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Probing the cosmological 21~cm global signal from the Antarctic ice sheet

Shijie Sun, Jiaqin Xu, Minquan Zhou, Shenzhe Xu, Fengquan Wu, Haoran Zhang, Juyong Zhang, Bin Ma, Zhaohui Shang, Xuelei Chen

TL;DR

This work evaluates the feasibility of measuring the cosmological 21 cm global signal from the Antarctic inland ice. It presents a compact, autonomous instrument featuring an elliptical-dipole antenna, a low-noise analog front end, a digital spectrometer, and self-calibration designed for extreme cold. RFI and ground-penetrating radar surveys indicate minimal interference and stable ground conditions, supporting clean spectral measurements. Field deployment demonstrates instrument performance and resilience, with sensitivities around 0.04–0.1 K and strong justification for Antarctica as a viable site for long-term cosmological 21 cm observations.

Abstract

The redshifted 21 cm line, arising from neutral hydrogen, offers a unique probe into the intergalactic medium and the first stars and galaxies formed in the early universe. However, detecting this signal is a challenging task because of artificial radio-frequency interference (RFI) and systematic errors such as ground effects. The interior of the Antarctic continent provides an excellent location to make such observations, with minimal RFI and relatively stable foreground signals. Moreover, a flat plateau in central Antarctica, with an ice cap over 2000 m deep, will show less ground reflection of radio waves, reducing the signal complexity in the area around the probing antenna. It may be advantageous to perform cosmological 21 cm experiments in Antarctica, and a 21 cm Antarctic global spectrum experiment can potentially be deployed on the Antarctic ice cap. We have performed preliminary instrumental design, system calibration, and implementation of such an instrument optimized for extreme cold and capable of long-term autonomous operation. This system shows the ability to effectively detect the 21~cm signal, confirming Antarctica as an excellent observational site for radio cosmology.

Probing the cosmological 21~cm global signal from the Antarctic ice sheet

TL;DR

This work evaluates the feasibility of measuring the cosmological 21 cm global signal from the Antarctic inland ice. It presents a compact, autonomous instrument featuring an elliptical-dipole antenna, a low-noise analog front end, a digital spectrometer, and self-calibration designed for extreme cold. RFI and ground-penetrating radar surveys indicate minimal interference and stable ground conditions, supporting clean spectral measurements. Field deployment demonstrates instrument performance and resilience, with sensitivities around 0.04–0.1 K and strong justification for Antarctica as a viable site for long-term cosmological 21 cm observations.

Abstract

The redshifted 21 cm line, arising from neutral hydrogen, offers a unique probe into the intergalactic medium and the first stars and galaxies formed in the early universe. However, detecting this signal is a challenging task because of artificial radio-frequency interference (RFI) and systematic errors such as ground effects. The interior of the Antarctic continent provides an excellent location to make such observations, with minimal RFI and relatively stable foreground signals. Moreover, a flat plateau in central Antarctica, with an ice cap over 2000 m deep, will show less ground reflection of radio waves, reducing the signal complexity in the area around the probing antenna. It may be advantageous to perform cosmological 21 cm experiments in Antarctica, and a 21 cm Antarctic global spectrum experiment can potentially be deployed on the Antarctic ice cap. We have performed preliminary instrumental design, system calibration, and implementation of such an instrument optimized for extreme cold and capable of long-term autonomous operation. This system shows the ability to effectively detect the 21~cm signal, confirming Antarctica as an excellent observational site for radio cosmology.

Paper Structure

This paper contains 17 sections, 17 figures, 1 table.

Table of Contents

  1. Introduction
  2. Antarctica for astronomy
  3. The advantages of Antarctica for astronomical observations
  4. Expedition, site selection and implementation
  5. Instrument design
  6. Antenna
  7. Receiver
  8. Analog Receiver
  9. Digital spectrometer
  10. Receiver chassis
  11. Power supply
  12. Design and implementation for extreme environment
  13. Results
  14. RFI measurement
  15. Ground effect
  16. ...and 2 more sections

Figures (17)

  • Figure 1: The beam-average sky temperature as a function of time for several latitudes.
  • Figure 2: A map of Antarctica, showing the position of Zhongshan Station (green dot), Kunlun Station (red dot), and the site of our 21 cm global spectrum experiment (blue dot) at (78°32'11"S, 77°0'50"E).
  • Figure 3: The Antarctic global spectrum measurement instrument.
  • Figure 4: The solar Panels.
  • Figure 5: Top: 3D-rendered model of the elliptical antenna. Bottom: The simulated far-field beam of the antenna for 6 frequency points in the range of 50 -- 100 MHz, and their corresponding 3 dB beamwidths.
  • ...and 12 more figures