Our universe is home to countless galaxies, each a grand collection of stars, gas, dust, and dark matter. These colossal structures are not static; rather, they are constantly in motion, engaging in a complex cosmic dance. Understanding how galaxies move involves considering both the universe’s overall behavior on the largest scales and gravitational interactions within smaller galactic neighborhoods.
The Universe is Expanding
On the grandest cosmic scales, galaxies are observed to be generally moving away from one another, a phenomenon not caused by their individual motion through space, but by the expansion of space itself. Imagine points on the surface of an inflating balloon; as the balloon expands, the points move farther apart, even though they are fixed on the surface. Similarly, galaxies are carried along as the fabric of the universe stretches.
Evidence supporting this expansion comes from “redshift.” Light from distant galaxies appears shifted toward the red end of the electromagnetic spectrum. This occurs because as light travels through expanding space, its wavelengths are stretched, making it appear redder to an observer. The more distant a galaxy, the greater its observed redshift, indicating it is moving away from us at a faster rate.
This relationship between a galaxy’s distance and its recession speed was first systematically observed by Edwin Hubble in the late 1920s. His observations led to Hubble’s Law, which states that a galaxy’s velocity away from us is directly proportional to its distance, meaning farther galaxies recede at higher speeds. The value of this proportionality, known as the Hubble constant, helps scientists understand the current rate of cosmic expansion.
The expansion is not centered on any particular point; instead, every point in the universe appears to be the center. This concept suggests the universe originated from an extremely dense state, a theory commonly referred to as the Big Bang. The ongoing expansion, evidenced by redshift, is fundamental to understanding galaxy movement.
Local Galactic Movements
While the universe’s large-scale expansion dictates that galaxies generally move apart, gravitational forces exert significant influence on smaller, local scales. Within galaxy clusters and groups, such as our own Local Group, the gravitational pull between galaxies can be strong enough to counteract the cosmic expansion. This allows galaxies to interact and move towards each other, rather than uniformly receding.
These local gravitational interactions can lead to galaxy collisions and mergers. Despite the term “collision,” these events are not head-on crashes between individual stars, as distances between stars are immense. Instead, they involve the gravitational interplay of entire galactic systems, causing them to pass through each other, distorting shapes, and eventually coalescing into a single, larger galaxy over billions of years.
A well-known example is the predicted interaction between our Milky Way and Andromeda galaxies, its closest large galactic neighbor. These two are moving towards each other, with models suggesting a 50% chance of colliding and merging within the next 10 billion years, likely transforming them into a single, more elliptical galaxy, sometimes called “Milkomeda.” Other merging galaxies, like the Antennae or Mice Galaxies, showcase these transformative processes.
Beyond these larger-scale interactions, individual galaxies also possess “peculiar velocities.” These motions deviate from the smooth Hubble flow, arising from gravitational tugs exerted by nearby matter, such as other galaxies or concentrations of dark matter. In dense regions like clusters, these peculiar velocities can be hundreds or even over a thousand kilometers per second, influencing local movements more significantly than cosmic expansion.
The Accelerating Expansion
A discovery in the late 1990s revealed that the universe’s expansion is not slowing down due to gravity, but is instead accelerating. This finding emerged from observations of distant Type Ia supernovae, which act as “standard candles” due to their consistent peak brightness. By measuring their apparent dimness, astronomers determine their distances. Comparing these distances with redshifts showed distant supernovae were farther away than expected, indicating the universe’s expansion was speeding up.
This discovery was made independently by two research teams, the Supernova Cosmology Project and the High-Z Supernova Search Team. The driving force behind this acceleration is attributed to a mysterious phenomenon known as dark energy. Dark energy is thought to be a form of energy inherent to space itself, exerting a repulsive force that pushes galaxies farther apart at an increasing rate.
While its exact nature remains unknown, dark energy is believed to make up approximately 70% of the universe’s total mass-energy content. Its influence becomes dominant on the largest scales, causing the expansion to accelerate. This ongoing acceleration has implications for the universe’s long-term future. It suggests galaxies will continue to move farther apart at an increasing pace, leading to a future where distant galaxies may recede so rapidly their light will be stretched beyond detection, making them effectively invisible. This scenario points towards an increasingly isolated universe.