6 Expansion of the Universe

Expansion on cosmic scales

On the largest scales, the average distance between galaxies that are not gravitationally bound to one another grows with time. This expansion does not mean that every object or the space within every object stretches: gravity and other forces hold together systems such as galaxies, solar systems, and atoms.

Earth is not the center of the expansion. An observer in any typical galaxy sees distant galaxies receding on average.

Scale factor and redshift

Cosmologists describe the changing size of the universe with the scale factor, usually written as $a$. It is conventionally set to $a=1$ today and was smaller in the past. As the universe expands, traveling light is stretched too; this effect is called cosmological redshift.

The redshift $z$ is related to the scale factor at the time the light was emitted by

For example, light emitted when $a=\frac{1}{3}$ has $z=2$, meaning its wavelength has been stretched to three times its original length. Redshift can also include contributions from an object’s motion relative to its surroundings, so astronomers account for local motions when studying cosmic expansion.

The Hubble relationship

For relatively nearby galaxies, the average recession speed $v$ is approximately proportional to distance $d$. This relationship, known as Hubble's law, is

Here, $H_0$, the Hubble constant, describes the present-day expansion rate and is commonly expressed in $\mathrm{km\,s^{-1}\,Mpc^{-1}}$. If $H_0$ is about $70\ \mathrm{km\,s^{-1}\,Mpc^{-1}}$, a galaxy $100\ \mathrm{Mpc}$ away has an average recession speed of about $7{,}000\ \mathrm{km\,s^{-1}}$.

This is a simple low-redshift approximation. At great distances, astronomers use a full cosmological model rather than treating speed as a simple measure of distance. The law describes large-scale expansion, not ordinary motion through space alone. In general relativity, sufficiently distant regions can have a rate of separation greater than the speed of light without locally moving through space faster than light.

The Hubble parameter changes over cosmic history, so $H_0$ is its value today, not an unchanging rate for all time.

Evidence and expansion-rate estimates

The expanding-universe picture developed through theoretical work, including that of Alexander Friedmann and Georges Lemaître, as well as observations of galaxies. In 1929, Edwin Hubble published evidence for a relationship between galaxy distance and recession.

Modern estimates of the expansion rate use multiple methods. Some results do not agree within their stated uncertainties; this discrepancy is called the Hubble tension.

The universe’s early history

The standard cosmological model describes the universe as having expanded from an early, hot, dense state about 13.8 billion years ago. The Big Bang describes the expansion of space itself; it was not an explosion from one location into pre-existing empty space. As the universe expanded, it cooled.

About 380,000 years later, the universe became transparent to light. That ancient light is observed today as the cosmic microwave background (CMB). Measurements of the CMB, together with other observations and a cosmological model, help scientists estimate the universe’s age and expansion history.

From slowing to acceleration

Gravity from matter slowed the expansion during much of cosmic history. Observations of distant supernovae in the late 1990s showed that expansion is accelerating in the more recent universe. In the standard model, this acceleration is attributed to dark energy, often represented by a cosmological constant.

The uncertain cosmic future

If dark energy behaves like a constant cosmological constant, the standard model predicts that the universe will keep expanding and that its expansion will continue to accelerate. Over immense timescales, galaxies beyond our gravitationally bound neighborhood would become increasingly difficult to observe. The cosmos would also grow colder and less able to support new activity, a broad future often called heat death.

This forecast depends on what dark energy is and how it changes. Some combinations of recent DESI measurements and other data have suggested that dark energy might evolve, but the evidence is not conclusive. DESI’s July 2026 analysis reported new measurements consistent with the standard model while noting that available results do not yet clearly distinguish it from evolving-dark-energy models. The universe’s ultimate fate therefore remains uncertain.