with changes in the pulsar's appearance that can be used to 'correct' for the shifts.

"Humanity's best clocks all need corrections, perhaps for the effects of changing temperature, atmospheric pressure, humidity or local magnetic field," says Lyne. "Here, we have found a potential means of correcting an astrophysical clock."

The discovery makes pulsars better tools for detecting gravitational waves--mysterious, powerful ripples which have not yet been directly observed, although widely believed to exist. The direct discovery of gravitational waves, which cause the distortion of space, could allow scientists to study the Universe shortly after the Big Bang and other violent events such as the merging of super-massive black holes.

"Many observatories around the world are attempting to use pulsars in order to detect the gravitational waves that are expected to be created by super-massive binary black holes in the Universe," says University of British Columbia astronomer Ingrid Stairs. "With our new technique we may be able to reveal the gravitational wave signals that are currently hidden because of the irregularities in the pulsar rotation."

"These changes are associated with a change in the shape of the pulse, or tick, emitted by the pulsar," says George Hobbs of the Australia Telescope National Facility. "Because of this, precision measurements of the pulse shape at any particular time indicate exactly what the slowdown rate is and allow the calculation of a 'correction'. This significantly improves their properties as clocks."

The scientists made their breakthrough using the 76-m Lovell radio telescope at the University of Manchester's Jodrell Bank Observatory. "These exciting results were only possible because of the quality and duration of the unique Lovell telescope pulsar timing database," said Ben Stappers of the University of Manchester.



The Daily Galaxy via jb.man.ac.uk University of British Columbia and Eurekalert.org

Image credit: Simulation of 2 coalescing black holes. Werner Berger

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