The observable universe spans roughly 93 billion light-years from one edge to the other, with Earth sitting near the center of that sphere. That number catches most people off guard, because the universe is only about 13.8 billion years old, and nothing travels faster than light. The mismatch between the age and the size is real, not a contradiction, and understanding why it exists reveals something fundamental about the nature of space itself. The full universe beyond what we can observe is almost certainly much larger, and may be infinite.
Why 93 Billion When the Universe Is Only 13.8 Billion Years Old
The most common reaction to the 93-billion-light-year figure is confusion. If light has only had 13.8 billion years to travel, how can anything be 46.5 billion light-years away from us in any direction? The answer is that space itself has been stretching the entire time that light has been in transit. A photon that left a distant galaxy shortly after the Big Bang has been traveling toward us for nearly the entire age of the universe, but the space it already crossed kept expanding behind it. By the time that photon arrives, the galaxy it came from has been carried much farther away by the cumulative expansion of space. One educational treatment of this problem frames it as the central puzzle students face: how can the cosmic microwave background radiation be roughly 90 billion light-years away when the universe is only about 13.8 billion years old?1IOP Publishing. Cosmological expansion
This expansion is not like an explosion pushing objects outward through a fixed background. Instead, the fabric of space between galaxies is growing. Galaxies are not racing through space away from us so much as the space between us and them is accumulating. The farther away something is, the more space there is between us, and the faster that accumulated expansion carries it away. This relationship between distance and apparent recession speed is described by the Hubble constant, which links the expansion velocity of objects to their distance from us.2PubMed Central. The Hubble Constant
At great enough distances, the rate of expansion between us and a distant object exceeds the speed of light. That sounds like it should be forbidden, but it is not. The speed-of-light limit applies to objects moving through space, not to the rate at which space itself stretches. Two galaxies can be receding from each other faster than light without either one breaking any rule of physics. They are standing still in their local patch of space while the space between them grows.
Comoving Distance and Why the Number Sounds So Large
When cosmologists say the observable universe is 93 billion light-years across, they are using a particular kind of measurement called the comoving distance. This is the distance between two points measured as if you could freeze the expansion of space at the present moment and lay down a ruler between them. It accounts for all the stretching that has happened since the light was emitted.
There is a different measure called the light-travel distance, which is simply how long the light was in transit, expressed as a distance. For the most distant objects we can see, the light-travel distance is close to 13.8 billion light-years, because the light has been traveling for nearly the entire age of the universe. But the comoving distance to those same objects, accounting for expansion, is about 46.5 billion light-years. These are both legitimate distance measurements, just answering different questions.3Budapest International Research in Exact Sciences (BirEx) Journal. Comoving Distance- Light Travel Distance
The 93-billion-light-year diameter is the comoving distance all the way across the observable sphere: 46.5 billion light-years in every direction from Earth, doubled. This is the standard figure used in cosmology when people talk about the “size” of the observable universe.
What Defines the Edge of the Observable Universe
The boundary of the observable universe is set by what cosmologists call the particle horizon. It is not a physical wall or barrier. It is the maximum distance from which light has had time to reach us since the beginning of the universe. Everything inside that sphere has had a chance to send a signal our way; everything beyond it has not. The particle horizon divides all matter into two groups: things that have already been observable and things that have not yet had time to make themselves known to us.4Monthly Notices of the Royal Astronomical Society. The cosmic horizon
The particle horizon is not permanent. It grows over time as light from more distant regions finally arrives. Every second, a thin shell of previously unobservable space enters our particle horizon. But this does not mean we will eventually see everything. The accelerating expansion of the universe, driven by dark energy, is pushing distant regions away from us faster than their light can close the gap. So while the particle horizon technically grows, the acceleration of expansion means many regions of the universe will never become visible to us.
In the standard cosmological model, there is also an event horizon, which represents the farthest distance from which a signal sent right now will ever be able to reach us. With the current rate of acceleration, that event horizon will settle at a fixed proper distance of about 5.1 gigaparsecs, or roughly 16.6 billion light-years.5The Astrophysical Journal. Dark Energy and the Observable Universe Anything beyond that distance, even if it emits a photon aimed directly at us starting today, will never reach us because the expansion of intervening space will always outrun the photon.
