The Big Bang Theory (CIE IGCSE Physics)

• 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

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