Hubble’s Law, named after the American astronomer Edwin Hubble, is a landmark discovery in modern cosmology. Published in 1929, his work provided the first strong observational evidence that the universe is not static. The law describes a direct, proportional relationship between the speed at which a galaxy appears to be moving away from us and its distance from Earth. This systematic relationship fundamentally altered humanity’s understanding of the cosmos and became the foundation for the theory of the expanding universe and the Big Bang model.
The Evidence Leading to the Law
The formulation of Hubble’s Law required two astronomical measurements: a galaxy’s velocity and its distance. Velocity measurement relied on redshift, a phenomenon studied by Vesto Slipher starting in 1912. Slipher observed that light from most galaxies was shifted toward the red end of the spectrum. This redshift is a manifestation of the Doppler effect, indicating that these galaxies are moving away from the observer; the greater the shift, the faster the recession.
Measuring distance was accomplished by Hubble using Cepheid variables. These pulsating giant stars have an intrinsic brightness (luminosity) directly related to the period of their light variations. By measuring the time a Cepheid takes to complete a cycle, astronomers determine its true luminosity. Comparing this known brightness to how dim the star appears from Earth allows for an accurate calculation of its distance. Hubble used this “standard candle” technique to establish distances for galaxies where Slipher had already measured the recessional velocity.
Defining the Hubble Relationship
When Edwin Hubble plotted the recessional velocities against the calculated distances, the resulting graph showed a clear, straight-line correlation. This linear relationship is mathematically expressed as Hubble’s Law: \(v = H_0d\). Here, \(v\) is the galaxy’s recessional velocity (in \(\text{km/s}\)) and \(d\) is the distance (in megaparsecs, \(\text{Mpc}\)). One megaparsec is approximately 3.26 million light-years.
The factor connecting these two quantities is \(H_0\), known as the Hubble Constant. It is the proportionality constant describing the rate at which recession velocity increases per unit of distance, with units of \(\text{km/s/Mpc}\). The value of \(H_0\) is currently subject to intense scientific scrutiny, a discrepancy known as the “Hubble Tension.” Measurements based on observations of the nearby universe, often using Cepheid variables, yield a value around 73.5 \(\text{km/s/Mpc}\). However, measurements derived from the Cosmic Microwave Background suggest a lower value, closer to 67.7 \(\text{km/s/Mpc}\). This disagreement suggests either an unknown measurement error or a need to revise the current standard model of cosmology.
The Concept of Cosmic Expansion
The most profound implication of Hubble’s Law is that the universe is actively expanding, not static. The proportional relationship between speed and distance means that the farther a galaxy is, the faster it appears to recede. This does not mean that Earth is the center of a cosmic explosion, but rather that space itself is stretching everywhere at once.
The expansion is considered to be homogeneous and isotropic on a large scale, a concept formalized in the Cosmological Principle. Homogeneous means the universe looks the same at every location, and isotropic means it looks the same in every direction. A common way to visualize this effect is to imagine spots painted on the surface of an inflating balloon. The galaxies are not flying through space but are being carried apart as the space between them expands. This stretching of space is the underlying cause of the cosmological redshift that Slipher first observed.
Calculating the Universe’s Age
The Hubble Constant provides a direct method for estimating the age of the universe. If one assumes that the expansion rate has been constant throughout time, the age can be approximated by taking the inverse of the Hubble Constant, a value known as the Hubble time (\(t_H = 1/H_0\)). This calculation essentially determines how long it would have taken for all the galaxies, moving at their current speeds, to have originated from a single point.
While the actual age calculation incorporates factors like the universe’s matter and energy density, the Hubble time provides a first-order estimate. Using the most precise cosmological models and measurements, the currently accepted age of the universe is about 13.8 billion years. The precision of this age estimate is directly dependent on the accuracy of the Hubble Constant measurement. The ongoing debate surrounding the exact value of \(H_0\) is a challenge because a faster expansion rate implies a younger universe, while a slower rate suggests an older one.