Cosmic expansion is the increase in distances between widely separated galaxies as the universe evolves, while Hubble’s Law describes how their recession speeds increase with distance. The law states that, for relatively nearby galaxies following the general cosmic expansion, recession velocity is approximately proportional to distance.
This relationship is one of the foundations of modern cosmology, the branch of astronomy and space science that investigates the universe as a whole. By measuring galaxy distances and the stretching of their light, scientists can reconstruct the history of cosmic expansion and investigate how that expansion may change in the future.
What Is Cosmic Expansion?
Cosmic expansion means that the distance between objects carried along with the large-scale expansion of the universe generally increases over time. It does not mean galaxies are all flying outward from a central location into empty space.
Instead, the distances between widely separated galaxies grow as the universe’s overall scale changes. This distinction helps explain why observers in different galaxies would measure similar expansion patterns.
Scientists have assembled several independent lines of evidence for this behavior. The measurements discussed in observational evidence for expansion include galaxy redshifts, distance measurements, and observations of the cosmic microwave background.
Expansion also influences scientific estimates of cosmic history. Researchers combine expansion measurements with other observations to develop estimates of the universe’s age, currently placing it at approximately 13.8 billion years.
Cosmic expansion is not necessarily constant throughout history. Observations indicate that its rate has changed, and theories of dark energy investigate why the expansion has been accelerating during the universe’s more recent history.
How Hubble’s Law Connects Distance and Recession Speed
Hubble’s Law expresses the relationship between the distance to a galaxy and its recession velocity, which is the rate at which its distance from an observer increases because of cosmic expansion.
The relationship is written as:
v = H₀ × d
- v: Recession velocity, usually expressed in kilometers per second.
- H₀: The Hubble constant, representing the present-day expansion rate.
- d: The galaxy’s present-day distance, commonly measured in megaparsecs.
One megaparsec equals approximately 3.26 million light-years. Astronomers use this unit because distances between galaxies are much greater than distances between stars within a galaxy.
The equation works particularly well as an approximation for galaxies at relatively modest cosmological distances. At greater distances, astronomers use more complete cosmological models that account for changes in the expansion rate over time.
A Practical Example of Hubble’s Law
Suppose astronomers observe a galaxy approximately 100 megaparsecs away. Using an illustrative Hubble constant of 70 kilometers per second per megaparsec, they can estimate its recession velocity.
v = 70 × 100 = 7,000 km/s
The calculation predicts a recession speed of approximately 7,000 kilometers per second due to cosmic expansion.
Now consider a second galaxy 200 megaparsecs away. Using the same expansion rate, its predicted recession velocity would be approximately 14,000 kilometers per second.
| Galaxy Distance | Predicted Recession Velocity |
|---|---|
| 50 megaparsecs | 3,500 km/s |
| 100 megaparsecs | 7,000 km/s |
| 200 megaparsecs | 14,000 km/s |
| 300 megaparsecs | 21,000 km/s |
These values illustrate the proportional relationship rather than measurements of specific galaxies. Real galaxies also have local motions caused by gravity, so their observed velocities may differ from the predictions.
How Astronomers Discovered the Expanding Universe
During the early twentieth century, astronomers began measuring the distances and spectra of galaxies beyond the Milky Way.
Vesto Slipher had already measured spectral shifts in spiral nebulae before Edwin Hubble established his distance-velocity relationship. Hubble studied light from distant galaxies and observed a phenomenon called redshift.
Working with galaxy distance estimates and previously measured radial velocities, Hubble published evidence in 1929 that more distant galaxies generally exhibited greater recession velocities.
Georges Lemaître had independently developed an expanding-universe interpretation and published a related distance-velocity relationship in 1927. Their contributions helped establish what is now also called the Hubble-Lemaître Law.
The discovery transformed cosmology by providing observational support for a universe whose large-scale structure changes over time.
How Redshift Reveals Cosmic Expansion
Redshift occurs when light is observed at a longer wavelength than the wavelength at which it was emitted. Astronomers measure it by comparing recognizable spectral features with their known laboratory wavelengths.
In an expanding universe, light traveling across cosmological distances is stretched as the scale of space increases. This produces a measurable shift toward longer wavelengths.
Different Causes of Redshift
Not all redshifts have the same physical explanation. Two important categories are:
- Cosmological redshift (due to expansion of space): Light wavelengths increase as the universe expands during the light’s journey.
- Doppler redshift (due to motion through space): Light shifts toward longer wavelengths when a source moves away from an observer.
Gravitational redshift is another phenomenon, occurring when light travels through differences in gravitational potential.
