Is the universe infinite?

How big is the part we can see?

The universe is about 13.8 billion years old, so you might expect to see 13.8 billion light-years in each direction and no farther. But space has been expanding the whole time. The light from the most distant places left them long ago, and since then the space between us has stretched. In a 2004 paper, astronomers Tamara Davis and Charles Lineweaver worked out where those places are now: about 46 billion light-years away. That distance is the radius of the observable universe. A NASA expert, speaking in NASA’s “We Asked a NASA Expert” series, rounds the full width to about 92 billion light-years.

Two things surprise people here. First, the bubble is centered on us only because we are the ones looking. Someone in a galaxy billions of light-years away would have their own bubble, centered on them. Second, the edge is not a place where anything stops. NASA’s Ask an Astrophysicist team explains that the universe’s age limits how much of it we can see. It does not tell us whether space itself is infinite. For a fuller tour of the sizes involved, see our story How Big Is the Universe, and Does It Have an Edge?

What does “flat” mean, and how was it measured?

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

Astronomers can draw such a triangle using the cosmic microwave background, the faint glow left from about 380,000 years after the Big Bang. The glow has hot and cold spots. NASA explains that if space is flat, the brightest spots should look about one degree across on the sky. Several experiments found about one degree. NASA’s WMAP spacecraft then pinned it down: by 2013, its team reported that space is flat with only a 0.4% margin of error.

The European Space Agency’s Planck satellite did even better. Its final 2018 results, combined with maps of how galaxies are spread out, show space is flat to within about 0.2%.

There is one honest wrinkle. Planck’s own microwave data, taken alone, leaned toward a slightly closed, ball-like shape. Adding other measurements pulled the result back to flat. In 2019, three researchers argued in Nature Astronomy that the closed-shape hint deserved serious attention. They wrote that future measurements are needed to settle whether it comes from hidden errors, new physics, or chance. Planck’s standard model of the universe still assumes flat space, but the question is being tested, not ignored.

Does flat mean infinite?

Not by itself. NASA’s WMAP team wrote that a flat universe “suggests” an infinite one. But they added that, since we can only see a finite part, all we can truly conclude is that the universe is much larger than what we can observe.

Here is why flat is not enough. Einstein’s theory of gravity describes the local shape of space: how it bends in each small region. It says nothing about how the pieces connect overall. That overall connection is called topology. A 2024 paper in Physical Review Letters by a team called the COMPACT Collaboration makes this point. It lists 18 possible flat shapes. One is ordinary endless space. Others are finite, or finite in some directions. The simplest finite one is the three-torus, a box where leaving through one wall brings you back through the opposite wall.

Analogy: think of that old video game screen, or the surface of a doughnut. You can travel forever without hitting an edge, yet the total area is finite. Where the analogy breaks: a doughnut’s surface curves in a bigger space around it, and a game screen sits inside a machine. A three-torus universe needs no outside space at all. Its flat space simply connects back to itself.

If our universe wrapped around on a scale smaller than our view, we would see repeats: the same pattern of hot and cold spots in two places on the sky. Planck searched for these and found none. For the simple shapes tested, any loop must be about as large as the whole observable universe, or larger. The COMPACT team adds a caution. Past searches, they write, have “far from exhausted” the possible shapes. The hunt goes on.

What is the strongest case each way?

The case for an infinite universe is simple. The data say flat, and the simplest flat space has no end. A 2009 study in Monthly Notices of the Royal Astronomical Society by Mihran Vardanyan, Roberto Trotta and Joe Silk weighed the evidence. They found the chance that the universe is spatially infinite lies between 67% and 98%, depending on the starting assumptions.

The strongest objection is that we may never be able to know. Space could curve so gently that no measurement could ever tell it from flat. Analogy: stand in a wide field. Earth is round, but your patch looks flat. (Where it breaks: on Earth you can travel far enough to see the curve, but we cannot travel beyond what we can see of the universe.) The same 2009 study argued that if the curvature is below a very tiny level, the geometry of the universe is “not knowable.” A gently closed universe would be finite. And a wrap-around shape larger than our view might leave no mark we can read. So even a perfect measurement of “flat” could leave the question open.

What do people often get wrong?

“The universe is 13.8 billion light-years wide.” That mixes up age and distance. Because space expanded while light traveled, the edge of what we can see is now about 46 billion light-years away.

“The universe has an edge out there.” The limit of our view is a horizon, not a wall. As far as anyone can tell, space has no edge, whether it is finite or infinite.

“The Big Bang happened at one spot, so the universe must be finite.” NASA’s Ask an Astrophysicist team explains that the Big Bang happened everywhere at once. It was the start of an expansion, not an explosion from a center.

“Flat means infinite.” As shown above, flat space can still be finite if it wraps around.

How can you check this yourself?

• Read NASA’s own words. The WMAP “Shape of the Universe” page (in the Internet Archive copy linked above) explains closed, open and flat in a few paragraphs.
• Find the Planck number. The Planck 2018 paper’s abstract gives the curvature result. Look for the words “consistent with a flat universe.”
• Watch for “observable.” When an article gives a size for “the universe,” check whether it means the part we can see. The two are very different.
• Ask what was tested. When you read “no sign of a wrap-around universe,” ask which shapes were searched. The COMPACT team’s paper shows many remain.

The answer stays the same after all this detail. The universe we can see is finite, and very large. Space is flat or very close to it. Whether the whole universe goes on forever is still an open question, and it may stay that way.