The Cosmic Microwave Background Radiation And Its Origin

In the series of Basics of Astrophysics, today is our twenty-sixth article based on a very important discovery of the cosmic microwave background radiation that bragged the 1978 Nobel prize in Physics to its discoverers. Let us begin by talking about some fundamental background of this amazing discovery.

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The fact that the universe is expanding is one of the most profound discoveries of the 20th century. This means that the universe was much smaller, denser and hotter in the distant past. At such high density and temperature, the universe was in the plasma state, and matter and radiation were in thermal equilibrium. Nothing could coagulate to form even the atoms or molecules of matter that we see today. Then, the universe expanded and both matter and radiation cooled. Today, the predicted age of the universe is 13.7 billion years. But after the initial 400,000 years, something amazing happened in the universe. In today's article, I am going to talk about the phenomenon that occurred long back but still continues to have its traces in the universe even today.

Detection of Weak Background Noise

An image of the cosmic microwave background radiation, taken by the European Space Agency (ESA)'s Planck satellite in 2013, shows the small variations across the sky.

In 1965, Bell Lab's Radio astronomers- Arno Penzias and Robert Wilson were trying to map signals from the milky way. While calibrating the horn radio antenna (designed to track the satellite echo), they detected a constant noise. This noise was due to radiation that was strongest in the microwave region of the radio spectrum. The radiation was unaffected by the antenna direction and was of cosmic origin. It is now called the cosmic microwave background (CMB) radiation.

What is Cosmic Microwave Background (CMB)?

The CMB radiation has a temperature of 2.7 K and its spectrum is a thermal black body curve. Since birth 13.7 billion years ago, the universe is continuously expanding and cooling. After 400,000 years from the start of the universe, it cooled down to a temperature of 3000 degree Celsius. At this point, the simplest atoms formed in the universe and the matter decoupled from the radiation. The light from this time has been traveling through space ever since. This is what gives us the cosmic background radiation.

The CMB radiation can be detected all around us from here on Earth or in space. It is like we can measure the afterglow of the Big Bang. Instead of seeing the afterglow at 3000 degrees, today we see it just at -270 degree Celsius (3K). This is due to the reason that the universe has stretched, which makes it appear much cooler.

CMB and the Big Bang Theory

Comic Microwave Background
Image of sky taken by COBE

Before the decoupling of matter and radiation, the universe was extremely dense and hot that it was actually opaque to all radiation. After the decoupling, it transitioned from an opaque to a transparent state. The residual radiation of that stage of the universe is what we call as the CMB radiation now. In 1992, a satellite named COBE measured the CMB over the whole sky. COBE has established that CMB is almost completely uniform, with an almost constant temperature over the whole sky. But then the question arises that if the universe was so uniform, how were the different structures such as stars and planets were formed?

The CMB radiation is not completely constant. There were tiny fluctuations, or ripples, in the temperature, at the level of just one part in 100,000. CMB radiation is evidence of the Big Bang origin of the universe. Even today, the investigation of the CMB radiation gives crucial insights to the beginning, expansion and the various occurring phenomena in the universe.

Author's Message

The CMB radiation is an important topic in Astrophysics. With this, we have entered the final phase of the series. In the last 4 articles, we will be discussing more general topics. Stay tuned!

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