Thirteen point eight billion years old, ninety-three billion light-years wide. Both numbers are right.
What if the figure that sounds impossible is the one that is correct? Our universe is 13.8 billion years old, and yet astronomers say the part of it we can see runs about 93 billion light-years from one side to the other. Welcome, curious reader. Sit with that tension for a moment. Read to the end, and the contradiction dissolves into some of the cleanest physics we have.
TL;DR — the direct answer. The observable universe spans roughly 93 billion light-years across, a radius near 46 billion light-years, even though only 13.8 billion years have passed since the Big Bang. Space itself expanded while ancient light traveled toward us, carrying the regions that first emitted that light far past any simple age-times-speed sum.
- How can the universe be wider than its own age?
- What does a light-year really measure?
- Why is the oldest light we see coming from 46 billion light-years away?
- Did anything travel faster than light?
- Is the observable universe the whole universe?
- Where does that leave the impossible number?
- Frequently asked questions
- Sources
How can the universe be wider than its own age?
It can, since space has been stretching the entire time light was in flight. The size of the observable universe feels like a typo the first time you meet it: 13.8 billion years of history, 93 billion light-years of width. The trap is picturing the cosmos as a fixed room with a stopwatch ticking inside it. Nothing about the room stayed fixed.
In the standard model of cosmology, the observable universe has a present radius of about 46 billion light-years, which gives a diameter close to 92 or 93 billion. NASA astrophysicist Amber Straughn quotes about 92 billion light-years as the common estimate; the 93-billion figure is the same result with slightly different rounding and parameters.
That number does not mean a single photon crossed 46 billion light-years of still space to reach us. It means the regions that sent us their earliest light now sit that far away, after the space between them and our planet grew during the trip.
- 13.8 Gyr — age of the universe
- ~46 Gly — radius we can observe
- ~93 Gly — diameter we can observe
- 3.2c — present recession speed of the CMB source
What does a light-year really measure?
A light-year measures distance, not time: it is how far light travels through empty space in one year. Multiply 13.8 billion years by the speed of light and you get 13.8 billion light-years. That answer is exact, and it is exactly right for the Solar System and the nearby cosmos, where expansion is too small to notice.
Reach across billions of light-years and the shortcut breaks. The distance between two far-apart regions kept changing while the photon was on its way, so the total ground covered no longer equals speed multiplied by travel time. The photon always moved at c locally. The map underneath it did not hold still.
Why is the oldest light we see coming from 46 billion light-years away?
The oldest light we can see is the cosmic microwave background, and its birthplace has since drifted to about 46 billion light-years from us. For roughly the first 380,000 years, ordinary matter was a hot ionized plasma that scattered photons over and over, keeping the cosmos opaque. When it cooled enough for electrons to settle onto atomic nuclei, the fog cleared and light finally ran free.
Those first free photons set out when their home regions were tucked in close to the matter that would later build the Milky Way. Each one kept sailing at light speed. Around it the scale of space kept growing, stretching the waves from a warm thermal glow at redshift near 1100 down to the faint microwave hiss that satellites read today.
So the surface we call the last scattering of light is our practical horizon. We cannot see past it, and its material now lies about 46 billion light-years off, which sets the radius of everything observable.
Did anything travel faster than light?
Nothing moved through space faster than light, and the distinction is the whole point. Special relativity forbids matter or information from outrunning light for any nearby observer. It places no such cap on the metric, the geometric rule that sets how fast the gap between two distant regions can widen.
Cosmic expansion is a change in that geometry, not galaxies rushing through a fixed void. Tamara Davis and Charles Lineweaver, working in the standard model with matter and dark-energy densities of 0.3 and 0.7 and a Hubble constant of 70, showed that we can watch galaxies whose recession speed has always topped the speed of light. The photon travels toward us locally while the space it crosses keeps changing.
The 46 versus 13.8 gap, worked through. If space were static, the farthest visible matter would sit at 13.8 billion light-years, the age times c. Integrate the same light path across an expanding scale factor and the emitting matter lands near 46 billion light-years instead, close to three times the light-travel distance. The ratio 46 divided by 13.8 is roughly 3.3, and Davis and Lineweaver note the particle horizon runs to about 3 times c times the age of the cosmos.
