How Large Is the Universe? From the Observable to the Total

The observable universe spans about 93 billion light-years in diameter, with a radius of roughly 46.5 billion light-years in every direction from Earth. That figure catches most people off guard, since the universe is only about 13.8 billion years old and nothing travels faster than light. The resolution lies in the expansion of space itself, which has been stretching the distances between objects ever since the Big Bang. But the observable universe is just the portion we can see. The total universe almost certainly extends far beyond that boundary, and how much farther is one of the most fascinating open questions in cosmology.

Why the Observable Universe Is Bigger Than You’d Expect

If the universe is 13.8 billion years old, you might assume that the farthest thing we could possibly see is 13.8 billion light-years away. Light has had 13.8 billion years to travel, so that should be the limit, right? The catch is that space has been expanding the entire time that light has been traveling toward us. A photon emitted by a distant galaxy billions of years ago has been crossing space that was simultaneously stretching beneath it, like walking on a conveyor belt that grows longer as you walk. By the time that photon arrives at our telescopes, the galaxy that emitted it has been carried much farther away by expansion.

The result is that the most distant objects whose light has reached us are now about 46.5 billion light-years away, giving the observable universe a full diameter of about 93 billion light-years.¹ This number is not a measurement of how far light has traveled in a straight, static line. It is a comoving distance, meaning it accounts for the cumulative expansion that has occurred since the light was first emitted. The edge of the observable universe is not a wall or a physical boundary. It is a horizon set by the finite age of the cosmos and the speed of light. Beyond it, light simply has not had enough time to reach us yet.

How Astronomers Measure Cosmic Distances

Pinning down the size of the observable universe requires reliable distance measurements across truly enormous scales. One of the most powerful tools astronomers use is the baryon acoustic oscillation, or BAO. In the early universe, sound waves rippled through the hot, dense plasma of matter and radiation. When the universe cooled enough for atoms to form, those waves froze in place, leaving a characteristic spacing imprinted in the distribution of galaxies. That spacing acts as a kind of cosmic ruler: because we know how large it should be from physics, we can compare its apparent size at different distances to figure out how much the universe has expanded.²

This “standard ruler” approach is conceptually similar to how you might judge how far away a familiar object is by how small it looks. The BAO scale is about 490 million light-years in the modern universe, and its imprint shows up reliably in galaxy surveys across billions of light-years.³ By measuring this ruler at many different cosmic epochs, astronomers can reconstruct the expansion history of the universe and, with it, the size of the observable sphere around us.

The Cosmic Event Horizon and What We Will Never See

The edge of the observable universe marks what we can see right now, but there is a separate and arguably more unsettling boundary: the cosmic event horizon. Because the expansion of space is accelerating (driven by dark energy), there are regions of the universe that are receding from us faster than light can bridge the gap. Light from those regions will never reach us, no matter how long we wait. In a universe dominated by dark energy, observers are permanently surrounded by this kind of horizon.⁴

What makes this concrete is that the event horizon is not infinitely far away. Objects with a present-day redshift greater than about 1.7 have already crossed our event horizon and are now completely out of causal contact with us. We can still see the light they emitted long ago, but no new signal from them will ever arrive.⁵ In practical terms, that means the majority of the galaxies we photograph through our most powerful telescopes are already unreachable. We are seeing their past, but their future is forever disconnected from ours.

This has a surprising consequence for the far future. As the universe continues to accelerate, the observable volume will effectively shrink. Galaxies that we can study today will redshift beyond detectability, and the imprints of early-universe processes will gradually “inflate away” during the late acceleration, much like how initial conditions were erased during the original cosmic inflation.⁴ Far-future astronomers will have access to a smaller, lonelier patch of the cosmos than we do now.

The Total Universe Beyond What We Can See

So the observable universe is large but finite. What about the total universe, the whole thing, including everything beyond the horizon? This is where the question shifts from measurement to inference, and the honest answer is that nobody knows for certain. But there are strong reasons to think it is vastly, perhaps incomprehensibly, larger than what we can observe.

