Category: GW Astronomy

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.

With LISA we will survey compact stellar-mass binary systems in our own galaxy and better understand the structure of the Milky Way.

Very soon after activation, LISA will detect gravitational wave signals from known nearby binary compact stars. Because of their known positions and periods they serve as “verification binaries” ensuring in particular, predictable LISA signals. Signals are also certain to appear from populations of numerous and various remnants in our galaxy, including white dwarfs and neutron stars, which are known to exist from electromagnetic observations.

Extrapolation of known samples predicts that LISA will detect several thousand binaries. For hundreds of these, LISA will determine the orbital periods, mass parameters and distance, a rich trove of information for detailed mapping of our home galaxy.

These discoveries will also shed light on the outcome of the common envelope phase, on the progenitors of type Ia supernovae, and on tidal and non-gravitational influences on orbits associated with the internal physics of the compact remnants themselves.

LISA will detect only the brightest and nearest binaries as individual sources; millions of others from across the Galaxy will blend together into a confusion foreground.

Confronting General Relativity with experimental measurements of gravity is one of the most important objectives of fundamental physics. The Gravitational Universe will permit major advances in the themes of strong gravity and cosmology.

General Relativity (GR) has passed all current tests in the weak field regime. The Gravitational Universe will permit unprecedented measurements of general relativity in the strong field regime.

The Gravitational Universe will explore relativistic gravity in the strong field, non-linear regime. It seems unlikely that any other methods will achieve the sensitivity of LISA to deviations of strong-field gravity by 2028. Unlike the ground-based instruments, LISA will have sufficient sensitivity to notice even small corrections to Einstein gravity.

LISA will map the spacetime around astrophysical black holes, yielding a battery of precision tests of GR in an entirely new regime. These have the potential to uncover hints about the nature of quantum gravity, as well as enabling measurements of properties of the universe on the largest scales.

The nature of gravity in the strong-field limit is so far largely unconstrained, leaving open several outstanding questions.

• Does gravity travel at the speed of light?
• Does the graviton have mass?
• How does gravitational information propagate: Are there more than two transverse modes of propagation?
• Does gravity couple to other dynamical fields, e.g., massless or massive scalars?
• What is the structure of spacetime just outside astrophysical black holes?
• Do their spacetimes contain horizons?
• Are astrophysical black holes described by the Kerr metric, as predicted by GR?

An outstanding way to answer these questions and learn about the fundamental nature of gravity is by observing the vibrations of the fabric of spacetime itself, for which coalescing binary black holes and Extreme Mass Ratio Inspirals are superior probes.

With LISA we will survey compact stellar-mass binary systems in our own galaxy and better understand the structure of the Milky Way.

Very soon after activation, LISA will detect gravitational wave signals from known nearby binary compact stars. Because of their known positions and periods they serve as “verification binaries” ensuring in particular, predictable LISA signals. Signals are also certain to appear from populations of numerous and various remnants in our galaxy, including white dwarfs and neutron stars, which are known to exist from electromagnetic observations.

Extrapolation of known samples predicts that LISA will detect several thousand binaries. For hundreds of these, LISA will determine the orbital periods, mass parameters and distance, a rich trove of information for detailed mapping of our home galaxy.

These discoveries will also shed light on the outcome of the common envelope phase, on the progenitors of type Ia supernovae, and on tidal and non-gravitational influences on orbits associated with the internal physics of the compact remnants themselves.

LISA will detect only the brightest and nearest binaries as individual sources; millions of others from across the Galaxy will blend together into a confusion foreground.

The present universe is in a phase in which both the star formation rate and Active Galactic Nuclei (AGN) activity are declining. In this after-noon cosmos LISA will observe quiescent massive black holes at the centers of galaxies within a volume of about 100 Mpc3.

LISA will offer the deepest view of galactic nuclei, exploring regions inaccessible to electromagnetic observations, by probing the dynamics of intrinsically dark, relic stars in the nearest environs of a massive black hole.

The probes used are the so-called Extreme Mass Ratio Inspirals EMRIs: a compact star (either a neutron star or a stellar-mass black hole) captured into a highly relativistic orbit around the massive black hole and spiralling through the strongest field regions a few Schwarzschild radii from the event horizon before plunging into the massive black hole. As the compact star weighs much less than the massive black hole, the mass ratio is extreme, and as the star-black hole pair is a binary, the inspiral phase is governed by the emission of gravitational waves.

Key questions can be addressed in the study of galactic nuclei with EMRIs:

• What is the distribution of stellar remnants at the galactic centres and what is the role of mass segregation and relaxation in determining the nature of the stellar populations around the nuclear Black holes in galaxies?
• What is the mass distribution of stellar relics?
• Are stellar-mass Black holes of 100 M⊙ or/and medium sized Black holes of 104 M⊙ present in galactic centres?

LISA will discover EMRI events, exploring the deepest regions of galactic nuclei, those near the horizons of black holes with masses close to the mass of the Black hole at our Galactic Centre, out to redshifts as large as z ~ 0.7.

In the Gravitational Universe, EMRIs are exquisite probes for testing stellar black hole populations in galactic nuclei. With LISA we will learn about the mass spectrum of stellar-mass black holes, which is largely unconstrained both theoretically and observationally. The measurement of even a few EMRIs will give astrophysicists a totally new and different way of probing dense stellar systems, determining the mechanisms that govern stellar dynamics in the galactic nuclei.