The universe is expanding
In 1929 Edwin Hubble showed that distant galaxies recede from us, and the farther they are, the faster they go. Run the film backwards and everything converges on a single origin.
Hubble–Lemaître law · 192913.8 billion years of history
Everything began
in a single moment.
A clear, serious guide to the Big Bang: what happened in the first fractions of a second, the evidence that lets us know it, and the questions science is still working to answer.
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13.8 Gyr
Age of the universe, from Planck satellite data
2.725 K
Temperature of the cosmic microwave background today
10−43 s
Planck time, the earliest moment physics can describe
380 kyr
Age when the universe first became transparent to light
The observable universe
A sphere 93 billion light-years across, with us at the centre of what we can see
Every observer sits at the centre of their own observable universe: the region from which light has had time to reach them since the Big Bang. Beyond that horizon, the universe continues, unseen.
93 GlyDiameter of the observable universe
~2 × 10¹²Galaxies within it, by current estimates
46.5 GlyDistance to the horizon in every direction
68 %Dark energy, driving its accelerating expansion
The first moments
From a hot, dense state to the first stars
The Big Bang is not an explosion in space. It is the expansion of space itself, starting from a state hotter and denser than anything that exists today.
Planck epoch
Gravity, space and time as we know them do not yet apply. No current theory describes this moment.
Inflation
Space expands by a factor of at least 10²⁶ in a tiny fraction of a second, smoothing the universe.
Quark epoch
A seething plasma of quarks, gluons and photons. The four fundamental forces take their separate forms.
Protons & neutrons
Quarks bind into hadrons. Matter narrowly wins over antimatter, leaving the particles everything is made of.
Nucleosynthesis
The first atomic nuclei form: about 75% hydrogen and 25% helium by mass, with traces of lithium.
Recombination
Electrons join nuclei to form atoms. Light travels freely for the first time: the cosmic microwave background.
First stars
After the dark ages, gravity gathers gas into the first stars and galaxies. The cosmos lights up.
A telescope is a time machine
To look far away is to look into the past
Light takes time to travel. Whatever you look at, you see it as it was when its light left. The farther the object, the older the image. Pick a target.
The Moon
1.3seconds ago
The light you see from the Moon left its surface 1.3 seconds ago. Even our closest neighbour is already a glimpse of the past.
Distance384,400 km
Light travel time1.3 s
Back thenYou had just started reading this sentence.
Why we know
Three independent lines of evidence
The Big Bang model is accepted not because it is elegant, but because very different measurements all point to the same story.
The afterglow is still here
Discovered by accident in 1965, the cosmic microwave background is light released 380,000 years after the beginning, cooled by expansion to 2.725 K and filling every direction of the sky.
Penzias & Wilson · Nobel Prize 1978The recipe of the elements
The observed amounts of hydrogen, helium and deuterium across the cosmos match what a hot early universe would have cooked in its first twenty minutes, to remarkable precision.
Primordial nucleosynthesisCommon misconceptions
Myths vs. facts
The Big Bang is one of the most misunderstood ideas in science. Most of the confusion comes from the name itself.
MythThe Big Bang was an explosion in space.
FactIt is the expansion of space itself. There was no empty space for anything to explode into: space, and distance, came with it.
MythIt happened at one point, the centre of the universe.
FactIt happened everywhere at once. Every galaxy sees the others receding, and none of them is the centre. There is no centre.
MythThe universe is expanding into something.
FactDistances between galaxies grow because space stretches, not because galaxies fly outward into a pre-existing void.
MythThe theory explains the very beginning.
FactIt describes how the universe evolved from an early hot, dense state. The instant t = 0 itself is beyond current physics.
MythNothing can recede faster than light, so distant galaxies cannot.
FactGalaxies beyond about 14 billion light-years do recede faster than light. Nothing moves through space faster than light; space itself has no speed limit.
Myth"Big Bang" was coined by its supporters.
FactFred Hoyle, its most famous opponent, used the phrase on BBC radio in 1949 as a dismissive nickname. It stuck.
