At What Speed Is the Universe Expanding?

The universe does not expand outward from a central point at one fixed speed. Instead, the distance between unbound galaxies generally increases over time. The rate at which that distance increases depends on how far apart the galaxies are, so a meaningful answer must give both an expansion rate and a distance.

Astronomers describe the present rate with the Hubble constant, usually written $H_0$. Its approximate value is $70$ kilometers per second per megaparsec (km/s/Mpc). That means that, on average, a galaxy in the general cosmic expansion is receding about $70$ km/s faster for every megaparsec of distance from us.

Expansion rate is not one speed

The Hubble law for the present universe is

$$v_{\text{rec}} = H_0 D,$$

where $D$ is a galaxy’s proper distance at the present cosmic time and $v_{\text{rec}}$ is its recession speed due to cosmic expansion. A megaparsec (Mpc) is about 3.26 million light-years.

Because distance appears in the equation, the recession speed has no single value for all galaxies. Nearby galaxies have small expansion-related recession speeds; more distant galaxies have larger ones. The number $H_0$ is a rate of expansion, not a speed by itself. Its units, km/s/Mpc, express how much additional recession speed corresponds to each unit of distance.

The value is approximate. Different measurement methods give somewhat different estimates, and the result depends on the cosmological model used to interpret observations. For a quick calculation, $H_0 \approx 70$ km/s/Mpc is a useful rounded present-day value.

A worked example

Suppose a distant galaxy is $100$ Mpc away and follows the average cosmic expansion, with its own motion relative to that flow neglected. Then

$$\begin{aligned} v_{\text{rec}} &\approx H_0D \\ &\approx (70\ \text{km/s/Mpc})(100\ \text{Mpc}) \\ &\approx 7{,}000\ \text{km/s}. \end{aligned}$$

The megaparsecs cancel, leaving a speed. If the distance were $200$ Mpc, the expansion-related recession speed would be about $14{,}000$ km/s. Real galaxies can also have peculiar velocities—motions caused by local gravitational influences—so their measured motion need not match the Hubble-law value exactly, especially at relatively small distances.

Can a galaxy recede faster than light?

Yes. In the standard expanding-universe description, the recession speed is the rate of change of proper distance across the universe at a given cosmic time. Since $v_{\text{rec}}=H_0D$, it exceeds the speed of light when the distance is greater than roughly $c/H_0$—about 14 billion light-years for $H_0\approx70$ km/s/Mpc.

This does not mean that the galaxy is locally racing past us through space faster than light. Special relativity forbids objects from overtaking light in their local surroundings; it does not impose the same limit on how quickly the distance between very widely separated locations can grow in an expanding spacetime. The comparison is between distances defined on a cosmological time slice, not a measurement of one object’s local speed past another.

The distance $c/H_0$ is called the Hubble radius. It is not the edge of the observable universe or a boundary beyond which nothing can be seen. Light from regions that are currently receding faster than light can still reach us as the expansion rate changes over cosmic history. At large distances, the relation between an observed redshift and a galaxy’s current recession speed also requires a cosmological model; it is not generally a simple Doppler-speed calculation.

What is actually expanding?

Cosmic expansion describes the large-scale increase in distance between locations that move with the average cosmic flow. It does not mean that every object, or every structure, grows in size. Galaxies, solar systems, and atoms are bound by forces that dominate over the expansion on those scales. The Milky Way and Andromeda, for example, are moving toward each other under gravity rather than simply separating with the Hubble flow.

Common mistakes

• Calling $H_0$ a speed: It is a rate with units of speed per distance. To get a recession speed, multiply it by a distance.
• Giving a speed without a distance: “The universe expands at 70 km/s” leaves out “per megaparsec.” A galaxy’s recession speed depends on how far away it is.
• Treating every redshift as an ordinary Doppler shift: For distant galaxies, observed redshift reflects the expansion history of space. Converting it to a current distance or recession speed requires a cosmological model.
• Assuming faster-than-light recession breaks relativity: The recession rate over cosmological distances is not a local speed through space, so it does not contradict the local light-speed limit.
• Assuming expansion pulls apart bound objects: The Hubble flow is a large-scale description. It does not make the size of a gravitationally bound galaxy increase at the same rate.