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Eugenio Coccia: The Quest for Gravitational Waves

March 15, 2013

A first detection of gravitational waves tests Einstein’s prediction. Another motivation for looking for gravitational waves is that one can look beyond the visible universe, to understand black holes, supernovae, GRBs. Third, one can look as back in time as a theorist can conceive, gaining information from the beginning of the universe.

The main features are two transversal polarization states, associated with massless, spin-2 particles (gravitons. The radiaotion is emitted by a time-varying quadrupole.

There are no laboratories to produce gravitational waves. A 1000 tons steel rotor at 4 Hz would give a luminosity of 10^-30 W: the comment of Einstein is “a practically vanishing value”. A collapse to neutron star with mass 1.4 M-sun gives 10^52 watts, and if this is in the galaxy this gives a 10^-18 power, in the virgo cluster 10^-21. This is a challenge to contemporary experimental physics.

Gravitational waves are emitted by coherent acceleration of a large portion of matter. They cannot be shielded, and they arrive to the detector in pristine condition. They can reveal features of their sources that other messengers can’t provide.

A pulsar in the sky has been shown to lose energy exactly as general relativity predicts. The orbital period decreases. So we are confident that GWs exist.

The methods of detection at very low frequency involve cosmic microwave background polarization. When the wave period is years, we go to pulsar timing. For higher frequency there are space interferometers, and present detector already taking data are at very high frequency.

We can learn a lot by detecting the waveforms of gravitational waves. For a supernova the information is a inner detailed dynamics of the SN. For spinning neutron stars, one gets the neutron star locations near the earth, and pulsar evolution. For coalescing binaries (two compact objects spiraling into one another) one can determine masses of the objects, distance to the system, measuring the Hubble constant. The stochastic background, relic of the early universe, provides a confirmation of the big bang, and inflation. It is also a unique probe to the Planck epoch. It might even provide the proof of existence of cosmic strings.

The speaker mentioned that every newly opened astronomical window has found unexpected results. Optical window was opened by Galilei, and the first surprise was Jupiter’s moons. Cosmic rays in 1912 provided us with the discovery of the muon. And so on. So the point is clear: if we open a window to study gravitational waves we are confident of new discoveries and surprises.

In the world there are several interferometric and resonant-mass detectors. Auriga and Nautilus are the only resonant-mass detector in operation now.

Forty years of attempts of detection have passed. There is a international committee for gravitational waves (GWIC). There is a lot of collaboration rather than competition, because of the need to join forces and data.

There was a phase change in the view. From 2005 on, we know that there are no such strong sources in the sky, need to be more conservative and try to detect the sources we imagine in the sky at the moment. A world-wide network of interferometers is composed of LIGO in Hanford and Livingston, VIRGO in Italy, GEoO600, and Kagra, a 3km interferomenter, is in preparation underground in the Kamioka mine.

Detecting such a tiny signal in presence of noise of all kinds mean isolation of mirrors from ground and acoustic sources. Need to use material that reduce thermal noise. The sensitivity is limted at low frequency by seismic noise, in the middle part by thermal noise. The progress in reducing the noise in LIGO can be illustrated with a graph of the strain as a function of frequency. The reduction versus time from 2001 to 2006 has allowed to go down by over three orders of magnitude.

What were the results of LIGO and VIRGO ? We have not yet a detection in our hands, but we think this is not far in the future. Data is analyzed jointly. Several year long science data runs have happened. Limits on gravitational waves from known millisecond pulsars have been set, and on compact binary coalescence rates in our local neighborhood. There are also limits in stochastic background of cosmological origin.

One can also think of searching for coincidence between neutrino telescopes and gravitational waves. A paper (Phys.Rev.Lett. 103 (2009) 031102) describes the concept.

After this first data, interferometers are off because they are preparing a second generation: advanced VIRGO and advanced LIGO. The sensitivity now is such that one could detect tens or hundreds of events per year by increasing the sensitivity by a factor of 10. This is possible with more powerful lasers, different topologies of interferometry, optimization, and putting all that we have learned to work. In 2015 this could be done and is the present goal.

The present array can be completed with a detector in India, LIGO-India. This is a 2-km interferometer that could be transported from the US to India. This improves the localization capability for a signal. It would also remove some blind spots in the sky.

So we are on the threshold of a new era of gravitational wave astrophysics. Sensitivities. But a third generation is already being thought of: underground detectors to remove noise and cryogenic, to reduce thermal noise. A 10km triangle underground can detect sources in a wide portion of the universe. This requires pushing forward the technology.

In space allows to escape from backgrounds at small frequencies. The LISA detector originally had arms of 5 million kms. Has been reduced because of costs. There will be savings in weight, launch cost. A demonstrator will be launched next year.

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