The regions that emitted the cosmic microwave background now recede at about 3.2 times the speed of light, and at the moment of emission that rate stood at 58.1 times c. We see them anyway. To close the case, the same authors compared supernova brightnesses against the rival idea that redshift is an ordinary special-relativistic Doppler shift, and ruled that interpretation out at the 23-sigma level.
Is the observable universe the whole universe?
No. The observable universe is only the patch whose light has had time to reach us since the beginning, a boundary centered on the observer and called the particle horizon. It travels with you. Someone in a distant galaxy sees their own sphere of the same radius, overlapping ours without matching it.
The 93-billion-light-year width is not an outer wall of the cosmos. It marks the present, model-dependent size of the region we could have received a signal from, and astronomers name several distances that get mixed up in headlines.
| Horizon | Approx. distance now | What it marks |
|---|---|---|
| Hubble sphere | ~14 billion ly | Distance where the recession speed equals the speed of light |
| Particle horizon (observable radius) | ~46 billion ly | Farthest matter whose light has reached us since the beginning |
| Event horizon | ~16 billion ly | Limit of any region we could still send a signal to |
Notice that the Hubble sphere sits well inside the observable radius, so galaxies beyond it recede faster than light and we still see them, the single fact most textbooks get wrong. The full universe is larger than our sphere by an amount nobody has measured, and it may well be infinite.
Where does that leave the impossible number?
Right where we found it, and no longer impossible. A 13.8-billion-year-old cosmos hands us a 93-billion-light-year view since the light-travel time and the present distance are two different quantities, split apart by an expansion that never paused. The naive multiplication holds in your neighborhood and fails across the void, and once you follow a single ancient photon through a stretching space, the two numbers stop fighting each other.
Here is the thought worth carrying home. Every horizon in this article is drawn around us, which means the edge of the observable universe is not a place out there but a limit built into where and when we happen to be looking. The cosmos is under no obligation to end where our line of sight does.
This article was written specifically for you by FreeAstroScience.com, where we explain complex scientific principles in simple terms. Come back and visit us, and never turn off your mind, for the sleep of reason breeds monsters.
Gerd Dani
Frequently asked questions
How big is the observable universe?
The observable universe measures roughly 93 billion light-years across, a radius near 46 billion light-years. That figure comes from the standard model of cosmology, where the regions that emitted the oldest light we detect have since been carried far away by cosmic expansion, long after their light set out toward us.
Why is the universe wider than its age in light-years?
Space itself expanded while light traveled. If the cosmos were static, 13.8 billion years of travel would cap distances at 13.8 billion light-years. Expansion stretched the gap between us and the emitting regions during the journey, so those regions now sit about 46 billion light-years away.
Did galaxies move faster than the speed of light?
No galaxy raced through space faster than light. Cosmic expansion stretches the distance between far-apart regions, and that recession rate can exceed the speed of light without breaking relativity. Davis and Lineweaver showed we routinely observe galaxies whose recession velocity has always been greater than light speed.
What is the cosmic microwave background?
The cosmic microwave background is the oldest light we can detect, released about 380,000 years after the Big Bang when the cooling universe let electrons bind to nuclei and become transparent. Those photons started from regions now roughly 46 billion light-years away, their wavelength stretched into microwaves by expansion.
Is the observable universe the same as the whole universe?
No. The observable universe is only the region whose light has had time to reach us since the beginning, a boundary centered on every observer called the particle horizon. The full universe is larger by an unknown amount, and the 93 billion light-year figure describes what we can see, not a physical edge.
Sources
- Davis, T. M. and Lineweaver, C. H. (2004). Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the universe. Publications of the Astronomical Society of Australia, 21(1), 97-109. arXiv:astro-ph/0310808. DOI: 10.1071/AS03040. arxiv.org/abs/astro-ph/0310808
- Meloni, D. (2026). Universo: spiegata l’ampiezza di 93 miliardi di anni luce. reccom.org, 23 July 2026. Includes the ~92 billion light-year estimate attributed to NASA astrophysicist Amber Straughn.
- Bennett, C. L. et al. (2003). First-Year WMAP observations, source of the Hubble constant value of 70 km per second per megaparsec used in the model above (cited within source 1).
