The universe is 13.8 billion years old, yet we can see 46 billion light-years in every direction. Space itself is stretching, and it is the one idea that explains everything else on this page.
A light-year is the distance light travels in a year: about 9.46 trillion km. Look up and you see the past. The Moon is 1.3 seconds ago. The Sun is 8 minutes ago. The nearest other star, Proxima Centauri, is 4.2 years ago. Andromeda, the nearest big galaxy, is 2.5 million years ago, when the first stone tools were being chipped in Africa. The more distant the object, the further back in time you look. An astronomer is also a historian.
To use the yardstick, you need to know how far things are. In 1912 Henrietta Leavitt, working as a human “computer” at Harvard, found that a type of pulsating star, the Cepheid variables, has a pulse rate that tells you its true brightness. Compare that with how bright it looks and you can calculate the distance. It was the first reliable step on what is now called the cosmic distance ladder.
From 1912 Vesto Slipher split the light of spiral nebulae (what we now call galaxies) into its rainbow of colours and found the whole pattern shifted towards red. Light from something moving away is stretched towards the red end, a bit like a siren dropping in pitch as an ambulance drives off. So the galaxies seemed to be moving away from us. In 1927 the Belgian priest and physicist Georges Lemaître worked out from Einstein's equations that the universe should be expanding, and that the further away a galaxy is, the faster it should recede. In 1929 Edwin Hubble, using Leavitt's stars to measure distances, confirmed it with data. Since 2018 the relationship has officially been called the Hubble–Lemaître law:
The speed of recession is proportional to distance. H0 is about 70 km/s for every megaparsec (3.26 million light-years). A galaxy 100 Mpc away recedes at about 7,000 km/s. Double the distance, double the speed.
This is not galaxies flying through space away from us. They hardly move through space at all. Space between them is growing. Think of a loaf of raisin bread rising in the oven: every raisin sees every other raisin moving away, and the further away, the faster. No raisin is the centre.
The light is stretched, too, as the space it travels through expands. Light is a wave, and the length of each wave sets its colour: short waves are blue, long waves are red. Stretch the waves and the colour slides towards red. This is cosmological redshift, and it is measured by z: wavelength observed = wavelength emitted × (1 + z). A galaxy at z = 1 had light that doubled in wavelength on the way here, and the universe was half its present size when the light left.
If you run the expansion backwards, everything converges on a hot, dense beginning. Setting that beginning at 1/H0 gives about 14 billion years, close to the measured age of 13.8 billion years.
For the first 380,000 years, the universe was a hot glowing fog of free electrons and nuclei that scattered light, like the inside of a star. When it cooled to about 3,000 °C, the electrons and nuclei combined into atoms, and the light was suddenly free to travel. That light is still arriving from every direction, now stretched by a factor of about 1,100 into microwaves at 2.725 K (about −270 °C, a few degrees above the lowest temperature possible). It is called the cosmic microwave background, the CMB.
It was found by accident in 1964 by two radio engineers, Arno Penzias and Robert Wilson at Bell Labs in New Jersey, who could not get rid of a faint hiss in their horn antenna. They cleaned out the pigeon droppings and the hiss was still there. They shared the 1978 Nobel Prize for the discovery.
The CMB is smooth to about one part in 100,000. Those tiny hot and cold spots are the seeds of every galaxy and cluster in the universe: slightly denser patches that gravity then pulled together. Satellites such as COBE, WMAP and Planck have mapped them in increasing detail.
Because the farthest light is the oldest and most redshifted, it arrives as infrared. The James Webb Space Telescope (launched on 25 December 2021) is built for infrared, which is why it can see the first galaxies. In 2024 it confirmed a galaxy at z = 14.3 whose light left when the universe was only about 290 million years old. Use the lab on the next tab to see the redshift for yourself.
Beyond the observable universe there is more universe, but its light has not reached us yet. The edge is not a wall. It is just as far as the light has had time to come. The whole thing may be much larger than what we can see, perhaps infinite.
How fast is the universe expanding now? Two very good methods give two different answers:
| Method | What it uses | H0 |
|---|---|---|
| Early universe | The CMB, from Planck, run forward with the standard model | about 67.4 |
| Late universe | Cepheids and supernovae in nearby galaxies | about 73 |
The gap is far too big to be a measurement error, according to the teams doing the work. This is called the Hubble tension. Either something is wrong with a measurement nobody has found, or our model of the universe is missing something. At the time of writing, the argument is not settled. If you want a problem that might be solved in your lifetime, this one is a good candidate.
The hydrogen-alpha line is red light at 656.3 nm, emitted by hydrogen atoms everywhere. Drag the redshift z to see what happens to it on its journey to us.
Each dot is a galaxy. Click any galaxy to make it “home”. Arrows show how fast every other galaxy is moving away from home. Then drag the scale slider to expand the space between them.
Pick an object. The light you see left it a long time ago.
Five questions.
A friend says: “The Big Bang was an explosion, so we should be able to point to where it happened.” Explain what is wrong with that picture, using the words space, expanding, centre and raisin bread.