Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one idea or one piece of evidence. Numbers come from NASA, ESA and OpenStax; the pictures are not to scale.
Tap the buttons and slide the sliders.
Replay both pictures. Which one has a middle and an edge? Which one happens everywhere at once?
An explosion has a centre and expands into pre-existing space. The Big Bang is the expansion of space itself: no centre, no edge.
Tap both buttons. Which one is how the universe really grew?
Tap a raisin to make it home, then drag time. Do far raisins move away faster? Try another home.
Pick any raisin as the origin: recession speed is always proportional to distance, so no raisin is the centre.
Slide time. Watch the raisins move apart as the dough grows.
The analogy breaks at the loaf’s edge: the universe has no edge or centre.
Drag the galaxy farther away. What happens to its light, and where does it land on the chart?
Move the galaxy: its redshift and recession speed grow with distance (about 70 km/s per megaparsec).
Colour shift exaggerated so you can see it.
Slide back to the first second. How hot was it? When was helium made? When did the fog clear?
The time axis is logarithmic, from 1 second to 13.8 billion years. Watch the temperature and the eras.
Slide back to the start. Does the universe get hotter or colder?
Time slider is logarithmic: each step is a much bigger jump in time.
Stretch the light until it becomes microwaves. Then switch your microwave glasses on and off.
Stretch the early universe's glow by about 1,100× to shift it into microwaves, and compare the microwave sky with the ordinary one.
Tap the glasses button. Can you see the old glow?
The warm and cool spots are exaggerated; the real ones differ by tiny amounts.
Drag the marker deeper. How old was the universe when that light left?
Farther objects are seen as they were earlier. Beyond the first galaxies lies the CMB, the “wall” we cannot see through.
Slide the marker. The farther you look, the older the light!
Stops evenly spaced, not to scale.
A long, long time ago, everything was packed close together, and it was super hot. Then space itself started to stretch. It is still stretching today!
The Big Bang was not a firework in one spot. It happened everywhere at once, even right where you are now. Space grew, like a stretchy blanket.
Think of raisin bread getting bigger in a warm oven. The dough grows, and the raisins move apart. Space is like the dough. Galaxies, giant groups of stars, are like the raisins.
At first, the universe was like a glowing, foggy cloud. Then it cooled, and the fog cleared. That old light is still all around us. Our eyes can't see it, but special antennas can.
What was there before the Big Bang? Scientists don't know yet. Maybe one day, you will help find out!
About 13.8 billion years ago, the universe was in an extremely hot, dense state, and it began to expand.12 Space itself has been stretching ever since, and as it stretched it cooled. First came tiny particles and light, then the first atoms, then stars and galaxies.16
The Big Bang was not an explosion in space. There was no single spot where it went off and no empty space outside for it to fill. It happened everywhere at once, including the place where you are sitting right now.1012 NASA's kids' site even suggests a better name might be the "Everywhere Stretch."1
The Big Bang theory is the scientific model of how our universe grew from that hot, dense beginning into the huge, cold cosmos we see today.2 It rests on two ideas: Einstein's general theory of relativity (our best theory of gravity) and the assumption that, on the very largest scales, the universe looks roughly the same everywhere and in every direction.212
The theory describes what happened after the start extremely well. It does not explain the very first instant, or what, if anything, came before.11
Imagine raisin bread rising in a warm kitchen. The raisins aren't swimming through the dough. The dough between them grows and carries them apart.39 Now pick any raisin as "home":
In the real universe, the raisins are galaxies and the dough is space. Gravity holds each galaxy together, so galaxies themselves (and you, and the Earth) don't stretch.9 The analogy has one flaw: a loaf has an edge and a middle. As far as we know, the universe has neither.9
In 1927, the Belgian priest and astronomer Georges Lemaître suggested that we live in an expanding universe.9 In 1929, Edwin Hubble announced that galaxies are moving away from us, and that the farther away a galaxy is, the faster it is going.311 Today this pattern is called the Hubble–Lemaître law.9
How could Hubble tell? From their light. Light travels as waves, and as space stretches, the waves traveling through it get stretched too. Longer waves look redder, so astronomers call this redshift.107 The farther away the galaxy, the longer its light has been traveling, and the more it has been stretched.
