Baryonic acoustic oscillations (BAO)
Illustration of one of the two primary cosmological probes of Euclid: Baryonic Acoustic Oscillations.
Galaxy clustering as a probe of the geometry of the Universe. The same acoustic features (Baryonic Acoustic Oscillations) seen in the cosmic microwave background (CMB) can be observed in the spatial distribution of galaxies, providing a standard cosmological ruler.
(The second Euclid cosmological probe is weak gravitational lensing.)