Hubble constant
The Hubble constant, denoted by H₀, quantifies the rate of expansion of the universe. It expresses the speed at which galaxies are receding from us as a function of their distance: for every additional megaparsec of distance, a galaxy recedes about 70 kilometers per second faster. It is one of the most important—and most debated—quantities in modern cosmology.
The Discovery of Expansion
In 1929, Edwin Hubble published a fundamental observation: all galaxies are moving away from ours, and the farther away they are, the faster they are moving away. This linear relationship between distance and recession velocity is known as Hubble’s law. It implies that the universe is expanding—a discovery that revolutionized cosmology and supported the Big Bang model.
How is it measured?
Two main methods are in contention. The first uses standard candles—objects of known luminosity, such as Type Ia supernovae or Cepheid variables—to measure distances directly. The second analyzes the cosmic microwave background, the thermal echo of the Big Bang, and extrapolates the value of H via the standard cosmological model.
The Hubble tension
The problem is that these two methods yield incompatible results. The direct measurement yields approximately 73 km/s/Mpc, while the analysis of the cosmic microwave background yields about 67 km/s/Mpc. This discrepancy, known as the Hubble tension, is too large to be attributed solely to measurement uncertainties. It could indicate a flaw in our standard cosmological model—and is one of the most active areas of research in theoretical physics.
Did you know?
The value of H₀ also allows us to estimate the age of the universe. By inverting the Hubble constant, we obtain the time elapsed since the Big Bang, which is approximately 13.8 billion years—in agreement with other independent measurements.
Frequently Asked Questions
Why is it called a constant if its value is debated?
The Hubble constant is constant throughout space at any given moment—all galaxies follow the same law. But it changes over cosmic time. The value H’ refers to its value today, and it is this precise value that is the subject of debate.
Can the Hubble constant change?
In cosmic time, yes. It was much larger right after the Big Bang. Today, under the influence of dark energy, the expansion is accelerating, which could alter its future value.
What does this have to do with black holes or galaxies?
The Hubble constant governs the overall expansion of the universe, not local movements. Nearby galaxies can move closer together under the influence of gravity—such as Andromeda toward the Milky Way—despite the general expansion.