How Big Is the Universe, and Does It Have an Edge?

The part of the universe we can see is enormous: a sphere about 93 billion light-years across, with us at its middle. (A NASA expert gives about 92 billion; estimates differ slightly.) A light-year is the distance light travels in one year, about 5.88 trillion miles, according to NASA. Beyond that sphere, the universe very likely keeps going. How far it goes, nobody knows: it may be finite, or it may go on forever.

As for an edge or a center, the best evidence says the universe has neither. That sounds impossible at first, so let’s take it one step at a time.

Why is the visible universe wider than 13.8 billion light-years?

The universe is about 13.8 billion years old, so you might expect to see only 13.8 billion light-years in any direction. But space has been expanding the whole time the light was on its way. Picture an ant walking along a rubber band while someone keeps stretching it: when the ant arrives, its starting point is much farther behind than the distance it walked.

The same thing happens to light: the most distant regions whose light reaches us are now about 46.5 billion light-years away, according to the National Radio Astronomy Observatory. Double that and you get about 93 billion light-years.

Does that break Einstein’s rule that nothing outruns light? No. Astronomers Tamara Davis and Charles Lineweaver showed in a 2003 paper that we can see galaxies whose distance from us grows faster than light, and they explained why that does not violate relativity. The galaxies are not racing through space; the space between us is growing.

The James Webb Space Telescope keeps pushing toward that horizon. It has seen a galaxy called MoM-z14 as it looked only 280 million years after the Big Bang, NASA reports. It appears, enlarged, in the box in the image above.

What does “observable” really mean?

The observable universe is not a wall but a horizon, like the one you see at sea. Its size is set by how far light has had time to travel to us, not by where the universe stops. Someone living in a galaxy billions of light-years away would have their own observable bubble, centered on them.

The oldest light anyone can catch is the cosmic microwave background, a faint glow released about 380,000 years after the Big Bang, according to NASA. We explain why telescopes cannot see past it in Can a Telescope See the Beginning of the Universe?

Is space flat or curved?

“Flat” here does not mean shaped like a pancake; it is about geometry. In flat space, the three angles of a giant triangle add up to 180 degrees, just like on paper. In a “closed” universe, like the surface of a ball, they add up to more, while in an “open” universe, curved more like a saddle, they add up to less.

Astronomers can draw such a triangle using the microwave background. Its hot and cold spots have a typical size of about one degree on the sky, which is what flat space predicts, as NASA explains. The Planck satellite, combined with maps of galaxies, found that space is flat to within about 0.2 percent.

One honest wrinkle: Planck’s own microwave data, taken alone, leaned toward a closed shape. Adding more measurements pulled the result firmly back toward flat.

Does anyone know if the universe is infinite?

This is a genuine open question, and a NASA expert put it simply: we don’t know whether the universe is finite or infinite. Flat space fits both options.

A flat universe could go on forever, or it could loop back on itself, like an old video game where walking off the right edge of the screen brings you back on the left. If our universe wrapped around on a scale smaller than what we can see, we would spot repeating patterns in the microwave background. Planck searched for them and found none. So, at least for the simple shapes they tested, any wrap-around loop must be roughly as large as our observable universe, or larger.

A gently curved universe is also still possible, as long as the curve is too slight to measure. Think of standing in a wide, flat field: Earth is round, but your patch looks flat. If the whole universe curves that gently, it must be far larger than the part we see.

Where is the center, and where is the edge?

The Big Bang was not an explosion at one spot that threw matter into empty space. As NASA’s astrophysicists put it, it happened everywhere at once, so it has no location. Two classic pictures help, and both have limits.

The balloon. Draw dots on a balloon and blow it up. Every dot sees every other dot moving away, and the surface has no edge and no center. The limit: a balloon has an inside and an outside, and the universe does not. There is no direction in space that matches “inward” or “outward” on the balloon. Also, the dots stretch as the balloon grows, but atoms, galaxies and groups of galaxies do not, because their own forces hold them together.

The raisin bread. As dough rises, the raisins drift apart without moving through the dough. Each raisin sees the farthest raisins moving away fastest, as Las Cumbres Observatory explains. The limit: a loaf has a crust and a middle, while the universe, as far as we can tell, has neither.

What would change the answer?

• Detecting curvature. If future surveys measured even a slight closed curve, space would be finite, like the surface of a ball, but in three dimensions.
• Finding repeating patterns. Matching patterns in the microwave background would reveal a wrap-around universe with a definite size.
• Accepting a limit. Because we cannot see beyond our observable bubble, a direct proof that the universe is infinite may stay out of reach, even if it is true.

It is strange and wonderful that we can measure the shape of space from the inside, using light that has traveled for almost the whole history of the universe. We know the size of what we can see. We do not know the size of all there is. For the story of how it all started, see What Happened in the First Second After the Big Bang? And keep asking.