Cosmic Explorer Observatory

Updated 29 July 2025

Cosmic Explorer Observatory is a next-generation gravitational-wave detector concept featuring two U.S. interferometers with 40 km and 20 km arms designed to significantly boost sensitivity.
It leverages advanced techniques like dual-recycled Fabry–Perot Michelson configurations and frequency-dependent squeezing to reduce quantum and thermal noise.
The observatory aims to propel multi-messenger astronomy and precision cosmology by enhancing event localization and statistical measurements of cosmic events.

Cosmic Explorer (CE) is a next-generation ground-based gravitational-wave observatory concept designed to extend the reach and scientific output of the current LIGO-Virgo-KAGRA network by an order of magnitude. Centered on a network of two U.S. facilities with 40 km and 20 km arms, and leveraging advanced interferometric technology, CE targets improved sensitivity over a broad frequency band (down to 5 Hz), enhanced noise suppression, and the capability to access gravitational-wave sources across cosmic time. By scaling up proven dual-recycled Fabry–Perot Michelson interferometer designs and incorporating deep technological and social considerations into its development, CE seeks to become central to global third-generation detector networks probing fundamental physics, cosmology, and multimessenger astrophysics.

1. Design Principles and Architecture

Cosmic Explorer is conceived as a network of two widely separated L-shaped interferometers located in the United States: one with 40 km arms and the other with 20 km arms (Daniel et al., 1 Oct 2024, Reitze et al., 2019). The arm lengths and their near-orthogonal configuration are fundamental to maximizing the detector's sensitivity, which scales as $(L\,\sin\theta)^\alpha$ with $\alpha\sim1.4-2.5$ , where $L$ is arm length and $\theta$ the opening angle (ideally 90°) (Daniel et al., 1 Oct 2024). Any deviations from the canonical geometry—such as length reduction by more than 2 km or opening angles outside the 65°–115° range—cause more than a 10% loss in sensitivity.

Key design features include:

Dual-recycled Fabry–Perot Michelson topology with 40 km (and 20 km) vacuum arms.
Room-temperature and cryogenic test-mass operation prospects: CE1 utilizes room-temperature, fused silica optics; CE2 aims for cryogenic silicon optics with 2 μm lasers (Reitze et al., 2019).
Frequency-dependent squeezing implemented via 4 km filter cavities to suppress quantum noise (Hall et al., 2020).
Massive test masses (up to 320 kg), advanced quadruple pendulum suspensions, and high circulating laser powers (1.5–3 MW) (Evans et al., 2023, Reitze et al., 2019).
Target strain sensitivity better than $10^{-23}/\sqrt{\mathrm{Hz}}$ down to 5 Hz, a roughly 10-fold improvement over Advanced LIGO (Hall et al., 2020, Evans et al., 2023).

Staged development is planned, beginning with CE1 (scaled LIGO-A+ technology) and advancing to CE2 (with further reductions in thermal and quantum noise, enabled by cryogenics and new optical substrates) (Reitze et al., 2019, Hall et al., 2020).

2. Sensitivity, Noise Sources, and Technological Advances

CE's unprecedented sensitivity across a broad frequency range is based on advances in multiple domains (Hall et al., 2020, Evans et al., 2023, Srivastava et al., 2022):

Seismic and Gravity Gradient Noise: Advanced seismic isolation (tenfold to hundredfold improvement at 1 Hz compared to current sites), multi-dimensional inertial sensing, and Newtonian noise suppression via seismic arrays and possible ground recesses (Hall et al., 2020).
Thermal Noise: Extended pendulum suspensions (4 m, up to 1500 kg), monolithic fused silica or silicon fibers/ribbons, and cryogenic silicon optics are employed to control Brownian, thermoelastic, and coating noise. For silicon optics, temperature must be controlled near the zero-crossing of the thermal expansion coefficient ( $|\alpha|\leq 4\times10^{-8}$ K $^{-1}$ ) (Hall et al., 2020).
Quantum Noise: Quantum radiation pressure dominates at low frequencies; shot noise at high frequencies. Frequency-dependent squeezing (4 km filter cavity, 6–10 dB improvement) is critical (Hall et al., 2020, Srivastava et al., 2022).
Residual Gas and Scattered-Light Noise: Vacuum quality is maintained with pressures limiting gas-induced phase and force noise. Mirror and beam tube design control scattered light to factors below the total noise budget (Hall et al., 2020).
Servo-Control Noise: Control systems are designed for suppression below $6\times10^{-24}/\sqrt{\mathrm{Hz}}$ with loop bandwidths of a few Hz.

Technological innovation focuses on scalable suspensions, improved sensors, and advanced optical and cryogenic engineering (Hall et al., 2020). Civil engineering is nontrivial: a 40 km arm on flat ground requires addressing up to 30 m in elevation differential due to Earth's curvature and could necessitate the displacement of tens of millions of cubic meters of earth if not sited on suitable terrain (Daniel et al., 1 Oct 2024).

