09 Sep From Moon’s Far Side, a Satellite to Look for the First Radio Signal

Why in the News?

A major development in radio astronomy and cosmology has brought renewed attention to the Moon’s far side as a possible location for studying the early Universe.

The proposed CosmoCube mission aims to detect the extremely faint redshifted 21-cm radio signal from neutral hydrogen. This signal could help scientists study the period before and during the formation of the first stars and galaxies.

Importantly for India, the topic is also connected with PRATUSH (Probing ReionizATion of the Universe using Signal from Hydrogen), a proposed Indian lunar-orbit radio astronomy experiment being developed by researchers associated with the Raman Research Institute.

UPSC Subject Mapping

 Understanding the Story: What is CosmoCube?

CosmoCube is a proposed compact satellite mission designed to observe the Universe from lunar orbit.

Its major objective is to detect the 21-centimetre signal associated with neutral hydrogen from the early Universe.

The mission proposes to make observations when the satellite moves behind the Moon, particularly on the far side, where the Moon can shield it from radio emissions originating from Earth.

Key Mission Facts

• Target: Redshifted 21-cm hydrogen signal
• Location: Lunar orbit, especially the far side of the Moon
• Scientific focus: Cosmic Dark Ages, Cosmic Dawn and Reionisation
• Major advantage: Protection from terrestrial radio interference
• Expected mission concept: A lightweight, compact satellite

According to the mission concept, observations could open a new window into a period of cosmic history that remains poorly understood.

The Timeline of the Early Universe

Understanding this topic becomes much easier through a simple timeline:

1. Big Bang

The Universe began approximately 13.8 billion years ago.

↓

2. Recombination

Around 380,000 years after the Big Bang, electrons combined with nuclei to form neutral atoms, particularly hydrogen.

The Universe became transparent enough for radiation to travel freely.

↓

3. Cosmic Dark Ages 🌑

This was a period before the formation of the first stars and galaxies.

The Universe contained large quantities of neutral hydrogen, but there were no stars producing visible light.

↓

4. Cosmic Dawn

The first stars and galaxies began to form.

Their radiation gradually changed the physical conditions of the Universe.

↓

5. Epoch of Reionisation

Radiation from early stars and galaxies ionised much of the neutral hydrogen.

Consequently, the Universe gradually transformed into the largely ionised cosmos we observe today.

What is the 21-cm Hydrogen Signal?

Neutral hydrogen can emit radiation at a wavelength of approximately 21 centimetres, corresponding to a frequency of about 1420 MHz in its rest frame.

However, because the Universe has expanded over billions of years, radiation originating in the early Universe gets redshifted.

Therefore, the original signal can now be observed at much lower frequencies.

In simple words:

The 21-cm signal acts like a cosmic time machine.

By studying its redshift, scientists can obtain information about different periods of the Universe’s history.

The signal can potentially reveal information about:

• Formation of the first stars ⭐
• Formation of the first galaxies 🌌
• Distribution of matter
• Evolution of neutral hydrogen
• Cosmic structure formation
• Reionisation of the Universe
• Possible properties of dark matter

Why Observe from the Far Side of the Moon?

Earth is a Very Noisy Place for Radio Astronomy

The faint hydrogen signal is extremely difficult to detect from Earth.

Several sources create interference:

 Human-made Radio Frequency Interference (RFI)

• Mobile communication
• FM radio
• Television transmission
• Radar
• Satellites
• Other communication technologies

 Earth’s Ionosphere

The ionosphere can affect low-frequency radio waves and complicate observations.

 Strong Foreground Radiation

Signals from our own galaxy can be vastly stronger than the faint cosmological 21-cm signal.

Consequently, detecting the ancient hydrogen signal is comparable to hearing a whisper in a stadium full of noise.

The Moon’s far side provides a natural shield from much of Earth’s radio interference.

The Moon as a Natural Radio Shield

When a satellite moves behind the Moon:

Earth 🌍 → Moon 🌕 → Satellite 🛰️

The Moon blocks direct terrestrial radio emissions.

Therefore, the satellite can potentially operate in a much quieter radio environment.

This makes the lunar far side strategically important for the future of:

• Radio astronomy
• Cosmology
• Low-frequency observations
• Search for signals from the early Universe

However, increasing lunar activity could eventually create new sources of radio interference even around the Moon. Therefore, protecting radio-quiet regions is becoming an important international scientific concern.

🇮🇳 India’s Connection: PRATUSH Mission

India is also pursuing research in this important field through PRATUSH.

PRATUSH stands for:

Probing ReionizATion of the Universe using Signal from Hydrogen

It is a proposed Indian radio astronomy experiment designed to study the redshifted 21-cm signal from the early Universe.

The mission concept is associated with the Raman Research Institute (RRI) and aims to use space-based observations, ultimately including observations from lunar orbit and the radio-quiet far side of the Moon.

Why is PRATUSH important?

It could help answer questions such as:

• When did the first stars form?
• How did the first galaxies emerge?
• How did the Cosmic Dawn occur?
• How did reionisation transform the Universe?

Thus, PRATUSH represents India’s potential contribution to frontier cosmology and lunar radio astronomy.

 Significance of the Development

1. Exploring the “Missing” Period of Cosmic History

Scientists know much about the Universe after galaxies formed.

However, the period between the early Universe and the emergence of the first luminous objects remains comparatively difficult to observe.

The 21-cm signal could provide information about this largely unexplored era.

2. Understanding the First Stars and Galaxies

The Cosmic Dawn represents the transition from a dark Universe to one containing luminous objects.

Therefore, detecting hydrogen signals from this period could help scientists understand when and how the first stars formed.

3. Understanding Dark Matter

The early Universe provides a relatively clean environment for testing cosmological models.

Moreover, the distribution and evolution of neutral hydrogen may help scientists investigate the role of dark matter in structure formation.

4. New Role for the Moon

Traditionally, the Moon has been viewed mainly as a destination for:

• Landing missions
• Human exploration
• Resource exploration

However, the Moon’s far side could also become a major platform for scientific astronomy.

Thus, lunar exploration may increasingly involve both space exploration and space-based observatories.

5. Importance of Radio Spectrum Protection

The growing use of satellites and communication technologies is increasing pressure on the radio spectrum.

Meanwhile, radio astronomy requires extremely quiet environments.

Therefore, the protection of radio-quiet zones—especially around the Moon—could become an important issue of global space governance.

 Key Challenges

1. Extremely Weak Signal

The 21-cm cosmological signal is exceptionally faint.

2. Foreground Contamination

Radiation from the Milky Way and other sources can overwhelm the desired signal.

3. Radio Frequency Interference

Human-made signals can contaminate scientific observations.

4. Increasing Lunar Activity

Future spacecraft and lunar infrastructure could introduce radio noise.

Consequently, scientists are concerned that the radio-quiet environment may not remain pristine indefinitely.

5. Technical Precision

The instruments require highly accurate calibration because even small errors can affect the measurement of such a faint signal.

UPSC Mains Practice Question

“The Moon is increasingly emerging not only as a destination for exploration but also as a platform for frontier scientific research.” Discuss with special reference to lunar far-side radio astronomy and the study of the early Universe.

(Answer in 250 words)