Low-frequency gravitational-wave science with eLISA/NGO

TL;DR

This study assesses the science performance of eLISA/NGO, a cost‑constrained space-based gravitational-wave observatory designed to operate in the 0.1 mHz–1 Hz band. It forecasts a rich science yield across MBH coalescences, EMRIs, and Galactic compact binaries, with detailed sensitivity modeling, population forecasts, and parameter-estimation capabilities. The work demonstrates eLISA’s potential to test general relativity in the strong-field regime, map MBH growth across cosmic history, and probe early‑Universe physics via stochastic backgrounds and standard sirens. The findings highlight eLISA’s capacity to address key questions in astrophysics, gravitation, and cosmology, even within a restricted mission design, and underscore the value of multi‑messenger follow‑ups with electromagnetic surveys.

Abstract

We review the expected science performance of the New Gravitational-Wave Observatory (NGO, a.k.a. eLISA), a mission under study by the European Space Agency for launch in the early 2020s. eLISA will survey the low-frequency gravitational-wave sky (from 0.1 mHz to 1 Hz), detecting and characterizing a broad variety of systems and events throughout the Universe, including the coalescences of massive black holes brought together by galaxy mergers; the inspirals of stellar-mass black holes and compact stars into central galactic black holes; several millions of ultracompact binaries, both detached and mass transferring, in the Galaxy; and possibly unforeseen sources such as the relic gravitational-wave radiation from the early Universe. eLISA's high signal-to-noise measurements will provide new insight into the structure and history of the Universe, and they will test general relativity in its strong-field dynamical regime.

Figures (8)

• Figure 1: eLISA equivalent-strain noise, averaged over source sky location and polarization, as a function of frequency. The solid red curve was obtained with the LISACode 2.0 simulator petiteau:2008PhRvD..77b3002P, while the dashed blue curve is plotted from Eq. (\ref{['eq:sens']}). For comparison, the dotted green curve shows the LISA sensitivity.
• Figure 2: Main figure: power spectral density of the stochastic GW foreground from Galactic binaries, before (blue) and after (red) the subtraction of individually resolvable systems, which are plotted as green and red/blue dots (for detached and mass-transferring systems). A few known verification binaries are shown as white dots. The solid/dashed black curves trace instrument noise alone/with confusion noise. Spectra are shown for the observable "$X$" of Time Delay Interferometry (see, e.g., PhysRevD.72.042003); subtraction is simulated for a two-year observation and threshold $\mathrm{SNR} = 7$; resolvable systems are placed a factor $\mathrm{SNR}^2$ above the combined instrument and confusion noise. Inset: time series of the residual foreground, which carries information about the number and distribution of binaries in the Galaxy.
• Figure 3: Left: constant-level contours of sky- and polarization-averaged SNR for equal-mass non-spinning binaries as a function of total rest mass $M_\mathrm{tot}$ and cosmological redshift $z$. The SNR includes inspiral, merger and ringdown. Right: SNR contours as a function of $M_\mathrm{tot}$ and mass ratio $q = m_1/m_2$.
• Figure 4: Left: distribution of expected SNR for MBH mergers as a function of $z$, computed from the SE/LE/SC/LC metacatalog (see main text). Right: likelihood for the mixing fraction $\mathcal{F}$, for an individual realization of mixed model $\mathcal{F}\, \mathbf{SE}+(1-\mathcal{F})\mathbf{LE}$ with $\mathcal{F}=0.45$ (see main text).
• Figure 5: Parameter-estimation accuracy (relative frequency of fractional or ab-so-lute errors over SE/LE/SC/LC metacatalog) for primary and secondary redshifted MBH masses and dimensionless spins ($m_1$ and $m_2$, $a_1/m_1$ and $a_2/m_2$, respectively), luminosity distance $D_L$ and sky position $\Delta\Omega$.