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LISA: The Future of Gravitational Waves
LISA is a concept of the construction of three satellites, separated by millions of km, which will form a high precision interferometer that senses gravitational waves by monitoring the minute changes in distance between free falling test masses inside the spacecraft. These three spacecrafts use the laser beams of the interferometry which go back and forth between the different satellites and so the signals are combined to search for gravitational wave signatures that come from distortions of spacetime and huge in-space events such as black holes and its remnants.
And so, LISA will be the first observatory in space to explore the Gravitational Universe and it will gather revolutionary information about the dark universe.
The last century has brought enormous progress in our understanding of the universe. We know the life cycles of stars, the structure of galaxies, the remnants of the big bang, and have a general understanding how the universe evolved. We have come remarkably far using electromagnetic radiation as our tool for observing the universe. But... Gravity is the engine behind the universe’s processes, and much of its action is dark. Opening a gravitational window on the universe will let us go further than any alternative.
Gravity has its own messenger: Gravitational Waves, ripples in the fabric of spacetime, which travel essentially undisturbed and let us peer deep into the formation of the first seed black holes, exploring redshifts, prior to the epoch of cosmic re-ionisation. The ground breaking discovery of Gravitational Waves by ground-based laser interferometric Gravitational Wave observatories in 2015 is changing astronomy, giving us access to the high-frequency regime of Gravitational Wave astronomy.
Already the first observation of Gravitational Waves brought a surprise, because the existence of such heavy stellar-origin binary black holes was not widely expected. But the low-frequency window below one Hertz will probably never be accessible from the ground. It is in this window that we expect to observe the heaviest and most diverse objects. Opening a gravitational window on the Universe in the low-frequency regime with the space-based detector LISA will let us go further than any alternative. Exquisite and unprecedented measurements of black hole masses and spins will make it possible to trace the history of black holes across all stages of galaxy evolution, and at the same time constrain any deviation from the Kerr metric of General relativity.
The survey mission LISA will be the first ever mission studying the entire universe with Gravitational Waves. LISA is an all-sky monitor and will offer a wide view on a dynamic cosmos using gravitational waves as new and unique messengers to unveil the Gravitational Universe: It provides the closest ever view of the early processes at TeV energies and can probe the entire universe from its smallest scales around singularities and black holes all the way to cosmological dimensions.
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The Gravitational Universe will open a new window in astronomy using powerful sources of gravitational waves to probe a universe that cannot be probed by other means. In the early years of this millennium, our view of the universe has been comfortably consolidated in some aspects, but also profoundly changed in others: General relativity underwent tough tests and ESA´s Planck mission delivered important cosmological findings, a new concept suggests that black holes and galaxies evolve jointly and numerous ultra-compact binary systems in the Milky way were discovered – exquisite laboratories for exploring the extremes of stellar evolution in binary systems.
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All of these advances were possible using only our first sense for observing the universe, electromagnetic radiation, tracing electromagnetic interactions of baryonic matter in the universe. However, almost all of the universe remains electromagnetically dark. On astronomical scales gravitation is the real engine of the universe. Listening to gravity will let us go further than any alternative. This is possible through the direct observation of gravitational waves. Gravitational waves only weakly interact with matter and travel largely undisturbed over cosmological distances. Their signature is a fractional squeezing of spacetime perpendicular to the direction of propagation. LISA will be the first ever mission surveying the entire universe with Gravitational Waves to address the Science Theme of the Gravitational Universe.
And for such a promising device, it will need a lot of new technology which is already being developed. The technology will be used to ensure that LISA has the best features to ensure its mission.
The main key feature of the LISA concept is a set of three heliocentric orbits maintaining a near-equilateral triangular formation, without the need for orbit corrections, with an appropriate launch strategy, the formation can be kept in an almost constant distance to the Earth of about 50 x 106 km so that the LISA formation will trail the Earth by about 20°.
These particular heliocentric orbits for the three spacecraft were chosen such that the triangular formation is maintained throughout the year, with the triangle appearing to rotate about the centre of the formation. The orbit is optimised to minimise the key variable parameters of so-called "arm breathing" and range change between the spacecraft, as both of these drive the complexity of the payload design. At the same time, it ensures that the distance to LISA from Earth is sufficiently small for communication purposes.
LISA will coherently measure the stretching and squeezing of spacetime, including frequency, phase, and polarisation. Hence it will shed light on the origin of gravitational waves — large-scale violent cosmic events — and trace the motions of distant matter directly. Compared to the Earth-bound gravitational wave observatories like LIGO and VIRGO, LISA addresses the much richer frequency range between 0.1 mHz and 1 Hz, which is inaccessible on Earth due to armlength limitations and terrestrial gravity gradient noise.
