Low-frequency gravitational-wave science with eLISA/NGO (1202.0839v2)

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.

• The paper demonstrates eLISA's capability by achieving strain sensitivities near 3.6×10⁻²⁴ to detect diverse low-frequency gravitational-wave sources.
• The methodology evaluates compact binaries, massive black hole mergers, and extreme-mass-ratio inspirals to advance our understanding of stellar evolution and galactic structure.
• The paper highlights eLISA's unique potential to test general relativity and probe cosmological phenomena through precise gravitational-wave measurements.

Low-frequency Gravitational-wave Science with eLISA/NGO

The paper provides a comprehensive review of the scientific potential of the New Gravitational-wave Observatory, also known as eLISA or NGO. This planned mission by the European Space Agency is poised to open the low-frequency gravitational-wave (GW) window from 0.1 mHz to 1 Hz, offering unmatched insights into various astrophysical phenomena and testing general relativity in previously inaccessible regimes.

Mission Overview and Sensitivity

The eLISA mission builds on the heritage of LISA, a collaboration that was originally between NASA and ESA. The refined eLISA architecture features reduced arm lengths of 1 million kilometers and a streamlined payload design involving one primary and two secondary spacecraft. These adaptations aim to balance scientific capability with budgetary constraints, enabling the detection and characterization of an extensive range of GW sources.

eLISA's sensitivity is pivotal, offering noise levels allowing for the detection of strain amplitudes around $3.6 \times 10^{-24}$ at optimal frequency regions. Such sensitivity ensures the detection of a multitude of stellar and galactic phenomena, achieving signal-to-noise ratios (SNR) of 1 for minimal sources and higher for substantial events.

Scientific Potential and Expected Discoveries

eLISA's scientific objectives encompass a wide array of sources and events that are critical for advancing our understanding of the universe. Some of the key areas of focus include:

• Compact Binaries in the Galaxy: eLISA is expected to identify thousands of ultra-compact binaries and disentangle the GW foreground created by their unresolved aggregate. This presents a novel opportunity to paper Galactic structure and binary evolution intricacies, such as the common-envelope phase and mass transfer stability.
• Massive Black Hole Binaries: eLISA's scope encompasses massive black hole (MBH) mergers, straddling masses generally inaccessible to electromagnetic (EM) observation. The mission conditions are conducive to determining MBH masses and spins with unprecedented accuracy, providing data essential for constraining MBH formation and accretion models.
• Extreme-mass-ratio Inspirals (EMRIs): EMRIs will allow precise measurements of individual MBH masses and spins, elucidating the dynamics and population statistics of dense stellar environments. eLISA's measurements may reveal the mechanisms powering MBH growth and constrain theories of compact object interactions in galactic nuclei.

Testing General Relativity and Fundamental Physics

eLISA holds promise for testing general relativity in the extreme gravitational fields present during inspirals and coalescences of massive objects. The precision with which eLISA can measure the phase and amplitude of GW signals offers a unique platform for verifying the Kerr nature of astrophysical black holes and probing alternative theories of gravity at unparalleled precision. The predicted performance underscores eLISA's potential to impose stringent constraints on models of quantum gravity and cosmological phenomena beyond the standard model.

Cosmological Implications and New Physics

The detection of background GWs originating from the early universe, potentially arising from cosmic strings, pre-big-bang scenarios, or other high-energy transitions, represents one of eLISA's scientific triumphs. Moreover, MBH binaries as standard sirens can provide an independent measure of cosmological parameters, complementing redshift-distance measures from traditional EM astronomy.

Conclusion

In summary, the eLISA/NGO mission promises to forge new paths in gravitational-wave astronomy through detailed investigation of a spectrum of complex astrophysical processes. While acknowledging budget constraints, the paper illustrates the substantial scientific return of the mission, which will test fundamental physics and unlock new insights into the cosmic dawn. Researchers in the field should anticipate eLISA as a keystone endeavor, capable of extending our grasp on gravitational phenomena exponentially.