3. Tunability, Configurations, and Network Synergy

Cosmic Explorer introduces tunable sensitivity leveraging signal extraction cavity (SEC) modifications (Srivastava et al., 2022):

Tuning is accomplished via the SEC macroscopic length $L_s$ and signal extraction mirror transmissivity $T_s$ , with the resonance frequency $f_s = c/(4\pi L_s)$ and bandwidth $\gamma_s \propto cT_s/(4L_s)$ .
High-frequency tuning (narrow sensitivity “notch” above 2 kHz) targets post-merger neutron star oscillations; low-frequency tuning (<500 Hz) favors inspiral, tidal, and ringdown measurability in neutron star and black hole mergers.
40 km detectors offer optimal broadband and low-frequency performance, while 20 km arms are advantageous for post-merger physics due to their higher free spectral range ( $f_\mathrm{FSR}\approx3.7$ kHz vs. $7.5$ kHz) (Srivastava et al., 2022, Giovanni, 16 May 2025).

CE's role within networks is crucial:

Combined operation with the Einstein Telescope (ET) and other next-generation observatories achieves angular resolutions down to sub-degree square for high-SNR binaries, critical for electromagnetic follow-up (Gupta et al., 2023).
Global geographic distribution (including proposed Southern Hemisphere detectors) maximizes duty cycle and localization precision for multi-messenger astronomy (Gardner et al., 2023).

4. Science Goals and Astrophysical Capabilities

CE is designed to address profound astrophysical and cosmological questions:

High-redshift reach ( $z\sim20$ –100 for black hole mergers) enables paper of the first stars (Population III) and primordial black holes (Gupta et al., 2023, Evans et al., 2023).
Increased event rates (potentially tens of thousands per year) provide unprecedented statistical power for population synthesis, merger distributions, and rates (Reitze et al., 2019, Evans et al., 2023).
BNS and BBH signals observed with SNR an order of magnitude higher than current detectors enable precise measurement of neutron star EoS (down to $\sim0.6\%$ uncertainty on $R_{1.4}$ with one year of data) (Finstad et al., 2022).
Sensitivity to dual-line gravitational-wave sources, e.g., double neutron star systems with detectable spin and orbital GW signatures, enables $\sim8\%$ relative accuracy in neutron star moment of inertia inference (Feng et al., 9 May 2025).
Enhanced sensitivity to stochastic backgrounds, including those from cosmological phase transitions, by probing high-frequency GW tails with sensitivity improvements of $10^{3}$ – $10^{4}$ over current-generation detectors in stochastic regime (Axen et al., 2018).

Template bank construction and search strategies for binary neutron star coalescences are robust: circular template banks are effective for eccentricities $e_7\leq0.004$ (CE1) and $e_7\leq0.003$ (CE2), with stochastic template placement required for higher-eccentricity signals (up to $e_7\approx0.05$ ) (Lenon et al., 2021). Computational costs for these searches, even at Cosmic Explorer scale, remain within reach of current resources.

5. Broader Implications: Cosmology, Multi-messenger Science, and Sociotechnical Integration

CE, as part of an international network, enables transformative cosmological and multi-messenger observations:

Gravitational-wave standard sire distance measurements for $H_0$ and dark energy equation of state; cosmic dipole estimation through GW number counts with $16\%$ precision after $\sim10^6$ BBH detections (Mastrogiovanni et al., 2022).
Combined LISA+CE observations extend the detectable phase transition parameter space in early-Universe models (Axen et al., 2018).
Early alerts for compact object mergers improve sky localization (to sub-degree fields), facilitating efficient electromagnetic and neutrino counterpart searches (Gupta et al., 2023, Gardner et al., 2023).
Network configuration, including Southern Hemisphere facilities, is critical for maximizing uptime, coverage, and localization, particularly during maintenance or upgrades (Gardner et al., 2023).
Site selection integrates technical, economic, and social criteria, emphasizing strong Indigenous and community engagement, minimal environmental disruption, and cross-disciplinary workforce development (Daniel et al., 1 Oct 2024).

6. Implementation, Challenges, and Future Prospects

Realizing CE requires resolving significant scientific, technical, and social challenges:

Stringent site selection based on tilt, arm length, and local topography to control noise coupling and civil engineering complexity (Daniel et al., 1 Oct 2024).
Subsystem R&D for cryogenic operation, low-loss coatings, squeezed light, seismic and Newtonian noise reduction, and advanced suspension materials (Hall et al., 2020, Reitze et al., 2019).
Comprehensive engagement with local and Indigenous communities through frameworks such as Free, Prior, and Informed Consent and adherence to international declarations (UNDRIP, ADRIP) (Daniel et al., 1 Oct 2024).
Long-term participation in global third-generation networks (ET, LIGO-India, Southern Hemisphere facilities) for optimal science return, policy coordination, and technological sharing (Gupta et al., 2023, Giovanni, 16 May 2025).

In conclusion, Cosmic Explorer is poised to deliver an order-of-magnitude advance in gravitational-wave astronomy through optimized interferometer scale, tunable configurations, state-of-the-art noise mitigation, and integration with a global detector network. Its anticipated sensitivity will enable the detection of both routine and rare gravitational-wave sources, open new observational windows on the early Universe and dense matter physics, and underpin precision cosmology, while its implementation is predicated on holistic scientific, engineering, and societal stewardship.