Lovely LISA

by Nicholas Mee on August 15, 2017

One of the amazing ideas to emerge from Einstein’s theory of general relativity was the possibility of gravitational waves rippling their way across the cosmos. It took a century to verify this prediction. Their existence was finally confirmed by LIGO (the Laser Interferometer Gravitational-wave Observatory) in September 2015, as described in this post: Cosmic Ripples.

What’s in a Name?

LIGO has so far detected three gravitational wave signals, all of which are due to black hole mergers in the distant universe. The image below shows an artist’s impression of the system that produced the third of these signals, detected on 4 January 2017. The black holes were 32 and 19 times the mass of the sun and were spinning in different planes, as depicted in the illustration which shows them just before their merger. The signal has been named GW170104. Guess why?

Artist’s impression of a close binary black hole system based on GW170104. Credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)

Lovely LISA

The detection of gravitational waves by LIGO was an incredible technological achievement. The European Space Agency (ESA) is planning to go one better by putting a gravitational wave detector in space. It is known as LISA (Laser Interferometer Space Antenna). There will be three spacecraft orbiting the sun in a triangular formation, consisting of a mother and two daughter craft each separated by a distance of 2.5 million kilometres. They will form a precision interferometer with lasers monitoring the distances between the mother and daughter craft. A passing gravitational wave will change these distances very slightly and this will be detected by the interferometer.

Schematic representation of LISA. Credit: NASA.

Paving the Way for LISA

The LISA Pathfinder mission was launched in December 2015 as a stepping stone to the LISA mission. It was devised to test the technology that will be used in LISA and demonstrate the feasibility of constructing an interferometer in space. LISA Pathfinder released two cubic test masses to float freely within the spacecraft and used its laser interferometer to measure their separation, as shown in the illustration below. It then monitored their positions to an unprecedented accuracy of less than one hundredth of a nanometre.

The goal was to show that test masses can be shielded from any stray internal and external forces and maintained in a state of almost perfect free-fall. Remarkably, the spacecraft avoids any contact with the test masses contained within its structure by sensing their motion and adjusting its own position using micro-thrusters to compensate. This is essential if the sensitivity of the interferometer is not to be destroyed by inevitable perturbations that the spacecraft will suffer. These arise from a number of sources including stray gas molecules within the craft, the solar wind and micro-meteoroid impacts.

The LISA Pathfinder mission released two cubic test masses to float freely in solar orbit while their separation was monitored by a precision interferometer. Credit: ESA/ATG medialab.

In the Pathfinder mission the test masses are located 40 centimetres apart, whereas the three LISA craft will be separated by millions of kilometres. The LISA interferometer will measure their separation just as accurately as the Pathfinder mission, so its sensitivity will scale up in proportion to its increased size. ESA announced in June this year that the technology trialled by LISA Pathfinder has performed beyond expectations, which means it will certainly be sensitive enough to detect gravitational waves when deployed by the LISA mission.

What Will We See?

LISA will greatly enhance our ability to study gravitational waves. It will detect signals invisible to LIGO and other ground based gravitational wave detectors, as it will be sensitive to gravitational waves with much longer wavelengths that are produced by much larger systems. Although the black holes that merged during the GW170104 event were very massive they were only 190 and 115 kilometres in diameter, with the merged black hole around 280 kilometres in diameter. These are very small objects by cosmic standards.

LISA will detect gravitational wave signals emanating from tightly bound binary systems containing two compact objects that may be white dwarfs, neutron stars or black holes orbiting each other prior to their merger. For instance, a binary black hole system such as GW170104 would be detected weeks or even months before the merger event. This will enable the position of the binary system to be located in the sky and the time of merger to be accurately predicted, which will greatly aid in visual identification of the merger event.

Supermassive Black Holes

There is a supermassive black hole of four million solar masses at the centre of our galaxy. Most, if not all, galaxies are thought to harbour a monster such as this within their core. LISA will be able to detect these beasts devouring nearby stars. It will also detect mergers of supermassive black holes. Such extremely violent and spectacular events must occasionally happen somewhere in the universe. We can look forward to finding out much more about them.

An artist’s impression of a supermassive black hole.
Credit: ESO/L. Calçada.

The creation and early growth of supermassive black holes is still not well understood. LISA should detect their birth pangs and provide important clues to how they formed and their relationship to quasars in the early universe. LISA will also help to improve models of the immediate aftermath of the Big Bang and the very early universe. As an important bonus, LISA will add to our knowledge of fundamental physics by providing stringent new tests for general relativity.

LISA is scheduled for launch in 2034 as part of ESA’s Cosmic Vision programme.

Further Information

There is more about the LISA mission on the website of the LISA consortium at: https://www.elisascience.org/

There is also information on the ESA website at: http://sci.esa.int/lisa/

 

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