Question 1

How do we know the universe is expanding?

Accepted Answer

From several independent lines of evidence that agree, the first and most direct being that **almost every distant galaxy is moving away from us**, faster the further away it is.

**The original observation.** In the 1920s, building on Vesto Slipher's measurements and Henrietta Leavitt's method for determining distance from variable stars, Edwin Hubble showed that a galaxy's **redshift** is proportional to its distance. Light from distant galaxies is shifted towards longer wavelengths, and the relationship is linear — twice as far, twice the shift.

**What redshift actually is here.** Not primarily a Doppler shift from galaxies flying through space. It is **space itself expanding** while the light is in transit, stretching the wave along the way. The distinction matters: galaxies are not rushing away into a pre-existing void, and there is no centre. Every observer anywhere sees the same recession, which is what an expanding space produces and what an explosion would not.

**The independent confirmations:**

**The cosmic microwave background**, discovered in 1965 — radiation filling the sky at 2.7 kelvin, the cooled remnant of a hot dense early universe. Its existence was predicted; its measured spectrum is the most perfect black body ever observed; and the tiny fluctuations in it match the structure we see today.

**The abundance of light elements.** The proportions of hydrogen, helium and lithium in the oldest material match what nuclear physics predicts for a hot dense phase, and match nothing else.

**Time dilation in supernovae.** Distant supernovae are observed to brighten and fade **more slowly**, by exactly the factor expansion predicts — a direct test.

**Galaxy evolution.** Distant galaxies, seen as they were long ago, look systematically different from nearby ones, which a static universe cannot explain.

**What came later.** In 1998, measurements of distant supernovae showed the expansion is **accelerating**, attributed to dark energy — currently unexplained. The rate of expansion measured by different methods also disagrees slightly, an unresolved problem known as the Hubble tension.

Question 2

What is dark matter, and how do we know it is there?

Accepted Answer

A form of matter that does not interact with light — it neither emits, absorbs nor reflects it — but which has mass and therefore gravity. We infer it because several completely independent

Question 3

How do we know how far away stars are?

Accepted Answer

Through a ladder of methods, each calibrated against the one below it — because no single technique works across the enormous range of distances involved, and the whole structure is only as sound as

Question 4

What is a light year, and why does looking at stars mean looking into the past?

Accepted Answer

A light year is a unit of distance, not time — the distance light travels in one year, roughly 9.46 trillion kilometres. The name misleads constantly, and it is the most common misunderstanding in

Question 5

What is an exoplanet, and how are they detected?

Accepted Answer

A planet orbiting a star other than the Sun. Thousands are now confirmed, and the striking thing is that almost none were detected by seeing them — planets are overwhelmingly outshone by their stars,

Question 6

How are gravitational waves detected?

Accepted Answer

By measuring a change in the length of a four-kilometre laser beam path smaller than a fraction of the width of a proton — an almost absurd measurement that took decades to make possible. What is

Question 7

What is the electromagnetic spectrum?

Accepted Answer

The electromagnetic spectrum is the full range of electromagnetic radiation — and the key insight is that radio waves, visible light, X-rays and everything between are the same phenomenon, differing

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