Astronomy
The Expansion of the Universe and the Hubble Constant
Quick fact
The current expansion rate (the Hubble constant) is about 70 km/s per megaparsec, meaning a galaxy 1 megaparsec away recedes at 70 km/s. But different measuring methods give slightly different values, a puzzle known as the 'Hubble tension'.
Why this is interesting
If everything in the universe is moving away from us, does that mean we're the center of it all? And why is the speed of that expansion still a mystery?
Read the full explanation
Understanding The Expansion of the Universe and the Hubble Constant
Imagine the universe as a balloon. When you inflate it, dots drawn on the surface move apart, and the more distant the dots, the faster they separate. The galaxies are like those dots: they aren't moving through space, but the space between them is stretching. Edwin Hubble observed that more distant galaxies appear more redshifted, meaning their light is stretched to longer wavelengths, indicating they are receding faster. This relationship is now called Hubble's Law: velocity = Hubble constant × distance. The Hubble constant is the current expansion rate, a number that tells us how fast the universe is growing today.
A deeper explanation
The expansion of the universe is a prediction of Einstein's general relativity, which describes gravity as the curvature of spacetime. When the universe is filled with matter and energy, spacetime itself cannot be static; it must either expand or contract. Hubble's observations provided the evidence for expansion, leading to the Big Bang theory—that the universe began from a hot, dense state and has been expanding ever since. The Hubble constant is not just a number; it sets the age of the universe and tells us how its expansion rate changes over time. However, the expansion rate depends on cosmic ingredients: if dark energy dominates, expansion accelerates. Surprisingly, measurements using nearby Cepheid variables (distance ladder) give a higher Hubble constant than measurements of the cosmic microwave background (early universe), suggesting new physics may be at play. This discrepancy, called the Hubble tension, is one of the biggest mysteries in cosmology, potentially pointing to unknown particles or evolving dark energy.