🔭 astrophysics

Quantifying the Impact of Lunar and Planetary Occultation on Experimental Global 21 cm Cosmology

This paper demonstrates that lunar occultations of high-power galactic regions during the next major lunar standstill can introduce significant foreground mismatches that severely disrupt global 21 cm signal recovery, whereas smaller bodies like Venus have negligible impact, thereby identifying specific lunar alignment scenarios that must be avoided to preserve experimental fidelity.

Original authors: Joe H. N. Pattison, Dominic J. Anstey, Eloy de Lera Acedo

Published 2026-02-25

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CC BY 4.0

Original authors: Joe H. N. Pattison, Dominic J. Anstey, Eloy de Lera Acedo

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Listening to the Universe's Baby Talk

Imagine the early Universe as a giant, quiet room where a baby (the first stars and gas) is just starting to cry. Astronomers want to hear that specific cry—a faint radio signal called the 21 cm signal—to understand how the Universe grew up.

However, the room is incredibly noisy. There are loud, booming speakers everywhere (our own Milky Way galaxy) and static from the walls (the Earth's atmosphere). The baby's cry is so quiet that it's like trying to hear a whisper while standing next to a jet engine.

To solve this, scientists use a clever trick called Forward Modeling. They build a super-accurate computer map of the "noise" (the galaxy and atmosphere) and then try to subtract it from what their antennas hear. If the subtraction is perfect, the baby's whisper remains. If the map is even slightly wrong, the whisper gets lost in the math.

The Problem: The Moon is a "Glitch" in the Map

This is where the Moon comes in. The Moon is huge in the sky (about the size of your thumb held at arm's length). It moves around, and sometimes it passes right in front of the loudest parts of the galaxy.

Think of the scientists' noise map like a Google Maps street view of a busy city.

The Map: Shows all the traffic, lights, and buildings (the galaxy's radio noise).
The Moon: Imagine a giant, opaque cloud suddenly drifting over the busiest intersection in the city.
The Glitch: The scientists' computer map doesn't know the cloud is there. It still thinks the traffic is flowing normally under the cloud.

When the scientists try to subtract the "expected" noise from the "actual" noise, they get confused. They subtract the wrong amount because the Moon blocked the noise they were expecting to see. This tiny mismatch creates a "ghost" in the data that looks exactly like the baby's whisper, or worse, it hides the whisper entirely.

The "Major Lunar Standstill" (The Perfect Storm)

The paper focuses on a specific time: 2025. The Moon has an 18.6-year cycle where it swings higher and lower in the sky. In 2025, it swings to its highest point (a "Major Lunar Standstill").

Because of this high swing, the Moon can now pass directly over the Galactic Bulge—the super-bright, super-loud center of our galaxy.

The Analogy: Imagine you are trying to listen to a whisper in a library. Usually, you avoid the loud children's section. But in 2025, a giant, silent bouncer (the Moon) walks right in front of the children's section. If your noise-canceling headphones (the computer model) don't know the bouncer is there, the sudden silence where the children should be screaming will look like a ghost whisper.

What the Scientists Found

The team ran thousands of simulations to see how bad this "glitch" could get. Here are their findings:

1. Location, Location, Location
It doesn't matter just that the Moon is there; it matters where it is.

The "Bad" Spot: If the Moon blocks the center of the galaxy and that spot is right in the middle of the telescope's "ears" (the part of the sky the antenna listens to most clearly), the error explodes. The signal recovery can fail by 115% (meaning the result is completely nonsense).
The "Okay" Spot: If the Moon blocks a quiet corner of the sky, or if it's far away from where the telescope is looking, the error is tiny (less than 1%).

2. The Size Matters (Moon vs. Venus)
The scientists asked: "What about smaller planets like Venus?"

The Analogy: If the Moon is a giant cloud blocking a highway, Venus is a single car.
The Result: Because Venus is so small, it only blocks a tiny speck of the "noise map." The computer model is so good at smoothing things out that it barely notices. The error is less than 2 parts per million. Verdict: You don't need to worry about Venus or other small planets ruining your data.

