Unveiling the Gravitational Universe at μ-Hz Frequencies (1908.11391v1)

Abstract: We propose a space-based interferometer surveying the gravitational wave (GW) sky in the milli-Hz to $\mu$-Hz frequency range. By the 2040s', the $\mu$-Hz frequency band, bracketed in between the Laser Interferometer Space Antenna (LISA) and pulsar timing arrays, will constitute the largest gap in the coverage of the astrophysically relevant GW spectrum. Yet many outstanding questions related to astrophysics and cosmology are best answered by GW observations in this band. We show that a $\mu$-Hz GW detector will be a truly overarching observatory for the scientific community at large, greatly extending the potential of LISA. Conceived to detect massive black hole binaries from their early inspiral with high signal-to-noise ratio, and low-frequency stellar binaries in the Galaxy, this instrument will be a cornerstone for multimessenger astronomy from the solar neighbourhood to the high-redshift Universe.

• The paper introduces a space-based interferometer to analyze μ-Hz gravitational waves and address fundamental astrophysical and cosmological questions.
• It leverages advanced LISA-derived technology to detect massive black hole and stellar binary inspirals with high signal-to-noise ratios.
• The design enhances multimessenger astronomy by simultaneously studying Galactic, extragalactic sources, and stochastic gravitational wave backgrounds.

Space-Based Gravitational Wave Detection at $\upmu$-Hz Frequencies

The paper proposes the development of a space-based interferometer designed to survey gravitational waves (GWs) in the $\upmu$-Hz frequency range, bridging the gap in the GW spectrum between the Laser Interferometer Space Antenna (LISA) and pulsar timing arrays (PTAs). This frequency band, set to become scientifically central by the 2040s, holds promise for answering fundamental questions in astrophysics and cosmology.

The intended $\upmu$-Hz detector could vastly extend LISA's capabilities, particularly in detecting massive black hole binaries (MBHBs) from early phases of inspiral, with high signal-to-noise ratio. It will also observe low-frequency stellar binaries within the Galaxy, enhancing multimessenger astronomy from local stellar neighborhoods to the distant Universe.

Implications and Observational Potential

The $\upmu$Ares, as proposed, demonstrates immense observational potential throughout various astronomical phenomena:

Galactic Sources:

• The ability to resolve approximately $10^5$ Galactic double white dwarfs (DWDs) down to $f \approx 10^{-4}$ Hz.
• Detection of approximately $100$ Galactic black hole binaries (BHBs) reaching frequencies $f \approx 10^{-5}$ Hz.
• Observations of mixed (CO + MS star), contact, and over-contact binaries within the previously inaccessible $10^{-5}$–$10^{-4}$ Hz range.
• Monitoring compact objects such as stellar mass black holes and brown dwarfs surrounding SgrA$^*$, potentially revealing their masses and dynamics.

Extragalactic Sources:

• Enhancements in detecting $10^3$ extragalactic BHBs and the evolution of MBHBs across cosmic history.
• Observations of extreme mass ratio inspirals (EMRIs) and intermediate mass ratio inspirals (IMRIs), offering insights into merging systems at higher redshifts.
• Investigation into bursts from massive direct seed formation events, and potential MBH interactions in lower-mass dwarf galaxies.

Stochastic Backgrounds:

• Analysis of confusion noise from DWDs over a broad frequency span.
• Detection and characterization of unresolved foregrounds from massive black hole binaries and the paper of cosmological stochastic gravitational wave backgrounds (SGWBs) with high sensitivity.

Scientific Context and Opportunities

The strategic focus on the $\upmu$-Hz band is motivated by several scientific opportunities:

Massive Black Holes and Early Universe:

• Understanding high-redshift quasar formation, probing black hole seeding models, and evaluating accretion processes.
• Investigating evolutionary mechanisms affecting MBH growth, validations of theoretical models on dark matter influences, and decay processes in nuclear astrophysics.

Multitelescopic and Multimodal Science:

• Improved synergy between GW and electromagnetic (EM) observations, leading to potentially richer multimessenger insights.
• Exploration of strong gravitational lensing effects on GW signals, providing cosmological constraints.
• Possible direct detection of specific strong-field phenomena due to exotic states like PBHs or dense stellar clusters.

Cosmological Surveys:

• Profiling the primordial Universe through the characterization of SGWB signals generated during early Universe phase transitions.
• Mapping high-z standard sirens, aiding in delineating the cosmic expansion history with improved precision from MBHB mergers.

Proposed Engineering Concept and Design

Achieving optimal performance in the $\upmu$-Hz band requires an innovative approach to spacecraft design and mission architecture:

• A heliocentric spacecraft arrangement, anchored by Martian orbital dynamics, potentially formed by two physicall non-coplanar triangular constellations, maximizing detection capabilities and source localization efficiency.
• Incorporation of technological advancements from LISA heritage in laser and telescope systems, enabling accentuated GW sensing with 10-watt lasers and 1-meter telescopes.

In conclusion, this transformative venture into the $\upmu$-Hz GW domain promises not only profound scientific revelations across astronomy and cosmology but also methodological advancements in space-based interferometry, ushering in an era of expansive astrophysical exploration.