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Big Bang Theory and its Evidence

Grade 9
Aug 20, 2022

Key Concepts

• Big Bang Theory

• Expanding Uiverse

• Evidences for the Big Bang


The Big Bang theory is an initial development of the universe. As per the Big Bang theory, the universe grew from an enormously small, very hot, and very dense state. From that time, it has expanded and become less dense and cooler. Astronomers use the Big Bang as an example to describe the creation of matter, space, and time, approximately 13.7 billion years ago. 

What proof is there to support the Big Bang theory?  

There are two most important scientific discoveries that provide great support for the Big Bang Theory:  

  1. Hubble’s discovery: In the 1920s Hubble’s discovery explained the relationship between a galaxy’s distance from Earth and its speed.  
  1. Cosmic Microwave Background Radiation: The invention in the 1960s of cosmic microwave background radiation.  
 1: Universe 

The Expanding Universe 

When scientists speak of the expanding universe, they are referring to the fact that the universe has been expanding in size since the Big Bang. But what exactly is growing in size? Galaxies, stars, planets, and the things that live on them, such as buildings, cars, and people, are not growing in size. Their size is determined by the strength of the fundamental forces that hold atoms and subatomic particles together, and this has not changed as far as we know. Instead, the space between galaxies is expanding – they are becoming more distant as space itself expands. 

What proof do we have for the expanding universe? 

The universe was thought to be static in the early twentieth century: it was always the same size, neither expanding nor contracting. However, in 1924, astronomer Edwin Hubble used a technique pioneered by Henrietta Leavitt to measure distances to distant celestial objects. Hubble measured the speeds of these objects using spectroscopic red-shift data and then graphed their distance from Earth against their speed. He noticed that the speed at which astronomical objects go apart is proportional to their separation. 

2: Hubble’s data (1929) 

Hubble’s invention of the relationship was afterwards used as confirmation that the universe is expanding. It’s like dots on the top of an expanding balloon; no matter which dot you choose, every other dot is shifting away from each other, and the dots too far away are moving the fastest. 


Of course, the expanding surface of a balloon is a two-dimensional shape, whereas the expanding universe is a three (or more) dimensional shape. Balloons also expand into existing space, whereas the universe encompasses all of space and thus cannot expand into anything.

3: Visualise the expansion of universe 


Hubble also realized that the distant objects he had been observing were located far beyond the Milky Way. This was the first-time humans discovered structures outside of the Milky Way. We now know that the objects observed by Hubble are galaxies, same to our own Milky Way galaxies, and that there are billions of them in the observable universe. 

Hubble’s invention added to the proof for the Big Bang theory, which was first suggested by Georges Lemaitre in 1927. It was a significant step forward for astronomy. 

Evidence supporting the Big Bang 

Blueshift and redshift 

When light passes through the drop of water, it separates into various color bands because white light is a combination of rainbow colors. This band of various colors is called the spectrum. The heated gases of stars produce light, and when this light passes through an outer atmosphere of a star, some light is absorbed. When scientists noticed a spectrum of this starlight, they saw the absorbed light had fallen from the spectrum and formed dark lines called absorption lines. When you look at a spectrum from a galaxy, you can see that the absorption line pattern does not exist at the same point in the spectrum. The position of the pattern looks shifted because the galaxies are moving away from each other and the space between them has increased. 

When the absorption lines of the spectrum shift towards the end of the blue color of the spectrum, it is called a blueshift. This means the galaxy is moving in the direction of us. If the absorption lines are shifted towards the red zone of the spectrum, then it is called a redshift; this means that the galaxy is moving away from us. The absorption lines of almost all galaxies are red shifted. As there is an expansion in space, the absorption lines show an expansion redshift. There is no center point to this enlargement. Suppose if we consider ourselves to be in the center of the galaxy, then we can see that the other galaxies are moving away from each other. 

 4: Spectrum 
5. Redshift 
6: Red shift and blue shift 

Astronomers think that in the past galaxies must have been close to each other. In the beginning, the universe was very small, hot, and dense, but after that, the universe expanded very quickly. This expansion of the universe is called the big bang, and it sent matter in all directions. 

The Cosmic Microwave Background Radiation (CMBR) 

The Big Bang theory states that the universe was initially very hot and dense. It cooled as it expanded (your refrigerator works on the same principle, expanding a liquid into a gas to cool the inside). Cosmologists calculated the theoretical temperature of today’s universe and began looking for evidence of it. 

Arno Penzias and Robert Wilson discovered CMBR accidentally in 1964 when they captured ‘noise’ in an antenna they had built to research how radio signals could be reflected off orbiting satellites. They initially mistook it for radio interference from nearby New York City, but it was eventually identified as radiation from beyond the Milky Way. 

