The Big Bang Theory
The Big Bang describes how the universe began 13.8 billion years ago in an extremely hot, dense state and has been expanding ever since.
Importantly:
The Big Bang was not an explosion in space
It was an expansion of space itself
Evidence for the Big Bang
Three major observations support this model:
Galaxy redshift
Distant galaxies are moving away from us — the farther they are, the faster they recede.Cosmic Microwave Background (CMB)
Faint radiation left over from the early universe, visible in all directions.Element abundance
The observed amounts of hydrogen and helium match predictions.
What Does “Expansion” Mean?
Expansion does not mean galaxies are moving through space away from a central point. Instead:
Space itself is stretching
Distances between galaxies increase
No centre or edge exists
A common analogy is dots on the surface of a balloon as it inflates.
The Observable Universe
Because light travels at a finite speed, we can only see part of the universe. This region is called the observable universe.
Beyond it, space may continue — but we cannot observe it yet.
Key Takeaways
The universe is expanding
The Big Bang explains cosmic origins
Expansion affects all large-scale distances
We can only observe part of the universe