Cosmology

Hubble Expansion

Space grows and carries the galaxies apart. Stand on any galaxy and every other one recedes at a speed in proportion to its distance, so no galaxy is the centre. Change what the universe is made of and watch the expansion slow, speed up or turn round, and send a photon across to see its wavelength stretched with space.

home galaxyselected galaxyrecession velocity v = H dHubble sphere, v = cphoton, coloured by wavelength

Drag to turn, scroll to zoom, click a galaxy to select it

Hubble rate H(t)—

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Universe

Matter Ω_m 0.31

Dark energy Ω_Λ 0.69

Hubble constant H₀ 67.7 km/s/Mpc

Cosmic time t 13.8 Gyr

Velocity arrowsHubble sphereComoving gridDivide out the expansion (comoving view)

Numbers

Scale factor a (today = 1)—

Hubble radius c / H—

Expansion is—

Selected: distance now—

Selected: recession speed—

Photon—

What to observe

Every arrow points straight away from home, and its length grows with distance: v = H d. Make another galaxy home and the picture is the same from there. There is no centre.
The galaxies themselves do not grow, only the space between them. Tick the comoving view, which divides the growth out: then nothing moves at all.
Beyond the purple Hubble sphere galaxies recede faster than light. That breaks no rule: nothing moves through space faster than light; space between grows.
Send light from a distant galaxy early on. At first the photon is carried away from us, then the Hubble sphere grows past it and it starts to gain ground, arriving stretched from blue into the infrared.
With matter only, the expansion slows forever; with enough matter it turns round into a crunch; with dark energy it slowed for 9 billion years and has been speeding up since.

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