The Big Bang Theory

The Big Bang theory — the universe began from a single, extremely hot and dense point and has been expanding and cooling ever since, for roughly 13.8 billion years.

It is an expansion, not an explosion into empty space. Space itself expanded, and there was no "outside" for it to expand into. The word "bang" is unhelpful and worth mentally ignoring.

Two pieces of evidence, and you need both:

1. Red shift of distant galaxies. Almost all galaxies are receding, and the more distant ones recede faster. Running that expansion backwards in time brings everything together at a single point.

2. Cosmic microwave background radiation (CMB). A faint microwave radiation arriving from every direction in space, corresponding to a temperature of about 2.7 K.

Why the CMB is the stronger evidence. It is the remnant of the intense radiation from the hot early universe, released when the universe first became transparent. As space expanded that radiation was stretched to much longer wavelengths, cooling from gamma rays down to microwaves. Its existence, its uniformity in every direction, and its precise temperature were predicted by the Big Bang theory before they were observed.

The Steady State theory was the main rival. It proposed that the universe has always existed and always looked much the same, with new matter created continuously as it expands so that the density never changes.

Steady State can account for red shift, since it also involves expansion. What it cannot account for is the CMB — there is no reason within the theory for a uniform microwave glow filling all of space at a particular temperature. That is why the discovery of the CMB in the 1960s effectively settled the argument in favour of the Big Bang.

This distinction is the question that comes up, so it is worth stating cleanly: red shift is consistent with both theories; the CMB is explained only by the Big Bang.

What the theory does not claim. It does not say what came before, or what caused it, or what the universe is expanding into. It describes how the universe has developed from a very early hot dense state, and the evidence supports that description well.

Example: the CMB was discovered accidentally by two radio astronomers who spent months trying to eliminate what they assumed was a fault in their antenna, including removing pigeons nesting in it. The signal was in every direction because it fills the universe.