The question of “how fast is the universe moving” involves two distinct types of motion. The primary motion is the expansion of space itself, which stretches the cosmic web and carries galaxies away from one another. This large-scale movement defines the universe’s overall growth rate. Separately, individual galaxies and clusters move within this expanding space, driven by local gravitational forces. Understanding the total speed requires quantifying both the uniform cosmic expansion and the localized movement of objects.
Measuring the Expansion Rate
The universe’s expansion rate is defined by the Hubble Constant (\(H_0\)), which relates a galaxy’s recession speed to its distance. Edwin Hubble first observed that light from distant galaxies was shifted toward the red end of the spectrum (redshift). This redshift is interpreted as a Doppler shift caused by galaxies moving away from Earth. Hubble’s Law states that the velocity of recession is directly proportional to the galaxy’s distance.
The Hubble Constant is expressed in units of kilometers per second per megaparsec (km/s/Mpc). A megaparsec is a unit of distance approximately \(3.26\) million light-years. This unit describes a speed increase for every unit of distance, meaning a galaxy twice as far away recedes twice as fast.
Current observations show a significant disagreement regarding the precise value of \(H_0\), known as the “Hubble Tension.” Measurements based on the early universe, specifically the Cosmic Microwave Background (CMB), yield a lower value, around \(67.4\) km/s/Mpc. In contrast, measurements from the late-time universe, using objects like Type Ia supernovae and Cepheid variable stars, suggest a higher value, often around \(73\) km/s/Mpc. This discrepancy points to either systematic errors in measurement methods or the need for new physics beyond the current standard model of cosmology.
Our Movement Through Space
While the Hubble Constant describes the overall cosmic expansion, objects like the Milky Way also have their own localized motion. This is called peculiar velocity, which is the speed of an object relative to the uniform expansion of the universe in its immediate vicinity. Peculiar velocity is driven by the gravitational pull of nearby structures, such as galaxy clusters and superclusters.
To measure this local speed, scientists use the Cosmic Microwave Background (CMB) as a universal frame of reference. The CMB is the faint afterglow radiation from the Big Bang. Our motion relative to this background causes a slight temperature difference, or dipole anisotropy, in the CMB due to the Doppler effect.
By analyzing this temperature variation, astronomers determined that the Milky Way galaxy and its neighbors in the Local Group are moving at a significant speed. The estimated velocity relative to the CMB is approximately \(600\) to \(630\) kilometers per second. This speed results from the combined gravitational tug of massive structures in our cosmic neighborhood, pulling us toward a region known as the Great Attractor.
The Accelerating Expansion
The speed of the universe’s expansion is not constant; it is increasing over time. The discovery of this accelerating expansion in the late 1990s was based on observing distant Type Ia supernovae. These bright stellar explosions serve as “standard candles” because they have a consistent peak luminosity.
Researchers compared the expected brightness of these supernovae with their observed, fainter-than-anticipated light. They determined that these distant objects were further away than expected if gravity were slowing the expansion. This implied that a mysterious, repulsive force must be counteracting gravity and driving cosmic acceleration over the last several billion years.
This unknown entity is named Dark Energy, estimated to constitute about \(68\%\) of the total mass-energy content of the universe. Dark Energy acts like a negative pressure, a property of empty space that pushes space apart. Its density remains constant even as the universe expands. Since Dark Energy’s influence dominates over the decreasing density of matter, it has caused the expansion to accelerate, starting approximately five billion years ago.
The long-term fate of the cosmos depends on Dark Energy’s exact nature. If its density remains constant, the universe will continue expanding at an increasing rate, leading to a cold, dark “Big Freeze.” In this scenario, galaxies outside our local group will recede beyond our observable horizon. Other theories suggest Dark Energy could evolve, potentially leading to a “Big Crunch” or a “Big Rip” that tears apart matter.
The Concept of Infinite Speed
A common point of confusion is that distant galaxies appear to recede faster than the speed of light, approximately \(299,792\) kilometers per second. The speed limit of light applies only to objects moving through space. The expansion of the universe, however, is the movement of space itself, a process not constrained by this limit.
The farther away an object is, the more stretching space exists between it and us, increasing its apparent recession speed proportionally. At extreme distances, this accumulated expansion can carry galaxies away at speeds exceeding the speed of light. This does not violate relativity because the galaxies are not moving through their local space faster than light; the space between us is stretching rapidly.
This effect defines the boundary of the observable universe, marking the distance beyond which light has not had time to reach us since the Big Bang. The accelerating expansion driven by Dark Energy means that light emitted today from certain distant galaxies will never reach us, as the intervening space expands too quickly. These galaxies are effectively receding from our perspective faster than the speed of light.