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Indiana University Bloomington

Gravitational Physics

Theory

Gravitational waves and dense matter

Einstein, almost 100 years ago, predicted gravitational waves. These are oscillations of space-time that have never been directly observed, despite intense efforts. Their observation is anticipated within the next five years at the Laser Interferometer Gravitational Wave Observatory (LIGO) or a similar European detector VIRGO. This historic discovery, and further measurements of gravitational waves, will provide new information on gravity, the properties of extremely dense matter, and the behavior of some of the most compact and energetic objects in the cosmos.

neutron star

Massive imperfection. A bump (red) on a super-dense and rapidly spinning neutron star (blue) can disturb space-time and radiate gravitational waves.

At Indiana University, we are working on the properties of dense matter that are important for the sources of gravitational waves (that are generated by the large scale motions of great masses) and the gravitational waveforms radiated. This is helping observers to better search for these waves. For example, we have carried out large-scale molecular dynamics (MD) simulations of the breaking strength of neutron star crust (the solid ash of a collapsed star). We find the crust to be the strongest material known, some ten billion times stronger than steel.

This great strength could support massive mountains on neutron stars. These mountains, on rapidly rotating stars, will radiate continuous gravitational waves that are strong enough to be detected with present equipment.

Please see the ScienceNow article “Neutron Stars: Billions of Times Stronger Than Steel

Symmetry Violations in Theory of Gravity

Alan Kostelecky is one of the world's leading authorities in the study of violations of Lorentz, CPT symmetries, and extensions to the Standard Model of particle physics and Einstein's theory of gravity, General Relativity.
More details.


Experiment

IUCF short-range gravity experiment

Sketch of the IUCF short-range gravity experiment.

The IUCF Short-Range Experiment

Newton's Law of gravity is called "universal," but is it necessarily so?  This is an experimental question. For test objects ranging in mass from a typical coin to a typical planet, and for distance scales ranging from a few microns to about a light-year, experiments and observations have shown that Newton's law makes an excellent prediction of the results.  For larger masses, there are deviations, which are well-described by Einstein's stunning achievement - the General Theory of Relativity.  But what about the case of larger distance scales?   This situation is still a mystery, because the mass distributions are unknown.  In fact, a special form of matter (the appropriately named "dark matter")  that cannot be seen with telescopes (optical, radio, or otherwise) has been postulated, to preserve Newton's law on astronomical scales.  And what of smaller distance scales?  This situation is also a mystery,  since the gravitational force is so feeble as to be barely detectable at this range.  We are used to feeling the "strong" effect of gravity, but that is only in the presence of the planet-sized object - the Earth - beneath our feet.  In fact, the force of gravity is at least thirty orders of magnitude weaker than the other forces we believe to be fundamental - electromagnetism and the nuclear forces.

Gravitational measurements in the micron range are an exceptional challenge given the competition from electromagnetism and other effects such as vibrations.  Several experiments have recently risen to meet that challenge, including one in progress at the Indiana University Cyclotron Facility (IUCF). This experiment uses a high-frequency technique, departing from the traditional torsion-pendulum approach pioneered by Cavendish over 200 years ago.  Planar, 1 kHz oscillators are used as test masses with a stiff conducting shield in between them to suppress backgrounds. The 1 kHz operational frequency is chosen since at this frequency it is possible to construct a simple passive vibration isolation system.  The challenge is well-motivated, since modifications to Newton's law at short range, arising from new elementary particles or even extra spacetime dimensions, are predicted from many models that attempt to describe gravity and the other fundamental interactions in the same theoretical framework.