How do we know the universe is expanding?

The universe is expanding due to the redshift of light from distant galaxies.

In the early 20th century, Edwin Hubble observed that the light from distant galaxies was shifted towards the red end of the spectrum. This redshift is caused by the Doppler effect, which occurs when an object is moving away from an observer. Hubble used this observation to conclude that the universe is expanding, as the redshift indicated that galaxies were moving away from us at increasing speeds.

Further evidence for the expanding universe came from the cosmic microwave background radiation (CMB). This is radiation left over from the Big Bang, and it is uniform in all directions. However, it is slightly cooler in some areas than others, indicating that the universe has expanded since the radiation was emitted.

The expansion of the universe is also supported by the observations of supernovae. Type Ia supernovae are particularly useful for this, as they have a consistent luminosity. By measuring the apparent brightness of these supernovae, astronomers can determine their distance from Earth. When this distance is compared to the redshift of the light, it becomes clear that the universe is expanding at an accelerating rate.

Overall, the evidence for the expanding universe is overwhelming. The redshift of light from distant galaxies, the CMB, and observations of supernovae all point to the same conclusion. This discovery has revolutionised our understanding of the universe and has led to the development of new theories, such as dark energy.