Big Bang Theory: Primordial Radiation

Key Concepts

• Big bang theory

• Cosmic Microwave Background (CMB)

• Importance of CMB

• Formation of CMB

• Discovery of CMB

• Evidence for the Big Bang

• Information we get from CMB

Introduction

According to Big Bang Theory, all of the energy and matter of the universe was in a strangely hot and dense state. Around 13.7 billion years ago, our universe started as a catastrophic explosion, which continued to expand, cool, and evolve to its present state. In the initial moments of this expansion, only energy and quarks (subatomic particles that are the building blocks of protons and neutrons) were present. Not until 380,000 years after the initial expansion did the universe cool adequately for electrons and protons to come together to form hydrogen and helium atoms. These are the lightest elements in the universe. For the first time, light traveled all through space. Ultimately, temperatures decreased adequately to allow clusters of matter to collect. This material produced huge clouds of dust and gases (nebulae), which rapidly developed into the first stars and galaxies. Our Sun and planetary system formed around 5 billion years ago (almost 9 billion years after the Big Bang).  

The Cosmic Microwave Background Radiation  

In 1965 scientists discovered that microwaves seem like coming from all directions in space. This radiation was predicted by the Big Bang Theory and is termed cosmic background radiation. Lately, thorough measurements of the cosmic background radiation have been done by an orbiting observatory known as the Wilkinson Microwave Anisotropy Probe (WMAP). A map of these measurements is shown in Figure 2. The bright areas on the image show regions where the density of matter was high and galaxies first formed. The WMAP data and other data indicate that the Big Bang occurred about 13.7 billion years ago. 

The Big Bang Theory thinks that the early universe was a very hot place, and as it expanded, the gas within the universe cooled. Therefore, the universe must be filled with radiation that is literally the leftover heat from the Big Bang, called the “cosmic microwave background“, or CMB. 

We cannot see the CMB with our naked eye. however, it is present everywhere in the universe. It is not visible to humans because it is very cold, only 2.725 degrees above absolute zero (- 459.67 ﹾF, or – 273.15 ﹾC). This means its radiation is highly visible in the microwave portion of the electromagnetic spectrum. 

Importance of Studying The Cosmic Microwave Background (CMB) 

As light travels at a finite speed, astronomers observing far objects are looking into the past. Most of the stars that are seen with the naked eye in the night sky are 10 to 100 light years away. Therefore, we see them as they were 10 to 100 years ago. We see Andromeda, the nearest and big galaxy, as it was around 2.5 million years ago. Astronomers are studying distant galaxies with the help of the Hubble Space Telescope. They can see them as they were simply a few billion years after the Big Bang. 

The CMB radiation was radiated 13.7 billion years ago, only a few hundred thousand years later the Big Bang, long before stars or galaxies ever existed. Therefore, by studying the complete physical properties of radiation, we can learn about conditions in the universe on very large scales at very early times, as the radiation we see today has travelled over such a huge distance. 

Formation of Cosmic Microwave Background (CMB) 

The universe started 13.8 billion years ago, and the CMB dates back to about 400,000 years after the Big Bang. That is because in the early phases of the universe when it was just one-hundred-millionth the size, it is today, its temperature was severe: 273 million degrees above absolute zero, this is according to NASA.  

Any atoms present at that time were immediately broken apart into tiny particles (protons and electrons). The radiation from the CMB in photons (particles corresponding to quantum of light, or another radiation) was scattered off the electrons. “Therefore, photons drifted all through the early universe, just as optical light drifts through a dense fog.” NASA wrote. 

Around 380,000 years after the Big Bang, the universe was sufficiently cool that hydrogen could form. Because the CMB photons are hardly impacted by hitting hydrogen, the photons move in straight lines. Cosmologists suggest a “surface of last scattering” when the CMB photons last hit matter; after that, the universe was very big. Thus, when we map the CMB, we are looking back in time to 380,000 years after the Big Bang, right after the universe was opaque or impenetrable to radiation. 

Discovery of CMB 

According to NASA, American cosmologist Ralph Apher in 1948 first anticipated the CMB when he was working with Robert Herman and George Gamow. This group was doing research associated with Big Bang nucleosynthesis, or the formation of elements in the universe besides the lightest isotope (type) of hydrogen. This type of hydrogen was produced very early in the universe’s history. 

However, the CMB was first noticed by accident. In 1965, two researchers with Bell Telephone Laboratories (Arno Penzias and Robert Wilson) were making a radio receiver and were confused by the noise it was picking up. They immediately understood the noise came consistently from all over the sky. At the same time, a team at Princeton University (headed by Robert Dicke) was trying to find the CMB. Dicke’s team got wind of the Bell experiment and understood the CMB had been discovered.  

Both teams immediately published papers in the Astrophysical Journal in 1965, with Penzias and Wilson telling about what they saw and Dicke’s team describing what it means in the context of the universe. (Soon after, Penzias and Wilson both got the 1978 Nobel Prize in physics). 

Evidence for the Big Bang 

In 1965, CMB radiation was discovered by chance. Penzias and Wilson, two radio astronomers in the United States, registered a signal in their radio telescope that could not be assigned to any particular source in the sky. 

The signal actually came from everywhere with the same strength (intensity), day or night, summer or winter. They decided that the signal had to come from out of our Galaxy. It came almost from the source of the universe. 

Scientists considered their discovery as strong proof for the ‘Big Bang’ theory. This theory predicted that the ‘shockwave’ of that primeval explosion would still be noticeable as a subtle ‘wallpaper’ coming from all over behind all galaxies, quasars, and galaxy clusters. 

Today, still the Big Bang model is the only model that is able to realistically explain the presence of the CMB. As per this model, the universe began with an extremely dense and hot phase that expanded and cooled itself; for several hundreds of thousands of years, the temperature was very high that neutral atoms could not form. 

Matter is comprised mostly of neutrons and charged particles (protons and electrons). Electrons interacted directly with the light particles, and hence light and matter were closely coupled at that time (that is, light could not move for a long distance in a straight line). Therefore light could not propagate, and the universe was opaque. 

What information do we get from THE COSMIC MICROWAVE BACKGROUND? 

The CMB is valuable to scientists as it helps us to learn about the formation of the early universe. It is at a constant temperature with only small fluctuations visible with precise telescopes. “By studying these fluctuations, the origin of galaxies and large-scale structures of galaxies can be identified by cosmologists, and they can determine the fundamental factors of the Big Bang theory,” This information is written by NASA.  

Summary

• As per the Big Bang theory, the universe is enlarging and its density, and temperature are decreasing.

• Our Sun and planetary system formed around 5 billion years ago (almost 9 billion years after the Big Bang).

• In 196S, scientists discovered that microwaves seem like coming from all directions in space.

• The Big Bang theory thinks that the early universe was a very hot place and that as it expanded, the gas within the universe cooled.

• The Cosmic Microwave Background (CMB) is the cooled residue of the first light that could ever move freely through the Universe.

• The universe started 13.8 billion years ago, and the CMB dates back to about 400,000 years after the Big Bang.

• American cosmologist Ralph Apher first anticipated the CMB in 1948 when he was doing work with Robert Herman and George Gamow.

• Today, still the Big Bang model is the only model that is able to realistically explain the presence of the CMB.

• The CMB is valuable to scientists as it helps us to learn about the formation of the early universe.