Two questions, one expansion
By now in this rung you have earned the central fact: space itself is expanding, distant galaxies recede in proportion to their distance, and their light is stretched to the red. Wind that film backward and everything was once hotter, denser, and closer together — the Big Bang. Two natural questions fall straight out of that picture, and this guide answers both. *When* did the expansion begin — how old is the universe? And *how far* can we see across it — how big is the part we can observe?
The two questions are tied together by the speed of light, the same finite speed that made every telescope a time machine back in the Foundations rung. Because light is fast but not instant, the age of the universe sets a hard limit on how far back we can see — and, with a twist we will untangle, on how far away we can see. The surprise of this guide is that the age and the size do not match in the naive way you would expect. Light has been travelling for 13.8 billion years, yet the edge of what we can observe lies far beyond 13.8 billion light-years. Both numbers are real, and the gap between them is the expansion of space caught in the act.
How old? Run the expansion backward
Start with the back-of-the-envelope estimate. If galaxies have always been flying apart at their present speeds, rewind the film and they all converge at one instant in the past; the time since that convergence is the Hubble time, and it is simply 1 over the Hubble constant. Plug in a Hubble constant of about 70 kilometres per second per megaparsec and the arithmetic gives roughly 14 billion years — astonishingly close to the right answer for what looks like a guess scribbled on an envelope.
Do the calculation properly and the answer is about 13.8 billion years. What makes us trust it is not one method but three that agree. First, the careful backward calculation just described, fed by the measured mix of matter and dark energy. Second, the oldest individual stars: the most ancient stars in globular clusters clock in at roughly 13 billion years by their own nuclear physics, and reassuringly they come out a little *younger* than the universe, never older. Third, and most precisely, the cosmic microwave background — the faint 2.7-kelvin glow from when the universe was about 380,000 years old — carries a detailed pattern of ripples whose geometry pins the age down to within a few tens of millions of years.
It is worth being clear about what "13.8 billion years" does and does not claim. It dates the *expansion* — the time since space began stretching from its hot, dense beginning — not a literal magical instant of creation; the theory describes the universe only from a tiny fraction of a second onward, where the physics is reliable. And the number is derived inside the standard model of cosmology and depends on the Hubble constant, so the unsettled value of that constant leaves a small honest wobble in the exact age, a thread we pick up at the end.
How big? The edge we can see
Now the size. The universe has an edge to our *view* — not an edge to the universe itself, but a horizon, the same way a sailor on a calm sea can only see so far. Because light travels at a finite speed and the universe has existed for only a finite time, light from beyond a certain distance simply has not had time to reach us yet. That boundary is the cosmic horizon, and everything inside it is the observable universe — the only part we can possibly study, with us at its centre purely because we are the ones doing the looking.
Here is the trap almost everyone falls into. The naive guess is that the horizon must sit 13.8 billion light-years away, since that is how long light has been on the road. But that ignores the very thing this whole rung is about: while that ancient light was crossing the cosmos, space itself kept expanding, carrying the regions that emitted it much farther away in the meantime. So the matter that released the cosmic microwave background — the most distant stuff we can detect — was nearby when its light set out, but now lies about 46 billion light-years from us. The observable universe is therefore roughly 93 billion light-years across, far larger than the light-travel distance alone would suggest.
light has travelled for: 13.8 billion years so naive radius would be: 13.8 billion light-years but space stretched meanwhile -> emitting matter now at: actual radius (today): ~46 billion light-years observable universe diameter: ~93 billion light-years
What the horizon is — and is not
It is easy to misread the cosmic horizon as a wall, or as the edge of the universe. It is neither. Nothing special happens out there; a galaxy sitting near our horizon sees a perfectly ordinary patch of cosmos around itself, with its *own* horizon stretching another 46 billion light-years past us in the opposite direction. The universe almost certainly carries on far beyond what we can see — possibly forever. The horizon is a fact about our *vantage point and the finite age of light*, not a feature of the cosmos. Every observer, everywhere, sits at the centre of their own observable sphere.
There is a subtle and genuinely strange consequence of the accelerating expansion, and it is worth meeting honestly. Galaxies beyond a certain distance are receding so fast — carried by growing space, not moving through it — that light they emit *today* will never reach us, no matter how long we wait. The space between us and them grows faster than their light can close the gap. This breaks no law, because nothing is travelling *through* space faster than light; it is space itself stretching. The melancholy upshot is that in the very far future, as expansion accelerates, distant galaxies will redden and fade from view, and an observer then will see a far emptier, more isolated sky than we do.
The concordance model — and its honest crack
Where do the precise numbers — 13.8 billion years, 46 billion light-years, the mix of ingredients — actually come from? They come from a single framework called the Lambda-CDM model, the standard model of cosmology. It is the simplest theory that explains nearly all the big observations at once, which is why it is also called the *concordance* model: many independent measurements concur on it. Its name is its recipe. Lambda is dark energy, the property of space driving expansion to accelerate; CDM is cold dark matter, unseen slow-moving matter that clumps under gravity to scaffold galaxies. The cosmic energy budget it implies is humbling: about 5% ordinary matter, 27% dark matter, 68% dark energy.
Be honest about what these labels mean. Dark matter and dark energy are names for our ignorance, not confirmed particles or settled physics. We infer dark matter from how its gravity bends galaxies and light, and dark energy from the accelerating expansion — but what either *is* remains genuinely open. Lambda-CDM is the best map we have, not the final word; the upcoming rungs on the dark sector and the early universe are where these open questions live.
And there is a real crack in this otherwise triumphant picture, one you should know about because it is live science, not a settled story: the Hubble tension. The age and size we have quoted hinge on the Hubble constant, the present expansion rate — but the two best ways of measuring it disagree, and the gap will not close. Reading it off the early universe (the microwave background, through Lambda-CDM) gives about 67 km/s/Mpc; measuring it directly in the nearby universe (climbing the distance ladder with Cepheid stars and supernovae) gives about 73. Both methods have been refined for years and shrunk their errors, yet the roughly 9% disagreement has grown *more* significant, not less.
Nobody yet knows what the tension means. It may be a subtle measurement error lurking in one method that careful work will eventually expose — or it may be a genuine signal that Lambda-CDM is incomplete, a hint of new physics in the early universe or in dark energy. This honest uncertainty is not a weakness of cosmology; it is the field working at its frontier. The age of about 13.8 billion years and the horizon at about 46 billion light-years are firm enough to build on, yet the tension is a standing reminder that even our best map of the cosmos is still being drawn.