The Big Bang Theory (Cambridge (CIE) IGCSE Physics)
Revision Note
Written by: Lindsay Gilmour
Reviewed by: Caroline Carroll
Did this video help you?
Evidence for the Big Bang Theory
Around 14 billion years ago, the Universe began from a single point that was extremely hot and dense
A giant explosion, known as the Big Bang, caused the Universe to expand outwards
As each point moved away from the others, the Universe began to cool
As a result of the initial explosion, the Universe continues to expand
The Big Bang Theory
Tracing the expansion of the Universe back to the beginning of time leads to the idea it must have begun with a “Big Bang”
Evidence from galactic redshift
Galactic redshift indicates that distant galaxies are moving away from us
If galaxies are moving away from us, this means the Universe must be expanding
Expansion of the Universe
Observations of light from galaxies show they are moving away from us which means the Universe is expanding
Redshift provides evidence for the Big Bang because:
1. Observations show that distant galaxies are all moving away from us
We see that light from glowing hydrogen in stars from distant galaxies is redshifted in comparison with light from glowing hydrogen on Earth
2. Observations show that the further away a galaxy is, the faster it is moving away from us
The spectra of light from more distant galaxies are more redshifted than closer galaxies due to the expansion of space itself
Galactic redshift spectra
The dark lines (representing glowing hydrogen) have shifted towards red wavelengths due to the stretching of light as it travelled through space that was expanding
Examiner Tips and Tricks
Make sure that you understand that the stretching of the wavelength of light is due to the expansion of the Universe, not the motion of stars and galaxies themselves.
This can be visualised by imagining a balloon with equally spaced points on it. The balloon represents space and the points represent galaxies.
When the balloon is deflated (i.e. the Universe was smaller), the points (galaxies) are closer together and are at an equal distance apart.
As the balloon (Universe) expands, all the points (galaxies) become further apart by the same amount.
This is because the space between the galaxies itself has expanded.
Cosmic microwave background radiation
Extended tier only
Cosmic microwave background radiation (CMBR) is a form of electromagnetic radiation that was emitted shortly after the beginning of the Universe
It is detected everywhere throughout the Universe
The CMBR map is the closest image that exists to a map of the Universe
It shows that the temperature of the Universe, and therefore the objects in it, are more or less uniformly spread out
CMBR map of the Universe
The CMBR map shows areas of higher and lower temperature in the Universe. Regions with higher temperatures have a higher concentration of galaxies, Suns and planets
Evidence from cosmic microwave background radiation
Cosmic microwave background radiation provides evidence for the Big Bang because:
1. Theory predicts the early Universe was extremely hot and dense
Therefore, CMBR would have initially existed as short-wavelength gamma radiation
The shorter wavelength in the past indicates the Universe must have been very hot in the beginning
2. CMBR is consistent with radiation that has been stretched over time
The Big Bang would have released a lot of energy in the form of extremely high-energy gamma radiation
As the Universe expanded, the wavelength of the radiation increased
Over time, it has been redshifted so much that it is now in the microwave region of the spectrum
3. CMBR can be interpreted as the radiation left over from the Big Bang
The CMBR is extremely uniform which indicates the Universe was initially much smaller than it is now
Redshift of CMBR
CMBR is a result of high-energy radiation being redshifted over billions of years
Worked Example
Describe and explain what can be deduced about the history of the Universe from the CMBR.
Answer:
Step 1: Recall the features of the CMBR
Microwave radiation is detected from all directions at a similar intensity
Step 2: State the source of this radiation
This is the radiation produced just after the formation of the Universe
Step 3: Describe how the wavelength has changed and explain why
When the Universe was formed, the radiation was high in energy and short in wavelength
Now it has less energy and a longer wavelength
This is because the Universe has expanded and cooled, causing the wavelength to increase
Step 4: Suggest what this tells us about the Universe in the beginning
This suggests the Universe was initially very small and very energetic and has been expanding since
Last updated:
You've read 0 of your 5 free revision notes this week
Sign up now. It’s free!
Did this page help you?