3. The Threshold of Disaster
They found a "tipping point." If the Moon causes a mismatch in the noise map of just 15 parts per million (a tiny fraction), the error in finding the baby's whisper jumps to 20%. If the Moon is in the "loud" part of the sky, that mismatch can jump to 180 parts per million, destroying the data completely.

The Solution: Don't Listen When the Moon is in the Way

The paper concludes with a simple piece of advice for astronomers: Be picky about when you listen.

Just as you wouldn't try to record a podcast during a thunderstorm, astronomers should avoid taking data when the Moon is:

High in the sky.
Directly in front of the center of the galaxy.
Directly in the "sweet spot" of their telescope's view.

By carefully checking the calendar and the Moon's path, they can skip the "bad nights" and ensure that when they do listen, they actually hear the Universe's baby talk, not a glitch caused by our nearest neighbor.

Summary

The Goal: Hear the faint radio signal of the early Universe.
The Enemy: The Moon blocking the bright galaxy, confusing the computer models used to filter out noise.
The Risk: In 2025, the Moon will swing high enough to block the galaxy's center, potentially ruining the data if not avoided.
The Fix: Don't observe when the Moon is in the "loud" part of the sky. Small planets like Venus are too tiny to matter.

1. Problem Statement

Global 21 cm cosmology aims to detect the redshifted hyperfine transition of neutral hydrogen to probe the early Universe (Cosmic Dawn and Epoch of Reionization). The primary challenge is that the 21 cm signal is approximately $10^5$ times fainter than galactic foregrounds (synchrotron radiation). Current detection strategies rely on forward modeling, where complex foreground models are simulated and subtracted to reveal the cosmological signal.

The core problem addressed in this paper is lunar occultation. As the Moon moves across the sky, it blocks (occults) high-power regions of the galactic foreground. If the data analysis pipeline does not account for this occultation, the foreground model used for subtraction will mismatch the actual observed sky. This mismatch introduces errors that can corrupt the recovery of the faint 21 cm signal. This issue is becoming critical as the Earth approaches a major lunar standstill (2025), where the Moon's declination reaches $\pm 28.7^\circ$ , allowing it to occult the Galactic Bulge (a high-power region) while simultaneously passing near the zenith of many 21 cm experiments.

2. Methodology

The authors utilized a forward modeling Bayesian analysis pipeline (developed for the REACH experiment) to simulate and quantify the impact of occultation.

Antenna Models: Two antenna types were simulated:
1. A vacuum-backed REACH-like dipole.
2. A 6 m conical log-spiral antenna.
  Both were modeled with precise directivity patterns (chromaticity) using CST Studio Suite.
Sky Modeling:
- Foregrounds: Generated using the 2008 Global Sky Model (GSM) at 408 MHz, transformed to 230 MHz to avoid the cosmological signal band.
- Horizon: Modeled as a blackbody (300 K) with a reflection coefficient of 0.6.
- Signal: A theoretical 21 cm signal (Gaussian profile) was injected across the entire sky.
Occultation Modeling:
- The Moon was modeled as a thermal emitter (weighted average of day/night temperatures: 280 K/50 K) and a reflector of solar radiation (albedo 0.1).
- The Moon's position and phase were tracked using AstroPy and PyAstronomy.
- Crucial Assumption: The foreground model used for signal recovery did not include the Moon. The "truth" simulation included the Moon, creating a deliberate mismatch to test the pipeline's robustness.
Analysis Metric: The quality of signal recovery was measured using the Root Mean Square Error (RMSE) between the injected signal and the recovered signal.
- Gold Standard: RMSE < 10% of signal depth.
- Spurious: RMSE > 20% of signal depth.
Error Quantification: Two error metrics were defined:
- $\epsilon_u$ : Unweighted fractional error in the sky map.
- $\epsilon_{bw}$ : Beam-weighted fractional error (accounting for antenna directivity).

