Light has a finite speed. When you look at the Sun, you see it as it was eight minutes ago. When you look at the nearest star outside our solar system, you see it as it was four years ago. Look at distant galaxies, and you are looking back in time.

The James Webb Space Telescope, launched in December 2021, was built to look the farthest back. Its mirror is 6.5 meters across — three times wider than Hubble's. It orbits a stable point a million miles from Earth, shielded from sunlight by a five-layer sunshield the size of a tennis court.

It only sees infrared light. This is the key. As the universe expands, light from distant objects gets stretched into longer wavelengths — a redshift. Visible light from the early universe is now infrared. Hubble couldn't see it. JWST was designed for nothing else.

In 2022, JWST released its first deep-field image: a single point of sky the size of a grain of sand held at arm's length. In the photo: thousands of galaxies. Some had been forming stars when the universe was 300 million years old — about 2% of its current age.

JWST has found galaxies older and bigger than current models predict. Some early galaxies look surprisingly mature for their age, forcing astronomers to revise the timeline of cosmic structure formation.

The telescope was supposed to last five years. Engineers think it will run twice that. For its remaining life, every image it sends back is, literally, a postcard from a younger universe — sent before there was a single human atom.