Chapter 6: The Expanding Universe

The Big Bang Theory         
Cosmology explains the universe began 13.8 billion years ago, expanding from a singularity into galaxies, stars, and planets.

The Big Bang Theory is the prevailing cosmological model that describes how the universe originated and evolved. It posits that around 13.8 billion years ago, the universe began from a singularity-a state of infinite density and temperature. In less than a second, this singularity expanded rapidly in an event known as the Big Bang, marking the birth of time, space, matter, and energy.

As the universe expanded, it cooled enough for subatomic particles-protons, neutrons, and electrons-to form. Within minutes, nuclear fusion created the lightest elements such as hydrogen, helium, and traces of lithium. For hundreds of thousands of years, the universe remained opaque until it cooled sufficiently for atoms to form. This process, known as recombination, allowed light to travel freely, leaving behind the Cosmic Microwave Background Radiation (CMB)-a faint glow still detectable today.

Over billions of years, gravity pulled matter together to form stars, galaxies, and eventually planetary systems. This large-scale structure is still evolving, with galaxies continuing to move apart due to the universe’s ongoing expansion.

Evidence Supporting the Big Bang Theory

1.       Cosmic Microwave Background Radiation (CMB): Discovered in 1965, it is the "afterglow" of the Big Bang and serves as direct evidence of a hot, dense early universe.

2.       Redshift of Galaxies: Edwin Hubble’s 1929 observations showed galaxies moving away from us, indicating that the universe is expanding.

3.       Elemental Abundance: The proportions of hydrogen, helium, and lithium predicted by the Big Bang nucleosynthesis match current observations.

Case Study: James Webb Space Telescope (JWST) and Early Galaxies

The James Webb Space Telescope (JWST), launched in 2021, has provided groundbreaking observations that refine our understanding of the early universe. JWST can peer back over 13 billion years, observing galaxies formed only a few hundred million years after the Big Bang.

·        Discovery of Early Galaxies: JWST identified galaxies that appear surprisingly massive and well-formed just 300–500 million years after the Big Bang, earlier than expected. This challenges current models of galaxy formation and suggests that stars and galaxies may have formed more rapidly than previously believed.

·        Study of Cosmic Dawn: By capturing infrared light, JWST detects faint signals from the universe’s first stars, providing insights into how the first elements heavier than helium were created.

·        Refinement of Expansion Models: These observations also help cosmologists test and refine the ΛCDM (Lambda Cold Dark Matter) model, the most widely accepted framework of cosmology.

This case study highlights how modern technology continues to test and expand the Big Bang theory, showing that science is not static but constantly evolving with new data.