Gravitational-Wave Lunar Observatory for Cosmology

Abstract: Several large-scale experimental facilities and space-missions are being suggested to probe the universe across the gravitational-wave (GW) spectrum. Here we propose Gravitational-wave Lunar Observatory for Cosmology (GLOC) - the first concept design in the NASA Artemis era for a GW observatory on the Moon. Using feasible interferometer technologies, we find that a lunar-based observatory is ideal for probing GW frequencies in the range between deci-Hz to 5 Hz, an astrophysically rich regime that is very challenging for both Earth- and space-based detectors. GLOC can survey binaries with neutron stars, stellar and intermediate-mass black holes to $\gtrsim 70\%$ of the observable volume of our universe without significant background contamination. The sensitivity at $\mathcal{O}(1~\mathrm{Hz})$ allows a unique window into calibrating Type Ia supernovae. At its ultimate sensitivity limits, GLOC would trace the Hubble expansion rate up to redshift $z \sim 3$ and test General Relativity and $\Lambda$CDM cosmology up to $z\sim 350$.

• The paper introduces the Gravitational-Wave Lunar Observatory for Cosmology (GLOC), a proposed observatory on the Moon designed to detect low-frequency gravitational waves (deci-Hz to 5 Hz).
• GLOC's unique lunar location and design will enable detection of gravitational waves in the deci-Hz to 5 Hz band, targeting sources like intermediate-mass black holes and binaries at high redshifts.
• This lunar observatory would complement ground- and space-based detectors, enhancing multiband observations and enabling precise calibration of Type Ia supernovae for cosmological measurements.

Gravitational-Wave Lunar Observatory for Cosmology (GLOC)

The exploration of the gravitational-wave (\textbf{GW}) spectrum has opened immense possibilities in the fields of fundamental physics, cosmology, and astronomy. Following the pioneering efforts of ground-based observatories such as LIGO and Virgo in the 10–1000 Hz frequency range, there is an imperative need to extend the sensitivity of GW detections to lower and previously underexplored frequencies. This concept paper introduces the Gravitational-Wave Lunar Observatory for Cosmology (\textbf{GLOC}), a project driven by the idea of establishing a GW observatory on the Moon to probe the deci-Hz to 5 Hz frequency band, which presents unique astrophysical opportunities that are inaccessible to current Earth- and planned space-based interferometers.

Conceptual Design of GLOC

The innovative proposal to position a GW detector on the Moon capitalizes on the unique environmental characteristics of the lunar surface. These include the negligible atmospheric interference and significantly lower seismic activity compared to Earth. Such an environment offers an unparalleled opportunity for achieving a low-frequency GW detection capability, essential for studying sources like neutron star and black hole binaries across vast cosmic distances. The key design feature of GLOC is an interferometer setup with an arm length of approximately 40 km, formed in a triangular configuration to maximize detection sensitivity and redundancy.

Scientific Prospects

1. Expanding the GW Frequency Range: GLOC aims to open the astrophysical window in the challenging deci-Hz to 5 Hz band, positioned between the capabilities of ground-based and space-based detectors. This range is particularly promising for the detection of intermediate-mass black holes (\textbf{IMBHs}), stellar-mass binaries, and primordial black holes, thereby enabling tests of General Relativity and cosmological models at unprecedented scales.

2. Astrophysical and Cosmological Investigations: The observatory is projected to detect gravitational signals from stellar and IMBH mergers from redshifts of $z \leq 70$, encompassing a significant portion of the observable universe. With GLOC's capabilities, researchers will be able to study the formation and evolution of massive black holes in the early universe and their potential connections to the seed black holes theorized to have formed in the aftermath of the Big Bang.

3. Multiband GW Astronomy: GLOC's complementary frequency coverage will allow for multiband observations, particularly with future Earth-based detectors like the Einstein Telescope and Cosmic Explorer, and planned space missions like LISA. Observations in multiple bands will enhance parameter estimation for GW sources, aid in sky localization, and further the understanding of the massive binary black hole formation channels.

4. Enhancements in Type Ia Supernovae Calibration: GLOC's sensitivity in the $\sim 1$ Hz region is crucial for calibrating Type Ia supernovae—a cornerstone of cosmological distance measurements—thus improving the precision of the Hubble constant and testing the $\Lambda$CDM cosmological model.

Implications and Future Directions

The construction of GLOC on the lunar surface presents technological and logistical challenges, demanding the development of highly sophisticated and durable instrumentation capable of withstanding the Moon's harsh environment. The strategic benefit of this endeavor is the long-term stability and operational capacity of a lunar-based observatory, devoid of atmospheric and human-induced noise perturbance.

By extending the observational horizon to a substantial fraction of the universe’s volume, GLOC promises to be instrumental in probing previously unreachable epochs of cosmic history. Its implementation will pave the way for novel investigations into the early universe, facilitating the direct measurement of phenomena that could refine foundational theories in physics.

In conclusion, the Gravitational-Wave Lunar Observatory for Cosmology represents a compelling and transformative vision for future GW astronomy. It is poised to augment our understanding of the universe on the largest scales imaginable, stimulating cross-disciplinary advancements in astrophysics and cosmology. As exploration and settlement of the Moon progress with initiatives like NASA's Artemis program, the potential for substantial scientific exploration accompanying human presence on the lunar surface becomes even more tangible.