How this instrument works
Hubble's law states that a galaxy's recession velocity is directly proportional to its distance: v = H₀ × d. Double the distance and the velocity doubles; there is no exponent and no offset, just a straight multiplication. The proportionality is not galaxies flying through static space — it is space itself stretching, carrying galaxies apart the way dots drawn on an inflating balloon spread from one another without any single dot actually crawling across the rubber. Edwin Hubble measured this in 1929 by comparing the redshift of light from dozens of nebulae against distances estimated from Cepheid variable stars, and the straight-line fit became the first observational evidence that the universe is expanding rather than static.
The constant H₀ carries the odd unit kilometres per second per megaparsec, mixing a speed with a distance because it is really an expansion rate: for every megaparsec of separation, space adds about 70 km/s of recession speed. That figure is not fixed by theory, it is measured, and different methods disagree by a few percent. Distance-ladder measurements built from Cepheids and Type Ia supernovae in the nearby universe give a higher value, near 73, while inferring H₀ from the cosmic microwave background, as the Planck satellite did, gives closer to 67. This unresolved gap is called the Hubble tension.
The law only holds at cosmological scale. Andromeda, a mere 0.78 Mpc away and gravitationally bound to the Milky Way inside the Local Group, is actually falling toward us at about 110 km/s — local gravity overwhelms the metric expansion the law describes. Run the relation in reverse, d/v = 1/H₀, and it gives a rough age for the universe: about 13.9 billion years for H₀ = 70, remarkably close to the 13.8 billion years found independently from the cosmic microwave background.
- Enter the expansion rate in the Hubble constant, km/s/Mpc field — 70 is the commonly used working value; recent measurements range from about 67 to 73.
- Enter the object's proper distance in the Distance, Mpc field. One megaparsec is roughly 3.26 million light-years.
- Read the Recession velocity, km/s field — the instrument multiplies the two inputs directly, with no rounding hidden along the way.
- Remember the scale the law applies to: it describes cosmological expansion, not gravitationally bound systems like our own galaxy cluster.
Worked example — a galaxy 100 megaparsecs away
Take a galaxy measured at 100 Mpc, about 326 million light-years, with a Hubble constant of 70 km/s/Mpc, the standard working value. The law gives v = H₀ × d = 70 × 100 = 7,000 km/s. That is the recession velocity attributable purely to cosmic expansion, over two percent of the speed of light, derived from nothing more exotic than a redshift measurement and a multiplication.
Push the distance to 200 Mpc with the same constant and the velocity doubles exactly, to 14,000 km/s — the direct proportionality that gives the law its name rather than a curve needing calibration at every point. Run 7,000 km/s back through v ⁄ H₀ and the same 100 Mpc falls out, which is exactly how astronomers turn a measured redshift into a distance for objects too far away for parallax or standard candles to reach.
Questions
What does the Hubble constant actually measure?
It is the universe's current expansion rate: how many kilometres per second of recession velocity appear for every megaparsec of distance. A value of 70 km/s/Mpc means a galaxy 1 Mpc away recedes at 70 km/s, one at 10 Mpc recedes at 700 km/s, and so on — a single number that sets the scale for the whole linear relation.
Why is recession velocity exactly proportional to distance?
Because the expansion is a property of space itself, applied uniformly everywhere. If every patch of space stretches by the same fractional amount per unit time, a point twice as far away accumulates twice as much stretching, and therefore twice the recession speed — no exponent or offset needed, just distance times a rate.
Does this mean the Milky Way sits at the center of the universe?
No. Any observer anywhere would see the same proportionality, with every other galaxy receding from them. It is a consequence of uniform expansion, not a special vantage point — the balloon-surface analogy makes the point exactly, since every dot on an inflating balloon sees every other dot moving away, with none of them at a true center.
Why doesn't Andromeda obey Hubble's law?
Because it is close enough to be gravitationally bound to the Milky Way rather than carried along by cosmic expansion. At 0.78 Mpc, the law would predict roughly 55 km/s of recession; instead Andromeda is approaching at about 110 km/s, since local gravity inside the Local Group dominates over the much weaker expansion term at that scale.
How does H₀ give an age for the universe?
Inverting the law, 1/H₀ has units of time and gives a rough age if the expansion rate had always been constant: for H₀ = 70 km/s/Mpc that works out to about 13.9 billion years, close to the 13.8-billion-year figure from detailed cosmic microwave background modelling, which also accounts for how the rate has changed over cosmic history.
Why do different methods give slightly different H₀ values?
This is the Hubble tension: distance-ladder measurements using nearby Cepheids and Type Ia supernovae consistently give around 73 km/s/Mpc, while inferring H₀ from the cosmic microwave background's structure gives around 67. The roughly eight percent gap remains unresolved and is one of the more actively debated puzzles in modern cosmology.