Big Bang & Hubble’s Law
The Big Bang theory describes the origin and evolution of the universe from an extraordinarily hot, dense state approximately 13.8 billion years ago. Rather than an explosion in space, the Big Bang was an expansion of space itself — every point in the universe moving away from every other point simultaneously. Think of dots drawn on a balloon: as the balloon inflates, every dot moves away from every other dot, not because the dots are moving through the rubber, but because the rubber itself is stretching. This is exactly how cosmic expansion works. One of the most powerful pieces of evidence for an expanding universe comes from the redshift of distant galaxies. When a light source moves away from an observer, its wavelengths are stretched to longer, redder values — a phenomenon called the Doppler effect applied to light. Edwin Hubble discovered in 1929 that virtually all distant galaxies show this redshift, and crucially, the farther away a galaxy is, the faster it appears to recede. This relationship, now called Hubble's Law, established that the universe is uniformly expanding in all directions. A third pillar of Big Bang cosmology is the Cosmic Microwave Background (CMB) — a faint glow of thermal radiation permeating the entire sky. Predicted theoretically in the 1940s and discovered accidentally in 1965, the CMB is the afterglow of the hot early universe, cooled over billions of years of expansion to just 2.725 K today. Its near-perfect uniformity and tiny temperature fluctuations (about 1 part in 100,000) match Big Bang predictions with extraordinary precision, making the CMB one of the strongest confirmations of modern cosmology.
- The Expanding Universe and Redshift
- Hubble's Law
- Running the Expansion Backwards: the Big Bang
- Cosmic Microwave Background (CMB)
- Big Bang Nucleosynthesis
The Expanding Universe and Redshift
When a galaxy moves away from us, light waves emitted by its stars are stretched. The observed wavelength λ_obs is longer than the emitted wavelength λ_emit. The cosmological redshift z is defined as z = (λ_obs − λ_emit) / λ_emit. For nearby galaxies moving at speeds much less than c, this redshift corresponds to a recession velocity v ≈ zc. Hubble plotted recession velocity against distance for dozens of galaxies and found a straight-line relationship — the hallmark of uniform expansion.
Hubble's Law
Hubble's Law states that a galaxy's recession velocity v is proportional to its distance d from us: v = H₀d. The constant of proportionality H₀ (the Hubble constant) is currently measured at approximately 70 km/s/Mpc (kilometres per second per megaparsec, where 1 Mpc ≈ 3.086 × 10²² m). This means a galaxy 1 Mpc away recedes at 70 km/s; a galaxy 100 Mpc away recedes at 7,000 km/s. Importantly, no single galaxy is the 'centre' — every observer in the universe sees the same law, because space itself expands uniformly.
Running the Expansion Backwards: the Big Bang
If every galaxy is moving away from every other today, then in the past all matter must have been closer together. Running cosmic expansion backwards in time leads to a moment of extreme density and temperature — the Big Bang singularity around 13.8 billion years ago. The age of the universe can be estimated from the Hubble time: t_H = 1/H₀ ≈ 14 billion years, which agrees well with independent age estimates from the oldest stars and radioactive dating.
Cosmic Microwave Background (CMB)
About 380,000 years after the Big Bang, the universe cooled enough for protons and electrons to combine into neutral hydrogen — an event called recombination. Before recombination, the universe was opaque (photons scattered constantly off free electrons); afterwards, photons streamed freely. Those photons form the CMB we observe today. Because the universe has expanded by a factor of about 1100 since recombination, the radiation has been redshifted from ~3000 K visible/infrared light to ~2.725 K microwave radiation. The CMB's near-perfect blackbody spectrum and its tiny anisotropies (mapped precisely by the WMAP and Planck satellites) provide a detailed snapshot of the early universe.
Big Bang Nucleosynthesis
In the first few minutes after the Big Bang, temperatures were high enough for nuclear fusion. The theory of Big Bang Nucleosynthesis (BBN) predicts that the early universe should have produced about 75% hydrogen and 25% helium by mass, with trace amounts of lithium and deuterium. Observations of the oldest, least-processed stars and gas clouds match these ratios precisely, providing yet another quantitative confirmation of the Big Bang model.
The Key Equation
- v — Recession velocity (km/s): The speed at which a galaxy moves away from us, inferred from its redshift.
- H_0 — Hubble constant (km/s/Mpc): The current rate of expansion of the universe; approximately 70 km/s per megaparsec.
- d — Distance (Mpc (megaparsecs)): The proper distance to the galaxy; 1 Mpc = 3.086 × 10²² m ≈ 3.26 million light-years.
Worked Example
Problem. A galaxy is observed to have a recession velocity of 2,100 km/s. Using H₀ = 70 km/s/Mpc, calculate (a) its distance in megaparsecs and (b) its distance in light-years.
Answer. The galaxy is approximately 30 Mpc (about 98 million light-years) away.
Origins · 1 minEdwin Hubble and how this was discovered
Hubble first demonstrated in 1923–24 that the Andromeda 'nebula' was a separate galaxy far outside the Milky Way, establishing that the universe extends far beyond our own galaxy. By 1929 he had measured the distances and recession velocities of dozens of galaxies, discovering the linear relationship now known as Hubble's Law. This provided the first observational evidence that the universe is expanding, lending crucial support to what became the Big Bang theory. Hubble trained as a lawyer at Oxford University before abandoning his legal career to return to astronomy. He also worked as a Spanish interpreter and briefly considered a professional boxing career before science won out.
Related Concepts
- Hertzsprung-Russell Diagram — Standard candles like Cepheid variables (calibrated using H-R diagram stellar physics) are essential for measuring galaxy distances needed to verify Hubble's Law.
- Doppler Effect — The cosmological redshift used to measure galaxy recession velocities is a direct application of the Doppler effect applied to light waves.
- Blackbody Radiation — The CMB has an almost perfect blackbody spectrum at 2.725 K; understanding blackbody radiation is essential to interpret what the CMB tells us about the early universe.
- Nuclear Fusion — Big Bang Nucleosynthesis — the formation of hydrogen, helium, and lithium in the first minutes — is governed by the same nuclear fusion physics that powers stars today.