The main reason is cosmic inflation, the brief period of exponentially rapid expansion thought to have occurred in the first fraction of a second after the Big Bang. During inflation, space expanded by an enormous factor in a vanishingly short time. The observable universe, enormous as it is, may represent only a tiny patch of the region that was inflated. The simplest inflationary models predict that the total universe could be many orders of magnitude larger than the observable part. Some estimates suggest the total could be at least 10^23 times the size of the observable universe, though numbers like this depend heavily on which model of inflation you adopt and how long inflation lasted.

One approach to constraining the total size uses the cosmic microwave background, the afterglow of the Big Bang. If the universe were smaller than the observable volume, or if it had a particular shape that caused it to wrap around on itself, we might see repeating patterns in the microwave background. Specifically, we would expect to find pairs of matching circles on opposite sides of the sky. A systematic search of the data found no such matched circles with a radius greater than 25 degrees, ruling out, for a wide range of possible shapes, any universe with a topology scale smaller than 24 gigaparsecs (about 78 billion light-years).⁶ That means even the lower bound on the total universe’s scale is already comparable to the observable universe, and it could be far larger.

Cosmic Topology and the Shape of Everything

The topology search mentioned above is worth dwelling on for a moment, because “the size of the universe” is tangled up with “the shape of the universe” in ways that are not obvious. When cosmologists talk about the universe being flat, they mean that its large-scale geometry follows the ordinary rules of Euclidean space: parallel lines stay parallel, the angles of a triangle add up to 180 degrees, and so on. Observations strongly support a flat or very nearly flat universe.

But flatness does not tell you whether the universe is infinite. A flat universe could extend forever in all directions, or it could be finite but wrap around on itself, like the surface of a torus. Think of an old video game where walking off the right edge of the screen puts you back on the left: the playing field is flat, but it has a finite area. Cosmologists test for this by looking for the repeating patterns in the microwave background described earlier. The non-detection of such patterns to scales of at least 24 gigaparsecs sets a lower bound on how big a finite universe would need to be.⁶ Most cosmologists interpret the data as favoring an extremely large or infinite universe, but a finite universe that is simply much bigger than the observable horizon has not been ruled out.

Eternal Inflation and Whether “the Universe” Is Even the Right Frame

Inflation does not just predict a larger universe. Many versions of the theory predict that inflation, once started, never completely stops. In these models, inflation ends in some regions (producing something like our observable universe), but continues in others, spawning new “pocket” universes in a process that goes on forever. This is the framework of eternal inflation.⁷

If eternal inflation is correct, the total reality may not be a single universe at all, but an endlessly self-generating collection of universe-sized regions, each with potentially different physical properties. Our observable universe would be one bubble in a foam of bubbles, and the question “how large is the universe?” would need rephrasing: “how large is our pocket universe?” might be the meaningful version. The answer would still be “far larger than the observable part,” but the overall structure could be literally infinite.

It is important to flag that eternal inflation is a theoretical prediction, not an observational finding. There is no direct test for other pocket universes. The idea follows naturally from the mathematics of many well-motivated inflation models, and it is taken seriously by a large fraction of theoretical cosmologists, but it remains in the realm of extrapolation. The event horizon described earlier places hard limits on how much of even our own pocket universe we can ever probe, let alone any others.

Holographic Limits on What We Can Know

There is an independent line of reasoning, rooted in quantum gravity rather than in traditional cosmology, that places its own bounds on the size of the universe we can meaningfully describe. The holographic principle, loosely stated, says that the maximum amount of information that can be contained in a region of space is proportional to the area of its boundary, not its volume. Applied to cosmology, this principle limits how much total “stuff” (including the imprint of early-universe processes) can exist inside a cosmological horizon.⁸

One practical consequence is an upper limit on the number of “e-folds” of inflation, a measure of how many times the universe doubled in size during the inflationary epoch. With reasonable assumptions, holographic bounds suggest an upper limit somewhere in the range of 65 to 146 e-folds, depending on the initial energy scale and reheating temperature. For a plausible simple cosmology, the limit lands around 85.⁹ For context, the minimum number of e-folds needed to solve the classic problems of Big Bang cosmology (the flatness and horizon problems) is about 60. So the holographic principle suggests inflation did not go on arbitrarily long, and the total universe, while vastly larger than the observable part, may not be infinite from the perspective of this framework.

