The Big Bang

The Big Bang is the best scientific explanation we have for how the universe began. According to the theory, about 13.8 billion years ago, everything that now exists (every star, planet, person and atom in the entire universe) was packed into a single point smaller than a full stop. Then, in less than a trillionth of a trillionth of a second, that point expanded outwards and the universe was born. It has been expanding and cooling ever since. The Big Bang is not a guess: it is supported by some of the strongest evidence in all of science.

  • When13.8 billion years agoWorked out from cosmic microwave background
  • First atomsapprox. 380,000 years laterLight could travel freely from this point
  • First starsapprox. 100 to 200 million yearsAfter the Big Bang
  • First galaxiesapprox. 400 million yearsAfter the Big Bang
  • Original temperatureapprox. 10³² °CRight after the bang
  • Temperature now2.725 K-270.42 °C, the leftover heat of the Big Bang

What the Big Bang really was

The first thing to know is that the Big Bang was not an explosion in space. It was an expansion of space itself. There was no "outside" for the Big Bang to expand into; space itself was being created as it expanded. There is no centre of the universe and no edge of the universe; every point looks like the centre because everything is rushing away from every other point.

The second thing is that the Big Bang does not explain why the universe started: only what happened once it got going. The first trillionth of a second is described by ideas like inflation, where the new universe doubled in size dozens of times in tiny fractions of a second. Before that, our current physics breaks down. Nobody yet knows what (if anything) came "before" the Big Bang, or whether the question even makes sense.

The timeline of the universe

Once the Big Bang got going, the universe went through a series of stages as it expanded and cooled.

  • 0 to 10&sup-3;&sup4; seconds: tiny, almost unimaginably hot, ruled by physics we do not understand yet.
  • approx. 10&sup-3;&sup6; seconds: inflation doubles the size of the universe over and over in a tiny fraction of a second.
  • approx. 1 second: protons and neutrons form.
  • approx. 3 minutes: the first atomic nuclei (hydrogen, helium and a trace of lithium) form.
  • approx. 380,000 years: the universe cools enough for the first complete atoms to form. Light can finally travel freely. The leftover glow of this moment is the cosmic microwave background.
  • approx. 100 to 200 million years: the first stars switch on, ending the cosmic "dark ages".
  • approx. 400 million years: the first galaxies form.
  • approx. 9 billion years: our Sun and the Solar System form.
  • approx. 13.8 billion years: today.

The cosmic microwave background

The strongest piece of evidence for the Big Bang is a faint glow that fills the entire sky in every direction, called the cosmic microwave background (or CMB). It was accidentally discovered in 1964 by two American radio engineers, Arno Penzias and Robert Wilson, who at first thought their antenna was picking up bird droppings. After cleaning the antenna and still finding the signal, they realised they were detecting the leftover heat of the Big Bang.

The CMB is the light that was released when the first atoms formed about 380,000 years after the Big Bang. Stretched out by 13.8 billion years of expanding space, that light is now microwave radiation with a temperature of only 2.725 K (just under 3 degrees above absolute zero). Spacecraft like COBE, WMAP and Planck have mapped this glow across the whole sky in incredible detail. Tiny variations in its temperature show the seeds of the first galaxies.

How we know it really happened

The Big Bang theory is supported by several completely independent pieces of evidence.

  • The universe is expanding: galaxies are moving away from us, and the further away they are, the faster they are moving (Hubble's law, 1929).
  • The cosmic microwave background: the leftover heat of the very early universe.
  • The abundance of the lightest elements: the universe is made of roughly 75% hydrogen, 25% helium, with a trace of lithium. Exactly what the Big Bang predicts.
  • The cosmic web: the giant scaffolding of galaxies and dark matter exactly matches simulations of how matter would clump up after a Big Bang.
  • The most distant galaxies look younger and hotter than nearby ones, as you would expect if you are looking further back in time.
Fact The static you used to see on an old analogue television tuned to an empty channel was partly the cosmic microwave background: roughly 1% of that noise came directly from the leftover heat of the Big Bang. So in a very small way, watching TV snow was watching the birth of the universe.

Who came up with the idea?

The Big Bang theory was first put forward in 1927 by a Belgian priest and physicist called Georges Lemaitre. He noticed that Einstein's theory of general relativity predicted an expanding universe and reasoned that, working backwards in time, the whole universe must have started from a single point (which he called the "primeval atom"). Einstein initially disliked the idea and told Lemaitre, "Your calculations are correct, but your physics is abominable."

Edwin Hubble's 1929 discovery that the universe really IS expanding made the idea suddenly respectable. The name "Big Bang" was actually invented as an insult by the British astronomer Fred Hoyle in a 1949 BBC radio interview: he did not believe the theory and called it the "big bang" sarcastically. The name stuck anyway, and within a few decades the Big Bang had become mainstream science.

Did you know? Some scientists are testing whether our Big Bang might just be one of many universes in a much bigger multiverse. Ideas like eternal inflation suggest that whenever inflation happens it might create a new pocket universe, with our entire universe being just one bubble of countless others. There is no direct evidence for the multiverse yet, but the idea takes the Big Bang theory and asks: what if it happens all the time?
Deeper dive: what happens in the first second of the universe?

The first second after the Big Bang was the most action-packed second in the history of the universe. Conditions changed so fast that scientists divide it up into a long list of separate epochs, some lasting only a tiny fraction of a second.

In the first 10-43 seconds (the Planck era) the universe was so hot and dense that all four fundamental forces of nature, gravity, electromagnetism, and the strong and weak nuclear forces, are believed to have been a single unified "superforce". Our current physics simply does not work in this period.

Then between 10-36 and 10-32 seconds, the universe went through a phase called inflation. In the time it takes light to cross an atom, the universe doubled in size dozens of times, going from much smaller than a proton to bigger than a grapefruit. Inflation smoothed out the early universe and stretched tiny quantum ripples into the seeds that would later become galaxies.

After inflation, the universe was a soup of matter, antimatter and energy. For reasons still not fully understood, matter slightly outnumbered antimatter; the two annihilated each other and left only the small surplus of matter behind. By one second after the Big Bang, the universe was full of protons, neutrons, electrons and photons, and the next stages, the formation of the first atomic nuclei in the first 3 minutes, were ready to begin.

For what happens after the Big Bang, see the expanding universe and the observable universe. For the invisible 95% of the universe, see dark matter and dark energy.