The European Space Agency (ESA; Paris, France) has reduced noise detection to ultralow levels in its LISA Pathfinder to set the stage for its successor, LISA, to detect gravitational waves from high-energy events in space. Their work is detailed in a paper in Physical Review Letters.
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The collision of two black holes, or the explosion of a dying star, or the dance of two super-dense neutron stars spinning wildly around one another can create a ripple in spacetime. Because the distortion effects are on the scale of a few millionths of a millionth of a meter over a distance of a million kilometers, all external noises need to be eliminated.
This concept has been proven on Earth with the Laser Interferometer Gravitational-Wave Observatory, LIGO, and the Virgo observatory. These facilities, operated by an international collaboration of more than a thousand scientists, comprise pairs of either three or four kilometer-long arms at 90 degrees to one another, each equipped with a laser beam and mirror system. As a gravitational wave passes through, the lengths of the arms are lengthened and shortened respectively by a minuscule fraction, tiny but enough to be noticeable as a change in the reflected laser pattern by the highly accurate equipment.
However, Earth detectors have limited space and they cannot escape external influences, ranging from vehicles passing by to local seismic activity. Their size is great for detecting high-frequency (10–1000 Hz) gravitational waves, like those coming from coalescing pairs of stellar-mass black holes or neutron stars, but isn't sensitive to lower frequency waves (0.00002–0.1 Hz) generated by supermassive black holes a million times more massive than the Sun. In addition, a cosmological background of gravitational waves covering the entire spectrum down to even lower frequencies (0.000000000000001 Hz) are thought to be produced by the formation of Universe itself in the theorized phase of 'inflation', the brief, accelerated expansion in its first moments 13.8 billion years ago.
To access the lower-frequency waves, the ESA is developing the Laser Interferometer Space Antenna or LISA, a three-satellite fleet that will create a triangular formation separated by 2.5 million kilometers and connected by laser beams, following Earth in orbit around the Sun. Such an endeavor, planned for launch in 2034, is pushing the boundaries of current technology.
Indeed, the key requirement for a space mission to measure any possible distortion caused by a passing gravitational wave is that it is isolated from all external and internal forces, which are present even in space, except gravity. To prove the fundamental concept of such a mission, ESA and its partners built the LISA Pathfinder, which successfully concluded last year, having demonstrated that offending internal and external 'noise' sources could indeed be removed to provide the quiet environment needed to make gravitational wave detections with the full-size LISA mission.
To achieve this, the technology demonstrator mission used two 2 kg free-falling cubes separated by 38 cm and linked by lasers. The spacecraft acted as a shield around them, protecting them from external sources. It maneuvered around them using tiny micro-newton thrusters to oppose solar radiation pressure and wind of particles, sensing the test mass motion and adjusting its own to compensate: essentially flying to within an accuracy of a few billionths of a meter and being able to sense the relative positions of the metal cubes to within a trillionth of a meter.
The mission already outperformed itself in the first week of operations, and now the final report card is in, showing that it even surpassed some of the requirements for its next-generation successor. The improvements focused on the lower frequencies, since at higher frequencies, between 60 mHz and 1 Hz, the mission’s precision was limited only by the sensing noise of the equipment used to monitor the position and orientation of the test masses.
After many more months in space, the data showed a 10-fold reduction in the effect of escaping residual gas pressure inside the spacecraft, which caused gas molecules to bounce off the cubes--just as gas bubbles in your fizzy drink bounce off ice cubes or the glass, and the drink eventually goes 'flat'.
More data also led to improved understanding of the small inertial force acting on the cubes caused by a combination of the shape of LISA Pathfinder's orbit and the effect of the noise in the signal of the startrackers used to orient it--improved control in LISA will eliminate this force further. A more accurate calculation of the electrostatic forces of the onboard electrical systems and magnetic fields has also now eliminated a systematic source of low-frequency noise.
Importantly, statistical analysis has allowed scientists to remove the effects of additional sporadic events to measure the noise at even lower frequencies than expected, down to 0.00002 Hz, essentially creating the quietest place in space. Overall, this proves that measurements at the low frequencies needed for LISA can be realized. It means that instead of only being able to detect a passing gravitational wave from a single event for a fraction of a second, LISA will be able to detect month- or even years-long chatter of multiple signals.
Furthermore, it will be sensitive to the first signs of a supermassive black hole merger, weeks before it has fully collided. This will give time to alert other ground- or space-based observatories so that they can also tune in to study the object at a range of other complementary wavelengths.
The mission will also likely uncover other currently unknown exotic sources of gravitational waves.
SOURCE: European Space Agency; http://www.esa.int/Our_Activities/Space_Science/ESA_creates_quietest_place_in_space
