Light from a distant galaxy arrives stretched: every spectral line is shifted toward the red. The fractional shift measures the recession speed, Δλ/λ ≈ v/c. Hubble found that speed grows in proportion to distance, v = H₀d, so the more distant a galaxy the faster it flees. Set the distance and watch the line slide red and the point climb the v–d line.
MissionSend the galaxy farther and watch its line slide deeper into the red as the recession speed climbs straight up the v = H₀d line.Streak 0Best 0
Compared with a galaxy at distance d, a galaxy at distance 2d recedes:
distance d200 Mpc
recession speed v = H₀d14000 km s⁻¹
redshift z = v/c0.047
observed λ of the 656 nm line687 nm
age of Universe ≈ 1/H₀14.0 Gyr
Because v = H₀d is a straight line through the origin, every galaxy sees all the others receding: the whole Universe is expanding. Running the expansion backwards points to a hot, dense beginning, the Big Bang, about 1/H₀ ago.