The Big Bang

The Big Bang Theory explains that the Universe began about 13.8 billion years ago as an extremely hot, dense state and has been expanding ever since. It was not a bomb exploding in space; it was space itself beginning to expand.

Expansion, not an explosion

Imagine dots on a balloon. As the balloon inflates, every dot moves away from every other dot. There is no special centre on the surface. In the Universe, galaxies are like the dots, and space is like the balloon’s rubber stretching.

Key evidence for the Big Bang

• Redshift of galaxies: Light from galaxies moving away is stretched to longer wavelengths (towards red). More distant galaxies show greater redshift, meaning the Universe is expanding.
• Hubble’s law: The speed a galaxy moves away is proportional to its distance. $$v = H_0 d \quad \text{and} \quad \text{age} \approx t = \frac{1}{H_0}$$ Here, H₀ is the Hubble constant.
• Cosmic Microwave Background Radiation (CMBR): A faint microwave glow is detected in every direction. It is the cooled “afterglow” from the early hot Universe, stretched into microwaves as space expanded.
• Measuring distances with supernovae: Very bright exploding stars (supernovae) act as standard candles. Their known brightness lets astronomers find how far away galaxies are, which supports Hubble’s law.

Simple timeline

• Very early: Universe extremely hot and dense; tiny particles formed.
• First minutes: Nuclei of hydrogen and helium formed.
• About 380,000 years: Atoms formed, light could travel freely; CMBR released.
• Hundreds of millions of years: First stars and galaxies formed.
• Today: Space continues to expand; distant galaxies move away faster because there is more space between us and them.