THE BIG BANG THEORY EXPLAINED – STORY OF ITS EMERGENCE

The Big Bang theory is the most widely accepted scientific theory for the origin of universe. It states that the universe came into existence 13.8 billion years ago by rapid expansion of an infinitely dense and hot point containing all the matter in highly condensed form. The Big Bang involved a sudden expansion of matter, energy and space from a single point forming the current picture of universe.

Ancient philosophers believed that the universe had and would exist forever. When most people were convinced by the idea of static universe, that was always fixed and permanent, a landmark discovery by Edwin Hubble in 1929 revolutionized the idea of universe’s origin.

Hubble observed that stars in all directions are moving rapidly away from us. This indicates that the universe is expanding. This means that objects were close together at earlier times. In fact, it appeared that about ten to twenty thousand million years ago all the matter in universe was compressed at a single point.

THE BIG BANG THEORY

According to this theory, about 13.8 billion years ago, all the matter and energy in the universe was concentrated at a singular point. This point was infinitely small. This single point had infinite density and the curvature of space-time was also infinite.

All scientific theories are based on the assumption that space-time is smooth and nearly flat. So all scientific laws would break at the big bang singularity, as the curvature of space-time was infinite back then. This means that existing physical laws could not be used to determine events before the big bang. We can say that time had a beginning at the big bang.

The infinitely small, dense and hot point began to expand rapidly, creating the universe as we observe today. The process of big bang happened in a sequence of different timelines.

As the expansion occurred, temperatures decreased and space enlarged. Eventually, galaxies and planets were born due to gravitational force in denser regions. Galaxies collapsed, contracted, got heated up to generate nuclear reactions forming stars.

The concept of Big Bang was first proposed by Georges Lemaitre in 1931. He suggested that the universe originated from a “primeval atom”. His idea was later reinforced by Edwin Hubble’s observation in 1929 that distant galaxies are constantly moving away from us. This proved that universe is expanding.

The initial expansion was followed by several phases to generate the current picture of universe. In the first fraction of second, the universe expanded at an extremely large rate (known as cosmic inflation). The temperature dropped rapidly. Then subatomic particles like quarks formed, then protons and neutrons. After three minutes, protons and neutrons fused to form hydrogen and helium. Over the next hundreds of millions years, particles in denser regions combined driven by gravitational force to form galaxies and planets.

EVIDENCES SUPPORTING BIG BANG

COSMIC MICROWAVE BACKGROUND RADIATION

The cosmic microwave background (CMB) is the faint, leftover thermal radiation from the big bang. As the universe expanded, the opaque, hot and dense plasma cooled until electrons and protons combined to form neutral hydrogen atoms. This allowed light to travel freely. Over 13.8 billion years, the expansion of universe has extended the spectrum of this light from visible wavelength to range of microwave wavelength.

The CMB was discovered by radio astronomers Arno Penzias and Robert Wilson in 1964. Currently, it has cooled to -270.45 degree celcius, just a little more than absolute zero. The CMB radiation distributed uniformly across the universe.

Scientists predicted that the shockwave of the primordial explosion during big bang would still be detectable as subtle background radiation coming from everywhere in the space. Today, the Big Bang model is the only one that is able to explain the presence of the CMB.

Since the time the cosmic microwave background radiation was emitted, the universe has expanded and cooled. The CMB has undergone the same process of expansion and cooling.

The microwave frequency of CMB is too long and cannot be seen with naked eye. These can however be detected using specially designed detectors.

COSMOLOGICAL REDSHIFT

Cosmological redshift is the shifting of light towards red end of the electromagnetic spectrum as it travels across an expanding universe. Light travels as a wave and its color is determined by its wavelength.

The visible light of the electromagnetic spectrum has colors of different wavelengths between 400 – 700 nm. Violet has highest frequency and shortest wavelength, while red has lowest frequency and longest wavelength (wavelength is inversely proportional to frequency). Red shifting of light therefore means that the frequency of light is decreasing (wavelength increasing).

Similar to the Doppler effect, when a light source moves towards us, its waves are compressed, shifting them to the blue end of the spectrum. When light is moving away from us, the light waves are stretched out and shifted towards red end of the spectrum.

The red shift of distant galaxies simply means that galaxies are moving away from us. In other words, the universe is expanding.

In 1929, Edwin Hubble proved that larger the distance of the galaxy, the faster was its movement away from us. The red shift was greater for distant galaxies (in fact directly proportional to the distance of galaxy from us). This is known as Hubble’s Law. This means that galaxies that are farther from us are moving away at a faster rate.

Since the universe has been expanding all the time, this means that all the objects were closer in the past and were exactly at the same place about 13.8 billion years ago at the time of Big Bang, compressed into a singular point of infinite density. The cosmological redshift phenomena provided a strong evidence supporting the Big Bang theory.