Ex Astris Scientia: Gravitational Waves

     In a previous post I briefly mentioned the Laser Interferometer Gravitational-wave Observatory (LIGO) designed to detect gravitational waves (GWs).  I want to expand on that and give an update on the status of GW detectors, especially LISA, but I thought it best to first include a discussion of GWs. There is a lot to say about this, and I'm trying to keep it as short as possible, so I appologize if I don't cover something thoroughly. For this reason, I have added extra links to easy-to-read sources in the list of references. So what are GWs and why do we care?
     GWs are ripples in spacetime, much like ripples in water, that are caused by the acceleration of compact, massive objects and propagate at the speed of light. They differ from the electromagnetic (EM) waves we currently use to observe the universe in that the EM waves propagate through space, but GWs are perturbations of spacetime itself (see this website: http://www.ligo.org/, which has a 7.5 minute video that shows very good computer simulations with a brief description of GWs as well as LIGO). So, while EM waves experience attenuation due to scattering and absorption through matter, GWs travel through matter with little attenuation. The problem is that they are very weak and so are difficult to detect. They are predicted by Einstein's General Relativity, and in 1993 Hulse and Taylor found indirect evidence for GWs with a radio pulsar (they got the Nobel prize for this), but so far none have ever been directly observed. And this is why we care about them.
     Direct observation of GWs would prove general relativity. According to Kip Thorne (1997) it would potentially allow for high-precision tests of GR, determination of Hubble's constant and the cosmological constant and a better understanding of the objects that cause them, such as neutron stars and black holes(more on this in the next section). Aside from compact objects, GWs would have also been produced from the Big Bang and so, like the Cosmic Microwave Background Radiation (look up WMAP for further info) that permeates the universe, GWs should also permeate the universe. This is referred to as the gravitational wave background, or GWB. So where observation by WMAP of the microwave background allowed astronomers to observe the early universe within hundreds of thousands of years after the Big Bang, direct observation of GWs would allow astronomers to observe the universe only seconds after the Big Bang.

                       Sources of GWs
     As mentioned above, GWs are produced from the acceleration of compact, massive objects. These objects are primarily neutron stars(NS) and stellar mass black holes(BH). A neutron star is one of the possible end stages of a star. It is the result of the collapse of a giant star after fusion has stopped. As the core collapses, protons combine with electrons to form neutrons, and what remains is a dense neutron core. If it is less than 3 solar masses, it remains a neutron star, otherwise it collapses into a black hole. These objects can come in binary systems: astronomers look for NS-NS binaries, NS-BH binaries and BH-BH binaries since it is known that these should produce GWs. These are what land-based interferometers such as LIGO are trying to detect.

Searching for GWs
Gamma Ray Bursts

Gamma ray bursts are associated with extreme core collapse and are also thought to result from a the merging (or coalescence) of a NS-NS binary or NS-BH binary, which should produce stronger GWs, so many astronomers believe that using gamma ray bursts is very promising (Abadie et al., 2012). In fact, some believe that with the next generation of GW observatories we should be able to directly detect and observe GWs within the next decade unless the theory is fundamentally flawed (Wen, 2011).
There is one other promising source of GWs for future detectors, and that is the detached whited dwarf binary J0651 +2844. According to Hermes et al. (2012), it is the loudest non-interacting binary in the mHz range, and that makes it an excellent verification source for future missions aimed at the direct detection of GWs. Hermes et al. (2012) also report that it is "the shortest period detached compact binary and the cleanest system to observe at optical wavelengths for orbital decay due to gravitational wave radiation." According to general relativity, any system emitting gravitational wave radiation should lose energy through this radiation. J0651 has a 12.75 minute orbital period and the orbit is decaying at a rate of about -0.31 s/yr, which is within the limits of what is predicted by general relativity.