speed away ≈ H₀ × distance · H₀ ≈ 70 km/s per megaparsec
A megaparsec is 3.26 million light-years. So a galaxy one megaparsec farther away moves away about 70 km/s faster.310
If galaxies are moving apart today, then in the past they were closer together. Run the film of the universe backwards and everything crowds together, getting denser and hotter.105 The part of the universe we can see today was once only a few millimetres across.2
| Time after the start | Temperature (roughly) | What was happening |
|---|---|---|
| A split second | Unknown | Possibly cosmic inflation: a burst of extremely fast stretching6 |
| 1 second | 10 billion °C | A soup of particles and light46 |
| 3 to 4 minutes | Under 1 billion K | Protons and neutrons fuse into helium and a little lithium10 |
| 380,000 years | About 3,000 K | Atoms form; the glowing fog clears (the microwave background)108 |
| A few hundred million years | Cold | The first stars and galaxies light up1011 |
| 13.8 billion years (now) | 2.7 K (the leftover glow) | Galaxies, stars, planets, you5 |
For a few minutes, the whole universe was hot enough to act like a fusion oven, but not for long. Only the lightest elements could form: hydrogen, helium and a trace of lithium.410 The theory predicts that about 24% of ordinary matter should be helium made in the Big Bang, and that is what astronomers measure.4 There's far more helium than stars could have made, and even the oldest stars contain lots of it.10
Heavier elements, like the carbon in your body and the iron in your blood, were made much later inside stars. You are literally made of stardust.4
For its first 380,000 years or so, the universe was a hot, glowing fog. Light couldn't travel far before bouncing off free electrons. When things cooled to about 3,000 K, electrons and nuclei joined into atoms, the fog cleared, and the light flew free.106
That light is still arriving from every direction. Space has stretched it into microwaves, so it now corresponds to a temperature of just 2.725 degrees above absolute zero.58 It was predicted in 1948 and found by accident in 1965, when Arno Penzias and Robert Wilson couldn't get rid of a faint hiss in a giant radio antenna. They won the 1978 Nobel Prize for it.5
The glow is even across the sky to better than one part in a thousand.5 Its tiny warmer and cooler patches, mapped by the COBE, WMAP and Planck satellites, were slightly denser and thinner regions: the seeds of all later stars and galaxies.810
Light takes time to reach us, so looking far away means looking into the past. We see the Andromeda galaxy as it was about 2.5 million years ago.5 NASA's Webb telescope has confirmed a galaxy seen as it was about 280 million years after the Big Bang.7 Behind the first galaxies sits the microwave glow itself, a "wall of light" from when the fog cleared. We can't see through it with light.5
The first stars lit up a few hundred million years after the Big Bang; see how stars are formed. And when the biggest stars die, some collapse into black holes.
Honest answer: nobody knows yet. No current theory reaches back earlier than about 10−43 of a second.10 Many cosmologists think the Big Bang was preceded by cosmic inflation, a fraction of a second when space expanded extremely fast. But NASA notes that scientists aren't sure what came before inflation or what powered it.6
There are other open questions too. Since 1998 we've known the expansion is speeding up. Scientists call the unknown cause dark energy, but nobody yet knows what it is.116 And Planck's detailed map shows a few small oddities that might one day need new physics.8 That's science working as it should: a very well-tested model with exciting edges still being explored.
About 13.8 billion years ago, the universe was in an extremely hot, dense state, and space itself began to stretch. As it stretched it cooled, letting atoms, then stars and galaxies form. It's still stretching today. It wasn't an explosion at one spot in empty space: it happened everywhere at once.
Nobody knows yet. Our physics can't yet describe the very first instant. Many cosmologists think a split second of extremely fast expansion called cosmic inflation came first, but NASA notes that scientists aren't sure what came before inflation or what powered it.
No scientific theory is ever 100% proven, but the Big Bang model is very well tested. Three independent kinds of evidence support it: galaxies moving apart (redshift), the amount of helium and other light elements, and the cosmic microwave background. There are still open puzzles, such as what dark energy is and some small oddities in the microwave map.
The Big Bang theory doesn't actually say the universe came from nothing. It describes how the universe grew and cooled from a hot, dense state. What, if anything, came before that state, and why it began, are still open questions.
By about three minutes in, the universe had cooled to around a billion degrees, cool enough for protons and neutrons to stick together. For a few minutes the whole universe acted like a fusion oven, making helium and a little lithium. That's why about a quarter of ordinary matter is helium.
Everywhere. The Big Bang wasn't an explosion from a centre into empty space; all of space was stretching at once. The spot where you are now was part of it, and there is no centre or edge of the universe to point to.
As far as we know, nothing. The expansion isn't the universe spreading into a bigger empty room; it's the distances within space growing. Like the dough in raisin bread, space itself stretches, and galaxies are carried apart.