Edwin Hubble, an American astronomer working in the 1920s, revolutionized cosmology. Before his work, many scientists believed the Milky Way constituted the entire universe, viewing “spiral nebulae” as mere gas clouds within our galaxy. Hubble’s meticulous observations proved these nebulae were, in fact, separate, massive collections of stars situated far beyond the Milky Way. This established the existence of countless galaxies and dramatically expanded the scale of the known universe. Hubble’s subsequent discovery was that the universe was not static but actively expanding. This finding provided the first opportunity to calculate an approximate age for the cosmos, requiring the measurement of galactic distances and their movement to determine the rate of expansion.
Establishing Cosmic Scale: Measuring Distance
The first challenge Hubble faced was establishing the true distance to these newly recognized galaxies, as accurate distances were necessary for calculating the universe’s expansion rate. He relied on a technique developed by astronomer Henrietta Leavitt, who studied Cepheid variables. These stars are massive, highly luminous, and pulse in brightness with a regular, predictable rhythm.
Leavitt discovered a direct relationship between a Cepheid’s period of pulsation and its absolute luminosity (intrinsic brightness). This relationship meant that observing the period allowed astronomers to determine the star’s true brightness. Since the apparent brightness of any light source diminishes predictably with distance, a Cepheid acts as a “standard candle” with a known brightness.
Hubble used the 100-inch Hooker telescope at Mount Wilson Observatory to locate these pulsating stars within the spiral nebulae. By measuring a Cepheid’s period in a distant galaxy, he determined its true brightness. Comparing this intrinsic brightness with how bright the star appeared from Earth allowed him to calculate its precise distance. This technique provided the first reliable step on the “cosmic distance ladder,” confirming that galaxies like Andromeda were independent “island universes” millions of light-years away.
Observing Galactic Motion: The Redshift Phenomenon
To estimate the universe’s age, Hubble needed to measure how fast these galaxies were moving. The method for measuring cosmic velocity relies on analyzing the light emitted by distant galaxies, a phenomenon called redshift, which uses the Doppler effect.
For light, the Doppler effect manifests as a shift in the observed color of a galaxy’s spectrum. If a light source moves away from the observer, its light waves are stretched, making the light appear redder (redshift). Conversely, if a source is moving toward the observer, the light waves are compressed, causing a blueshift. Astronomers use a spectroscope to split the galaxy’s light, looking for shifts in the characteristic lines produced by elements like hydrogen and calcium.
The necessary velocity measurements were largely provided by the preceding work of astronomer Vesto Slipher. Slipher’s observations demonstrated that the vast majority of spiral nebulae exhibited a redshift, indicating they were receding from Earth at high speeds. Hubble combined these velocity data points with his newly determined distance measurements. This synthesis of distance and velocity information was crucial before the fundamental law of cosmic expansion could be established.
Formulating the Expansion Rate (Hubble’s Law)
The culmination of Hubble’s work involved plotting the distance measurements against the recessional velocity data. A clear and unmistakable pattern emerged: the farther a galaxy was from the Milky Way, the faster it appeared to be moving away. This demonstrated a direct, linear relationship between a galaxy’s distance and its recession velocity.
This proportional relationship became formalized in the equation v = H0d, known as Hubble’s Law. In this formula, v is the galaxy’s recessional velocity, d is its distance from the observer, and H0 is the constant of proportionality, called the Hubble Constant. The Hubble Constant represents the current rate of the universe’s expansion.
This discovery provided the first observational evidence for an expanding universe. Hubble’s Law showed that space itself was expanding, carrying the galaxies along with it. The calculation of this constant, H0, was the scientific achievement that paved the way for an age estimate.
Translating the Constant into an Age Estimate
The final step in estimating the universe’s age involved a mathematical inversion of the Hubble Constant. Hubble reasoned that if the universe expands at a constant rate, running the expansion backward should lead to the initial moment of the Big Bang, known as the Hubble Time.
The calculation is straightforward: since velocity equals distance divided by time (v = d/t), the Hubble Law can be rearranged. Because H0 is the ratio of velocity to distance (H0 = v/d), the time since the expansion began is the reciprocal of the Hubble Constant, or t = 1/H0. This calculation provided a rough estimate for the age of the cosmos.
Hubble’s initial calculations yielded an age of approximately 1.8 billion years, which was problematic because geological evidence suggested the Earth itself was older. The error was later traced back to the distance measurements involving Cepheid variables. Astronomers subsequently discovered there were two distinct types of Cepheid stars, each with a different period-luminosity relationship, a distinction Hubble could not make. This misclassification led him to significantly underestimate galactic distances, resulting in a Hubble Constant that was too large and an age estimate that was too short. This early calculation nonetheless established the foundational method for determining the age of the universe.