Tuesday, July 24, 2012

Detecting Gravitational Waves in Space?


The LISA Space-Craft. Credit: AEI/Milde Marketing/Exozet.
 Gravitational waves are extremely weak vibrations of the space-time. The fabric of space-time is very stiff. By the time they are close to Earth, the gravitational wave strain for some of the strongest sources is less than 10-20.   These waves travel through the universe at the speed of light and pass unaffected by obstacles. Some are believed to carry information from the beginning of the universe. This post is inspired by the LISA Symposium I attended in May 2012 in Paris. I have waited longer than I should have before writing. So, I cannot give too much detail. I will summarize what I remember from that symposium almost three months after the meeting. The talks are available online if you are looking for more details or for other aspects of the gravitational wave science that were presented and are not mentioned here.



What I learned in Paris
My brother, Mihai, and I
The US side: Since LISA was not selected to fly either in the US or in Europe, there was a call to re-examine the LISA concept from NASA with the aim to develop a concept for a cheaper mission. A series of potential space based gravitational wave detectors emerged. However, most were LISA-like, and all cost estimates were around one billion dollars. They showed it is so far not possible to make a cheaper gravitational wave mission in space.  It did emerge that perhaps there could be multiple LISA-type missions in the future launched by different countries.

The European side: 
Attendants of the Paris meeting. Proof that we were there.

 Even though eLISA/NGO was not selected as the next large European mission, its science was selected unanimously by ESA reviewers as the best among the three proposed large missions. The technology preparation for this mission is going forward with the LISA Pathfinder, which is half a billion Euros enterprise with a planned launch of 2014/2015. So, LISA is not 'dead'. It is going forward with the LISA Pathfinder.  If the Pathfinder is successful, it seems unavoidable that a LISA mission will fly afterwards. The waiting and the preparation process is extremely difficult for any mission. The first LISA-concept was proposed in the 1980s. The Pathfinder will be launched in 2015+ and the mission itself maybe in 2026+. I have only been to one LISA Symposium so far. The enthusiasm built is beautiful to watch, but I also felt some of the pain of the many people who have studied this mission for most of their lives and may not even live long enough to see it fly.

Most space missions take very long to plan and to be launched. This is natural. However, to me as a human it is very difficult to imagine the 2030s. How much of the technology developed now will still be useful then? In 2032 I will be 60.  I've turned 30 in September 2012. I cannot imagine myself being that old yet...perhaps I'll look like my mother, but how will the world around me look? will we have quantum computers by then? will that change our way of life entirely?

Other countries
China and India seemed interested to launch the satellite for free, i.e., in exchange for technology on building the launch vehicle. At this point it is unclear if a future LISA will fly from Russia, the US, China, or India. China has some LISA-type research going on. They plan a pathfinder that will fly in a low orbit to measure the geoid of the Earth. The geoid is called the true figure of the Earth - it the equipotential surface at sea level.  A LISA-type mission will not be in a low orbit and so their naming scheme is confusing. However, the European pathfinder contains an accelerometer that is several orders of magnitude more precise than the 'state of art' accelerometers used in geophysics and, if flown in a low orbit, could potentially measure the geoid. A more accurate geoid measurement will help us map Earth's sub-structure, i.e., find water, oil reservoirs, predict  Earthquakes, etc, and so flying such a satellite would be of high impact.

What about Earth-based detectors? Pulsar Timing?
Scientists have built several gravitational wave detectors on Earth: two LIGO detectors in the US and VIRGO in Italy. They were operational until last year, and they are currently being upgraded. We had an updates from the LIGO and VIRGO teams, and it seemed that the design sensitivity is expected to be reached later ~ 2018 and that they expect a few sources per year. Other detectors are being planned in India, Japan and Australia, but there was not much discussion about that.

Pulsars are also natural gravitational wave detectors. If a gravitational wave passes between a pulsar and the Earth, it will affect the timing between pulses. However, it is important to remember that Ground based, space based and pulsar timing gravitational wave detectors are all sensitive to different parts of the frequency spectrum, which makes these three different techniques complementary and not in competition with each other.

Why build a detector in space?
When a gravitational wave passes by a detector, it compresses and expands the space around the detector by very tiny amounts. On Earth, the compression and expansion measurements are contaminated by deep noise due to the activities going on around the detector, e.g., people cutting trees nearby, traffic, Earthquakes. This noise is very difficult to remove from the data, but scientists have made significant progress in that direction in previous LIGO and VIRGO runs.  However, no gravitational wave have been found to date. The data from the Earth-based detectors is not public in order to avoid false claims of detection and loss of credibility.

On the other hand, space is a quiet environment. However, building a detector in space is much more difficult than building one on Earth. It has to work perfectly from the beginning.  Once it has flown it is very expensive to impossible to fly up there to fix it or make adjustments. So, scientists chose to build the Earth-based detectors first. The hope in the gravitational wave community is that the space based gravitational wave detectors will be built and will fly in the next three decades. We expect to detect many gravitational waves with both Earth-based and space based detectors that will open new windows to the universe, and we hope to hear some of the many untold parts of its history.

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