Author: Jan Scharein

Gravitational waves are ripples in the fabric of space-time generated by some of the most powerful astrophysical events – such as exploding stars and collisions of two black holes at the centres of galaxies. Gravitational waves travel at the speed of light through the universe, unhindered by intervening mass – to gravitational waves the universe is transparent. That is why gravitational waves are the cosmic messengers that allow us to explore the so far dark side of the universe.

In September 2015, the first direct detection of gravitational waves by the ground-based LIGO Observatories added a new sense to our perception of the Universe: for the first time we were able to LISTEN to the Universe because gravitational waves are in some respects similar to sound waves. Hence gravitational wave astronomy complements our understanding of the Universe and its evolution.

Gravitational waves measured by LISA, a large mission in space, will allow us, for example, to trace the formation, growth, and merger history of massive black holes. Also it will enable us to confront General Relativity with observations, and it will probe new physics and cosmology with gravitational waves.

Gravity rules

Albert Einstein revealed that gravity rules the universe. According to his theory of general relativity gravity determines the curvature of space-time – the deformation of space-time depends on the position of masses and on their acceleration. Accelerating masses thus create gravitational waves – distortions in space-time that ripple outwards like waves on a pond.

In a pure gravitational environment all masses follow space-time along the straightest paths possible through this curved arena. The “straight line” to curved space-time is called a “geodesic”. Gravitational waves are waves in the fabric of space-time. They change approximately parallel geodesics, pushing them together and then pulling them apart. Two free falling test masses will experience this as an oscillating change in the relative distance to each other. ESA´s technology test mission LISA Pathfinder (LPF) has demonstrated this effect.

Gravitational waves within general relativity

With his theories of relativity Einstein laid the foundation for much of modern physics. Fundamental ideas of his theories are:

• The speed of light is constant.
• Space, time and gravity are strongly connected to each other.
• Space and time are flexible. They change according to circumfluent mass: stationary mass changes space.
• Accelerating masses create gravitational waves – distortions in space-time that ripple outwards like waves on a pond.

In General Relativity gravity is no longer simply a force that pulls falling apples to the ground. Instead, gravity is geometry. The presence of matter alters the geometry of space and time, and the geometry in turn determines how matter and light moves. Einstein predicted in 1916 that space-time would be curved and that matter and light follow the curvature of space-time (geodesics). This was experimentally observed as early as in 1919 by Arthur Eddington.

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External Links

Learn more about Einstein’s Theories of Relativity at Einstein Online

Black hole seeds probably began to form at high redshift, z ~ 15 – 20, and then participated in the assembly process of galaxies.

Black Hole seeds proceeded through (hundreds to thousands of) hierarchical mergers of smaller protogalaxies. When two galaxies merge into one, their central black holes sink to the centre of the new galaxy, usually find each other, form binaries, inspiral, and coalesce becoming the loudest sources of gravitational waves.

Supermassive black holes grow mostly by accretion, but a substantial number of inspiral and coalescence events are likely to be observed by LISA each year.

LISA will detect coalescence events of massive binary black holes in a wide interval of redshifts and masses extending back to early protogalaxies at z ~ 15. The intense accretion phase that supermassive black holes experience in the QSO epoch erase information on how and when the black holes formed. LISA will be able to directly map and mark the loci where massive black holes form by studying gravitational waves emitted during their coalescence following the merger of their hosts.

Gravitational waves can travel undisturbed and provide clean tracers of the properties of the first black holes: their masses, the time of formation, their number density. This is the information needed to constrain how black holes formed and evolved in the first galaxies.

LISA will discover black holes formed at early epochs of our universe. The early black holes grew over cosmic time to generate the supermassive black holes present in most galactic nuclei today.

Optical, radio and X-ray astronomical observations have provided evidence that nearly all bright galaxies house a supermassive black hole in their centre, weighing millions to billions suns. LISA will help to discover the evolution of these giants and follow their formation routes from the very beginning.

Luminous quasi- stellar objects (QSOs), powered by accretion onto a supermassive black hole, existed just a few hundred million years after the Big Bang. Astronomers have observed galaxies containing double black holes that will eventually collide and merge.

From the very beginning of structure formation black holes and galaxy mergers are an integral part of the evolution of our Universe. In Cold Dark Matter cosmology, galaxies form small and grow over time through mergers and accretions.

The exact formation routes of the first black hole “seeds” are unknown, but black holes must have evolved inside their host galaxies in similar fashion, starting from humble masses of hundreds to possibly several 10⁵ M⊙ up to few billion solar masses in giant galaxies today. During their evolution to a supermassive black hole, black holes are expected to transit through mass intervals observable by LISA.