How big is the universe, and how far can we see?

What the phenomenon is

Two numbers get quoted together and appear to fight each other. The universe is 13.8 billion years old: the Planck collaboration's final cosmological parameters give an age of 13.797 billion years, with an uncertainty of 23 million, and a Hubble constant of 67.4 kilometres per second per megaparsec. And the observable universe is about 93 billion light years across.

That second figure comes from the distance to the particle horizon. Tamara Davis and Charles Lineweaver, in a 2004 paper written specifically to clear up confusions about cosmological horizons, put it at about 46 billion light years: light that has been travelling towards us since the beginning was emitted from positions that are now roughly 46 billion light years away. Double that for a diameter.

The popular claim

The objection writes itself, and it is everywhere. Nothing travels faster than light. The universe is 13.8 billion years old. So nothing we can see should be more than 13.8 billion light years away, and 46 billion is impossible — therefore the age is wrong, or the expanding universe is wrong.

Two other claims travel with it: that telescopes are looking at the edge of the universe, and that because the observable sphere is centred on us, we must be at the centre of everything. All three are natural readings of the numbers. All three are wrong, and Davis and Lineweaver wrote their paper partly because versions of them appear in the professional literature too, not only in popular accounts.

The scientific explanation

The first mistake is treating how long ago as the same thing as how far away now. Light from a distant galaxy travelled for, say, twelve billion years — but while it was in flight, the space it was crossing kept expanding, so the galaxy that emitted it has been carried much further away in the meantime. The travel time and the present distance are different quantities, and for the most distant sources they differ by a factor of about three: Davis and Lineweaver note that the ratio of the radius of the observable universe to the age of the universe is roughly three to one.

This also means we routinely observe galaxies that are receding faster than light. Their paper states directly that we can observe galaxies that have, and always have had, recession velocities greater than the speed of light, and explains why this does not contradict special relativity: the motion is not within any observer's inertial frame, no observer overtakes a light beam, and everyone measures light locally at exactly c.

The second mistake is thinking the particle horizon is what we actually see. It is not. The effective limit is the cosmic microwave background, at a redshift of about 1100, because the early universe was opaque and we cannot see past the surface where it cleared. Planck's final results describe that last-scattering surface as the universe about 380,000 years after the Big Bang. The matter that emitted the microwave background is now receding from us at about 3.2 times the speed of light, and was receding at around 58 times the speed of light when it emitted that light.

NASA's Astronomy Picture of the Day puts the answer to "how far can we see" in one sentence: in light, the farthest we can see comes from the cosmic microwave background. Some neutrinos and gravitational waves around us come from further out, but there is no technology yet that can detect them.

The farthest individual object confirmed so far is a galaxy, MoM-z14, at a spectroscopic redshift of 14.44 — seen as it was about 280 million years after the Big Bang. It was confirmed with JWST's NIRSpec prism spectroscopy, and the paper was published in The Open Journal of Astrophysics on 30 January 2026.

As for being at the centre: every observer anywhere has their own observable sphere, centred on themselves, because light has had the same amount of time to reach everyone. The centre is a fact about the observer, not about the universe.

What is still debated

The honest answer to how big the whole universe is: nobody knows, and it may not be knowable from inside. What can be measured is the shape. Combining Planck with baryon acoustic oscillation data gives a curvature parameter of 0.0007 with an uncertainty of 0.0019 — the collaboration describes this as spatially flat to an accuracy of 0.2 per cent. A universe that is exactly flat and simply connected is infinite, but measurement can only ever give limits, never exactness.

The strongest published limit of this kind comes from Mihran Vardanyan, Roberto Trotta and Joseph Silk, writing in Monthly Notices of the Royal Astronomical Society Letters in 2011. Using Bayesian model averaging over cosmological models, they put the curvature radius of the universe at more than 42 gigaparsecs at 99 per cent confidence, which corresponds to at least 251 Hubble spheres. In other words: the whole is at least a couple of hundred times the volume we can see, and may well be infinite.

The numbers also move a little as cosmology argues with itself. The Planck paper discusses tensions between its parameters and some other measurements, and is candid that inflation, dark energy and dark matter are not understood at any fundamental level. Horizon distances are computed within a model; change the model's parameters and the horizon shifts by a few per cent. The qualitative picture — a visible sphere tens of billions of light years in radius, inside something much larger — does not shift.

Verdict

How big is the universe, explained as plainly as the science allows: the part we can see is measured, and the rest is not. Real or Legend has filed this as confirmed. The size of the observable universe is a measured quantity with published sources: about 46 billion light years in radius, about 93 billion across, bounded in practice by the microwave background rather than by the horizon itself. The size of the whole universe is not a measured quantity, and a page that gives you a single confident number for it is telling you something nobody knows.