Galaxies move constantly, driven by two distinct forces. A galaxy is a massive collection of stars, gas, and dark matter. This movement is influenced both by the local gravitational pull of neighboring galaxies and by the overall expansion of the cosmos. Understanding galactic motion requires differentiating between movement through space and motion caused by the stretching of space itself.
Detecting Galactic Motion
Astronomers measure galactic movement by analyzing the light they emit, using the Doppler effect. This effect describes the change in a wave’s frequency relative to an observer moving toward or away from the source.
When applied to light, the Doppler effect reveals if a galaxy is moving toward or away from the observer. If a galaxy moves away, its light waves are stretched, shifting them toward the red, longer-wavelength end of the spectrum, a phenomenon called Redshift. Conversely, if a galaxy is approaching, its light waves are compressed, shifting them toward the blue, shorter-wavelength end, known as Blueshift.
By comparing the observed wavelengths of spectral lines to their known, stationary wavelengths, astronomers calculate the galaxy’s velocity along the line of sight. The magnitude of the shift directly correlates to the speed of the galaxy relative to the Milky Way.
Gravitational Interactions and Local Motion
Galaxies are bound together by gravity into groups, clusters, and superclusters. A galaxy’s motion within these structures is governed by the collective gravitational pull of its neighbors, resulting in peculiar velocity. This motion is independent of the universe’s overall expansion and can be directed in any direction.
For instance, the Milky Way belongs to the Local Group, which includes the Andromeda Galaxy. Andromeda is rushing toward the Milky Way at approximately 110 kilometers per second. This approach is confirmed by the blueshift detected in Andromeda’s light, indicating that gravitational attraction overcomes the universe’s general expansion.
This local gravitational interaction will eventually lead to a galactic merger, predicted in about 4.5 billion years. The entire Local Group is also being pulled toward a much larger, dense region known as the Virgo Cluster. Peculiar velocities near large concentrations of mass highlight gravity’s influence on local cosmic dynamics.
The Universe’s Expansion
While local gravity dictates the movement of nearby galaxies, the motion of galaxies on the largest scales is dominated by the expansion of the universe itself. This expansion is not a movement of galaxies flying through a stationary space but rather a uniform stretching of the fabric of space-time between them. The galaxies themselves are essentially being carried along as the space between them grows larger.
The relationship between a galaxy’s distance and its speed of recession is described by Hubble’s Law. This law states that the farther a galaxy is from us, the faster it appears to be moving away. This observation does not imply that the Milky Way is at the center of the universe, but rather that every observer in every galaxy would witness the same effect.
A helpful visualization is imagining dots drawn on the surface of an inflating balloon. As the balloon expands, the distance between all the dots increases. The dots, representing galaxies, are not moving across the surface, but the surface itself is expanding.
The expansion of the universe is not constant; it is currently accelerating, driven by a mysterious force known as dark energy. This acceleration means the recession velocity of distant galaxies is increasing over time. This global, expansion-driven motion is the primary reason nearly all distant galaxies exhibit Redshift.
The Concept of Absolute Motion
In the framework of general relativity, there is no single, fixed reference point in the universe, making the concept of absolute motion scientifically problematic. Any velocity measurement is inherently relative to the chosen frame of reference. However, the Cosmic Microwave Background (CMB) offers a universal backdrop against which movement can be measured.
The CMB is the residual heat radiation left over from the early universe that permeates all of space. Although the CMB is almost perfectly uniform, scientists observe a slight temperature difference, or dipole anisotropy, across the sky. This subtle shift results from the Doppler effect caused by our motion relative to the CMB’s reference frame.
By measuring this anisotropy, astronomers determined that the Milky Way is moving relative to this universal reference frame at approximately 600 kilometers per second. This motion is largely directed toward a massive, gravitationally dense region known as the Great Attractor. While the CMB does not establish an absolute center for the universe, it provides a powerful tool for measuring our galaxy’s true velocity through the cosmos.