What type of celestial object did Hubble observe to discover cosmic acceleration?

Hubble observed Type Ia supernovae to help discover that the universe's expansion is accelerating.

Type Ia supernovae are unusually useful because their peak brightness can be standardized. By comparing how bright one appears with how bright it should be, astronomers can estimate its distance. Their spectra also reveal how quickly their host galaxies are moving away, allowing researchers to compare distance with cosmic redshift.

In 1998, two independent teams studied distant Type Ia supernovae and found them fainter—and therefore farther away—than expected in a universe whose expansion was slowing under gravity. The result implied that cosmic expansion has been speeding up, leading to the concept of dark energy. The discovery earned the 2011 Nobel Prize in Physics for Saul Perlmutter, Brian Schmidt, and Adam Riess.

Cepheid variables are commonly confused with this discovery because they help calibrate the cosmic distance ladder. They were important to distance measurement, but the accelerating-expansion evidence came from Type Ia supernovae.