What the Universe Is Made Of

Part 3: What Is the Universe Made Of?

Now read the budget. Measurements — above all the cosmic microwave background and distant supernovae — pin down each $\Omega$:

• Radiation, $\Omega_r \approx 9\times10^{-5}$: negligible today, though it dominated the early universe.
• Ordinary (baryonic) matter, $\Omega_b \approx 0.05$: everything the periodic table is made of — stars, gas, planets, you.
• Dark matter, $\Omega_{dm} \approx 0.26$: the gravitating mass we weighed two readings ago, now placed in the cosmic budget.
• Dark energy, $\Omega_\Lambda \approx 0.69$: the dominant component, driving the acceleration we meet in Part 5.

Two readings ago we showed, galaxy by galaxy, that dark matter outweighs the baryons several-fold and promised a single number for the cosmic budget. Here it is: ordinary matter is only about 5% of the critical density, dark matter about 26%, so dark matter outweighs ordinary matter roughly five to one across the whole universe. Together, $\Omega_m = \Omega_b + \Omega_{dm} \approx 0.31$.

The cosmic microwave background (CMB) is the single most powerful witness. Early on the universe was hot, dense, and ionized, and photons scattered constantly off free electrons. As it expanded and cooled, electrons combined with nuclei into neutral atoms — an era called recombination — and the universe became transparent. That released light, stretched by expansion into microwaves, fills the sky today at $2.73~\mathrm{K}$. The exact pattern of its tiny temperature ripples measures the baryon density and the matter density precisely, and — as we will see next — the geometry of space.