The Expanding Universe
One of the most surprising discoveries of the 20th century was that the universe is not standing still. Every galaxy in the universe is moving away from every other galaxy. The further apart they are, the faster they are moving. This is called the expansion of the universe, and it is one of the strongest pieces of evidence for the Big Bang. Even stranger, this expansion is not slowing down. It is actually speeding up, pushed by a mysterious force called dark energy.
- Discovered byEdwin HubbleIn 1929
- Rule of expansionHubble's Lawv = H×d (speed = constant × distance)
- Current expansion rateapprox. 73 km/s per MpcA galaxy 3.3 million light years away moves at 73 km/s
- Universe is expandingFaster every yearNot slowing as expected
- Cause of accelerationDark energyapprox. 68% of all the energy in the universe
- Observable universe93 billion light yearsLarger than its age in light years due to expansion
What does "expanding" mean?
It is easy to picture expansion the wrong way. Galaxies are not flying outwards through space from some central point like fragments of an exploding bomb. Instead, space itself is stretching, carrying the galaxies along like raisins baked into a rising loaf of bread. Every raisin moves away from every other raisin, but no raisin is the centre.
Another good picture is the surface of a balloon. Stick a bunch of dots on a balloon to represent galaxies, then blow up the balloon. From any dot, every other dot moves away. There is no centre of the expansion (the centre of the balloon is below the surface, not on it). And the dots are not moving across the rubber: the rubber itself is stretching beneath them.
How we know: red-shift
The expansion shows up as a tiny shift in the colour of the light from distant galaxies. As space stretches, it stretches the light waves passing through it. Long-wavelength red light gets stretched even more red, an effect called red-shift. Astronomers can compare the dark lines in a galaxy's spectrum (which are produced by specific elements at specific wavelengths) with where those lines should appear, and work out exactly how much the light has been stretched.
The bigger the red-shift, the further away the galaxy is and the faster it is moving from us. The most distant galaxies seen by the James Webb Space Telescope have red-shifts of over 13, meaning their light has been stretched to roughly 14 times its original wavelength on its 13-billion-year journey to us.
Hubble's Law
In 1929 the American astronomer Edwin Hubble measured the distances and red-shifts of several dozen galaxies. He found that the further a galaxy is, the faster it is moving away from us, and that the relationship is straight-line: double the distance, double the speed. The mathematical version is called Hubble's Law:
v = H0 × d
where v is the recession speed, d is the distance, and H0 (the Hubble constant) is roughly 73 km/s per megaparsec. That means a galaxy that is 3.3 million light years away is moving from us at about 73 km/s. A galaxy 33 million light years away is moving from us at about 730 km/s. And so on.
The accelerating universe
For decades, astronomers expected the expansion to be gradually slowing down over time, because the gravity of all the matter in the universe should be pulling everything back together. In 1998, two independent teams led by Saul Perlmutter, Brian Schmidt and Adam Riess set out to measure exactly how much the expansion was slowing.
They got a complete shock. The expansion is not slowing down at all. It is speeding up. Something invisible is pushing galaxies apart faster and faster. The three astronomers won the 2011 Nobel Prize in Physics for the discovery. The mysterious thing doing the pushing was named dark energy, and it now appears to make up about 68% of everything in the universe. Nobody yet knows what it is.
What does the expansion mean for us?
The expansion of the universe is so gentle that on small scales it has almost no effect. Galaxy clusters are held together by gravity easily; the Milky Way is not getting any bigger; the Solar System is not flying apart; your bedroom is not stretching while you sleep. Expansion only matters across the huge distances between distant galaxy clusters, where there is so much empty space between them that the gentle stretch adds up to noticeable speeds.
In the very far future, however, the accelerating expansion will start to matter. In tens of billions of years, dark energy will have pushed almost all the other galaxies in the universe so far away that they will be moving from us faster than the speed of light (which is allowed for the expansion of space itself), and we will no longer be able to see them at all. The future universe will look much emptier than ours does.
Deeper dive: the Hubble tension and the future of cosmology
Astronomers measure the Hubble constant (the current expansion rate of the universe) in two main ways. The "early universe" method uses the precise pattern of tiny ripples in the cosmic microwave background, plus our best theory of how the universe evolved. This gives a Hubble constant of about 67.4 km/s/Mpc. The "late universe" method uses Cepheid variable stars in nearby galaxies and Type Ia supernovae in galaxies further out, building a "distance ladder" to galaxies whose red-shifts are known. This gives a Hubble constant of about 73 km/s/Mpc.
Those numbers should agree, but they do not. The gap is currently around 8% and the error bars on each measurement are too small for both to be right. The disagreement is called the "Hubble tension" and it is a major puzzle in modern cosmology. Either one of the measurements is wrong in a way we have not yet found, or the standard model of cosmology itself is missing a piece of physics.
Several major new experiments are running right now to try to settle the question. The European Space Agency's Euclid spacecraft, launched in 2023, and the upcoming Nancy Grace Roman Space Telescope will both make new independent measurements. If the tension survives the next decade of better data, it could be the first crack in our current understanding of the universe.
For where this all started, see the Big Bang. For what we can and cannot see, visit the observable universe. For the mysterious push, see dark energy.