The answer to whether galaxies are moving is yes, but the motion is far more complex than simple travel through space. The entire cosmos is a dynamic environment, constantly rearranging itself due to gravitational forces and the continuous expansion of the fabric of spacetime itself. This universal movement is not uniform, involving both local gravitational pulls and a larger-scale flow. Understanding this galactic movement requires distinguishing between the motion of individual galaxies and the overall expansion that governs the space between them. The evidence for this universal dynamism reveals a universe that is not static but rather constantly evolving.
The Observational Evidence of Galactic Movement
The primary method scientists use to confirm and measure the motion of distant galaxies involves analyzing the light they emit. This technique relies on the Doppler effect, the same phenomenon that causes the pitch of an ambulance siren to change as it moves toward or away from an observer. When applied to light waves, an object moving away from us has its light stretched to longer, redder wavelengths, known as redshift. Conversely, an object moving toward us has its light compressed to shorter, bluer wavelengths, resulting in a blueshift.
Pioneering work in the early 20th century by astronomer Vesto Slipher provided the first concrete evidence of widespread galactic motion. Slipher used a spectrograph to measure the velocity of what were then called “spiral nebulae,” finding that the vast majority exhibited significant redshift, indicating they were receding from the Milky Way at high speeds. This finding suggested that these nebulae were separate, distant galaxies moving away from us.
A few years later, Edwin Hubble correlated these measured recession velocities with the distances to the galaxies, which he determined using variable stars called Cepheids. Hubble’s key finding was that a galaxy’s recession velocity is directly proportional to its distance from us. The farther away a galaxy is, the faster it appears to be moving away, a relationship that provided the first observational basis for a systematically expanding universe.
Local Motion Versus Universal Expansion
The movement of any given galaxy is actually a combination of two distinct types of motion that operate on different cosmic scales. The first is peculiar velocity, which is the motion of a galaxy relative to the smooth, large-scale flow of the expanding universe. This local movement is caused entirely by the gravitational pull of nearby matter, such as neighboring galaxies, clusters, and superclusters.
Peculiar velocity explains why a few nearby galaxies, like the Andromeda galaxy, exhibit a blueshift, meaning they are moving toward the Milky Way. The gravitational bond between the Milky Way and Andromeda is strong enough to overcome the expansion of space at this relatively small distance. Our two galaxies are on a collision course, with gravity dominating the expansion between them.
The second, and more dominant, type of movement is recessional velocity, which is the motion caused by the expansion of space itself. This universal expansion is not a movement of galaxies through space, but rather the stretching of the space between them. To visualize this, one can imagine dots painted on the surface of an inflating balloon; the distance between them increases as the balloon expands.
Recessional velocity is responsible for the redshift observed in almost all distant galaxies. For galaxies beyond our local gravitational neighborhood, the expansion of space completely overwhelms local gravitational attraction, causing them to move away from us and each other at increasing speeds.
The Driving Force Behind Accelerated Movement
While early astronomers confirmed the universe was expanding, observations made in the late 1990s revealed a stunning and unexpected twist: the expansion is actually speeding up. This acceleration implies the existence of a mysterious, repulsive force counteracting gravity, which scientists have named dark energy. This force is thought to dominate the cosmos, making up an estimated 68% of the total energy and matter budget of the universe.
The discovery of this acceleration was made by observing Type Ia supernovae, which act as “standard candles” for measuring cosmic distances. By comparing the expected brightness of these stellar explosions with their observed brightness and their redshift, researchers found that distant supernovae were fainter than they should have been. This suggested that the galaxies hosting these supernovae were farther away than predicted, meaning the expansion had been accelerating over time.
Dark energy can be conceptualized as an intrinsic energy density of space itself, exerting a uniform negative pressure that permeates the entire universe. Unlike matter and radiation, whose densities decrease as the universe expands, the density of dark energy remains nearly constant. As space grows, the total amount of dark energy increases, leading to a stronger repulsive influence that pushes galaxy clusters apart at a faster rate.
If the acceleration continues indefinitely, galaxies that are not gravitationally bound to each other will eventually recede so quickly that their light will never reach us, moving beyond our observable horizon. This scenario could lead to a “Big Freeze,” where the universe becomes cold, dark, and increasingly empty as all distant celestial objects disappear from view.