The Expanding Universe

Finite age: we can only see light that has had time to reach us, so only a limited number of stars contribute.
Finite speed of light: the farther you look, the further back in time you see.
Expansion and redshift: light from far away is stretched to longer, invisible wavelengths.

How we know the universe began hot and dense 13.8 billion years ago and is expanding, why that expansion seems to be speeding up, and what astronomers are still arguing about in 2026: dark matter, dark energy and the Hubble tension.

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