Evidence for the Big Bang (WJEC GCSE Physics): Revision Note

Evidence for the Big Bang

• 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”

• The Big Bang theory is very well supported by two main pieces of evidence:

  1. Cosmological redshift

  • This provides strong evidence for the expansion of the Universe

  2. Cosmic Microwave Background Radiation (CMBR)

  • This provides evidence for the expansion of the Universe and that it had a hot beginning

Evidence from Cosmological Redshift

• Cosmological redshift provides evidence for the Big Bang because:

  1. Observations show that distant galaxies are all moving away from us

  • We see dark lines on the absorption spectra from distant galaxies are shifted to the red end of the spectrum when compared with light from similar sources in the laboratory

  2. Hubble's law shows that the further away the galaxy, the faster it is moving away from us

  • The dark lines from more distant galaxies are more redshifted due to the expansion of space itself

  3. This suggests that the Universe began at a single point and has been expanding outwards ever since

  • If we use Hubble's law and go back to time t = 0, this suggests that galaxies must have been closer together in the past

Cosmological Redshift Spectra

The dark lines have shifted towards red wavelengths due to the stretching of light as it travelled through space that was expanding

Cosmic Microwave Background Radiation

• Cosmic microwave background radiation (CMBR) is the radiation 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