age of the universe
How old is everything? Not the Earth (4.5 billion years), not the Sun, but the universe itself — the time since the Big Bang, since space began expanding from its hot, dense beginning. The age of the universe is that single number, and pinning it down has been one of cosmology's defining achievements. The current best value is about 13.8 billion years.
There are two independent ways to reach this number, and their agreement is what gives confidence. One is to run the expansion backward: knowing how fast the universe expands today and how that rate changed over time (from the Friedmann equations and the measured mix of matter and dark energy), you can compute the time back to when the scale factor was zero. The other is to find the oldest objects in the universe — the most ancient stars in globular clusters, which by their physics cannot be older than the universe — and these come out to roughly 13 billion years, comfortably less than but close to 13.8 billion. The most precise figure comes from the cosmic microwave background: its detailed pattern of ripples fixes the universe's age to within a few tens of millions of years.
The age matters as a fundamental anchor and a sanity check. Any proposed cosmology that produced stars older than the universe would be immediately wrong, and indeed an earlier crisis arose when some star ages seemed to exceed the then-estimated cosmic age — resolved by better measurements. The honest caveats: the 13.8-billion-year figure is derived within the Lambda-CDM model, and it depends on the Hubble constant, so the unresolved Hubble tension translates into a small lingering uncertainty in the exact age. And it dates the expansion, not a literal 'moment of creation' — the theory describes the universe from very shortly after the beginning onward.
Compress the 13.8 billion years into a single calendar year. The Big Bang is January 1; the Milky Way forms in spring; the Sun and Earth appear in early September; the first humans show up minutes before midnight on December 31, and recorded history is the last second.
Two independent methods — expansion and oldest stars — agree on about 13.8 billion years.
The 13.8-billion-year age is model-dependent (Lambda-CDM) and tied to the Hubble constant, so the unresolved Hubble tension leaves a small uncertainty. It dates the expansion, not a literal instant of creation.