The Big Bang, Quasars & Cosmology — Quick Summary

Hubble's law & expansion

Redshift comes from expanding space stretching light — not galaxies moving through static space.

Hubble found recessional velocity ∝ distance:

H = Hubble constant ≈65 km s⁻¹Mpc⁻¹ (accepted range 60–70), found from the gradient of a v–d graph. Assuming H is constant, age ≈ 1/H ≈ 15 billion years.

The Big Bang theory

A smaller universe in the past means denser and hotter matter (V∝T) — implying an extremely hot, dense origin that expanded and cooled. The rival steady state theory (now discredited) proposed an eternal, unchanging universe with matter continuously created to keep density constant.

Evidence: CMBR

CMBR matches a black-body spectrum almost perfectly — peak in the microwave region.

Predicted (~3 K) before discovery; found accidentally by Penzias & Wilson (1964) as uniform microwave noise. Uniform to ±0.003 K across the sky → universe is homogeneous and isotropic. Originates from recombination (~100,000 yr after the Big Bang), when the universe cooled enough for atoms to form and became transparent to light.

Evidence: H/He abundance

Big Bang nucleosynthesis (t≈100 s) fused free neutrons into helium; the predicted proton:neutron ratio (~7:1) gives 12 hydrogen nuclei for every helium nucleus (3:1 by mass) — matching observation, a major success of the theory.

Dark energy

Distant Type 1a supernovae are dimmer than expected — the expansion is accelerating.

Distant Type 1a supernovae appear dimmer than their redshift predicts, showing that the expansion is accelerating, not slowing under gravity as expected. An unknown dark energy (~68% of the universe's mass-energy) is proposed to drive this — distinct from dark matter.

Quasars

Extremely bright, compact, hugely redshifted objects powered by supermassive black holes with actively feeding accretion discs. All known quasars are very distant (and therefore ancient) — none exist locally, showing the universe has changed over time, contradicting the steady state model.