How Astronomers Actually Measure Cosmic Distances
Measuring distances on the scale of billions of light-years is not straightforward. Nobody is pointing a ruler at a galaxy. Instead, cosmologists use a toolkit sometimes called the cosmic distance ladder, where each rung builds on the one below it. Nearby distances are measured using parallax, the tiny apparent shift in a star’s position as Earth orbits the Sun. Farther out, certain types of stars and supernovae serve as “standard candles” because their true brightness is known, so comparing that to how bright they appear tells you how far away they are. At still greater distances, a phenomenon called baryon acoustic oscillations acts as a standard ruler.
Baryon acoustic oscillations are a leftover imprint from the early universe, when sound waves propagated through the hot, dense plasma that filled all of space. Those waves froze in place when the universe cooled enough for atoms to form, leaving a characteristic spacing in the distribution of galaxies. Because the physical size of that spacing is well understood from theory, finding it in galaxy surveys tells astronomers how far away those galaxies are.6Nature Astronomy. Evidence for baryon acoustic oscillations from galaxy–ellipticity correlations This ruler has been used to trace the expansion history of the universe out to billions of light-years.7Journal of Cosmology and Astroparticle Physics. Optimal reconstruction of baryon acoustic oscillations for DESI 2024
These different techniques can be cross-checked against each other. The distance ladder is calibrated using both the local expansion rate as a nearby anchor and the sound-horizon scale from the cosmic microwave background as a far-away anchor, creating what is sometimes called an inverse distance ladder that works from large scales down rather than small scales up.8Monthly Notices of the Royal Astronomical Society. Calibrating the cosmic distance scale ladder: the role of the sound-horizon scale and the local expansion rate as distance anchors Newer work extends this approach by using quasars and other objects as distance markers at redshifts that were previously out of reach, testing whether the standard cosmological model holds up at greater distances.9Journal of Cosmology and Astroparticle Physics. Probing alternative cosmologies through the inverse distance ladder
The Observable Universe Versus the Whole Universe
Everything discussed so far applies to the observable universe, the 93-billion-light-year-wide sphere centered on our location. But the universe almost certainly extends far beyond what we can see. The observable part is just our information bubble, defined by the finite speed of light and the finite age of the cosmos. There is no reason to think the universe ends at the edge of our observational reach, any more than the ocean ends at the horizon you can see from the beach.
How much larger the whole universe might be is genuinely unknown. It could be modestly larger than what we observe, or it could be infinite. The answer depends on the overall geometry and topology of space, and current data can only set lower bounds. One analysis of curvature measurements found, with high confidence, that the universe contains at least five times the volume of a single Hubble sphere, which is roughly the observable volume.10Monthly Notices of the Royal Astronomical Society. How flat can you get? A model comparison perspective on the curvature of the Universe That is a conservative lower limit; most cosmologists suspect the true extent is vastly greater.
Is the Universe Flat, Curved, or Infinite
The shape of the universe on the largest scales depends on its curvature. If space has zero curvature, it is flat and extends infinitely in all directions. If it has positive curvature, it curves back on itself like the surface of a sphere, making it finite even though it has no edges. If it has negative curvature, it is saddle-shaped and also infinite. Every major measurement to date suggests the curvature is extremely close to zero, consistent with a flat, infinite universe. Depending on the statistical priors used, the probability that the universe is spatially infinite has been estimated at between 67 and 98 percent, with odds on the order of 50 to 1 in favor of flatness compared to a closed model.10Monthly Notices of the Royal Astronomical Society. How flat can you get? A model comparison perspective on the curvature of the Universe
That said, the evidence is not airtight. Some studies using gravitational lensing data have found hints of positive curvature, which would favor a closed, finite universe. One analysis of strong gravitational lensing found that its estimates consistently favored a closed model at a high confidence level, a result in tension with the near-perfect flatness inferred from the cosmic microwave background.11The Astrophysical Journal. Curvature from Strong Gravitational Lensing: A Spatially Closed Universe or Systematics? Whether that tension reflects a genuine detection of curvature or systematic errors in the lensing data is still debated. The mainstream consensus tilts heavily toward flatness, but with the honest caveat that we are trying to measure the shape of something enormously larger than our observational reach. Even future experiments pushing measurement precision down to about four ten-thousandths of a percent will face a hard limit: if the true curvature is below a certain threshold, it becomes indistinguishable from zero and the geometry of the universe is simply unknowable.10Monthly Notices of the Royal Astronomical Society. How flat can you get? A model comparison perspective on the curvature of the Universe
Could the Universe Wrap Around on Itself
Flatness and finiteness are not the same thing. Even a perfectly flat universe could be finite if its topology is unusual. Imagine living on the surface of a cylinder or torus: locally the surface is flat, but if you travel far enough in one direction, you come back to where you started. Some cosmologists have investigated whether the universe might have a compact topology like this, where space repeats itself. If it did, you might see the same distant objects in multiple directions on the sky, or find telltale patterns in the cosmic microwave background.
