Nobody knows yet whether the universe goes on forever. The part we can see has a limit, about 92 billion light-years across, set by how far light has had time to travel, and the real universe is almost certainly bigger than that.1NASA. How Big is Space? We Asked a NASA Expert: Episode: 61 Even a finite universe probably wouldn’t have an edge or a wall, because space could curve or wrap back on itself. So the question is less about finding a boundary than about measuring the shape of space itself.
How Big Is the Part of the Universe We Can See
Astronomers call the part of space we can see the “observable universe.”2BBC Sky at Night Magazine. How big is the Universe? This is what astronomers think the size of the cosmos is, and how they worked it out Light travels at a finite speed, so we can only see objects whose light has had time to reach us.3Study.com. Observable vs. Entire Universe: Overview, Size & Difference That makes the observable universe a sphere with Earth at the center, and we can’t see past its boundary, called the cosmic horizon.4Study.com. Observable vs. Entire Universe: Overview, Size & Difference A light-year is the distance light travels in a year, nearly 6 trillion miles. Across, the observable universe is estimated at about 92 billion light-years.5NASA. How Big is Space? We Asked a NASA Expert: Episode: 61
That may seem too big for a universe about 13.8 billion years old. But space has been expanding while that light traveled, so the objects that sent it are now much farther away than they were.
The horizon is a limit of sight, like the horizon at sea, not a wall. Any observer anywhere would see their own bubble centered on themselves, and there’s no reason to think space just stops beyond ours.6BBC Sky at Night Magazine. How big is the Universe? This is what astronomers think the size of the cosmos is, and how they worked it out
The 92 billion light-year figure describes only what we can see. The question of finite or infinite is about everything, including regions we can never see, and the whole is very likely larger than the visible part.7NASA. How Big is Space? We Asked a NASA Expert: Episode: 61
The figure is a diameter, so the distance from us to the horizon is about half of it, roughly 46 billion light-years in each direction. And because far-off light left long ago, looking farther out is also looking further back in time. The farthest limit of sight isn’t a place you could travel to. It’s a moment in the past, close to the beginning of the universe’s visible history.
The bubble also keeps growing. As time passes, light from farther regions has had more time to arrive, so we slowly take in more space. Expansion is carrying distant regions away at the same time, so this doesn’t mean we’ll eventually see everything. The number in headlines is a snapshot of today’s bubble, not a permanent size.
Can Something Be Finite Without an Edge
Shape and size are linked. A flat space that doesn’t wrap around would be infinite in extent.8Wikipedia. Shape of the universe A curved space, like a 3D version of a sphere’s surface, is finite yet has no boundary.9Wikipedia. Shape of the universe Even a perfectly flat space can be finite if it wraps around.10Wikipedia. Shape of the universe So flat doesn’t automatically mean infinite, which is why measuring shape alone can’t settle the question. On a curved space, travel far enough in one direction and you’d come back to where you started.
The wraparound version works like an old arcade game, where leaving the right side of the screen brings you in from the left. Such a universe does that in all three directions. Space would be flat everywhere but still have a finite volume, and in principle you might see the same galaxy in two opposite directions.
A hard edge is unlikely. An edge would be a special place unlike everywhere else, yet observations show the universe looks broadly the same in every direction. Scientists consider wrapping or curving more natural than a wall. That’s why “what is outside it?” may have no meaning: the universe wouldn’t sit inside anything, it would just be all of space.
Finite also doesn’t mean small. A finite universe’s total volume could still be enormously larger than the part we can see. If it were that big, it would look and behave like an infinite universe in every observation we could make, so the real question is whether its size is within reach of our instruments.
Measuring Whether Space Is Flat or Curved
The cosmic microwave background is leftover light from the early universe that has crossed space for billions of years.11AAS Nova. Measuring the Curvature of the Universe with Cosmic Clocks If space is curved, the patterns in that light get bent in a measurable way, like seeing through a lens. The Planck satellite’s first reading of this light leaned slightly toward a closed, curved universe.12AAS Nova. Measuring the Curvature of the Universe with Cosmic Clocks A later reanalysis found the same data fit a flat universe, and this light alone can’t pin curvature tightly.13Monthly Notices of the Royal Astronomical Society. The evidence for a spatially flat Universe
The oldest light is a good probe because it has crossed nearly the whole visible universe, so any bending of space along the way adds up over a huge distance. It carries a pattern of slightly warmer and cooler spots whose typical size is known from physics. Their apparent size reveals the geometry: in curved space they’d look larger or smaller than in flat space.
In flat space a triangle’s angles add up to 180 degrees; on a curved surface they add up to more or less. Cosmic patterns act as giant triangles that astronomers can measure.
