What the Universe Is Made Of

Part 5: What Is Its Fate?

The budget is not fixed forever, because the components dilute differently as space expands. Matter thins as volume grows, $\rho_m \propto a^{-3}$; radiation thins faster, $\rho_r \propto a^{-4}$ (an extra factor from its wavelength stretching); but dark energy holds nearly constant density, $\rho_\Lambda \approx \text{const}$, as space expands. Whichever component dominates at a given epoch drives the expansion.

Early on, matter and radiation dominated, and their gravity decelerated the expansion — the pull of all that density slowing the stretch. But as space expanded, matter thinned while dark energy did not. A few billion years ago (around redshift $z \approx 0.6$), dark energy’s repulsion grew strong enough to overcome matter’s pull, and the expansion began to accelerate; dark energy comes to outweigh matter’s density entirely a little later, by $z \approx 0.3$. Distant Type Ia supernovae — the same standard candles from the distance ladder — revealed this in 1998: they are fainter, hence farther, than a decelerating universe would place them.