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Author: Jan Scharein

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Compact stellar-mass binaries

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.

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Gravitational Wave Sources

A New Astronomy

Pulsar vaporising a star remnant

Artists impression of a pulsar vaporising a star remnant.

What powered the Big Bang, what happens to space and time in black holes and what is the mysterious dark energy accelerating the expansion of the Universe? Gravitational wave signals will provide rich new information about the behaviour, structure and history of the physical universe, and about physics itself.

LISA will measure gravitational wave signals from a wide range of sources that are of strong interest for a deeper understanding of the cosmos.

LISA data is expected to shed light on

  • the astrophysics of black holes and galaxy formation,
  • merging massive black holes in galaxies at all distances,
  • massive black holes swallowing smaller compact objects like neutron stars,
  • known binary compact stars and stellar remnants,
  • members of known populations of more distant binaries,
  • probably other sources, possibly including relics of the extremely early Big Bang, which are as yet unknown,

and it will provide exceptionally strong tests of the predictions of general relativity. These unique dynamical tests are based on mergers of two massive black holes with maximally warped vacuum spacetimes travelling nearly at the speed of light and interacting strongly with each other. This system will allow us to test the full non-linear dynamics of gravitational theory.

In the same way that accelerated electric charges generate electromagnetic radiation, accelerated mass and energy generate gravitational radiation. The periodic motion of a system of mass M and size R at a (luminosity) distance D creates gravitational waves with a strain amplitude of about

h ~ (GM/(Rc2 ))2 (R/D)

with a frequency determined by the frequency of the motion. The shapes and strengths of the observed waves give us details about the structure and behaviour of the system that produced them.
LISA will open the gravitational wave window in space and measure gravitational radiation, from about 0.1 mHz to 100 mHz, a band where the Universe is richly populated by strong sources of gravitational waves.

External Links

Multimedia

GW-SOURCES – Gallery

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Gravitational Wave Physics and Astronomy Workshop (GWPAW) 2022, December 5-9, Melbourne

Registration and Abstract submissions are now open. Gravitational Wave Physics and Astronomy Workshop (GWPAW) 2022. The conference will be an in-person event taking place in Melbourne Australia, 5-9 December 2022. Hybrid participation will be possible for those who are unable to attend in person. Further details including registration and the exact conference venue will be finalised shortly.

Detailed information can be found on the GWPAW2022 website , and will be regularly updated with event details:

www.gwpaw2022.org

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The Laws of Nature

Confronting general relativity with high-precision measurements of strong gravity

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.

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Ultra-compact binaries in the Milky Way

Capturing the sound of millions of tight binaries with 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.

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Extreme Mass Ratio Inspirals

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.

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Astrophysical Black Holes

Exploring the early cosmic dawn

Probing cosmic dawn and high noon

Galaxy – Merging NGC 6240

Two galaxies on collision course.

LISA observations will probe massive black holes over a very wide range of redshift, covering essentially all important epochs in their evolutionary history. LISA will offer a unique, new way to probe both cosmic dawn and high noon, to address a num­ber of unanswered questions:

  • When did the first Black holes form in pre-galactic haloes? What is their initial mass and spin?
  • What is the mechanism of black hole formation in galactic nuclei? How do black holes evolve over cosmic time due to accretion and mergers?
  • What can we learn about galaxy hierarchical assembly?

To answer these questions LISA will discover the first black hole seeds out to redshifts of order 20, in the cosmic dark ages before reionisation, and determine their masses and spins, using gravity alone.

LISA will also study the evolution of massive black holes by tracking their merger history during cosmic dawn and high noon. To this end, it is important to precisely measure their mass, spin and redshift over a wide, yet unexplored range.

Intermediate to massive black holes with masses in the interval between 104 M⊙ and 107 M⊙ will be detected by LISA, to explore for the first time the low-mass end of the massive Black hole population, at cosmic times as early as z ~ 10.

The Gravitational Universe will make it possible to survey the vast majority of all coalescing massive black hole binaries throughout the whole universe. This will expose an unseen population of objects which will potentially carry precious information about the black hole population as a whole. It will provide both the widest and deepest survey of the sky ever, since gravitational wave detectors are non-directional in nature, and operate as non-pointed and weakly directional full-sky monitors. The range of black hole redshifts and masses that will be explored is complementary to the space explored by electromagnetic observations.

LISA will detect all binary black hole mergers even when the black holes are not active. With this unbiased and complete survey, it will be possible to investigate the link between the growing seed population with the rich population of active supermassive black holes evolving during cosmic dawn and high noon, probing the light end of the mass function at the largest redshifts.

Black hole coalescence events will illuminate the physical processes of black hole formation and feeding. While the mass distribution carries information about the seeds, the spin distribution charts the properties of the accretion flows, whether they are chaotic or coherent. Gravitational wave observations alone will be able to distinguish between the different massive Black hole formation and evolution scenarios.

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External Group “AEI Hannover”

Leadership: Karsten Danzmann
Location: Germany
Institution: Max Planck Institute for Gravitational Physics, Hannover

Areas Of Works / Expertise:
Interferometry, Phasemeter, Optics, Data Analysis, TDI, LISA Science, LISA performance, LISA Pathfinder, System architecture

About

The LISA group at AEI Hannover commits to perform the lead role and overall coordination of the Consortium. This includes coordinating the development of the Consortium Provided Items and the engineering activities associated with the design of the MOSA.

contact
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Henry Wegener (2022) “Analysis of Tilt-to-Length Coupling in the GRACE Follow-On Laser Ranging Interferometer”

Institute: Albert Einstein Institute & University of Hannover

DOI
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Michael A. Keim, Valeriya Korol, Elena M. Rossi (2022) “The Large Magellanic Cloud Revealed in Gravitational Waves with LISA”

Future exploration of the Large Magellanic Cloud (LMC) with LISA will uncover a rich population of binary systems emitting detectable gravitational waves, offering valuable insights into the LMC’s properties and giving a more complete understanding of the Milky Way’s largest satellite galaxy.
Zenodo: https://zenodo.org/record/6918083

DOI