The accelerating expansion of the Universe

What just happened?

Type Ia supernovae are "standard candles" — they always explode with almost the same absolute brightness, so from their observed brightness we read off the distance. The most distant ones turned out fainter than a constant expansion rate predicts — that is, they are farther than they should be.

Hubble's law:

— a galaxy's recession velocity grows proportionally to its distance. $H_0 \approx 70\ \mathrm{km/s/Mpc}$ is the Hubble constant. On a $v(d)$ plot this gives a straight line.

Why is this proof of acceleration?

The most distant supernova points deviate upward from the Hubble line, and the deviation grows with distance:

The scale factor $a(t)$ grows ever faster ($\ddot a > 0$) — the expansion is accelerating. This observation (SN Ia) won the 2011 Nobel Prize; it is driven by dark energy.

Why you need this for the exam:

Hubble's law $v = H_0 d$ plus the idea of standard candles solve problems about the expansion of the Universe and distance measurement. The trick: a straight line = constant rate; points deviating upward = acceleration.

Remember: distant Type Ia supernovae lie above the Hubble line, and the farther they are the larger the deviation — that is how we know the expansion of the Universe is accelerating (dark energy).