First Light

Cold start the universe and run it forward. For fifteen seconds you watch fourteen billion years go by: a blinding-hot fog, a sudden clearing, a long dark, then the first stars switch on and the structure you live inside falls together.

The concept

The early universe was a hot, dense, opaque plasma, light could not travel without scattering off free electrons. As space expanded it cooled, and at about 380,000 years the electrons bound to nuclei. The fog cleared in an instant, and that flash, stretched by expansion into microwaves, still fills the sky as the cosmic microwave background. Everything after is gravity slowly assembling the dark.

The math

Expansion is tracked by the scale factor $a(t)$, set to 1 today. Light emitted when the universe was smaller arrives stretched, redshifted by

$1 + z = \frac{1}{a}$

so the CMB, emitted at $a \approx 1/1100$, reaches us redshifted by $z \approx 1100$, its 3000 K glow cooled to 2.7 K. How $a(t)$ grows follows the Friedmann equation,

$\left(\frac{\dot a}{a}\right)^2 = \frac{8\pi G}{3}\rho - \frac{k}{a^2} + \frac{\Lambda}{3}$

with radiation, then matter, then the dark energy $\Lambda$ taking turns dominating $\rho$ and reshaping the expansion.

Why it stays strange

The CMB is not a metaphor for the Big Bang. It is the actual light of it, the oldest thing any instrument can see, and its faint temperature ripples are the seeds every galaxy grew from. You can detect it as static on an untuned analogue television: a few percent of that snow is the universe's first light, arriving right now.

Further reading

• Weinberg, The First Three Minutes.
• Penzias and Wilson (1965), the accidental discovery of the CMB.

@book{weinberg1977first,
  author    = {Weinberg, Steven},
  title     = {The First Three Minutes},
  publisher = {Basic Books},
  year      = {1977}
}