Introduction

The cosmic microwave background radiation (CMB) is a remnant of the Big Bang, and it permeates the universe. However, what about its interaction with Earth and how it is perceived on our planet? This article delves into the fascinating aspects of the CMB and its journey through Earth, exploring how it is heard and received.

Is Cosmic Microwave Background Radiation Flowing Through Earth?

There is a common misconception that the CMB actually flows through solid rock. In reality, it does not pass through solid materials, including the Earth's crust. Instead, the CMB interacts with the Earth in a different, more subtle way. Microwave radiation, which includes the CMB, encounters significant absorption by the Earth’s atmosphere, and eventually, by the Earth’s surface. Let us explore these concepts further.

First Detection and Listening to CMB

The CMB was first discovered by Arno Penzias and Robert Wilson in 1964 using a radio telescope in Holmdel, New Jersey. They observed an unexplained source of static in their receiver, which turned out to be the CMB. In fact, you can listen to the CMB using a radio or an analog TV set if you tune to a spot without broadcast signals. Around one percent of the static you hear or see is the CMB. This static is not just a random noise; it is a valuable piece of evidence about the early universe.

The Journey of CMB Through the Atmosphere

The CMB is primarily at the microwave frequency, which aligns with its name. The Earth’s atmosphere, however, is not entirely transparent to microwaves due to absorption by molecules like H2O and CO2. This absorption causes a significant amount of the CMB to be either absorbed or reflected by the atmosphere, making it difficult to observe from the ground without specialized equipment.

For that reason, many CMB experiments are conducted either in space, on rockets or balloons, or at very high altitudes and dry climates such as the South Pole and the Atacama Desert in Chile. The first detection of the CMB on Earth was indeed from New Jersey, emphasizing the challenges of ground-based observation.

Microwaves on Earth's Surface and Interior

Once microwaves managed to pass through the atmosphere, they are absorbed by the Earth's surface. The optical depth of microwaves is high, meaning that the mean free path is short. The surface of the Earth, including the ocean and crust, is dense and absorbs most of the microwaves that reach it. This absorption is essential for the thermal equilibrium of the Earth.

However, does this mean that there are no microwaves inside the Earth's crust? Far from it. The interior of the Earth is in thermal equilibrium and supports its own radiation bath. The microwaves inside the Earth are not cosmogenic but are emitted by the rocks and promptly reabsorbed. These internal emissions are part of the thermal balance of the Earth but do not constitute the CMB from the Big Bang.

Conclusion

The cosmic microwave background radiation, though not passing through Earth in the traditional sense, interacts with our planet in unique and interesting ways. Understanding these interactions not only enhances our knowledge of the universe but also provides insights into the thermal behavior of Earth itself. From the first detection by Penzias and Wilson to the intricate dynamics within our planet's atmosphere and surface, the CMB continues to be a fascinating subject for study and exploration.

References

[1] Penzias, A. A., Wilson, R. W. (1965). A Measurement of Excess Antenna Temperature at 4080 Mc/s. The Astrophysical Journal, 142, 419-421.