The universe is expanding, which involves the stretching of the very fabric of space itself. Determining the rate of this cosmic growth is complex because the answer is not a single speed, but a rate that depends on distance. This expansion is quantifiable and is currently increasing, driven by a mysterious, pervasive force that continues to reshape the cosmos.
The Measured Rate of Expansion
The present-day rate of the universe’s expansion is defined by the Hubble Constant (\(H_0\)). This constant is not a conventional measure of speed, but rather a rate of speed per unit of distance, expressed in kilometers per second per megaparsec (km/s/Mpc). A megaparsec (Mpc) is an immense distance, equating to approximately 3.26 million light-years. The Hubble Constant thus describes how much faster a distant object moves away from us for every additional Mpc it is away.
The exact value of the Hubble Constant is a major subject of ongoing research, leading to the “Hubble Tension.” Measurements based on the local, more recent universe (using supernovae and Cepheid variables) tend to cluster around 73 to 74 km/s/Mpc. Conversely, measurements derived from the Cosmic Microwave Background—light left over from the early universe—suggest a slower rate, closer to 67 km/s/Mpc.
The discrepancy between these “early universe” and “late universe” values persists, suggesting either unaccounted-for measurement errors or a gap in our current understanding of cosmology. Despite this disagreement, the current expansion rate is known to hover around 70 km/s/Mpc.
What Cosmic Expansion Actually Means
Cosmic expansion is the metric expansion of space itself, fundamentally different from objects moving through space. The distance between gravitationally unbound objects, such as distant galaxies, increases because the space between them is growing. The galaxies remain stationary within their local space, but the space between them stretches.
A common visualization is dots drawn on an inflating balloon, representing galaxies. As the balloon inflates, the dots move farther apart because the surface beneath them expands. There is no central point of origin, and every distant galaxy appears to be moving away from every other distant galaxy.
The expansion only affects the universe on the largest scales, specifically beyond galaxy clusters. On smaller, local scales, gravity overpowers the weak repulsive effect of expanding space. Therefore, structures like our solar system, the Milky Way, and galaxy clusters are held together by gravity and do not expand.
Why the Expansion Rate Is Increasing
For much of the 20th century, cosmologists expected the universe’s expansion to slow down due to the collective gravitational pull of all matter. Gravity should have caused the rate of expansion to decelerate over time. However, observations made in 1998 using distant Type Ia supernovae revealed that the expansion of the universe is actually accelerating.
This discovery indicated that a mysterious, repulsive force was overcoming gravity. Scientists named this entity “Dark Energy,” theorized to be a form of energy inherent in the fabric of space itself. Unlike matter, which dilutes as space expands, the density of Dark Energy remains nearly constant, meaning more energy appears as new space is created.
Dark Energy now accounts for approximately 68% of the total mass-energy content of the universe and is the dominant force dictating its evolution. The presence of this pervasive force means the expansion is not slowing down as expected, but instead getting faster. This acceleration began about five billion years ago and ensures that the rate of separation between distant galaxies will continue to increase, determining the long-term fate of the universe.