Big Bang theory
Astrophysics & Cosmology - OCR A-Level Physics
Key Definition
Big Bang theory
The theory that the universe began from an extremely hot, dense state approximately 13.8 billion years ago and has been expanding and cooling ever since.
The theory that the universe began from an extremely hot, dense state approximately 13.8 billion years ago and has been expanding and cooling ever since.
- Evidence 1: Hubble's lawThe recessional velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. of a galaxy is directly proportional to its distance from us, indicating the universe is expanding. and galactic redshiftAn increase in the observed wavelength of light compared to the emitted wavelength, caused by the source moving away from the observer or by the expansion of space.. All distant galaxies show redshiftAn increase in the observed wavelength of light compared to the emitted wavelength, caused by the source moving away from the observer or by the expansion of space., and the further away they are, the greater the redshiftAn increase in the observed wavelength of light compared to the emitted wavelength, caused by the source moving away from the observer or by the expansion of space.. This is consistent with an expanding universe that was once much smaller and denser.
- Evidence 2: Cosmic microwave backgroundNearly uniform microwave radiation from all directions with a black body spectrum corresponding to approximately 2.7 K. The cooled remnant of radiation from the early universe. (CMB) radiation. Discovered by Penzias and Wilson in 1965. It is a nearly uniform microwave radiation from all directions with a black body spectrum corresponding to $T \approx 2.7$ K.
- The CMB is the cooled remnant of the very hot radiation that filled the early universe. As space expanded, the wavelengths stretched (redshifted) from gamma rays to microwaves.
- The CMB has tiny temperature fluctuations (~1 part in 100,000) that correspond to densityMass per unit volume of a material. Measured in kg m⁻³. variations in the early universe, which seeded galaxy formation.
- Evidence 3: Relative abundance of light elements.The observed proportions of hydrogen (~75%) and helium (~25%) match predictions from Big Bang nucleosynthesis. The observed ratio of hydrogen to helium (~75:25 by mass) matches predictions of Big Bang nucleosynthesis, where light nuclei formed in the first few minutes.
Examiner Tips and Tricks
For a 6-mark 'describe the evidence for the Big Bang' question, cover at least two pieces of evidence (CMB + redshift) with specific details: CMB temperature (2.7 K), who discovered it (Penzias and Wilson), what Hubble's lawThe recessional velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. of a galaxy is directly proportional to its distance from us, indicating the universe is expanding. shows (v \propto d), and what these observations imply about the universe's history.