Why the visible universe reaches 46.5 billion light-years in every direction
Light has been travelling for at most 13.8 billion years, so you might expect the visible universe to end 13.8 billion light-years away. Its edge actually lies about 46.5 billion light-years off in every direction. The difference comes from space itself stretching while that ancient light was on its way.
The observable universe is the sphere around us from which light has had time to arrive since the cosmic expansion began. Observable here has nothing to do with telescope power; it is a hard limit set by the speed of light and the age of the cosmos. The universe as a whole may be vastly bigger, possibly infinite, or finite yet edgeless, like the surface of a ball in more dimensions.
The oldest light we catch, the cosmic microwave background, left its source about 380,000 years after the Big Bang. The matter that emitted it was then only around 42 million light-years from the stuff that would become us. Expansion has since carried it roughly 46 billion light-years away, so the photon has travelled 13.8 billion years across a gap that kept widening. Measured today, the observable universe is about 93 billion light-years across.
The future is less generous than you might think. Because expansion is accelerating, a signal sent right now can only ever reach us from an event closer than about 16 billion light-years; anything farther is permanently out of contact, and that same distance marks how far a light-speed traveller setting off today could ever get. Galaxies near the edge will seem to slow and freeze, their light growing ever redder and fainter until they effectively vanish. There is also a future visibility limit, about 62 billion light-years, beyond which nothing will ever come into view.
What lies within is still staggering. Estimates run as high as 2 trillion galaxies, and more stars, and perhaps more Earth-like planets, than there are grains of sand on all Earth's beaches, though some counts are in the hundreds of billions of galaxies. Counting ordinary matter only, leaving out dark matter and dark energy, and treating it all as hydrogen gives the Eddington number, a figure for the total atoms in view.
Source: Observable universe