Understanding redshift and cosmic motion helps astronomers distinguish the effects of local movement from the expansion of the universe.
For cosmological redshift, the observed wavelength is related to the emitted wavelength through the expansion of the universe between emission and observation. At large redshifts, converting that measurement into distance requires a cosmological model rather than simply applying the ordinary Doppler formula.
What Is the Hubble Constant?
The Hubble constant, written H₀, measures the present-day rate of cosmic expansion. Its units are kilometers per second per megaparsec, indicating how much recession velocity increases for each additional megaparsec of distance.
A commonly used approximate value is 70 km/s/Mpc. However, precise measurements using different methods have produced results that do not fully agree.
Measurements based on the cosmic microwave background within the standard cosmological model suggest a value near 67 to 68 km/s/Mpc. Some measurements using nearby distance indicators, including Cepheid variable stars and Type Ia supernovae, suggest values around 73 km/s/Mpc.
This disagreement is known as the Hubble tension. Researchers continue investigating whether it reflects unrecognized measurement uncertainties, limitations in cosmological models, or previously unknown physics.
Why the Expansion of the Universe Is Accelerating
During the late 1990s, observations of distant Type Ia supernovae revealed evidence that cosmic expansion had been accelerating rather than continuously slowing under gravity.
The standard cosmological model explains this acceleration using dark energy, a component associated with the large-scale behavior of spacetime.
In the simplest model, dark energy is represented by a cosmological constant. Its energy density remains constant as space expands, allowing its influence on cosmic expansion to become increasingly significant as matter becomes more spread out.
Scientists estimate that dark energy accounts for roughly 68% of the universe’s total energy density in the standard cosmological model. Its physical nature remains uncertain, and astronomers continue testing whether its properties change over time.
Does Cosmic Expansion Affect Galaxies, Planets, and Atoms?
Cosmic expansion does not cause every object in the universe to grow. Systems held together by sufficiently strong gravitational, electromagnetic, or nuclear forces do not simply expand along with the universe.
- Galaxies: Stars within gravitationally bound galaxies remain associated with their galaxies rather than separating because of cosmic expansion.
- Solar systems: Planetary orbits are governed primarily by local gravity, not the expansion described by Hubble’s Law.
- Planets and atoms: Electromagnetic and other binding forces maintain their structures.
- Distant galaxy groups: Widely separated systems that are not gravitationally bound generally become farther apart as the universe expands.
The Andromeda Galaxy provides a useful exception to the general recession pattern. It is approaching the Milky Way because their mutual gravitational attraction and local motions overcome the expansion effect at their separation.
Does the Universe Have a Center of Expansion?
In the standard cosmological model, the universe has no preferred central location from which all galaxies are moving outward. Observers in different locations would see broadly similar expansion patterns when examining sufficiently large cosmic distances.
Imagine dots marked on the surface of an inflating balloon. As the balloon expands, the distances between the dots increase, but no dot on the surface represents the center of that expansion.
This analogy illustrates increasing separation, although the universe is not understood to be a balloon expanding into an external surrounding space. The mathematical description concerns changes in distances within spacetime itself.
Frequently Asked Questions
Can galaxies recede faster than the speed of light?
Yes. At sufficiently large cosmological distances, recession rates associated with the expansion of space can exceed the speed of light. This does not violate special relativity because the recession rate describes changing distance across expanding spacetime, not an object’s local motion through space faster than light.
Is Hubble’s Law proof of the Big Bang?
Hubble’s Law provides strong observational evidence that the universe is expanding. Combined with the cosmic microwave background and measured abundances of light elements, it supports the Big Bang model, which describes the universe’s evolution from an earlier hot, dense state. The law alone does not establish every detail of that model.
Will the universe continue expanding forever?
Current observations are consistent with continued expansion under the standard cosmological model, particularly if dark energy behaves like a cosmological constant. However, the universe’s ultimate future depends on the properties of dark energy and other cosmological factors that scientists are still investigating.
Cosmic expansion explains why widely separated galaxies generally grow farther apart, while Hubble’s Law provides a measurable relationship between their distances and recession velocities. Together with redshift observations and measurements of the cosmic microwave background, this relationship allows astronomers to test models of the universe’s history. Improving measurements of the Hubble constant and dark energy may reveal whether our current understanding of cosmic expansion is complete.
Sources: Edwin Hubble, “A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae” (1929); Planck Collaboration, “Planck 2018 Results. VI. Cosmological Parameters” (2020); NASA, Imagine the Universe! educational resources on the expanding universe and Hubble’s Law.