Pulsar Timing Arrays

A less expensive way to detect GWs than building giant interferometers is to use pulsars. Pulsars are rapidly rotating, highly magnetized neutron stars. The strong magnetic field accelerates electrons to high velocities and produces radiation in radio, optical, x-ray and gamma wavelengths and this radiation travels out along the axis of the magnetic field, like a beam, that appears to observers on Earth like a pulse as the star rotates. They are ideal for use in detecting GWs because of their precision timing. They are the most stable natural standards of astronomical time and give us a unique opportunity to detect the low-frequency GWB (Potapov, 2010). GWs distort the EM signals produced by the pulsars, causing fluctuations in the times of arrival. When many pulsars are observed over time, patterns in the fluctuations can reveal gravitational waves. This collection of pulsars is called a pulsar timing array.

More on GW Detectors
LIGO is what is known as a Michelson interferometer that consists of two observatories, one in Washington and one in Louisiana, and each is connected to a corner station by L-shaped arms. I highly recommend this website, http://www.ligo.org/. LIGO is designed to detect GWs from 40 Hz to several kHz, with maximum sensitivity at 150 Hz. Virgo is an Italian GW obsrvatory similar to LIGO. Neither has been able to detect GWs yet. Improvements to both, called Advanced LIGO and Advanced Virgo, are in progress that would increase the sensitivity by a factor of ten. Advanced LIGO is scheduled to be fully operational by 2014 and Advanced Virgo by 2015. The Laser Interferometer Space Antenna (LISA) is a space-based interferometer designed to detect GWs in a frequency range of 10^-4 to 10^-1 Hz. It began as a joint NASA-ESA project and is supposed to be the best chance to observe GWs from the Big Bang. They have been planning it for quite some time, and according to the NASA website, should have been operational by now. Unfortunately, there were funding issues (and possibly other problems) that caused delays. NASA has not updated their website so keep in mind that if you visit the pages pertaining to LISA, it is incorrect. Other websites of unknown reliability also claim LISA will be operational in the near future, but according to the ESA's contact person, Oliver Jennrich, LISA will not be operational before 2027. I have been interested in this project since I learned about it several years ago, so I am quite disappointed that it is still so far from completion.

NOTE: I realize I tried to put a lot of information here so I highly recommend looking through my source list. The Kip Thorne link is especially good for detailed information without being too technical; in particular, try reading the introduction and section 4 (about the interferometers and GW detection). It discusses clearly and in detail some things I only touched on.

References:

LIGO
General: http://www.ligo.org/

For fun: http://www.einsteinathome.org/ (similar to SETI, you can allow your computer's idle time to help with the search for spinning neutron stars to detect GWs)

Technical: https://www.advancedligo.mit.edu/overview.html

LISA

general info: Oliver Jennrich - ESA contact
ESA website LISA mission homepage:
http://www.rssd.esa.int/index.php?project=LISA&page=index

Specs and technical:

http://www.rssd.esa.int/SYS/docs/ll_transfers/LISA_Science_Requirements.pdf

Gravitational waves and pulsars:
general info

NASA website:     http://imagine.gsfc.nasa.gov/docs/features/topics/gwaves/gwaves.html
         http://imagine.gsfc.nasa.gov/docs/science/know_l2/pulsars.html
  Square Kilometre Array:

http://www.skatelescope.org/the-science/gravity-einstein-pulsars/pulsars-gravitational-waves/

1993 Nobel prize info: http://cosmictimes.gsfc.nasa.gov/online_edition/1993Cosmic/nobel.html

Technical:
Abadie, J. et al., ApJ, 760:12 (18pp), 2012 November 20

SEARCH FOR GRAVITATIONAL WAVES ASSOCIATED WITH GAMMA-RAY BURSTS DURING LIGO SCIENCE RUN 6 AND VIRGO SCIENCE RUNS 2 AND 3

doi:10.1088/0004-637X/760/1/12

Hermes, J.J. et al., 2012, RAPID ORBITAL DECAY IN THE 12.75-MINUTE BINARY WHITE DWARF J0651+2844

arXiv:1208.5051 [astro-ph.SR]