eLISA: Astrophysics and cosmology in the millihertz regime

TL;DR

eLISA opens a new observational window by detecting millihertz gravitational waves, enabling robust studies of ultra-compact binaries, massive black hole growth, and extreme-mass-ratio inspirals. The paper outlines mission design, sensitivity, and waveform models, and demonstrates how GW observations will test general relativity in the strong-field regime while constraining black hole demographics and cosmic history. It also discusses prospects for cosmology via standard sirens and potential new physics from early-Universe backgrounds, providing a framework to discriminate between competing formation and evolution scenarios. Collectively, eLISA promises transformative insights into galaxy formation, black hole evolution, stellar dynamics in galactic nuclei, and fundamental physics.

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.

Figures (27)

• Figure 1: The eLISA orbits: The constellation is shown trailing the Earth by about 20 degrees (or $5 \times 10^{10} \textrm{km}$) and is inclined by 60 degrees with respect to the ecliptic. The trailing angle will vary over the course of the mission duration from 10 degrees to 25 degrees. The separation between the spacecraft is $L = 1 \times 10^9$ m.
• Figure 2: The constellation of the three eLISA spacecraft constitutes the science instrument. The central spacecraft harbors two send/receive laser ranging terminals, while the end spacecraft has one each. The laser in the end spacecraft is phase-locked to the incoming laser light. The blue dots indicate where interferometric measurements are taken. The sketch leaves out the test mass interferometers for clarity.
• Figure 3: Partition of the eLISA measurement. Each measurement between two test masses is broken up into three different measurements: two between the respective test mass and the spacecraft and one between the two spacecraft (S/C). As the noise in the measurement is dominated by the shot noise in the S/C-S/C measurement, the noise penalty for the partitioning of the measurement is negligible. The blue (solid) dots indicate where the interferometric measurements are taken.
• Figure 4: Sensitivity of eLISA (averaged over all sky locations and polarisations) versus frequency: the solid red curve is obtained numerically using the simulator LISACode 2.0 petiteau:2008PhRvD..77b3002P and the dashed blue curve is the analytic approximation based on equation \ref{['eq.sens']}. For a reference, we also depict the sensitivity curve of LISA (dotted, green curve).
• Figure 5: Artist impression of a detached double white dwarf binary (left) and an interacting binary in which a neutron star accretes material from a white dwarf donor. The Earth is shown to set the scale. Courtesy BinSim by Rob Hynes.