Penzias and Wilson observed cosmic microwave background radiation (CMBR), which is leftover heat radiation from the Big Bang. Today, CMBR is extremely cold due to the universe’s expansion and cooling. It’s only 2.725 Kelvin (-270.4 °C), or 2.725 degrees Celsius above absolute zero. 

Cosmic microwave background radiation pervades the entire universe and can be detected in all parts of the sky at any time of day or night. 

Learning about CMBR 

COBE, the Cosmic Background Explorer satellite, was launched in 1992 to search for small variations in CMBR temperature. The Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft was launched in 2001 to measure variations more precisely. Cosmologists believe that small temperature variations in the CMBR are caused by differences in matter density in the early universe. Different densities resulted in the formation of galaxies and stars. 

7: Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft 

The WMAP poll 

The WMAP survey produce  d a detailed temperature map of the entire sky after nine years of observation. The colors represent minute variations in the temperature of the background radiation. These are the locations where galaxies formed. 

According to the WMAP survey, CMBR is nearly the same in all directions. Red spots are slightly warmer than blue spots, but the difference is only about 0.0002 of a degree. 

The WMAP survey provides compelling evidence in favor of the Big Bang theory. The radiation pattern resembles what astrophysicists expect for the universe if it originated in a dense state and expanded to its current size. 

 8: Wilkinson Microwave Anisotropy  

Combination of elements 

Some chemical elements produced shortly after the Big Bang. Elements such as hydrogen and helium The Big Bang theory predicts how much of each element was created in the beginning of the universe. The amount of each chemical seen by astronomers when looking at very old galaxies and stars agrees with the Big Bang theory. 

This proof cannot be found in new stars, like the Sun. This is due to the fact that newer stars contain chemical elements produced by older stars. As a result, the chemical composition of new stars differs greatly from that of stars that existed shortly after the Big Bang. 

9: Sun 

A look back in time 

The alternative to Big Bang theory is the Steady State theory. According to this theory, the universe does not change significantly over time. 

As we know that light takes a very long time to travel across the universe. So, when we look at distant galaxies, we are also looking back in time.  

As a result, we can see that very old galaxies differ greatly from newer galaxies. This demonstrates that the universe has changed. This evidence is more consistent with the Big Bang theory than with the Steady State theory. 

Summary of Proof 

The Big Bang was followed by a period of extremely rapid expansion and cooling known as ‘inflation.’ In March 2014, scientists from the Harvard-Smithsonian Center for Astrophysics discovered evidence to support inflation theory while studying the characteristics of cosmic microwave background radiation. Their discoveries at the Amundsen-Scott South Pole Station are thought to reflect gravitational waves’ early impact on the universe following the Big Bang. 

The universe became transparent to light about 380 000 years after the Big Bang. Gravity slowed the expansion of the universe for the next billion years. 

10: Big Bang (Inflation) 

What is the universe made of? 

Stars and galaxies are almost made of hydrogen and helium gas, with little amounts of heavier elements. Hydrogen, helium, and the other elements form the stars, galaxies, and other objects that can be identified with various types of telescopes. But, over the past many decades, astronomers have found proof for another type of matter that can not be identified with telescopes. This type of matter is known as dark matter and can be identified by only its gravitational effects on ordinary matter. Observations have shown that there is around five to six times as much dark matter in the universe as ordinary matter. 

Dark matter is made up of particles that do not absorb, reflect, or emit light and thus cannot be detected using electromagnetic radiation.  

Dark matter is material that cannot be seen with the naked eye.  

We know dark matter exists because of the effect it has on objects that we can see with our own eyes. 

 7: Dark matter 

Dark Energy  

All matter present in the universe exerts a gravitational force on all other matter, and also on dark matter. These gravitational forces could be strong enough to slow down the expansion of the universe. But, in the late 1990s, astronomers have found that the rate of expansion was increasing.  Astronomers postulated that an unidentified repulsive force was causing the expansion to speed up and it is due to the presence of a type of energy known as dark energy. According to astronomers that of all the matter and energy in the universe, we see around 75% is dark energy, around 20% is dark matter, and only around 5% is ordinary matter. 


• The Big Bang theory explains the initial development of the universe.

• Astronomers use the Big Bang as a very good example to describe the creation of matter, space, and time approximately 13.7 billion years ago.

• Hubble noticed that galaxies have a tendency to move away from the Earth.

• In 1965 scientists discovered that microwaves appear to come from all the directions in space.

• Astronomers use redshift and blueshift to determine how near and far the object is from the Earth.

• According to the WMAP survey, CMBR is nearly the same in all directions.

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

• Galaxies and stars are formed by the collection of matter into clumps due to gravity.

• Dark matter does not give off light.

• The presence of a sort of energy known as dark energy is driving the expansion to speed up due to an undetermined repulsive force.



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