DEX: A European Vision for Low-Frequency Radio Cosmology from the Lunar Far Side

DEX: A European Vision for Low-Frequency Radio Cosmology from the Lunar Far Side. J. LazendicGalloway1, C.D. Brinkerink2, M.J. Arts3,1, M.J. Bentum1, A.J. Boonstra3, B. Cecconi4, A. Fialkov5,6, J. Garcia Gutiérrez7, S. Ghosh8, J. Grenouilleau9, L.I. Gurvits10,11, M. Klein-Wolt2, L.V.E. Koopmans8, Z. Paragi10, D. Prinsloo1,3, R.T. Rajan11, E. Rouillé4, M. Ruiter3, J.A. Tauber12, H.K. Vedantham3, A. Vecchio2,4, C.J.C. Vertegaal1, J.C.F. Zandboer1, P. Zucca3, 1Eindhoven University of Technology, 2Radboud University, 3ASTRON, 4LIRA, Observatoire de Paris-PSL, 5Institute of Astronomy, University of Cambridge, 6Kavli Institute for Cosmology, Cambridge, 7ESA/ESTEC/TEC-SYE, 8University of Groningen, 9ESA/ESTEC/HRE-E, 10Joint Institute for VLBI ERIC, 11Delft University of Technology, 12Leiden Observatory, Leiden University Introduction: The far side of the Moon offers the most pristine electromagnetic environment in the inner Solar System. Shielded from terrestrial RFI and free from ionospheric distortions, it provides the only viable location for observing the Universe at ultra-long wavelengths (<30 MHz). The Dark Ages Explorer (DEX) is a European-led initiative to harness this unique lunar environment to explore one of the last uncharted epochs of cosmic history—the Dark Ages and the Cosmic Dawn—through ultralow frequency radio observations. DEX is designed to detect the highly redshifted 21-cm line emission from neutral hydrogen in the intergalactic medium, providing the first direct window into the evolution of structure before and during the formation of the first stars, galaxies, and black holes. This cosmological signal, spanning redshifts z ≈ 10–100, contains information about the thermal and ionisation history of the Universe, the physics of dark matter, and the nature of primordial density fluctuations. As ground-based observatories are limited to redshifts z ≲ 25 due to ionospheric absorption and RFI, a lunar-based platform is essential to accessing the full 21-cm cosmology frontier. This presentation outlines the DEX concept, developed through a European Space Agency Concurrent Design Facility (CDF) study, which evaluated the scientific potential, mission architecture, and technological feasibility of deploying a filled-aperture, low-frequency radio interferometer on the lunar far side. The study leveraged realistic constraints based on ESA’s Argonaut lunar lander and explored a modular, scalable deployment path toward a scientifically transformative lunar observatory. DEX is envisioned to operate in two complementary modes. First, it will target the global signal—the sky-averaged brightness temperature of the 21-cm line—which contains imprints of the first radiative processes in the Universe. Second, it aims to measure the angular power spectrum of spatial fluctuations in the 21-cm brightness temperature, revealing the structure of the cosmic web at early times and constraining fundamental cosmological parameters. A scalable array architecture enables progressive enhancement of sensitivity and redshift reach. The lunar far side is not just a passive platform for DEX—it is a critical enabler. In addition to providing a radio-quiet zone, the Moon’s slow rotation (~28 Earth days) allows long integration times with stable sky coverage, ideal for deep radio observations. The mid-latitude landing sites considered, such as inside Tsiolkovsky crater, strike a balance between minimised Earthshine diffraction, optimal sky visibility, and terrain smoothness suitable for array deployment. Beyond its primary cosmology goals, DEX supports a wide suite of secondary science cases, showcasing the scientific return of a lunar radio observatory. These include: Heliophysics: DEX will monitor solar radio bursts (e.g., Type II and III) and track coronal mass ejections, providing critical information about solar energetic particles and shock dynamics in the heliosphere, free from Earth’s interference. Planetary science: The array will detect cyclotron maser emissions from planetary magnetospheres (especially Jupiter and Saturn), offering insights into magnetospheric dynamics and enabling long-term remote sensing not possible with flyby missions alone. Exoplanets: DEX has the potential to detect coherent radio emission from magnetised exoplanets, revealing magnetic field strengths and rotation rates, and possibly enabling the discovery of exomoons via star-planet interaction signatures. Radio transients: Ultra-long wavelength observations of Fast Radio Bursts (FRBs) and other coherent astrophysical transients could push our understanding of these enigmatic phenomena to new redshift regimes, offering a tool for probing the intergalactic medium and fundamental physics. In summary, the DEX concept offers a scalable and effective path toward transforming the lunar far side into a cornerstone of observational cosmology. By harnessing the unique lunar environment, DEX not only opens a new observational window on the Universe but also acts as a technology pathfinder for sustained, science-driven infrastructure on the Moon.