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AQA A-level Physics The Big Bang theory

The Big Bang theory

This page covers the following topics:

1. Red-shift
2. Dark mass and energy
3. Hubble's law
4. Evidence for the Big Bang

Redshift is the apparent shift of wavelength of light as the source is moving away from us. When the source is at a constant distance from the person, the waves emitted will have a constant wavelength, and thus a constant colour. If the source is moved away, the distance the person will see between the two next wavefronts will be greater than the original and thus the wavelength of the waves received by the person will be greater, and thus the wave was shifted towards the red end of the light spectrum. Redshift acts as evidence for the Big Bang theory. This is because the redshift observed from distant stars and galaxies acts as evidence that the universe is moving away from us, and thus expanding. This means that a "Big Bang" must have occured so that all matter is constantly moving away from the initial point.

Red-shift

Distant galaxies are moving away increasingly faster, and thus the expansion of the Universe seems to be acceleration. This is a phenomenon which cannot be currently explained, thus scientists have stated that the cause of this is something called dark energy. Not much is understood about dark energy other than the fact that it causes the Universe to expand increasingly faster. Dark energy is approximated to account for 68% of the Universe. Another thing which cannot be explained is the fact that galaxies rotate quicker than they should be, according to the mass of their stars. This suggests that the Universe holds a mass which has yet to be detected by humans. This was given the name of dark matter. It is approximated that 27% of the Universe is made up of dark matter. Overall, only 5% of the Universe is made of matter that can be understood by humans.

Dark mass and energy

Research found that the speed of a galaxy is directly proportional to its distance from the Earth. The greater the distance of a galaxy from Earth, the greater the redshift which was observed and thus the greater the velocity at which the galaxy is travelling. This is known as Hubble's law and is given by v = Hd, where v is the velocity of a galaxy, d is the distance to the galaxy and H is Hubble's constant, given by 2.3 Γ— 10⁻¹⁸ s⁻¹. The age of the Universe can be approximated by 1/H.

Hubble's law

The red-shifted light received from distant galaxies provides evidence for the Big Bang model, as it suggests that the Universe is currently expanding. Cosmic Microwave background radiation (CMBR) has been detected coming from all directions in space and has a very low temperature. CMBR is said to be the remains of the energy procuded in the Big Bang and is spread thinly across the Universe, thus providing more evidence for this model. Lastly, nearly all nuclear matter in the Universe is helium and hydrogen, which is consistent with the Big Bang model. This is because during the Big Bang, hydrogen was converted into helium due to nuclear fusion. As the Universe expanded, it cooled down, and therefore fusion was no longer possible. The ratio of hydrogen to helium after the initial fusion was 3:1, which is consistent with today's distribution of matter.

Evidence for the Big Bang

1

Describe the phenomenon about the roation of galaxies which cannot be understood and why.

Describe the phenomenon about the roation of galaxies which cannot be understood and why.

2

Explain why the idea of dark energy arose.

Explain why the idea of dark energy arose.

3

Explain how CMBR provides evidence for the Big Bang model.

Explain how CMBR provides evidence for the Big Bang model.

4

Explain what redshift is.

Explain what redshift is.

5

Describe what happens to the velocity of a galaxy if its distance from Earth is doubled.

Describe what happens to the velocity of a galaxy if its distance from Earth is doubled.

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