What the Universe Is Made Of

Section 1 of 7

Concept Throughline

By the end of this reading, you will be able to:

Read
the Friedmann equation as a model — what sets the expansion rate, and what each term means
Compute
the critical density and interpret each density parameter $\Omega_i$ as a fraction of it
State
the cosmic energy budget — radiation, ordinary matter, dark matter, dark energy
Connect
the total density to the geometry of space (flat, closed, or open)
Explain
why the expansion decelerated under matter and now accelerates under dark energy

Concept Throughline

One equation governs the whole universe. You do not have to solve it — you have to read it. It tells you what the cosmos is made of, what shape it has, and how it will end.

For three modules you have practiced a single disposition: an equation is not a wall to climb but a story to read. You read hydrostatic equilibrium for the structure of a star; you read the mass–luminosity relation for the main sequence. Now you read the one equation that governs everything — the Friedmann equation — and it answers the three biggest questions at once: what is the universe made of, what is its shape, and what is its fate?

Observable

The microwave sky and exploding stars

The cosmic microwave background is uniform and almost perfectly flat-geometry; Type Ia supernovae at large distance are fainter than a coasting universe predicts.

Model

The Friedmann equation and its budget

$H^2 = (8\pi G/3)\rho - kc^2/a^2$ . Divide by $H^2$ and measure each density against the critical density: $1 = \Omega_r + \Omega_m + \Omega_\Lambda + \Omega_k$ .

Inference

Composition, shape, and fate

The universe is about $5\%$ ordinary matter, $26\%$ dark matter, $69\%$ dark energy; spatially flat; and switching from deceleration to acceleration.