The origin of the Universe

​​In a nutshell

The Big Bang theory suggests a giant explosion created the Universe and as a result it is continually expanding. The Steady State theory suggests that the Universe has always existed and the density of galaxies is always constant. The Doppler effect is when the pitch of a sound changes as a source moves towards and away from an observer. Red shift is the light wave version of the Doppler effect.

The origin of the Universe

The Big Bang theory

The Big Bang theory was first suggested in the $$1920s$$​ and says that the Universe started $$13.8\ billion\ years\ ago$$​ as a very small, hot and dense region. All the energy was concentrated in this region and it 'exploded' to create the Universe.

The result of this explosion is that the Universe continuously expands. This means each point in the Universe moves away from each other point.

This can be visualised by imagining a deflated balloon with dots drawn on it. As the balloon is blown up, the dots get further and further away from each other. This is like the galaxies in the Universe.

The Steady State theory

The Steady State theory says that the Universe has always existed.

As the Universe expands, new galaxies are formed in the gaps between the old galaxies moving apart. This means that the density of galaxies in the Universe remains constant through time. 

The Steady State theory is no longer widely accepted due to conflicting evidence. This evidence instead provides support for the Big Bang theory, which is accepted by most modern astronomers.

A. Big Bang theory B. Steady State theory

Cosmic microwave background radiation

The evidence to disprove the Steady State theory came in the form of cosmic microwave background (CMB) radiation.

The Big Bang suggests that there were large amount of radiation released during the Big Bang. The wavelength of this radiation has increased due to the expansion of the Universe and is now in the microwave part of the electromagnetic spectrum.

These microwaves are observed as CMB signals coming from all regions of the sky. The slight variations in microwave wavelengths are due to tiny fluctuations in the density of the Universe in the early stages. These variations went on to form galaxies.

Curiosity: Two radio astronomers accidentally discovered CMB radiation in $$1964$$​. At first they thought it might be interference from pigeons nesting near the equipment!

The Doppler effect and red shift

The Doppler effect can be seen when a source of waves moves with respect to an observer. For example, for sound waves this is when the pitch of a sound changes as a sound source moves past an observer. The pitch of a sound wave is related to its frequency and wavelength so these also change.

As a moving sound source moves towards an observer, the observed sound's wavelength gets shorter and therefore the frequency increases. This means the observed pitch becomes higher.

As a moving sound source moves away from an observer, the observed sound's wavelength gets longer and therefore the frequency decreases. This means the observed pitch becomes lower.

1. Source 2. The wavelength in front of the source is shorter. 3. The source is moving forward.

Note: The Doppler effect can be observed for any type of waves. The frequency and wavelength change the same way as they do for sound waves!

Example

When an ambulance moves towards a car, the siren appears to be higher pitched. When it passes the car and moves away, the siren appears to be lower pitch. If both the car and the ambulance were stuck in a traffic jam, the siren would appear at a normal pitch!

​​Light

The Doppler effect also occurs for electromagnetic waves. The wavelength of observed light from a visible light source moving away from an observer becomes longer. This corresponds to a shift in the visible light observed towards the red end of the spectrum. Observed light from a visible light source moving towards an observer would shift towards the blue end.

​Note: The Doppler effect only occurs when the moving object travels at a similar speed to the speed of the emitted waves. This is the speed of sound for sound waves and the speed of light for red shift. When someone walks at a normal pace towards an observer, the pitch of their voice doesn't change and their white t-shirt won't turn red!​

As every point in the Universe is continuously expanding, the celestial bodies observed from Earth are moving away. This means that light observed from Earth is red-shifted depending on how fast the bodies are moving away. 

This was observed by Edwin Hubble in the $$1920s$$​, which confirmed the theory that galaxies were moving away from the Earth. Light from more distant galaxies is more greatly red-shifted. This is because a source that travels closer to the speed of the emitted waves (in this case the speed of light) has a greater shift in observed wavelengths.