3. Key Contributions

Quantification of Thresholds: The study establishes specific error thresholds where signal recovery fails. It identifies that a base map error of 15 parts per million (ppm) or a beam-weighted error of 40–50 ppm can degrade signal recovery significantly.
Decoupling Location vs. Beam: The paper demonstrates that the impact of occultation is not solely determined by the Moon's position in the sky, but by the interaction between the occulted region and the antenna beam's directivity.
Planetary Occultation Analysis: It proves that bodies smaller than the Moon (e.g., Venus) do not cause significant signal disruption via thermal/solar reflection mechanisms, unlike the Moon.
Operational Guidelines: Provides specific advice for the upcoming major lunar standstill (2025), identifying observing windows that must be avoided to preserve data fidelity.

4. Key Results

A. Numerical Testing (Static Simulations)

Correlation: There is a strong correlation ( $\rho = 0.763$ for dipoles) between beam-weighted sky map error ( $\epsilon_{bw}$ ) and signal recovery RMSE.
Thresholds:
- An occultation-induced error of ~15 ppm (unweighted) or ~40 ppm (beam-weighted) reliably leads to an RMSE of 15% or higher.
- Errors exceeding 180 ppm (beam-weighted) can lead to RMSEs of 115%, rendering the signal unrecoverable.
Nuance: A simple sky map error metric is insufficient. If the Moon occults the Galactic Bulge (high power) but is outside the antenna's main beam, the error is lower than if it occults a lower-power region directly under the beam's peak directivity.

B. Realistic Simulations (Dynamic Scenarios)

The authors simulated observations at four global sites (Karoo, Dandiganahalli, Inyarrimanha, Deep Springs) during the 2025 major lunar standstill.

Worst-Case Scenario: When the Moon is near the Galactic Center and near the antenna zenith (high directivity), the RMSE spikes to 30–38% (Dipole) and up to 59% (Log Spiral). This occurs when the Moon occults the most powerful part of the sky while the antenna is most sensitive to it.
High Error, Low Impact: In cases where the Moon is near the Galactic Center but far from the zenith (low directivity), the unweighted error ( $\epsilon_u$ ) was high (~179 ppm), but the beam-weighted error was lower. The Log Spiral antenna recovered the signal acceptably, while the Dipole struggled (RMSE ~20%).
Best-Case Scenario: When the Moon occults low-power regions of the sky (away from the Galactic Center), the RMSE remained below the 10% gold standard, even if the Moon was near zenith.

C. Planetary Occultation (Venus)

A simulation placing a Venus-sized object (1 arcminute diameter) in the Galactic Bulge resulted in an unweighted error of 1.5 ppm and beam-weighted errors of <5 ppm.
Result: The RMSE was <10% for both antenna types.
Conclusion: Planetary occultation is negligible for global 21 cm cosmology regarding thermal/reflected emission. (Note: Non-thermal radio interference from satellites or planets is a separate issue not covered here).

5. Significance and Implications

Observing Strategy: The paper provides a critical operational guide for experiments like REACH, EDGES, and SARAS. As the Moon reaches its maximum declination in 2025, it will align with the Galactic Bulge. Experiments must avoid observing windows where the Moon transits high-power galactic regions while simultaneously passing through the high-directivity (zenith) region of the antenna beam.
Modeling Requirements: The study highlights that forward modeling pipelines must be extremely precise. A mismatch as small as 15 ppm in the foreground model can double the error in signal recovery.
Future Research: While thermal occultation is manageable via scheduling, the authors note that Radio Frequency Interference (RFI) reflected from the Moon (Earthshine) and non-thermal emissions from smaller bodies (e.g., Jupiter, satellites) remain open challenges requiring future investigation.

In summary, the paper concludes that while lunar occultation poses a severe threat to 21 cm signal recovery if ignored, it is a manageable systematic error through careful scheduling and avoidance of specific high-risk alignment windows during the major lunar standstill.