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Expansion of the Universe

Physics (Year 12) - The Standard Model

Dev Lohar

Big Bang Theory

The Big Bang theory is the leading explanation for the origin and evolution of the universe. It states that the universe was once in an extremely hot and dense state and has been expanding over time. The theory is supported by a wealth of observational evidence, including redshift, Hubble’s law, cosmic microwave background radiation, and the relative abundance of different elements throughout the universe. These phenomenon will be further explored below.

Doppler effect

The Doppler effect is the phenomenon that occurs when a moving object emits waves, such as sound or light waves. As the object moves, the waves are compressed or stretched, which changes their frequency and wavelength. This can be observed as a change in the pitch of a sound wave or the colour of a light wave.

In the case of the expansion of the universe, the Doppler effect is observed as a shift in the frequency of light waves emitted by distant galaxies. As the universe expands, the fabric of space itself is stretched, which causes the light waves to become stretched as well. This results in a shift towards the red end of the spectrum, a phenomenon known as redshift.

The degree of redshift observed in a galaxy’s light is directly proportional to its distance from Earth; the more distant the galaxy, the greater the effect. By measuring the redshift of a galaxy, astronomers can determine its distance and, therefore, its rate of recession. This has allowed scientists to map the expansion of the universe and provide strong evidence for the Big Bang theory.

Hubble’s law

Hubble’s law is a fundamental relationship that also describes the expansion of the universe. It states that the distance between two distance galaxies is directly proportional to their recessional velocity (ie. the speed at which they are moving away). In simpler terms, the farther apart the two galaxies are, the faster they are moving away from each other.

This law was observed in 1929 by astronomer Edwin Hubble and through his observations, he was able to find a linear relationship between the distance of a galaxy and its recessional velocity. This relationship is expressed as:

An interesting use of the Hubble’s constant is that the reciprocal of it (ie. 1/H_0 ) gives the approximate age of the universe.

Cosmic microwave background radiation (CMBR)

Cosmic microwave background radiation, or CMBR, is the left over radiation which was emitted during the Big Bang. It’s called ‘microwave’ radiation because it has a very long wavelength, and it’s called ‘background’ radiation because it can be found everywhere in the universe. CMBR acts as evidence for the expansion of the universe because it has uniform temperature and that is important because it is a characteristic that is expected from radiation that was created in the early universe ad has been expanding along with it.

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