A separate analysis focused on the cosmological event horizon arrives at a similar conclusion from a different direction: the dark-energy-driven acceleration of expansion limits the observable e-folds to roughly 60, regardless of how long inflation actually lasted.⁴ The convergence of these two very different approaches is striking: whether you reason from quantum information theory or from the behavior of dark energy, you end up with a consistent picture of how much of the universe’s inflationary history is accessible to observation.

The Local Supervoid and Why “Nearby” Scale Matters Too

When most people think about the size of the universe, they picture the very largest scales. But there are interesting structure questions at intermediate scales too, and they are surprisingly relevant to the overall picture. Observational studies suggest that the region around our galaxy, out to a few hundred megaparsecs (roughly a billion light-years), is significantly less dense than the cosmic average. This underdense region has been called the Keenan-Barger-Cowie void, or the local supervoid.¹⁰

Why does this matter for understanding the universe’s size? If we happen to live inside a large underdense region, the local expansion rate would appear faster than the cosmic average, because matter flows outward from voids. This is directly relevant to the “Hubble tension,” the persistent disagreement between the expansion rate measured locally (using nearby supernovae and other objects) and the rate inferred from the cosmic microwave background. Some researchers have explored whether accounting for the KBC void could reduce this tension.¹¹

The Hubble tension matters for the “size of the universe” question because the expansion rate is one of the key inputs for calculating how big the observable universe is. If the true expansion history differs from our current best model, the 93-billion-light-year figure could shift. The tension has not been fully resolved, and the local supervoid is one of several competing explanations, but it is a reminder that even our best measurement of the observable universe has some model dependence built in.

Neutrino and Gravitational Wave Horizons

Almost everything we know about the universe comes from light: visible, infrared, microwave, radio, X-ray, gamma. But light has a hard cutoff. The earliest light we can see comes from about 380,000 years after the Big Bang, when the universe cooled enough for atoms to form and space became transparent. Before that, the universe was an opaque plasma, and no photon from that era survives in a useful form.

Neutrinos and gravitational waves, however, interact with matter far more weakly and could in principle carry information from much earlier times. The cosmic neutrino background is predicted to have decoupled from the rest of the universe about one second after the Big Bang, far earlier than the microwave background. Detecting it directly would let us probe an era invisible to any telescope that relies on light.¹² We have not yet detected the cosmic neutrino background directly, though its existence is strongly supported by indirect evidence. If we eventually do, it would push our observational horizon back toward a slightly larger sphere than the one defined by photons alone.

Gravitational waves could go even further. In principle, gravitational waves generated during inflation itself could carry imprints from the very first instants of the universe, from epochs that no particle-based signal can reach. Detecting a primordial gravitational wave background is a major goal of current and planned experiments. If successful, it would not technically make the “observable universe” bigger in the way we usually define it (that definition is based on the particle horizon for photons), but it would extend the effective reach of our observational tools deeper into cosmic history and closer to the true beginning.

Putting Numbers in Perspective

The observable universe contains something on the order of two trillion galaxies, each with hundreds of billions of stars. Its volume is staggering. And yet, if the simplest inflationary models are even roughly correct, the observable universe is to the total universe what a single grain of sand is to the Sahara, or perhaps something far more extreme than even that analogy captures. The topology constraints from the microwave background already tell us the total universe is at least as large as the observable one, and quite possibly many orders of magnitude larger.⁶

One reason this question stays open is that the tools we have for probing the total size are all indirect. We can look for curvature in the geometry of spacetime (a slightly curved universe implies a finite total size), we can search for repeating topology patterns, and we can extrapolate from inflationary theory. All of these approaches agree that the total universe is at minimum much larger than what we can see. But whether it is finite or genuinely infinite remains an open question, and it is not clear that any observation, in principle, could settle the matter. The cosmic event horizon ensures that information from the vast majority of the universe will never reach us, and the accelerating expansion means this situation will only get worse with time.

There is something both humbling and clarifying about this. The observable universe, as large as it is, is the only part we can study, test theories against, and meaningfully discuss with data. Everything beyond the horizon is inference, shaped by how much we trust our models of inflation, dark energy, and fundamental physics. The 93-billion-light-year number is impressively well-measured. The total size may forever remain a question that physics can constrain but not fully answer.