Evidence for the Big Bang (AQA A Level Physics)

• Around 13.7 billion years ago, all the matter in the Universe exploded from a hot, dense singularity
• Since then, the matter has been expanding and cooling from this single point to form the Universe that exists today
  • 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've began with a “big bang”

• There are three key pieces of evidence to support the Big Bang theory:

1. Galactic redshift & Hubble’s law

• • This provides strong evidence for the expansion of the Universe

2. Cosmic Microwave Background Radiation (CMBR)

• • This provides evidence that the Universe had a hot beginning

3. The relative abundance of hydrogen and helium

• • This provides evidence the Universe was once far hotter and denser than it is now

Evidence from Galactic Redshift

• Redshift provides evidence that the Universe is expanding because:

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

• • Light spectra from distant galaxies show redshift
  • The light waves stretch (i.e. wavelength increases) due to the expansion of the universe

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

• • Observations show that the further away a galaxy is, the greater the redshift
  • From Hubble's law the further away a galaxy is, the faster its recession speed

3. This suggests that at some point in the past, all galaxies must have been at the same point

• • Extrapolating Hubble's law back to time t = 0 suggests that matter must have been closer together in the past

Comparing the light spectrum produced from the Sun and a distant galaxy

Cosmic Microwave Background Radiation

• Cosmic microwave background radiation (CMBR) is the radiation detected in all parts of the Universe
• The spectrum of CMBR shows a peak in the microwave region that corresponds to a temperature of 2.7 K
• The CMBR is found to be extremely uniform throughout the Universe

CMBR Map of the Universe

The CMBR map with areas of higher and lower temperature

• The CMBR map is the closest image that exists to a map of the Universe
• The different colours represent extremely small fluctuations in temperature
• The redder regions represent slightly warmer temperatures indicating a higher density of galaxies
• The bluer regions represent slightly cooler temperatures indicating a lower density of galaxies

Evidence from Cosmic Microwave Background Radiation

•  Cosmic microwave background radiation provides evidence that the Universe had a hot beginning because:

1. Theory predicts the existence of uniform black body radiation that peaks in the microwave region

• • CMBR is isotropic, meaning it can be detected coming from all directions equally
  • Measurements show it perfectly fits a black-body profile with a peak wavelength in the microwave region
  • It is extremely uniform which indicates the Universe was initially much smaller than it is now

2. CMBR is consistent with radiation that has been redshifted over time

• • The wavelength of the CMBR has been redshifted by an expanding and cooling Universe
  • The shorter wavelength in the past indicates the Universe must have been very hot in the beginning

3. CMBR can be interpreted as the radiation left over from the Big Bang

• • CMBR was emitted when the Universe cooled sufficiently for matter and radiation to ‘decouple’ i.e. when protons and electrons combined into neutral atoms and photons were released
  • The emitted radiation would have been extremely high-energy gamma which has been redshifted into the microwave region as the Universe has expanded

Redshift of CMBR

CMBR is a result of high-energy radiation being redshifted over billions of years

Relative Abundance of Hydrogen & Helium

• At the time of the Big Bang, the Universe would have been extremely hot
• Initially, it would have been too hot for even protons and neutrons to exist

• The Universe would have quickly cooled to the point where protons and neutrons could exist freely
• However, at this point, free neutrons would start to decay into protons
• As a result, the ratio of protons to neutrons would increase rapidly

• For a short time, it would have been hot enough for hydrogen nuclei to fuse into helium nuclei
• The neutrons bound in helium nuclei would then become stable

Nucleosynthesis of Hydrogen & Helium

• Initially, the ratio of protons to neutrons is 1:1
  • In a sample of 16 nucleons, there would be 8 protons and 8 neutrons
  • If 6 neutrons decay into protons, then there would be 14 protons and 2 neutrons

• The ratio of protons to neutrons becomes 7:1
  • During fusion, 2 neutrons combine with 2 protons to form a helium nucleus, i.e. 4 nucleons in total
  • Out of 16 nucleons in total, 25% of the total mass (4 nucleons) makes up a helium nucleus and 75% of the total mass (12 protons) makes up hydrogen nuclei
  • Therefore, the relative abundance of hydrogen to helium in the early Universe is 3:1

Evidence from the Relative Abundance of Hydrogen & Helium

• The relative abundance of hydrogen and helium provides evidence for the Universe being hotter and denser in the past because:

1. Theory predicts hydrogen fused into helium nuclei in the early Universe

• • The Big Bang theory suggests that a very brief period of hydrogen fusion occurred when the Universe was very young and hot, resulting in the production of helium

2. Fusion would have stopped before heavier nuclei were created

• • As the Universe expanded and cooled, fusion reactions would have stopped before the creation of larger nuclei could have taken place

3. The predicted relative abundances of hydrogen and helium are consistent with observation

• • Calculations predict the early Universe would've consisted of hydrogen and helium nuclei in the ratio of 3:1
  • This is consistent with observations of some of the Universe’s oldest objects (73% hydrogen, 25% helium and 2% everything else)