Scrub cosmic time from the Big Bang to today and watch two spheres grow at different rates: the naive "light traveled at speed c" distance, and the real comoving particle horizon - the actual edge of the observable universe. Click a reference shell (Milky Way, Local Group, Virgo galaxy cluster, Laniakea, Hubble radius, particle horizon) to see its real distance.
Observable Universe Horizon 3D Explorer
Scrub cosmic time from the Big Bang to today and watch why the observable universe's real edge - about 46.5 billion light-years away - is more than three times as far as the 13.8 billion light-years a naive "light traveled at speed c for the age of the universe" guess would give.
The universe is 13.787 billion years old (Planck 2018), so light from the earliest moments has literally been traveling for almost that entire span. But the source that emitted that light has not stayed put - space itself has been expanding the whole time the light was in transit, so by the time we see it, that source is now far beyond where a simple speed-times-time calculation would place it.
- Two growing spheres driven by one "Cosmic time" slider: the naive light-travel-time distance (green wireframe) and the real comoving particle-horizon distance (orange, translucent)
- Six labeled reference shells on a compressed log scale - Milky Way, Local Group, Virgo galaxy cluster, Laniakea Supercluster, Hubble radius, and the particle horizon itself
- A toggle switches the facts-panel headline between comoving distance and light-travel-time distance for the same cosmic moment
- Facts panel lists the age of the universe, the 46.5 Gly horizon radius, the 14.4 Gly Hubble radius, and two competing published galaxy-count estimates
- Runs fully in the browser with the vendored three.js engine - no account, no upload; your last view mode is remembered on your device
| Quantity | Value |
|---|---|
| Age of the universe today | 13.787 billion years (Planck 2018) |
| Comoving particle-horizon radius (today) | about 46.5 billion light-years (diameter about 93 Gly) |
| Naive light-travel distance (today) | about 13.8 billion light-years |
| Hubble radius (c / H0, H0 = 67.4 km/s/Mpc) | about 14.4 billion light-years |
| Galaxy count estimate (Conselice et al. 2016) | about 2 trillion |
| Galaxy count estimate (New Horizons, 2021) | about 200 billion (competing, lower) |
Why the horizon is bigger than the light-travel guess
Picture every galaxy as a raisin baked into a loaf of bread that is rising in the oven. As the loaf expands, every raisin moves further from every other raisin - not because the raisins are flying through the dough, but because the dough itself is stretching between them. Light leaving a distant galaxy 13 billion years ago set off toward us across a much smaller universe; by the time that light arrives today, the space it crossed has stretched, and the galaxy that emitted it is now much further away than the light's travel time alone would suggest. That gap between the 13.8 billion light-year naive guess and the real 46.5 billion light-year comoving distance is the direct, measurable signature of that stretching.
The Cosmic Microwave Background - the oldest light we can see, emitted about 380,000 years after the Big Bang - shows the same effect at its most extreme: its light has been traveling for almost the full age of the universe (about 13.8 billion years), yet the matter that emitted it is now, in comoving terms, about 46 billion light-years away. Two competing published estimates for how many galaxies fill that volume - about 2 trillion (Conselice et al. 2016) versus a lower revision of about 200 billion (New Horizons, 2021) - are not yet settled, so this explorer presents both rather than picking a winner.
For the Hubble Law recession-speed relationship (v = H0 d) that this horizon accompanies, see the Expanding Universe 3D Explorer; for how astronomers measure distance rung by rung with parallax and Cepheid variables in the first place, see the Cosmic Distance Ladder 3D Explorer. This page is the one visualizing the resulting horizon geometry - why the observable universe's radius so exceeds the naive light-travel guess.
Everything renders on your device with WebGL. The 3D engine loads once (about 0.7 MB) and is cached, and nothing about your visit is sent to a server.
This is an educational approximation, not a numerical integration of the Friedmann equations: the comoving-horizon growth curve is a stylized, monotonic curve tuned to reach the real 46.5 billion light-year figure at the real 13.787 billion year age, and the six reference shells use real characteristic distances compressed onto one log scale for readability.
Frequently Asked Questions
Why is the observable universe 46.5 billion light-years across if it is only 13.8 billion years old?
Because space itself has been expanding the whole time light from distant galaxies was traveling toward us. The light's travel time really is close to the universe's age, but the source that emitted it has since moved much further away as the space between us and it stretched - so the comoving distance today (about 46.5 billion light-years) is far larger than the naive light-travel guess (about 13.8 billion light-years).
What is the "particle horizon"?
It is the comoving distance to the farthest light that could possibly have reached us since the Big Bang - the true edge of the observable universe. Its radius today is about 46.5 billion light-years, giving a diameter of about 93 billion light-years.
What is the Hubble radius, and is it the edge of the universe?
The Hubble radius (c / H0) is about 14.4 billion light-years, using H0 = 67.4 km/s/Mpc from Planck 2018 data. It is a characteristic cosmological distance, not a hard boundary - the observable universe's real edge, the particle horizon, reaches more than three times further.
How old is the universe, and how precisely is that known?
About 13.787 billion years, plus or minus 0.020 billion years, per the Planck Collaboration's 2018 cosmological parameters (published 2020). It is one of the most firmly established numbers in cosmology.
How far away is the Cosmic Microwave Background?
Its light has been traveling for almost the entire age of the universe, about 13.8 billion years. But the matter that emitted it is now, in comoving terms, about 46 billion light-years away, because that matter's redshift is about 1090 - it has been carried outward by 13.8 billion years of cosmic expansion since it emitted that light.
How many galaxies are in the observable universe?
Two competing published estimates exist, not a single settled number: about 2 trillion, from Conselice et al. (2016), extrapolated from the galaxy stellar mass function; or a lower revision of about 200 billion, from Lauer, Postman, Weaver, and colleagues (2021), based on New Horizons spacecraft measurements of the cosmic optical background finding less background light than 2 trillion faint galaxies would produce. This explorer presents both rather than picking a winner.
How does this differ from the Expanding Universe explorer?
The Expanding Universe 3D Explorer shows the Hubble Law recession-speed relationship (v = H0 d) as a stretching grid of galaxy markers. This page shows the resulting horizon geometry - why the observable universe's radius (46.5 billion light-years) so exceeds the naive light-travel-time guess (13.8 billion light-years) implied by its age. They are complementary views of the same expansion.
Is this a real cosmological simulation?
No. It is an educational approximation, not a numerical integration of the Friedmann equations. The comoving-horizon growth curve is a stylized, monotonic curve tuned to reach the real 46.5 billion light-year figure at the real 13.787 billion year age, and the six reference shells use real characteristic distances compressed onto one log scale for readability.