Galaxies and Cosmology — The Big Bang, Expansion, and the Dark Universe

Lecture 4 Overview: From Stars to the Whole Universe

Stars (Lecture 3) gather into galaxies, and galaxies together make up the universe. This lecture tackles the biggest question of all — how did the universe begin, and where is it headed?

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1. Galaxies — Cities of Stars

A galaxy is a massive structure of hundreds of billions of stars, gas, and dust, bound together by gravity.

Classified by shape

• Spiral galaxies: with arm structures (the Milky Way, Andromeda)
• Elliptical galaxies: round or oval, dominated by older stars
• Irregular galaxies: no clearly defined shape

The Milky Way

• A barred spiral galaxy, about 100,000 light-years across
• The Sun sits in the outer suburbs, about 26,000 light-years from the center

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2. The Big Bang — the Universe’s Beginning

The Big Bang theory holds that the universe began about 13.8 billion years ago from a single point and rapidly expanded from there.

Key evidence for the Big Bang:

1. Cosmic expansion — galaxies are moving apart from each other (Hubble’s law)
2. The cosmic microwave background (CMB) — the “afterglow” of the Big Bang (about 2.7K)
3. The ratio of light elements — the observed ratio of hydrogen to helium matches predictions

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3. Cosmic Expansion and Redshift

More distant galaxies recede faster (Hubble’s law). Light from a receding object gets stretched to longer wavelengths and appears redder — this is redshift.

Nearby galaxy → small redshift → recedes slowly
Distant galaxy → large redshift → recedes quickly
→ direct evidence that the universe is expanding

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4. Dark Matter and Dark Energy — 95% of the Universe

Remarkably, the ordinary matter we know — stars, planets, our own bodies — makes up only about 5% of the universe.

Component	Share	What we know
Ordinary matter	About 5%	Stars, gas, and everything we’re familiar with
Dark matter	About 27%	Doesn’t interact with light; detected only through gravity
Dark energy	About 68%	An unknown force accelerating the universe’s expansion

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5. The Future of the Universe

Because dark energy is accelerating the expansion, the leading scenario is that the universe will expand and cool forever (the “Big Freeze”). But since we don’t know what dark energy actually is, that future isn’t set in stone.

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Key Takeaways

1. Galaxies come in spiral, elliptical, and irregular forms; the Milky Way is a barred spiral galaxy.
2. The evidence for the Big Bang (about 13.8 billion years ago) includes cosmic expansion, the cosmic microwave background, and the observed ratio of elements.
3. The Big Bang wasn’t an explosion — it was the expansion of space itself.
4. Redshift is direct evidence of cosmic expansion.
5. About 95% of the universe is made up of the still-mysterious dark matter and dark energy.

Next lecture: We turn our attention down, to the ground beneath our feet — Earth. We’ll look at Earth’s internal structure and plate tectonics to understand why earthquakes, volcanoes, and continental drift happen.