What proof do scientists have that the universe is expanding?

The observation of galaxies moving away from one another, as indicated by the redshift of their light, is one of the main pieces of evidence supporting the theory that the universe is expanding. The further away galaxies are from us, and the faster they appear to be receding, the more redshift indicates that they are moving in that direction. Other lines of evidence supporting the theory of an expanding universe include the observation of uniform temperature distributions in cosmic microwave background radiation, which is consistent with an expanding universe, and theoretical models such as the Big Bang theory.

Scientists have several lines of evidence to support the theory of the universe's expansion:

1. Hubble's Law: Observations of distant galaxies show that they are moving away from us, and the farther away they are, the faster they appear to be moving. This relationship between a galaxy's distance and its velocity, known as Hubble's law, provides strong evidence for the expansion of the universe.

2. Cosmic Microwave Background Radiation (CMB): The CMB is the remnant radiation from the early universe, and it is nearly uniform in all directions. However, slight variations in temperature across the sky indicate regions of slightly different densities in the early universe, which are attributed to quantum fluctuations during the Big Bang. The expansion of the universe has stretched and cooled this radiation, resulting in its detection as microwave radiation today.

3. Redshift of Spectral Lines: When astronomers observe light from distant galaxies, they find that the spectral lines in the light are shifted toward longer (redder) wavelengths. This redshift indicates that the galaxies are moving away from us, consistent with an expanding universe.

4. Observations of Cosmic Structures: Large-scale structures in the universe, such as galaxy clusters and superclusters, exhibit patterns that are consistent with the predictions of an expanding universe. These structures have formed and evolved over billions of years due to the gravitational interactions of matter within an expanding space.

5. Observations of Supernovae: Studies of distant supernovae have revealed that the universe's expansion is not only continuing but also accelerating. This unexpected discovery, made in the late 1990s, provided strong evidence for the existence of dark energy, a mysterious force driving the accelerated expansion of the universe.

These pieces of evidence, combined with theoretical frameworks such as general relativity, provide a robust foundation for the understanding that the universe is expanding.