GAINESVILLE, Fla. — Like a celestial top, the spinning neutron star known as the Crab Pulsar is slowing, a phenomenon that astronomers have yet to fully understand.
Now, researchers with the Laser Interferometer Gravitational Wave Observatory Scientific Collaboration — an international collaboration headed by a University of Florida physicist — have ruled out one long-hypothesized cause: emission of gravitational waves.
"We can now say definitively that gravitational waves play only a minor role at best in this phenomenon," says David Reitze, a UF professor of physics and spokesperson for the collaboration. "'Our measurements tell us that no more than 4 percent of the energy loss of the pulsar is caused by the emission of gravitational waves.'"
The Crab Pulsar is located in the Crab Nebula, one of the most famous objects in the sky. Astronomers have long known that the pulsar spins about 30 times per second, and that the rate is slowing. Pulsars are almost perfect shaped, tiny, extremely dense spheres made almost entirely of neutrons. The Crab Pulsar contains more mass than the sun, yet has a radius of only 10 kilometers, or about 6.2 miles.
Scientists have proposed a number of hypotheses for the physical mechanism behind the spin "braking," including such phenomena as asymmetric particle emission, magnetic dipole radiation and gravitational-wave emission.
Gravitational waves are ripples in the fabric of space and time predicted by Einstein's general theory of relativity but never observed.
The hypothesis was that the spinning Crab Pulsar might generate gravitational waves as a result of even a tiny deformation of its shape. Such a deformation might result from physical strain on the pulsar's semi-solid crust, or its enormous magnetic field.
Researchers at the gravitational wave observatory known as LIGO used the observatory's network of interferometers — essentially, extremely sensitive rangefinders that can detect extremely small motions indicative of gravitational waves — to test this hypothesis.
The LIGO network includes observatories in Hanford, Wash., and Livingston, La.
When the scientists analyzed data gathered from the Crab Nebula region, they found no evidence of gravitational waves, ruling it out as a cause for the Pulsar braking.
"The physics world has been waiting eagerly for scientific results from LIGO," said Joseph Taylor, an astronomer and professor of physics at Princeton University who won the Nobel Prize for indirect detection of gravitational waves. "It is exciting that we now know something concrete about how nearly spherical a neutron star must be, and we have definite limits on the strength of its internal magnetic field."
The University of Florida has been a major participant in the LIGO project since 1996. UF researchers designed and built the input optics for the LIGO interferometers. These optics are a major subsystem of LIGO, providing crucial stabilization of the laser beam used to detect gravitational waves in the ultra-high precision interferometers.
In addition to Reitze, University of Florida professors Guido Mueller and David Tanner led the development and construction effort of the optics subsystem. UF faculty members Guenakh Mitselmakher, Sergei Klimenko and Bernard Whiting, meanwhile, continue to play key roles in LIGO's data analysis.
"LIGO is one of the most sensitive instruments ever built, able to detect motions smaller than the size of an atomic nucleus," said Tanner. "It's very interesting that even with such a sensitive instrument, we have yet to detect gravitational waves, even though they were predicted by Einstein's theory more than 90 years ago."
LIGO is funded by the National Science Foundation and operated jointly by Caltech and the Massachusetts Institute of Technology. More than 600 scientists from universities in the U.S. and 11 other countries participate in the project.
The next major milestone is the Advanced LIGO Project, slated for operation in 2014. Advanced LIGO, which will utilize the infrastructure of the LIGO observatories, will be 10 times more sensitive. UF is also a major participant in Advanced LIGO.
"Bursts of gravitational waves may be produced by a number of astrophysical events, including merging black holes or neutron stars," said Mitselmakher, a distinguished professor of physics and director of the UF Institute for High Energy Physics and Astrophysics. "It is anticipated that Advanced LIGO will be able to discover such events, if their frequency is as predicted by theory."