How do we know the Universe is expanding?

The understanding that the Universe is expanding is grounded in several key observations and scientific principles. Here are the primary reasons and evidence supporting this concept:

Redshift of Distant Galaxies
The most direct evidence comes from observing the redshift of light from distant galaxies. When astronomers observe these galaxies, they notice that their light is shifted towards the red end of the spectrum. This phenomenon, known as the redshift, indicates that these galaxies are moving away from us. The farther away a galaxy is, the greater its redshift, suggesting that the Universe is expanding uniformly in all directions.

Hubble's Law
Edwin Hubble, in 1929, formulated what is now known as Hubble's Law. By measuring the redshifts of various galaxies and their distances, Hubble found a linear relationship: the farther a galaxy is, the faster it appears to be moving away. This discovery provided strong evidence for an expanding Universe, as it implies that space itself is stretching.

Cosmic Microwave Background Radiation (CMB)
The CMB is the afterglow of the Big Bang, discovered in 1965 by Arno Penzias and Robert Wilson. This radiation provides a snapshot of the early Universe, just 380,000 years after the Big Bang. The uniformity and spectrum of the CMB match predictions for an expanding Universe that started from a hot, dense state and has been cooling as it expands.

Large Scale Structure of the Universe
The distribution of galaxies and clusters of galaxies forms a web-like structure across the Universe. Computer simulations of the Universe's growth, based on the expansion model, match the observed large-scale structure. This congruence supports the idea that the Universe has been expanding and evolving over billions of years.

General Relativity and Friedmann Equations
Einstein's theory of General Relativity provides a framework for understanding the dynamics of the Universe. The Friedmann equations, derived from General Relativity, describe how the Universe expands. These equations predict an expanding Universe given the observed amount of matter and energy.

Observations of Type Ia Supernovae
Type Ia supernovae are used as standard candles in cosmology because of their consistent intrinsic brightness. Observations of these supernovae in distant galaxies show that they are fainter than expected in a non-expanding Universe. This indicates that the Universe is expanding, and the rate of expansion has been accelerating over the past few billion years, a discovery that led to the concept of dark energy.

Baryon Acoustic Oscillations (BAO)
BAO are regular, periodic fluctuations in the density of the visible baryonic matter (normal matter) of the Universe. These oscillations left an imprint in the distribution of galaxies and the CMB. By measuring the scale of these fluctuations, astronomers can infer the expansion history of the Universe.

Quasar Absorption Lines
Quasars, extremely luminous active galactic nuclei, can be used to study the intergalactic medium. The absorption lines in the light from quasars, caused by intervening clouds of gas, show redshift patterns consistent with an expanding Universe. These lines help trace the history of cosmic expansion.

In summary, the evidence for an expanding Universe is robust and multi-faceted, coming from the redshift of galaxies, the CMB, large-scale structure, Type Ia supernovae, and theoretical frameworks like General Relativity. These observations and theories together paint a consistent picture of a Universe that has been expanding since the Big Bang.