Was the early universe hotter or cooler than the universe today?

What is the cosmic microwave background?

State how radiation temperature scales with scale factor.

State the temperature-redshift relation.

If the present CMB temperature is \(2.7\,\mathrm K\), estimate the temperature at \(z=9\).

Why did the universe cool as it expanded?

Why could neutral atoms form only after cooling?

What is primordial nucleosynthesis?

Why does radiation density fall faster than matter density during expansion?

If \(T_0=2.7\,\mathrm K\), estimate the redshift when the temperature was \(2700\,\mathrm K\).

Explain why the CMB is strong evidence for a hot early universe.

Why did photons decouple from matter after atoms formed?

Use \(E\sim k_BT\) to explain why high temperature allows particle creation.

Why did light nuclei not form immediately at the earliest times?

What is matter-radiation equality?

Derive why \(T(z)=T_0(1+z)\) from scale factor scaling.

If the scale factor doubles, what happens to CMB photon wavelength, photon energy, and temperature?

Why are small CMB temperature anisotropies important?

Explain how expansion connects particle physics to cosmology in the early universe.

Why does the early universe provide tests of physics beyond direct laboratory energies?