Edwin Hubble · the universe is expanding — and expansion is the ultimate ruler
Every galaxy is rushing away, and the farther it is, the faster it recedes. This single pattern reaches to the edge of the observable universe.
Hubble's velocity–distance diagram: galaxy recession speed rises linearly with distance. The slope is H₀. Measure a galaxy's redshift → get recession speed → divide by H₀ → get distance.
Redshift: absorption lines in a galaxy's spectrum appear shifted toward longer wavelengths. The fractional shift gives recession speed, and Hubble's Law converts speed to distance.
After measuring many galaxies with Cepheids, Edwin Hubble noticed a pattern in their light. Almost every galaxy's spectrum was redshifted — its absorption lines displaced toward longer, redder wavelengths, exactly as sound from a receding ambulance drops in pitch.
More strikingly, the more distant a galaxy, the more strongly its light was shifted. This pointed to one conclusion: the universe itself is expanding — not galaxies flying through space, but space itself stretching, carrying galaxies apart like raisins in a rising cake.
v = H₀ × d · recession speed = Hubble constant × distance
Turn this around: measure a galaxy's redshift (easy, from any spectrum), divide by H₀ (calibrated using Cepheid distances), and you have its distance. This works for galaxies billions of light-years away — far beyond where any Cepheid could ever be seen. The top rung reaches to the edge of the observable universe: 46 billion light-years.
H₀ must be calibrated using Cepheid distances (Rung 6), which rest on parallax (Rung 5), which rest on 1 AU (Rung 4), which rests on Kepler (Rung 3), which rests on two eclipse types (Rung 2), which rests on a stick in Egypt (Rung 1). Remove any rung and the ladder collapses.
⚑ Assumptions that made it work
Redshift is caused by cosmic expansion, not something else. Alternative explanations — tired light or exotic physics — were proposed. The coherent picture from general relativity makes expansion the overwhelming consensus.
The Cosmological Principle: the universe looks the same in all directions and from all locations. If the universe were lumpy or had special directions, Hubble's Law would be a local accident. Modern observations support this on large scales.
H₀ is constant across space. Currently there is a serious unresolved disagreement — the Hubble tension — between H₀ measured from the local distance ladder and H₀ inferred from the cosmic microwave background. They disagree by about 8–10%, and no one yet knows why.
General relativity correctly describes cosmic expansion. GR has passed every observational test, but the physics of dark energy — which drives accelerating expansion — is almost entirely unknown.
✋ Try it yourself — stretch a rainbow
Redshift sounds like something only a mountaintop telescope could measure. Both halves of it are
on your kitchen table.
First, split some light. Take a CD or DVD, shiny side up, and tilt it under a
lamp until a rainbow runs across it. The disc's microscopic grooves do the same job as the prism in
a real spectrograph. Now compare what different lamps give you:
Daylight from a window, or an old-fashioned filament bulb — a smooth,
unbroken band, every colour blending into the next.
An energy-saving bulb or a fluorescent tube — the same band, but with a few
bright stripes standing out of it.
An orange street lamp, if there is one outside — nearly all the light packed
into one narrow stripe.
Why that matters: the stripes are the giveaway. Each one is made
by one particular element, and it always lands in exactly the same place — sodium's stripe is the
same orange in a street lamp, in a candle flame with salt in it, and in the Sun. A plain rainbow
carries no such marks, so a plain rainbow tells you nothing about motion. You cannot measure a
shift in something that has no lines in it. Hubble could only do what he did because starlight
comes ruled with these lines.
Now stretch one. Take a wide rubber band or a strip cut from a balloon, and
draw a row of lines on it with a marker — not evenly spaced; copy the pattern below, or invent one.
Hold it slack, look at it, then stretch it and look again.
What you have just done: that is a redshift, and it is not a
metaphor. Light from a distant galaxy is not travelling through a universe that sits still — space
itself has been stretching the whole time the light was on its way, and the wave stretches with it,
exactly like the marks on your rubber band. Longer waves are redder waves. And notice the part that
gave Hubble his law for free: the marks that started farther apart moved farther. Stretch
by a tenth, and every gap grows by a tenth. That is why the more distant galaxy is always the more
strongly shifted one.
And the honest catch: the ambulance siren on this page is a good
first picture, but it is not quite this. A siren drops in pitch because the ambulance is
moving through the air. The galaxies are mostly not moving through anything — the space
between us and them is getting longer. Your rubber band is the better model of the two, because
nothing on it is running anywhere. It is the band itself that grew.
Where this leads next. Wind the expansion backwards far enough and everything
was closer, denser and hotter — which sounds like it proves there was a beginning. It does not, on
its own: an expanding universe with no start at all was a serious rival idea for thirty years. Two
further pieces of evidence are what settle it.
How the universe began →
— the three pieces of evidence, what each one alone would allow, and where the honest answer is
that nobody knows.
⏳ Historical Anchor
When Hubble Discovered the Expanding Universe...
📉The Wall Street Crash happened in October 1929 — the same year Hubble published his law! Millions of people suddenly lost all their savings overnight, starting the Great Depression.
🐭Mickey Mouse had appeared for the very first time just one year earlier, in 1928, in a cartoon called Steamboat Willie. He spoke his first words and immediately became world famous.
🎬Talking movies were completely brand new — The Jazz Singer (1927) had just proved that films could have sound. Before that, all films were silent, with a pianist playing music live in the cinema!
👗Women wore cloche hats (like a bell pulled down over the head), bobbed hair, and loose "flapper" dresses. Men wore wide-shouldered suits with wide lapels and fedora hats.
🛩️Charles Lindbergh had just crossed the Atlantic solo in a tiny plane in 1927 — taking 33½ hours. When he landed in Paris, 150,000 people were waiting. The whole world celebrated for days.
