The CMB Radiation

The CMB: How An Accidental Discovery Became a Key To Understanding The Universe?

Our universe is glutted with several stray radiations. We are exposed to radiations every day, from sources such as minerals in the ground, radioactive radiations, medical x-rays,  electromagnetic radiations from stars around us, radiations from telephone towers, and many more. However, another major category of radiation is present in every nook and corner around us. Well, I’m talking about the Cosmic Microwave Background radiation, also known as CMB radiation, that is prevalent in the universe since the time it was in its cradle.

What Is CMB Radiation?

According to the Big Bang theory, the early universe was a scorching place. About 400,000 years after the big bang, it was engulfed with a dense soup of hot ionized plasma. At these conditions of temperature, electrons and protons couldn’t coalesce into atoms. As we know, every object around us, be it the star-like sun, a toaster, or a metallic rod, has a thermal spectrum or a blackbody spectrum of electromagnetic radiation associated with it. Similarly, a thermal spectrum of radiation was also associated with thick plasma soup.

But as there were no neutral atoms, these photons of electromagnetic radiations emitted by the super-hot plasma couldn’t travel very far before being bounced back by the free electrons. However, as time went by, the universe underwent rapid inflation and expansion. As it expanded, the plasma within it diluted and cooled down. Eventually, when it cooled below 3000 K,  neutral atoms finally formed. With no free electrons to redirect the photons, the universe became transparent for the first time. This allowed photons emitted by hot plasma to now travel freely.

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The universe eventually got filled with radiation that was simply the remnant heat leftover from the Big Bang. And this afterglow of the big bang is what we call CMB or the Cosmic Microwave Background. Basically, the CMB represents the heat leftover from the Big Bang, which has been streaming through space for the last 14 billion years, just like the heat from a sun-warmed rock gets reradiated at night.

Graph of cosmic microwave background(CMB) spectrum measured by the FIRAS instrument on the COBE. It is the most precisely measured black body spectrum in nature. (Image: Wikipedia)
Graph of cosmic microwave background spectrum measured by the FIRAS instrument on the COBE. It is the most precisely measured black body spectrum in nature.

Now, you might be wondering why it is called the cosmic “Microwave” background. The answer is related to CMB’s energy. Although, when this cosmic background light was released billions of years ago, it was as hot and bright as the surface of a star. However,  as the universe expanded, space got stretched by a factor of a thousand since then. Eventually, the wavelength of CMB photons also got stretched due to a phenomenon called cosmological redshift. Due to expansion, the CMB radiation has cooled to its present-day temperature, which is extremely cold, just 2.725 degrees above absolute zero. And the energy and wavelength corresponding to this temperature lies in the microwave part of the electromagnetic spectrum, and so is the name.

Although we can’t see the CMB with our naked eye, it is present everywhere in the universe. In fact, if we could see microwaves, the entire sky would appear to glow with a brightness that is uniform in every direction. 

Discovery of CMB Radiation:

Ralph Alpherin first predicted the CMB radiation’s existence in 1948 while he was doing some research related to Big Bang Nucleosynthesis. However, practically it was found accidentally for the first time in 1965 when two researchers, Arno Penzias and Robert Wilson, associated with Bell Telephone Laboratories, were trying hard to create a radio receiver. While doing so, the duo was puzzled by an unknown noise that their receiver was picking up. Soon, they realized that the noise was coming uniformly from all over the sky and was nothing other than the CMB radiation that Ralph had earlier postulated.

The CMB: How An Accidental Discovery Became a Key To Understanding The Universe? 2
The Holmdel Horn Antenna on which Penzias and Wilson discovered the CMB radiation

Simultaneously, a team of researchers led by Robert Dicke at Princeton University tried to find the CMB radiation. Within no time,  Dicke’s team learned about the Bell experiment and realized the CMB had been found. Soon after, both the teams published individual papers in the Astrophysical Journal in 1965. In their publications, Penzias and Wilson focused on what they saw,  while Dicke’s team explained CMB according to what it means in the context of the universe. In 1978, Penzias and Wilson received the 1978 Nobel Prize in physics for their grand discovery.

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Significance of CMB Radiation:

The CMB radiation was emitted 13.7 billion years ago, only a few hundred thousand years after the Big Bang. Thus, by studying the detailed physical properties of this radiation, we can learn about conditions that prevailed in the universe on enormous scales at very early times because the CMB radiation that we see today has traveled over such a large distance since then.

CMB is at a uniform temperature with only small fluctuations, varying by no more than one part in ten thousand. These fluctuations are visible only with very precise telescopes. A knowledge of these fluctuations enables the cosmologists to extract information regarding galaxies’ origin and large-scale structures of galaxies.

For instance, the first space-based full-sky map of CMB radiation was plotted by NASA’s Cosmic Background Explorer (COBE) mission. This map came to be known as the “baby picture” of the universe. It confirmed the Big Bang theory predictions and showed hints of cosmic structure that were not seen before. Another detailed map came in 2003 via Wilkinson Microwave Anisotropy Probe (WMAP). This picture revealed the universe’s age to be about 13.7 billion years. It also revealed that the oldest stars started shining about 200 million years after the Big Bang, far earlier than predicted. This map also spotted a “cold spot” that was bigger than earlier expected.

The Cosmic Microwave Background temperature fluctuations as measured by WMAP. The colors represent the tiny temperature fluctuations. Red regions are warmer and blue regions are colder by about 0.0002 degrees. (Image:wikipedia)
The Cosmic Microwave Background temperature fluctuations as measured by WMAP. The colors represent the tiny temperature fluctuations. Red regions are warmer and blue regions are colder by about 0.0002 degrees.

This was followed by another set of CMB data collected by the European Space Agency’s Planck space telescope, which showed the CMB’s highest precision picture yet. The CMB data has also hinted at dark matter and dark energy that is mysteriously accelerating the expansion of the universe.

Undoubtedly, the Cosmic Microwave Background is one of the most significant tools used by scientists to learn how the early universe was formed and is definitely expected to reveal a lot more exciting stuff in the years to come.

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