LIGO

LIGO ( Laser Interferometer Gravitational Wave Observatory / Laser Interferometer Gravitational Wave Observatory ) is an observatory, with the aid of gravitational waves to be detected. Originally founded in 1992 by Kip Thorne, Ronald Drever ( Caltech ) and Rainer Weiss (MIT ), the project hundreds of scientists in more than 40 institutions now employs worldwide.

LIGO consists of two observatories located in Hanford (Washington ) and Livingston (Louisiana ).

The data obtained with LIGO are evaluated by several groups according to types of possible sources of gravitational waves. These are:

• Stochastic sources (eg, from the Big Bang )
• Pulsars ( sources with continuous emission of radiation )
• Merger of two compact objects such as neutron stars and black holes
• Collapsing stars ( supernovae )
• Exotic sources

• 5.1 Enhanced LIGO
• 5.2 Advanced LIGO

Task

The main task of LIGO is the direct measurement of gravitational waves of cosmic origin. These waves are predicted by Albert Einstein's general theory of relativity, but could still not be detected directly in spite of numerous experimental tests.

An indirect indication of the existence of these waves there by the discovered by Russell Hulse in 1974 double pulsar PSR 1913 16. The variations in the orbit of this binary system agree exactly with the predictions of general relativity to the emission of gravitational waves. For this discovery, Russell Hulse received the Nobel Prize for Physics in 1993.

The direct detection of gravitational waves is a very active research area in modern physics, as it could make it possible to astronomy in the electromagnetic spectrum and the neutrino astronomy, a completely new type of astronomy. Therefore, it was tried in the 1960s, using resonance cylinders to measure gravitational waves, led by Joseph Weber. In the 1970s, then the possibility of using interferometers for this search was realized by Rainer Weiss.

In 1992, LIGO was founded, the construction work on the two detectors were completed in 1999. After initial testing and fine tuning of the systems the first scientific measurement period took place in August 2002. The end of 2007 ended the already fifth measuring period, after two years of data were obtained with a so far unique sensitivity. Gravitational waves could be detected in these data but have not yet.

Observatories

LIGO operates two observatories in Hanford (Washington) (46 ° 27 ' 28 " N, 119 ° 24' 35" W46.457777777778 - 119.40972222222 ) and Livingston (Louisiana ) (30 ° 33 ' 45 "N, 90 ° 46 '30 " W30.5625 - 90 775 ) and are located about 3,000 kilometers apart. Since gravitational waves propagate at the speed of light, it can be concluded in the sky from the propagation time difference between two measured in these observatories signals on the position of the actual source. It also enables numerous underground faults that only propagate at the speed of sound ( such as vibrations, distant earthquakes, etc. ) are excluded.

Each observatory having an L-shaped ultra-high vacuum system with a side length of respectively four kilometers, in which a laser interferometer is housed. The observatory in Hanford has a second, housed in the same vacuum system interferometer with a side length of two kilometers.

Operation

In the mutually perpendicular arms of the Michelson interferometer observatories are housed in which the laser beams perform the actual measurement.

At the main station of the Observatory ( the corner of the L, at which the two arms cross ) a stabilized laser beam from ten watts of power is first sent through a mirror which, although it makes the laser light in the system, but not in the reverse direction (power recycling mirror). Characterized the power of the laser light into the interferometer, and thus, the sensitivity is increased.

Thereafter, the beam is incident on a beam splitter on which the beam is split and each half is sent into one of the two or four kilometers arms. Accommodated in each arm is a Fabry -Perot interferometer, consisting of two mirrors ( one of which is semi-transparent ) so that the light is about 75 times that route passes before it passes through the semitransparent mirror and then incident on the beam splitter. By this technique, the multiple reflections, the effective barrel length of the light is increased, which in turn increases the sensitivity of the instrument.

At the beam splitter in the corner station both partial beams are directed onto a photodiode that measures the intensity of light arriving there. The interferometer, especially the adjustable mirror at the ends of the two arms, is now set so that the two partial beams just wipe (see interference ) and thus ideally no light arrives at the photodiode. By a number of external and internal factors but this is not permanent possible so that the entire system must be constantly adjusted to achieve the cancellation of the two partial beams.

Crossing a gravitational wave observatory, the relative lengths of the arms of the interferometer to change. While being stretched one arm, the other arm is shortened. This causes a phase shift of the two partial waves of the laser light and the interference changes the intensity of the measured light, which is stored electronically measured and for further evaluation on a hard drive.

By using a combination of mirrors, the laser intensity and the Fabry-Perot cavity within the system observatories are able to measure a difference between the two arm lengths of which corresponds to about 1/1000 of a proton diameter. Despite the incredible sensitivity of the instrument probably needs further improvements to allow the direct detection of a gravitational wave can be provided.

The entire measurement technique is therefore very sensitive to external influences such as movements in the ground ( remote earthquakes, waves on remote beaches ), weather -related effects (wind), road traffic as well as to internal factors such as thermal motions of the atoms in the mirrors scattered in the tunnels light, etc. the task of data analysts, it is then, among other things, to filter out the gravitational signal from all this noise.

Signals

There are a number of signals to be searched. These can be grouped into continuous signals ( search for pulsars and cosmic gravitational background radiation ) and transient signals (merger of compact objects and unclassifiable outbreaks ). These four signals can be but also by modeling the signal classification ( see table).

Pulsars

Pulsars are neutron stars with a strong magnetic field and rotate up to 500 revolutions per second around its own axis. If these pulsars asymmetries in their mass distribution (eg by a small bump on the surface ), they emit, according to the theory of gravitational waves, which the rotation frequency decreases. As an example of the Crab pulsar be mentioned, which rotates about 30 times per second.

To search for signals from unknown pulsars can participate even any means of the Einstein @ home project on your home PC. It is performed by the BOINC software and is free of charge.

Gravitational wave background radiation

Many models to the universe say strong gravitational waves ahead incurred shortly after the Big Bang. These gravitational waves have a broad range and makes it possible for detection of these waves much more time to look into the history of the universe than is possible with the cosmic microwave background radiation.

Merger of compact objects

Circling be two compact objects, such as two neutron stars or two black holes ( or combinations thereof), they also emit gravitational waves according to the theory. Thus, the system loses energy, so that approach both body slowly. This stronger gravitational waves are radiated, so that speeds up this process both bodies collide and merge into a black hole.

This was indirectly demonstrated in the double pulsar PSR 1913 mentioned above 16, and the measurements exactly fit the predictions of general relativity. Although the two bodies approach each year in this system by about 3.5 meters, both neutron stars merge until about 300 million years ago.

The expected signals for such a scenario can be calculated very accurately analytically, so that a targeted search can be performed for such gravitational waves in the data.

Bursts

Burst signals are short, unmodeled signals such as those in a supernova, could arise the collapse of a very heavy star. Such signals may also highlighted by the fusion of two very heavy black holes form or by effects of cosmic strings.

Unknown sources

It is also possible that gravitational waves are found by astronomical sources that were not previously considered, or the previously completely unknown to us.

Future

Enhanced LIGO

Since 7 July 2009, a further measurement period is officially used, with modernized and improved instruments (enhanced LIGO ) is approximately twice as large as is the range of the measurements before. This means that the eight times the volume of the universe can be overheard by gravitational waves. In this measuring period of the French- Italian Virgo detector is again included.

Advanced LIGO

Since about two years of measurement with enhanced LIGO instruments are again extensively improved, so that the sensitivity to be improved by a factor of 10 compared to the source terminal. In other words, the 1000-fold volume with the same sensitivity are investigated. This conversion is currently ( December 2013 ) has not yet been completed.