The people
Six minds that changed how we see the beginning
The Big Bang was not one discovery but a chain of them, over half a century, by people who often did not believe what they were finding.
Henrietta Swan Leavitt
Discovered in 1912 that certain pulsating stars, Cepheids, reveal their true brightness through their rhythm. This gave astronomy its first reliable cosmic ruler, without which Hubble could not have measured anything.
Georges Lemaître
In 1927, from Einstein's equations, derived that the universe must be expanding, two years before Hubble's data. In 1931 proposed the "primeval atom", the first version of the Big Bang.
Edwin Hubble
Showed in 1924 that Andromeda is a galaxy far outside our own, then in 1929 that galaxies recede faster the farther they are. The universe was suddenly enormous, and growing.
George Gamow & Ralph Alpher
Calculated how a hot early universe would forge hydrogen and helium in its first minutes, and predicted that a faint afterglow, a few degrees above absolute zero, should still fill the sky.
Arno Penzias & Robert Wilson
Trying to remove a stubborn hiss from a radio antenna, they found it came from every direction in the sky. It was Gamow's predicted afterglow. Nobel Prize in Physics, 1978.
Vera Rubin
In the 1970s measured that stars at the edges of galaxies orbit far too fast for the visible matter to hold them. Her work made dark matter, most of the mass in the universe, impossible to ignore.
The frontier
What we still do not know
Ordinary matter, everything you have ever seen or touched, makes up about 5% of the universe. The rest is an open problem.
What came before?
The Big Bang describes the evolution of the universe from an extremely early state. Whether "before" is even a meaningful question is itself under debate.
What drove inflation?
Inflation explains why the universe is so uniform, but the field responsible has not been identified, and its signature in the sky is still being searched for.
What is dark matter?
About 27% of the cosmos is matter that neither emits nor absorbs light. We see its gravity everywhere, but not a single particle of it has been detected directly.
Why is expansion accelerating?
Since 1998 we know the expansion is speeding up, attributed to dark energy, roughly 68% of everything. Its nature is one of the deepest puzzles in physics.
Frequently asked
Short answers to common questions
What is the Big Bang?
The Big Bang is the scientific model describing how the universe expanded from an extremely hot, dense early state about 13.8 billion years ago. It is not an explosion in space but the expansion of space itself, and it is supported by the observed expansion of the universe, the cosmic microwave background and the abundance of light elements.
How old is the universe?
About 13.8 billion years. The most precise value, 13.787 ± 0.020 billion years, comes from the Planck satellite measurements of the cosmic microwave background published in 2018.
What was there before the Big Bang?
Nobody knows. In the standard model, time as we measure it begins with the Big Bang, so "before" may not be a meaningful question. Several hypotheses exist, such as eternal inflation, a bouncing universe or quantum gravity scenarios, but none is confirmed by observation.
What is the cosmic microwave background?
The cosmic microwave background (CMB) is light released about 380,000 years after the Big Bang, when the universe cooled enough for atoms to form and became transparent. Stretched by expansion, it now reaches us as microwaves with a temperature of 2.725 kelvin, uniform across the whole sky.
What is the evidence for the Big Bang?
Three independent observations: distant galaxies recede faster the farther they are (Hubble–Lemaître law, 1929); the cosmic microwave background fills the sky at 2.725 K (discovered 1965); and the measured amounts of hydrogen, helium and deuterium match what a hot early universe would have produced in its first minutes.
What is cosmic inflation?
Inflation is a proposed period of extremely rapid expansion between roughly 10⁻³⁶ and 10⁻³² seconds after the Big Bang, during which space grew by a factor of at least 10²⁶. It explains why the universe is so uniform and flat, but the field that drove it has not been identified.
What are dark matter and dark energy?
Ordinary matter is only about 5% of the universe. Dark matter, about 27%, is matter detected only through its gravity. Dark energy, about 68%, is whatever causes the expansion of the universe to accelerate. The nature of both is unknown.
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