The universe is known to be expanding primarily because of the observation of galaxies moving away from us, with their velocity increasing with distance. This phenomenon is observed through the redshift of light from distant galaxies, first discovered by Edwin Hubble in the late 1920s. The redshift occurs because the wavelength of light stretches as objects move farther away, shifting toward the red end of the spectrum. This relationship between distance and velocity is known as Hubble's Law and strongly supports the idea of a uniformly expanding universe. Additional evidence comes from several other observations:
- Type Ia supernovae studies in the late 1990s showed that the expansion of the universe is accelerating, as these supernovae appeared dimmer and farther away than expected.
- The cosmic microwave background (CMB) radiation reveals fluctuations consistent with the universe having been much smaller and denser.
- Large-scale structure surveys show a distribution of galaxies that fits the predictions for an expanding universe.
- Baryon acoustic oscillations and weak gravitational lensing also provide independent support for an expanding cosmos.
- The abundance ratios of light elements like hydrogen and helium align with the processes expected in an expanding early universe.
Together, these diverse lines of evidence make the expansion of the universe a well-established and foundational concept in modern cosmology.
