Cosmology: The Big Bang & the Expanding Universe

Astronomy Fundamentals

Chapter 8 · Cosmology: The Big Bang & the Expanding Universe

Chapter 7 zoomed out to the Milky Way's own real structure. This chapter zooms out one final, enormous step further — to the universe as a whole, its real evidence for expansion, and an honest look at how much of its actual composition remains genuinely unknown.

Redshift: Real, Direct Evidence for Expansion

The same real spectroscopy tools from Chapter 1 — absorption lines revealing a star's own composition — also reveal motion. When a distant galaxy is moving away from Earth, its own real spectral lines shift toward longer, redder wavelengths, a real Doppler-like effect called redshift. The genuinely striking real pattern: almost every distant galaxy shows this redshift, and more distant galaxies show a real, greater redshift — direct, measurable evidence that the universe itself is expanding.

Hubble's Law — And a Real, Corrected Credit

Edwin Hubble published the real relationship between a galaxy's distance and its recession velocity in 1929, combining his own distance measurements with redshift data gathered by other astronomers:

v = H₀ · D

where v is recession velocity, D is distance, and H₀ is the Hubble constant.

A real, formally corrected credit

Belgian priest and astronomer Georges Lemaître actually published this same relationship two years earlier, in 1927, deriving it directly from the equations of general relativity — before Hubble's own observational confirmation. In 2018, the International Astronomical Union formally voted to rename the relationship the Hubble-Lemaître law, a real, deliberate correction restoring Lemaître's own priority to the historical record.

The Cosmic Microwave Background: Real, Accidental Evidence

On 20 May 1964, Arno Penzias and Robert Wilson, working at Bell Telephone Laboratories in Holmdel, New Jersey, discovered the Cosmic Microwave Background almost entirely by accident, while using a radio antenna originally built for satellite communication experiments. They found a persistent, unexplained excess antenna temperature of about 4.2 K that they genuinely couldn't account for. When word reached Princeton physicist Robert Dicke — whose own team had been actively searching for exactly this signal — he reportedly said, "Boys, we've been scooped." Penzias and Wilson received the real 1978 Nobel Prize in Physics for the discovery. The CMB's own real, precisely measured temperature today is 2.72548 ± 0.00057 K — the genuine afterglow of the early universe, redshifted and cooled by billions of years of real cosmic expansion.

The Hubble Tension: A Real, Currently Unresolved Discrepancy

A genuinely open scientific problem, not a settled number

Two real, independent methods for measuring the Hubble constant today give meaningfully different answers. "Late universe" measurements, using distance-ladder techniques on nearby objects, give a real value around 73 km/s/Mpc. "Early universe" measurements, based on Cosmic Microwave Background analysis, give a real value around 67.7 km/s/Mpc. These two real results disagree at a statistical significance greater than 5 sigma — a genuinely serious, unresolved tension that current cosmology has not yet explained, rather than a minor rounding difference.

What the Universe Is Actually Made Of

The real, current Lambda-CDM cosmological model breaks the universe's total mass-energy content down into three components: dark energy at roughly 68.3%, dark matter at roughly 26.5%, and ordinary (baryonic) matter — everything directly observable, including every star, planet, and galaxy covered in this course so far — at only about 4.9%.

An honest admission worth sitting with

Dark matter and dark energy are defined almost entirely by their real, measured gravitational and cosmological effects — not by any confirmed understanding of what they physically are. Extensive real searches for dark matter particles have so far produced no well-agreed detection, and dark energy's own real value is extremely small compared to what basic vacuum-energy theory would predict, a genuine, unresolved mismatch. In real, honest terms: the Lambda-CDM model describes the universe's own large-scale behavior with remarkable precision, while the fundamental physical nature of roughly 95% of its total content remains genuinely unknown.

The Universe's Real Composition

Component	Real Share	How Well Understood?
Dark energy	~68.3%	Genuinely poorly understood
Dark matter	~26.5%	Detected only by gravitational effect
Ordinary matter	~4.9%	Well understood — everything directly observable

Hands-On Exercises

Exercise 1

Using Hubble's Law (v = H₀ · D) with a Hubble constant of H₀ = 70 km/s/Mpc, calculate the real recession velocity of a galaxy measured to be 150 Mpc away.

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Exercise 2

Explain, using this chapter's own real reasoning, why redshift observed consistently across nearly every distant galaxy — with more distant galaxies showing greater redshift — counts as direct evidence for cosmic expansion, rather than evidence that our own galaxy just happens to sit at the center of a cloud of galaxies all independently moving away from us.

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Exercise 3

The Hubble tension shows two independent measurement methods disagreeing at a statistical significance greater than 5 sigma. Explain why this level of disagreement is treated as a genuine, serious scientific problem rather than something likely explained by ordinary measurement error.

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Chapter 8 Quick Reference

Redshift, growing with distance, is real, direct evidence for cosmic expansion
Hubble's Law (v = H₀D) — published by Hubble in 1929, but derived earlier by Lemaître in 1927; renamed the real Hubble-Lemaître law by the IAU in 2018
The Cosmic Microwave Background was discovered accidentally in 1964 by Penzias and Wilson (1978 Nobel Prize); its real measured temperature is 2.72548 K
The real Hubble tension — early-universe (~67.7 km/s/Mpc) vs. late-universe (~73 km/s/Mpc) measurements disagreeing at >5σ — remains a genuinely unresolved problem
The real Lambda-CDM model: ~68.3% dark energy, ~26.5% dark matter, ~4.9% ordinary matter — with the true physical nature of dark matter and dark energy still genuinely unknown
Next chapter: Exoplanets & the Search for Life