Every distant galaxy is moving away from us, and the farther it is, the faster it goes. That sounds like we sit at the centre of an explosion. Click any other galaxy to stand there instead, and you will see exactly the same thing.
Click any galaxy in the field to make it home. The plot redraws from there, and the slope comes out the same.
Galaxies sit on a grid that is being stretched, like raisins in rising dough or dots on an inflating balloon. Nothing moves through space here; the space between them grows. Every distance is multiplied by the same scale factor, so a galaxy twice as far away gains twice as much distance each second.
That is Hubble's law: speed = H × distance. The plot shows each galaxy's speed away from home against its distance. Small random motions scatter the points, as they do in real data, but the fitted slope stays put.
The Big Bang was not an explosion at one point in space. It happened everywhere at once, and every point has an equal claim to be its centre. The galaxies themselves do not stretch, because gravity holds them together; only the gaps between them grow.
Redshift here is not a Doppler shift from motion through space. The light's wavelength stretches by exactly the factor the universe grew while it travelled.
The model uses steady "coasting" growth, where the age is exactly 1/H. The real universe slowed down, then sped up, and the two nearly cancel: its age is 13.8 billion years, against 1/H₀ ≈ 14.0.
One straight line on a plot turned into a map of the cosmos, a clock for its age, and one of the biggest open puzzles in physics.
Measure a galaxy's redshift and Hubble's law gives its distance. Surveys such as SDSS and DESI have done this for millions of galaxies and quasars, mapping the cosmic web of filaments and voids in three dimensions.
In the demo: read the plot sideways: speed in, distance out.Light released 380,000 years after the Big Bang, when the universe was 1,100 times smaller, left as a 3,000 K orange glow. Stretched by the same factor, it arrives today as 2.7 K microwaves, mapped in detail by COBE, WMAP and Planck.
In the demo: wavelength grows by the same factor as the universe.Nearby supernovae and Cepheid stars give H₀ ≈ 73 km/s/Mpc. The early universe, read from the microwave background, predicts about 67. The gap has survived years of checks and may point to new physics.
In the demo: slide H₀ from 67 to 73 and watch the age shift by more than a billion years.In 1998 two teams used distant Type Ia supernovae as standard candles and found them dimmer than expected: the expansion is speeding up. The discovery won the 2011 Nobel Prize, and the cause, called dark energy, is still unknown.
In the demo: a changing slope over time is how acceleration shows up.The first galaxies' ultraviolet and visible light has been stretched more than ten times, into the infrared. That is why the James Webb Space Telescope observes in infrared, and how it has found galaxies seen as they were about 300 million years after the Big Bang.
In the demo: push the emission slider down and the blue line leaves the visible.1/H₀ gives a ballpark age for the universe of about 14 billion years. It must be older than its oldest stars, and globular cluster stars dated at 12 to 13 billion years agree, a cross-check between two completely different methods.
In the demo: 1 / slope reads the age straight off the plot.