Lunar radio telescopes like TSUKUYOMI could detect faint 21-cm hydrogen signals from cosmic Dark Ages, revealing dark matter properties and early structure formation.
University of Tsukuba researchers simulated 21-centimeter radio emissions from neutral hydrogen during the universe’s Dark Ages (400 million to 100 million years post-Big Bang), revealing these faint signals carry signatures of dark matter properties. Global signal brightness temperatures reach ~1 millikelvin with similar-magnitude variations induced by dark matter clumping. Japan’s TSUKUYOMI lunar telescope project targeting 1–50 MHz frequencies aims to detect these signals by 2030, potentially unlocking dark matter’s mass and velocity.
The Curious Physics of the Cosmic Dark Ages
The cosmic Dark Ages spanned 100 million years following recombination at z~1100 when protons and electrons combined into neutral hydrogen, plunging the universe into lightlessness until first stars ignited at z~20–30. During this epoch, collisional coupling between hydrogen atoms and surrounding particles drove the 21-cm hyperfine transition signal, producing faint absorption against the cosmic microwave background with predicted brightness temperatures Tb~−40 mK at z~75. This signal encodes pure structure formation physics uncontaminated by astrophysical processes like star formation or reionization, probing ~10^12 Fourier modes compared to ~10^6 from CMB—offering unprecedented constraints on primordial density fluctuations, inflationary non-Gaussianity (fNL), and dark matter properties across scales 0.1–10 Mpc inaccessible to other probes.
What Happens During Dark Matter-Hydrogen Interactions
Park and colleagues’ Nature Astronomy simulations model cold dark matter (CDM) versus warm dark matter (WDM) scenarios, demonstrating that subgalactic-scale dark matter clumping modulates the 21-cm brightness temperature field through gravitational influence on gas density and velocity distributions. CDM models with particle masses ≥10 GeV produce hierarchical clumping down to Earth-mass halos, whereas WDM with keV-scale masses suppresses small-scale structure through free-streaming, creating distinguishable signatures in the 21-cm power spectrum and bispectrum. The simulations achieved ~1 mK global signal amplitude with dark-matter-induced variations of comparable magnitude, implying that broad-band observations spanning ~45 MHz frequency ranges could constrain dark matter particle masses and velocities by measuring spatial fluctuations in the Dark Ages signal. Alternative exotic models—including millicharged dark matter interacting electromagnetically with baryons or dark matter decay producing excess radio backgrounds—produce characteristic deviations reaching ≥4.7σ detection significance with 1,000-hour integrations.
Why It Matters for Dark Matter Detection
Detecting the Dark Ages 21-cm signal circumvents limitations plaguing terrestrial dark matter searches: underground detectors like LUX-ZEPLIN probe only WIMPs with specific interaction cross-sections, while collider experiments constrain particle physics models indirectly. The 21-cm approach probes dark matter through its gravitational effects on primordial hydrogen, model-independently testing CDM versus alternatives (WDM, self-interacting, ultra-light axion-like) across redshifts z~30–150 when structure formation initiated. Lunar observations avoid Earth’s ionosphere (opaque below 10 MHz) and radio-frequency interference plaguing ground-based experiments, enabling clean measurements of the global signal’s spectral shape that encodes dark matter free-streaming scales, acoustic oscillations, and relative baryon-dark-matter velocities imprinted at recombination.
Observational Challenges in 21-cm Detection
Extracting millikelvin Dark Ages signals requires suppressing foreground contamination exceeding the cosmological signal by 10^4–10^5 factors, including Galactic synchrotron emission (~10^4 K at 50 MHz), extragalactic radio sources, and CMB itself (~2.7 K). Advanced statistical filtering techniques developed for the Murchison Widefield Array isolate probable hydrogen emission by modeling foreground smooth-spectrum components, though ground-based efforts remain limited to z<20 due to ionospheric absorption. Lunar farside deployment eliminates terrestrial interference and ionospheric distortion while providing stable thermal environments and radio-quiet zones during lunar night (14 Earth days), crucial for detecting faint cosmological signals. Antenna mutual coupling, beam calibration uncertainties, and galactic foreground residuals after removal impose scale cuts limiting recoverable Fourier modes, though Fisher forecasts suggest lunar arrays could still measure primordial non-Gaussianity at fNL~10^-2, competitive with Planck CMB constraints.
Link to Japan’s TSUKUYOMI Lunar Program
TSUKUYOMI (named for the Shinto moon deity) plans staged deployment beginning with single-antenna prototypes in 2027–2028, expanding to 10+ antennas by the 2030s under Japan’s Artemis program participation. The baseline design employs 5-meter dipole antennas 7 meters above lunar regolith, targeting 1–50 MHz (z~27–280 for 21-cm), with spectrometers achieving 62.5 kHz resolution using Xilinx RFSoC digital back-ends derived from JUICE spacecraft heritage. Projected 8,000-hour integration times yield 5.5–7.7 mK sensitivities at 15 MHz, sufficient for 5.9σ detection of standard Dark Ages signals and distinguishing exotic dark matter models at ≥2.2σ. Complementary international efforts include NASA’s FARSIDE (128 antennas, 1–40 MHz) and LuSEE-Night (single 6-meter dipole launching 2026), collectively establishing multi-facility verification pathways for cosmological detections.
What the Future Holds for Lunar Radio Astronomy
Expanding from initial single-antenna deployments to distributed arrays with 100,000+ elements spanning 100 km^2 would achieve sample-variance-limited observations probing ~10^10 independent Fourier modes of the Dark Ages 21-cm field. Such arrays enable tomographic mapping of structure formation across z~30–150, measuring the evolution of dark matter clustering, baryon-dark matter relative velocities (imprinted at decoupling), and primordial perturbations on scales where inflation predictions diverge from standard ΛCDM. Future missions incorporating on-board calibration sources, interferometric baselines for angular resolution, and machine-learning foreground subtraction will refine systematic error budgets below current ~1 mK levels. Coordinated observations across TSUKUYOMI, FARSIDE, and CosmoCube (lunar-orbiting CubeSat at z~13–150) will provide independent confirmations and extended frequency coverage mapping reionization’s onset alongside Dark Ages physics.
Why This Discovery Is So Exciting for Cosmology
Simulating achievable Dark Ages 21-cm signals demonstrates lunar telescopes’ capability to conclusively test dark matter nature—potentially resolving decades-old debates between CDM, WDM, and exotic alternatives through direct observation rather than indirect inference. The ~1 mK signal amplitude with comparable dark-matter-induced fluctuations implies realistic detection prospects within the 2030s, transforming dark matter searches from underground laboratories to cosmic archaeology. Successfully measuring the global signal across ~45 MHz bandwidths would constrain dark matter particle masses, free-streaming scales, and self-interaction cross-sections independent of astrophysical uncertainties dominating Cosmic Dawn (z~20–6) interpretations. This represents humanity’s first direct observational access to the universe’s first 100 million years—an epoch encoding cosmology’s fundamental parameters free from the complex astrophysics complicating later cosmic epochs.
Conclusion
Japan’s TSUKUYOMI and complementary lunar radio programs promise revolutionary dark matter constraints by detecting faint 21-centimeter hydrogen signals from the cosmic Dark Ages, an epoch inaccessible to ground-based telescopes. Simulations demonstrating millikelvin signal strengths with dark-matter-induced variations validate detection feasibility, positioning 2030s lunar deployments to finally reveal whether dark matter is cold, warm, or requires exotic physics beyond standard models. Explore more about astronomy and space discoveries on our YouTube channel, So Join NSN Today.
