Hubble’s Law states that galaxies are moving away from us at speeds proportional to their distance. The farther a galaxy is, the faster it recedes. This simple relationship, expressed as velocity equals the Hubble constant times distance (v = H₀ × d), is the foundational evidence that the universe is expanding.
The Equation and What It Means
The law is written as v = H₀ × d, where v is a galaxy’s recessional velocity in kilometers per second, d is its distance in megaparsecs (one megaparsec equals about 3.26 million light-years), and H₀ is the Hubble constant, a number that describes how fast the universe is expanding per unit of distance.
What makes this law so striking is its linearity. A galaxy twice as far away moves twice as fast. One ten times farther moves ten times faster. This pattern isn’t because galaxies are flying through space like shrapnel from an explosion. Instead, the space between galaxies is itself stretching. A useful analogy is a raisin cake rising in the oven: as the dough expands, every raisin moves away from every other raisin, and raisins that started farther apart separate faster, not because they’re moving through the dough but because more dough is expanding between them.
Why Farther Means Faster
The proportional relationship between distance and speed follows directly from uniform expansion. If every stretch of space grows at the same rate, then more space between two objects means more total stretching. Imagine a rubber band with marks every centimeter. If the band doubles in length, marks that were 1 cm apart are now 2 cm apart, but marks that were 10 cm apart are now 20 cm apart. The farther apart they started, the greater the distance they gained in the same amount of time.
This is why Hubble’s Law doesn’t point to a center of the universe. Every observer, no matter which galaxy they sit in, would see the same pattern: all other galaxies receding, with more distant ones receding faster. The expansion is a property of space itself, not motion through it.
Redshift: How We Measure Expansion
Astronomers detect this expansion by analyzing the light from distant galaxies. As space stretches, the wavelength of light traveling through it stretches too, shifting it toward the red end of the spectrum. This cosmological redshift is distinct from the ordinary Doppler effect you experience when a siren changes pitch as it passes you. The Doppler effect comes from motion through space and can shift light toward either red or blue depending on direction. Cosmological redshift is always positive because the universe is always expanding.
The amount of redshift directly reflects how much the universe has grown since the light was emitted. A galaxy whose light has a redshift of 1 emitted that light when the universe was half its current size. In practice, a galaxy’s total redshift has both a cosmological component from expansion and a smaller component from the galaxy’s own movement relative to its surroundings, but for distant galaxies the expansion dominates.
Measuring the Distances
Getting velocity from redshift is relatively straightforward. Measuring distance is the hard part, and astronomers rely on objects called standard candles, things whose true brightness is already known. If you know how bright something actually is and measure how bright it appears from Earth, you can calculate how far away it must be.
Cepheid variable stars are one of the most important standard candles. These stars pulse in brightness on a regular cycle, and the length of that cycle is directly tied to the star’s true luminosity. A Cepheid that pulses slowly is more luminous than one that pulses quickly. The most luminous Cepheids shine 40,000 times brighter than the Sun, making them visible in galaxies millions of parsecs away. Edwin Hubble himself used Cepheids in the 1920s to establish that other galaxies existed and that they were receding.
For galaxies too far away for individual stars to be resolved, astronomers use Type Ia supernovae. These occur when a specific type of dead star accumulates enough material to explode, and because the conditions are nearly identical each time, these explosions all reach roughly the same peak brightness: about 4 billion times the luminosity of the Sun. That consistency makes them visible across billions of light-years and reliable as distance markers.
The Hubble Constant and Its Disputed Value
The slope of the line when you plot galaxy distance against recession velocity gives you the Hubble constant, H₀. Its value tells you the current expansion rate: how many additional kilometers per second of recession speed you get for each megaparsec of distance. A higher Hubble constant means a faster-expanding universe.
The problem is that two different methods of measuring H₀ give two different answers, and neither seems to be wrong. Observations of the cosmic microwave background, the radiation left over from the early universe, yield a value of about 67 to 68 km/s/Mpc. Telescope measurements of nearby galaxies using Cepheids and supernovae consistently land higher, around 72 to 73 km/s/Mpc. The James Webb Space Telescope’s largest study of this question found a value of 72.6 km/s/Mpc, nearly identical to what the Hubble Space Telescope measured for the same galaxies.
This gap, known as the Hubble tension, is statistically significant. Combining multiple independent approaches to measuring expansion in the nearby universe produces a disagreement with early-universe values between 4 and nearly 6 standard deviations, well beyond what could be explained by measurement error. Either there’s an undetected systematic problem in one set of measurements, or something about our understanding of the universe’s evolution is incomplete.
Estimating the Age of the Universe
One of Hubble’s Law’s most powerful applications is estimating how old the universe is. If you imagine rewinding the expansion, running the film backward until everything was in the same place, the time that takes is roughly the inverse of the Hubble constant. Using an H₀ of 72 km/s/Mpc, this “Hubble time” comes out to about 13.6 billion years.
The actual age depends on what the universe is made of and how its expansion has changed over time. In a universe full of matter and no dark energy, gravity would have slowed the expansion, meaning things were moving faster in the past and the true age would be shorter, around 9 billion years. In a universe with dark energy accelerating the expansion, the age turns out to be slightly longer. The best current estimate, accounting for all of these factors, is 13.8 billion years.
Why It’s Now Called the Hubble-Lemaître Law
In 2018, the International Astronomical Union voted to officially rename the law the Hubble-Lemaître Law, with 78% of members in favor. The change recognized Belgian astronomer Georges Lemaître, who independently derived the relationship between galaxy distance and recession velocity and published his work in 1927, two years before Hubble’s more famous 1929 paper. Lemaître’s original publication appeared in a less widely read French-language journal, and his contribution went largely unrecognized for decades. The renaming was intended to honor both scientists’ roles in establishing one of modern cosmology’s most fundamental discoveries.