The science

The universe is about 13.8 billion years old, yet the edge of the part we can see, the observable universe, is about 46 billion light-years away. Nothing outran light: the distance kept growing while the light was on its way.

The oldest light we can see, the cosmic microwave background, left glowing gas about 370,000 years after the Big Bang (Planck's figures put the peak there; it's often rounded to 380,000). That gas was then only about 41 million light-years from the patch of space that would become our galaxy. For the light's first 4 billion years, the space between it and us grew faster than it could close the gap, carrying it out to nearly 6 billion light-years before it could start heading in. It reached us 13.8 billion years after it set out. Today, the gas that sent it is about 45 billion light-years away.

Because the expansion is speeding up, light that galaxies emit today from beyond about 17 billion light-years will never reach us. We still see their old light, just never what they look like now. That boundary already encloses only about 5% of the observable universe's volume.

All numbers are computed for flat ΛCDM with Planck 2018 parameters (H0 = 67.4 km/s/Mpc, Ωm = 0.315); older parameter sets give 46.5 billion light-years for the edge. The chart is to scale: time across, the light's distance from us up. The growing universe above it is not: its distances are squashed to fit on screen, the same way for the gas and the light, so the light turns back when the chart says it does. The sphere at the start and end is drawn as it is today, with the 17-billion-light-year boundary to scale inside it.

Imagery: the oldest light itself, the cosmic microwave background as mapped by WMAP (NASA / WMAP Science Team, 9-year map).