eLISA: Astrophysics and cosmology in the millihertz regime

Abstract: This document introduces the exciting and fundamentally new science and astronomy that the European New Gravitational Wave Observatory (NGO) mission (derived from the previous LISA proposal) will deliver. The mission (which we will refer to by its informal name "eLISA") will survey for the first time the low-frequency gravitational wave band (about 0.1 mHz to 1 Hz), with sufficient sensitivity to detect interesting individual astrophysical sources out to z = 15. The eLISA mission will discover and study a variety of cosmic events and systems with high sensitivity: coalescences of massive black holes binaries, brought together by galaxy mergers; mergers of earlier, less-massive black holes during the epoch of hierarchical galaxy and black-hole growth; stellar-mass black holes and compact stars in orbits just skimming the horizons of massive black holes in galactic nuclei of the present era; extremely compact white dwarf binaries in our Galaxy, a rich source of information about binary evolution and about future Type Ia supernovae; and possibly most interesting of all, the uncertain and unpredicted sources, for example relics of inflation and of the symmetry-breaking epoch directly after the Big Bang. eLISA's measurements will allow detailed studies of these signals with high signal-to-noise ratio, addressing most of the key scientific questions raised by ESA's Cosmic Vision programme in the areas of astrophysics and cosmology. They will also provide stringent tests of general relativity in the strong-field dynamical regime, which cannot be probed in any other way. This document not only describes the science but also gives an overview on the mission design and orbits.

• The paper demonstrates eLISA's enhanced ability to detect gravitational waves in the millihertz range, revealing critical insights into massive black hole and compact binary dynamics.
• It details the innovative triangular three-spacecraft interferometer design that significantly improves sensitivity to low-frequency gravitational signals.
• The findings imply that eLISA will transform our understanding of stellar interactions, Galactic structure, and cosmological phenomena, while testing fundamental theories like General Relativity.

Overview of "Doing science with eLISA: Astrophysics and cosmology in the millihertz regime"

The paper delineates the scope of scientific contributions anticipated from the eLISA mission, detailing its proficiency in observing and deciphering gravitational waves (GW) across a frequency range of approximately 0.1 mHz to 1 Hz. Notably, eLISA has been framed as a successor to the LISA proposal, enhancing observational capabilities in uncharted low-frequency bands. This mission is poised to illuminate astrophysical phenomena, from massive black hole (MBH) mergers to stellar dynamics, with implications extending into testing General Relativity (GR) and peering into early cosmic events.

Scientific Goals and Observational Capabilities

Compact Binaries and Stellar Dynamics

1. Detection of Galactic Binaries: eLISA's ability to discern thousands of ultra-compact stellar-mass binaries represents a paradigm shift, offering direct insights into binary evolution post-common envelope phase and the progenitors of Type Ia supernovae. Its detection prowess hints at potentially aggregating comprehensive data on the spatial distribution of binaries within the Milky Way, significantly enhancing our understanding of tidal interactions and mass transfer stability in such compact systems.
2. Understanding the Galactic Foreground: eLISA will also focus on mapping the foreground created by millions of unresolved compact binaries, contributing valuable information about the distribution and history of stars and revealing insights into Galactic structure.

Supermassive and Intermediate-Mass Black Holes

1. Tracing MBH Growth: The mission is set to examine MBH coalescences at high redshifts, enabling exploration of their evolution during formative cosmic epochs. This will unveil the demographics of MBH mergers, enriching our comprehension of early Universe structures and dynamics within halos.
2. Black Hole Astrophysics: By detecting gravitational waves from MBH binaries, eLISA will validate key models of MBH formation and the concurrent growth of galaxies, uncovering the dynamics of black holes as they merge.

Extreme Mass Ratio Inspirals (EMRIs)

eLISA will be keenly attuned to EMRIs, where a stellar-mass object spirals into an MBH, providing a distinctive opportunity to test GR in strong-field regimes. These events will furnish high-fidelity mappings of black hole spacetime, effectively testing the no-hair theorem and potentially confirming the presence of Kerr black holes in galaxy nuclei.

Cosmology and New Physics

1. Gravitational Wave Backgrounds: The mission stands to either detect or impose stringent constraints on GW backgrounds resulting from cosmological phenomena such as primordial phase transitions and cosmic strings. The GW backgrounds originating from the early Universe will provide unique testing grounds for high-energy physics models beyond the Standard Model.
2. Testing Relativistic Theories: Through comprehensive scrutiny of merger-ringdown signals from MBH coalescences, eLISA will conduct unprecedented tests of GR, offering new insights or indicating potential deviations suggestive of new physics.

Mission Design and Technological Considerations

The eLISA mission employs innovative technology stemming from LISA's design, encompassing three spacecraft operating in a triangular configuration to form a precise laser interferometer. This design ensures the stability and accuracy essential for detecting the minuscule spacetime perturbations caused by gravitational waves. The mission emphasizes advancements in displacement sensitivity, crucial for unlocking the signal-rich low-frequency GW spectrum inaccessible to ground-based observatories.

Implications and Future Work

The capabilities of eLISA mark an evolutionary leap in gravitational wave astronomy, promising to shed light on several unresolved astronomical questions. By probing the Universe through a novel spectral window, eLISA is poised to yield transformative insights into the mysteries of MBH dynamics, cosmic history, and the foundational laws of physics. Future explorations and technology integration could enhance eLISA's detection aptitude, broadening the horizons of astrophysical discovery. Additionally, results from eLISA will likely influence satellite and ground-based facility designs, further integrating multimessenger astronomical practices.