What Is the Big Bang Theory?

The theory describes the evolution of the universe from about a tiny fraction of a second after its beginning up to the present, roughly 13.8 billion years later. It says that everything was once far denser and hotter, and that space itself has been expanding.

It does not describe the moment of origin. The physics we have breaks down at extremely high densities, so what happened at the very start, or whether that question is even well formed, remains open. The theory is about what came after, and that part is very well tested.

Three independent observations support it strongly:

• •Distant galaxies are moving away from us, with speed proportional to distance
• •The cosmic microwave background, faint radiation left from when the universe cooled enough to become transparent
• •The measured proportions of hydrogen, helium and lithium, which match predictions from the first few minutes

This is the strongest single piece of evidence. About 380,000 years after the beginning, the universe cooled enough for atoms to form and light to travel freely. That light is still arriving, stretched by expansion into microwaves.

It was found accidentally in 1965 by radio astronomers who could not get rid of a persistent hiss in their antenna. Later satellites mapped it in detail, revealing tiny temperature variations that match what the theory predicts for the seeds of galaxies.

Several common images are wrong. The big bang was not an explosion at a point in existing space. It was an expansion of space itself, happening everywhere at once, so there is no centre to point to.

Nor are galaxies flying outwards through space like debris. The space between them is stretching. A useful mental image is dots on a balloon being inflated: every dot moves away from every other, and no dot is the centre.

Plenty. About 95 percent of the contents of the universe is dark matter and dark energy, neither of which is understood. There is also active debate over measurements of the expansion rate, where different methods give results that do not quite agree.

These are real open problems, not signs the framework is collapsing. The big bang theory is the best supported account we have of cosmic history, and the disagreements are about details within it rather than whether it is broadly right.

The name was coined by the astronomer Fred Hoyle during a radio broadcast in 1949, and Hoyle did not believe the theory. He favoured a rival steady state model in which the universe had no beginning and matter was continuously created.

Whether he intended the name as mockery is disputed, and he later said he was simply trying to draw a vivid contrast for listeners. Either way the label stuck to the theory it was meant to distinguish itself from, and the discovery of the cosmic microwave background in 1965 effectively ended the steady state model.

A common puzzle goes like this. If the universe is about 13.8 billion years old and nothing travels faster than light, surely we can only see objects up to 13.8 billion light years away. In fact the most distant regions we can observe are now estimated to be about 46 billion light years from us, giving an observable universe roughly 93 billion light years across. The difference comes from the expansion of space while the light was still travelling towards us.

Light that left a distant region long ago had to cross space that kept stretching during its journey. By the time it reaches us, the region it came from has been carried much farther away. This does not break the speed limit of relativity, which applies to objects moving through space rather than to the growth of space itself. The observable universe is simply the part from which light has had time to reach us. The whole universe may be far larger, and nobody knows whether it is finite or infinite.

One of the theory's most impressive successes can be checked with simple arithmetic. In the first second or so, protons and neutrons were constantly converting into each other. As the universe cooled, this stopped, and free neutrons began slowly decaying. By the time conditions allowed nuclei to hold together, a few minutes in, there was roughly one neutron for every seven protons. Take 16 particles in that ratio: 2 neutrons and 14 protons. The 2 neutrons combine with 2 protons to make one helium-4 nucleus, and the other 12 protons stay as hydrogen nuclei.

That helium nucleus holds 4 of the 16 units of mass, so about a quarter of all ordinary matter by mass should be helium, and that is very close to what astronomers measure in the oldest stars and in gas clouds containing very few heavy elements. Stars make helium too, but nowhere near enough to explain so much of it everywhere. The same reasoning explains why the big bang made almost nothing heavier than lithium. There are no stable nuclei with five or eight particles, and the universe cooled and thinned out too quickly for those gaps to be bridged.

The expanding universe was proposed before it was firmly observed. In 1927 Georges Lemaître, a Belgian physicist who was also a Catholic priest, showed that Einstein's equations allowed an expanding universe and linked this to the redshifts of galaxies. Two years later Edwin Hubble published measurements showing that more distant galaxies recede faster. Because Lemaître's paper appeared in a little-read Belgian journal, Hubble received most of the credit for decades, and in 2018 the International Astronomical Union recommended calling the relationship the Hubble-Lemaître law. Lemaître also suggested that the universe began from a single 'primeval atom', an early version of the big bang idea.

The law lets you estimate the universe's age with one division. Today's expansion rate is about 70 kilometres per second for every megaparsec of distance, where a megaparsec is roughly 3.26 million light years. A galaxy 100 megaparsecs away therefore recedes at about 7,000 kilometres per second. Run that motion backwards at a constant speed and every galaxy meets us at the same moment, about 14 billion years ago. The close match with 13.8 billion is partly lucky, since the expansion rate has changed over time. Hubble's own early value was about seven times too large, implying an age of only around 2 billion years, which clashed with geology.

The big bang theory says the universe began hot and dense and has been expanding for 13.8 billion years, supported by galaxy motions, leftover radiation and the abundance of light elements, and it describes what happened after the beginning rather than the beginning itself.