All of this of course maintaining a “Drag Free Operation”, which will allow the spacecraft to follow the test masses, while shielding the test masses from spurious forces.
And so, the spacecraft are actively controlled to remain centered on the test masses along the interferometric axes, without applying forces on the test masses on these axes.
With this new tech, Drag-free operation reduces time-varying disturbances to the test masses caused by force gradients arising in a spacecraft that is moving with respect to the test masses.
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Another great feature is LISA's Interferometric laser ranging scheme, which is very similar to systems used for radar-tracking;
For LISA, the direct reflection of laser light, such as in a normal Michelson interferometer, is not possible due to the large distance of million km between the spacecraft: Diffraction expands the laser beam so much that for each Watt of laserpower sent, only about 250 pW are received. Direct reflection would thus result in an attenuation factor of about 6.25 x 10-20, yielding about one photon in every three days. Therefore, lasers at the end of each arm operate in a “transponder” mode.
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A laser beam is sent out from one spacecraft to another. The laser in the far spacecraft is then phase-locked to the incoming beam thus returning a high-power phase replica. The returned beam is received by the initial spacecraft and its phase in turn compared with the phase of the local laser. A unique feature of LISA interferometry is the virtual elimination of the effects of laser frequency noise, which would otherwise couple to the science signal through the sizable armlength difference. Stabilization to a reference cavity, as built into the payload, is not enough to suppress it completely.
And the last main feature of LISA itself is its sensitivity which will ensure observation in its frequency window of every gravitational wave source, without the need for targeting individual sources, its coherent observation mode will allow the distinction of overlapping signals and locate them. LISA is designed to measure gravitational radiation over a broad band at low frequencies, from about 0.1 mHz to 1 Hz, a band where the Universe is richly populated by strong sources of gravitational waves. LISA can sense waves from the densest regions of matter, the earliest stages of the Big Bang, and the most extreme warping of spacetime near black holes. In particular, LISA can observe objects that are shielded from electromagnetic observations by other stars or dust, such as binary systems close to or beyond the galactic center.
LISA Pathfinder
There is also LISA Pathfinder which paves the way for the LISA mission by testing in space the very concept of gravitational wave detection. LISA Pathfinder was already launched on December 3, 2015 as a proof-of-concept that tests key technologies that are specially developed for the LISA mission. After a picture perfect start, a journey to its destination some 1.5 million kilometers from Earth towards the Sun, and a successful release of the test masses,
LISA Pathfinder began its job as a space laboratory on 1 March. This spacecraft placed two test masses in a nearly perfect gravitational free-fall, and controls and measures their relative motion with unprecedented accuracy. This is achieved through innovative technologies comprising inertial sensors, an optical metrology system, a drag-free control system and micro-Newton thruster system. The test-masses and their environment are the quietest place in the solar system.
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With LISA Pathfinder, scientists have created the quietest place known to humankind. Its performance exceeds all expectations by far. Observing the most perfect free fall ever created was only made possible by reducing and eliminating all other sources of disturbance.
The first results, presented in June 2016, show that the two test masses at the heart of the spacecraft are falling freely through space under the influence of gravity alone. They are unperturbed by other external forces, to a precision more than five times better than originally required. The results show that the test masses are almost motionless with respect to each other, with a relative acceleration lower than 1 part in ten millionths of a billionth of Earth's gravitational acceleration.
All these technologies are not only essential for LISA, they also lie at the heart of any future space-based test of Einstein's General Relativity. The successful demonstration of the mission’s key technologies thus opens the door to the development of a large space observatory capable of detecting gravitational waves emanating from a wide range of exotic objects in the Universe.
And so, LPF demonstrates the fundamental technologies needed to build a gravitational wave observatory in space.
During the 5 planned years of researching, LISA will collect and process terabytes of data using thousands of CPUs which will be publicly available after a short period to scientists and curious people in the form of a catalogue that includes the identified sources and their parameters as well as the basic strain measurements and the software tools to analyse those data streams. Transient events, such as massive black hole mergers, will be announced as soon as possible to allow the community to plan for electromagnetic co-observation of a merger event. LISA is scheduled to launch in the early 2030 and it will make history, with powerful collaborations such as NASA, LIGO, ESA and the global scientific community itself which is the biggest of them all.