Searches for those patterns in data from the WMAP satellite found no evidence of a compact topology. The data were compatible with an infinite, non-repeating universe, and any fundamental repeating cell would need to be at least 1.2 times the distance to the surface where the cosmic microwave background was emitted to avoid being detected.12The Astrophysical Journal. Constraints on the Topology of the Universe from the Wilkinson Microwave Anisotropy Probe First-Year Sky Maps The WMAP data did show unusually low power in the largest-scale temperature fluctuations, which some had speculated might be a sign of finite topology, but the expected correlations for a compact universe simply were not there. Subsequent data from the Planck satellite tightened these limits further, pushing any possible repeating scale well beyond the observable horizon.
The Multiverse Angle
If inflation, the brief burst of exponential expansion thought to have occurred in the universe’s first fraction of a second, is the correct picture of cosmic origins, then the universe may be far larger than even the most generous finite estimates. Many inflationary models predict that inflation is eternal: it ends in our local region, allowing normal matter and galaxies to form, but continues elsewhere, spawning an endless landscape of separate regions. In this framework, the result is a multiverse containing an infinite number of spatially infinite pocket universes, each potentially with different physical properties.13Journal of Cosmology and Astroparticle Physics. Probabilities in the inflationary multiverse
Whether the multiverse is real science or unfalsifiable speculation is one of the livelier debates in theoretical physics. The core difficulty is that these other pocket universes, if they exist, are separated from us by inflating space and can never be observed. That makes it hard to test the idea directly. Still, the multiverse prediction emerges naturally from the same inflationary physics that successfully explains many observed features of our universe, like its flatness and the pattern of temperature fluctuations in the cosmic microwave background. It is less a wild add-on and more an uncomfortable consequence of a theory that works well in the parts we can check.
What the Numbers Mean in Practice
There are roughly two trillion galaxies within the observable universe, each with hundreds of billions of stars. The 93-billion-light-year diameter contains all of this and represents every photon, every gravitational wave, every particle that has had time to reach us since the beginning. But the number itself is a snapshot. It was smaller in the past and continues to grow, both because light from more distant regions keeps arriving and because ongoing expansion stretches the comoving distances between objects.
In practical terms, the universe you can see is not the universe you can visit. Even at the speed of light, the accelerating expansion guarantees that the vast majority of what you observe today will eventually fade beyond the event horizon. Galaxies you can photograph right now will, in the far future, redshift out of visibility as the expansion carries them away. The observable universe will not shrink in terms of its particle horizon, but it will empty out as fewer and fewer objects remain within communicable range. The knowable cosmos is, in a real sense, a closing window.
The Hubble Tension and Why the Exact Number Keeps Shifting
You might notice that different sources give slightly different figures for the size of the observable universe, sometimes 92 billion light-years, sometimes 93, sometimes 93.6. The reason is that the exact number depends on the values plugged into the cosmological model, and one of those values, the Hubble constant, is currently disputed. Two independent ways of measuring how fast the universe is expanding give answers that do not quite agree. Measurements from the local universe using supernovae and other nearby markers give a somewhat higher expansion rate than measurements inferred from the cosmic microwave background and the early universe.
This discrepancy, known as the Hubble tension, has persisted for years and has not been explained away by measurement errors. Some researchers are exploring whether small changes in the expansion history of the universe could reconcile the two numbers.14Universe. The Linear Response of the CMB-Inferred Hubble Constant to Perturbations in the Expansion History If the true expansion rate turns out to be at the higher end, the observable universe would be slightly smaller in comoving distance; if at the lower end, slightly larger. The difference amounts to a few percent at most and does not change the basic picture, but it is a genuine open problem in cosmology. Resolving it could also reveal something unexpected about the physics of expansion itself, which would affect much more than just a diameter estimate.