Author: Joffrey Rieger

Supermassive black hole binaries are the strongest sources of gravitational radiation in the universe. From them, we will learn more about galactic formation and structure.

When two black holes are locked in orbit they emit gravitational waves which causes them to lose energy, leading to an inspiral and collision of the system.

The emitted gravitational waves contain information about the two bodies and the surrounding spacetime. Ground-based gravitational-wave detectors have now observed many binaries whose components are stellar-mass black holes, weighing tens of solar masses.

LISA will detect binaries whose components are supermassive black holes, weighing up to several tens of millions of solar masses. Information from their gravitational waves will provide us with rich insight into galactic formation and structure, across the observable Universe.

LISA explores the populations of stellar-mass compact objects in galactic nuclei and their dynamics. It will capture gravitational wave signals from inspiraling stellar-mass compact objects into massive black holes in galactic nuclei.

Large populations of stellar-mass black holes, neutron stars, and white dwarf stars are expected to be present in the galactic centres. All these compact objects might reach the last stable orbit around the massive black hole before being torn apart by the hole´s gravity. Some of these objects will move on nearly-radial orbits around the central black hole, others will be scattered in, close to the massive black hole, following resonant interactions with other stars present in the surroundings.
All of them will be captured on relativistic orbits.

The resulting binaries, with very small mass ratios, will become more and more compact by losing energy to gravitational radiation and later coalesce in what are called EMRIs – Extreme Mass Ration Inspirals. The strongest signals will come from stellar-mass black holes. Because they are heavier than other stars they will be more concentrated near the centres of galaxies.

LISA will have the capability of detecting EMRI signals up to redshift z ~ 0.5 – 0.7, and of measuring the mass and spin of the massive black hole with high accuracy, and in galaxies of all types. Thus it will provide a census of the massive black hole populations in the near universe in a mass interval around 10⁴ M_⊙ – 10⁶ M_⊙ for which electromagnetic observations are incomplete or even missing.

See a movie of an EMRI event (Credit: Steve Drasco).

LISA´s detections will provide unique information also on the mass spectrum of stellar black holes, in particular their upper mass limit, which is unconstrained observationally and for which we have poor knowledge from stellar evolution models.

EMRI detections will further carry information, albeit indirect, into the dynamical processes involving the various stellar populations present in the extreme environments of a galactic nucleus. LISA also has the potential of discovering a coalescence event involving two middleweight black hole of thousands suns in colliding star clusters, in the local universe. This would resolve the longstanding issue on the presence of central black holes in stellar systems as the old globular clusters.

LISA, the Laser Interferometer Space Antenna, will be the first Gravitational Wave Observatory in space. It will allow us to observe the entire universe including areas that are inaccessible by other means.

With LISA we will be able to look back at the very beginning of the universe, investigate its evolution further and learn much about its structure.

LISA will complement earth-bound gravitational wave observatories and other astronomical instruments, and significantly enrich Multimessenger astronomy.

LISA´s core technologies were already successfully tested with the LISA Pathfinder mission. Now, an international consortium of scientists together with ESA and NASA is working towards LISA´s launch.

The First Gravitational Wave Observatory in Space

The first direct detection of gravitational waves in 2015, was a major scientific breakthrough. It opened the era of gravitational wave astronomy. Here on Earth gravitational wave detectors can observe high-frequency gravitational waves. They originate from objects with small masses, a few 10’s that of the Sun.

Sources with much larger masses, such as the mergers of massive black holes at the centres of galaxies, produce signals at much lower frequencies, undetectable on Earth. LISA will detect such signals and allow us unique insights in our universe – its beginning and evolution, its population, maybe the big bang and so-far unknown components.

Mission Concept

The LISA mission consists of three spacecraft orbiting the Sun in a triangular configuration. The three satellites, separated by 2.5 million km, will be connected by laser arms forming a high precision instrument – the first laser interferometer in space. It will sense gravitational waves by monitoring the minute changes in distance between free falling test masses inside the spacecraft.

LISA will be an astronomical observatory of unprecedented versatility and range.

LISA Pathfinder successfully tested core technologies

LISA´s core technologies were successfully tested with the LISA Pathfinder mission and already inspired other projects. One example is the Laser Ranging Instrument: Developed for LISA and tested on LISA Pathfinder it currently flies on the GRACE Follow-On mission and helps observing the Earth´s water movements across the planet – an important contribution to climate research.

What makes LISA so exciting?

Gravity is the dominant force in the universe

Gravity is an incredible powerful force. It drives many of the universe’s processes but so far, much of its action is invisible. This is why we want to measure gravity´s messengers: gravitational waves.

Gravitational waves are ripples in the fabric of space-time. They travel undisturbed through the universe and contain unique information. LISA will be the first ever misson to detect gravitational waves from space and thus observe the entire universe. LISA will listen to gravity and let us go further than any other astronomical method.

LISA will be a milestone for Multimessenger astronomy

Gravitational waves are fundamentally different from electromagnetic waves such as visible light, infrared or x-rays. This is why LISA will perfectly complement traditional astronomical observations.

LISA will open the gravitational wave window in space and measure gravitational radiation over a broad band of frequencies. This will enrich our knowledge about the Universe substantially.

New technology: LISA will consist of sciencecraft

The classical distinction between a spacecraft and its payload doesn’t fit the LISA mission well. Usually, a spacecraft provides the infrastructure for the scientific instruments, the payload, it carries. LISA is different because each of its spacecraft is part of the scientific instrument – it protects key elements, the free-falling test masses, from disturbances.

The LISA spacecraft must thus be designed and built with special requirements in mind. The importance of the co-design and the co-operation of spacecraft and payload is captured in the term “sciencecraft”.

Only in space: The long-baseline interferometer

LISA is so valuable for astronomers all over the world because it will measure low-frequency gravitational waves that cannot be observed from earth due to armlength limitations and terrestrial gravity gradient noise.

The ideal instrument for measuring low-frequency gravitational waves is a laser interferometer with an arm length as large as possible and long integration times. Hence LISA can be thought of as a high precision Michelson interferometer in space with an arm length of 2.5 mil­lion km. The arm length has been carefully chosen to allow observation of most of the interesting sources of gravitational waves in the target frequency band.

LISA will be an astronomical observatory of unprecedented versatility and range.

LISA´s key features and sensitivity

Key features of LISA are interferometric measurement of distances, long baselines of 2.5 x 10⁶ km, drag free sciencecraft based on inertial sensors, and the familiar “cartwheel”-orbits. Compared to the Earth-bound gravitational wave observatories LISA addresses the much richer frequency range between 0.1 mHz and 1 Hz.

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