hubble-constant

Two carefully checked ways of measuring how fast the universe expands disagree by roughly 8 to 10 percent, a gap near 5 sigma. Either a hidden error remains, or standard cosmology is incomplete.

Hubble's 1929 redshift observation tied galaxy speed to distance. See what the 24-galaxy data showed, why his constant was off 7x, and the tension today.

In 1929, Edwin Hubble showed that galaxies recede from us at speeds proportional to their distance, the first direct evidence that the universe is expanding. The slope of that relation, the Hubble constant, is still debated: two precise methods disagree by about 5 sigma.

From parallax to supernovae, astronomers chain overlapping methods together to measure distances from the nearest stars to galaxies billions of light-years away. Here is how each rung of the cosmic distance ladder works, and why its top rung is now in dispute.

The universe is 13.8 billion years old. But how do we know? Three independent methods — the cosmic microwave background, the oldest stars, and Hubble's constant — converge on the same answer. Here's the evidence.

The observable universe is 93 billion light-years across. The full universe is bigger — possibly infinitely so. Here's what we can measure, what we can't, and why.