Galaxies are massive systems in constant motion, revealing the physics governing the universe. A galaxy’s movement is determined by two distinct influences. On small scales, gravity dominates, causing galaxies to pull toward one another and orbit common centers of mass. On large scales, the movement is a consequence of the cosmos expanding, carrying galaxies apart over vast distances.
Local Motion Driven by Gravity
Within gravitationally bound structures, such as galaxy groups and clusters, the movement of galaxies is directed by the gravitational pull of their neighbors. These local interactions overpower the general expansion of the universe, causing systems to remain clustered or draw closer together. Our Milky Way Galaxy resides in the Local Group, which contains over 50 galaxies, including the Andromeda Galaxy (M31). The Milky Way and Andromeda are approaching each other at approximately 123 kilometers per second, a motion that will lead to a collision in about 4.5 billion years. Galaxies in these groups orbit a shared center of mass, much like planets orbiting a star.
Universal Expansion and Hubble’s Law
Beyond galaxy groups, the primary force driving galactic movement is the expansion of the universe itself. On immense scales, the space between galaxy clusters is physically stretching, increasing the distance between them over time. This phenomenon is described by Hubble’s Law, which states that a galaxy’s recession velocity is directly proportional to its distance from us. The galaxies themselves are not accelerating through space; rather, the fabric of spacetime between them is expanding. This cosmic stretching causes nearly all distant galaxies to appear to recede from the Milky Way.
The rate of expansion is measured by the Hubble constant, estimated to be around 70 kilometers per second per megaparsec. Observations of distant supernovae show this expansion is accelerating, not slowing down. This acceleration is attributed to Dark Energy, an unknown repulsive force permeating all of space. Dark Energy is thought to make up approximately 68% of the total energy density of the universe.
The Influence of the Cosmic Web
Galaxies are organized into a vast architecture known as the Cosmic Web. This large-scale structure consists of dense clusters and groups connected by long filaments, which surround immense, empty regions called voids. Gravity has channeled matter along these filaments, creating a framework that dictates the flow of galaxies. The gravitational field of this web causes galaxies to have “peculiar velocity,” which is their true velocity relative to the smooth flow of cosmic expansion.
Our Local Group is part of the Laniakea Supercluster and is being drawn toward an enormous concentration of mass called the Great Attractor. This localized region of mass, estimated to be equivalent to about 10^16 solar masses, pulls the Milky Way and surrounding galaxies toward it at speeds of hundreds of kilometers per second. Peculiar velocities can reach up to plus or minus 700 kilometers per second, significantly altering a galaxy’s movement from the uniform expansion defined by Hubble’s Law.
Observing Galactic Movement
Astronomers measure galactic movement using the Doppler effect applied to light, which manifests as redshift or blueshift. Redshift occurs when a light source moves away, stretching the waves toward the red end of the spectrum. Conversely, blueshift occurs when the source moves toward the observer, compressing the waves toward the blue end. Redshift is the primary tool used to confirm Hubble’s Law and measure recession velocities caused by cosmic expansion.
By analyzing the spectral lines in a distant galaxy, astronomers can precisely calculate its speed and distance. Blueshift is much rarer and indicates local, gravity-dominated movement, such as the Andromeda Galaxy approaching the Milky Way. These measurements allow scientists to separate a galaxy’s overall motion into the velocity due to expansion and the peculiar velocity due to local gravitational forces.