“Flat” here doesn’t mean a pancake. It means space follows the ordinary geometry taught in school, where parallel lines stay parallel. Space is three-dimensional, so flat space is just the familiar kind we picture around us. The alternatives are a positively curved shape, like a sphere’s surface, or a negatively curved one, like a saddle.
Astronomers also measure a regular spacing in how galaxies cluster across the sky, a kind of cosmic ruler.14arXiv. The Baryon Oscillation Spectroscopic Survey: Precision measurements of the absolute cosmic distance scale Combined with the oldest light, this shows space is flat to within about 0.2 percent.15Monthly Notices of the Royal Astronomical Society. The evidence for a spatially flat Universe Separate measurements of nearer galaxies agree with flat space too.16Physical Review D. Constraining the curvature density parameter in cosmology If space is curved, the curve is so gentle that the whole universe would be vastly bigger than what we see. That 0.2 percent is a limit, not a measurement of zero curvature: measurements can’t tell a perfectly flat universe from one with a curve smaller than that margin.
Flat-looking doesn’t prove infinite. Earth looks flat across a parking lot though it’s a sphere, and a huge curved universe would look flat across our small visible patch. A flat universe could also still wrap around, as described above.
Looking for Signs That Space Wraps Around
Scientists have checked. If the universe wraps around and is small enough, the oldest light would show pairs of matching circles in opposite parts of the sky.17Physical Review D. Circles-in-the-sky searches and observable cosmic topology in the inflationary limit The shell of oldest light would overlap itself, and each overlap appears twice.18Classical and Quantum Gravity. Reconstructing the global topology of the universe from the cosmic microwave background An early claim of matching circles turned out to be a software error.19arXiv. The Topology of the Universe Searches since then found that the shortest trip around the universe through us must exceed about 98.5 percent of the visible universe’s width.20arXiv. The Topology of the Universe
The search rules out wraparounds small enough for the oldest light to have crossed the universe more than once, because those would have left circles. It leaves open a universe that wraps around at a scale a bit larger than the part we can see. So the result says the universe is not small, not that it is infinite.
The latest data fit a flat, infinite universe best.21Universe. The Status of Cosmic Topology after Planck Data They remain consistent with finite shapes, including a soccer-ball-like twelve-sided shape and a flat wraparound one.22Universe. The Status of Cosmic Topology after Planck Data
The twelve-sided idea drew attention because early satellite data showed weaker large-scale patterns than expected, and a finite twelve-sided space could naturally explain that. It remains possible but unproven.
If It Is Infinite, Are There Copies of You
Some cosmologists propose that an infinite space contains endless regions the size of our visible universe.23arXiv. The Level I Multiverse is not the same as the Level III Multiverse A leading theory of the early universe suggests endless “pocket universes,” each possibly infinite.24Journal of Cosmology and Astroparticle Physics. Probabilities in the inflationary multiverse From there, some argue every history physics allows would repeat infinitely many times.25PubMed Central. The cosmological model of eternal inflation and the transition from chance to biological evolution in the history of life But these regions lie beyond any horizon, so they can’t be observed.26BBC Sky at Night Magazine. How big is the Universe? This is what astronomers think the size of the cosmos is, and how they worked it out
The copies follow from a counting argument. Quantum physics limits how finely one region can differ from another, so only a finite number of distinct arrangements of matter fit in a bubble that size. With endless bubbles and finitely many possible arrangements, any arrangement with a nonzero chance of occurring would have to appear again and again. That is the argument for copies of Earth, and it depends entirely on the assumptions fed into it. It’s reasoning from math, not observation.
It also doesn’t say you could meet or contact a copy, since they’d be unimaginably far beyond our horizon. It doesn’t apply if the universe is finite, or if far regions differ in their basic laws. It’s a consequence of one set of assumptions, not a prediction astronomers are trying to confirm.
Will We Ever Know the Answer
The universe’s expansion is speeding up. As a result, observers end up surrounded by horizons that limit how much of the universe they can ever reach.27Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics. Observational implications of cosmological event horizons Distant galaxies will slowly slip out of view over billions of years.
Some things could still settle it. Finding genuine matching circles would be strong evidence for a finite, wraparound universe. A clear measured curve would point to a finite, closed universe.28Wikipedia. Shape of the universe But if space is flat and doesn’t wrap at visible scales, observation can’t prove it is infinite.
A finite universe could be confirmed by a single positive clue, such as a repeated pattern or a measurable curve. An infinite universe would be shown only by the absence of such clues, and absence is always open to the reply that we just didn’t look far enough. The best we can do for infinity is push the possible size of a finite universe higher and higher. Scientists default to the simplest model, flat and infinite, while staying open to the alternatives.
Future surveys will help. Larger galaxy maps will tighten the cosmic ruler and shrink the margin around flat. If the margin shrank and still held a small curve, it would point to a finite universe; if it shrank toward zero, a finite universe would have to be bigger still.