Three independent lines of evidence support the Big Bang: redshift, CMBR, and hydrogen-helium abundance

The Big Bang theory

• Around 13.7 billion years ago, all matter in the Universe exploded outward from a hot, dense singularity.
• Since then, matter has been expanding and cooling from this single point to form the Universe that exists today.
• Three key pieces of evidence support this theory:

1. Galactic redshift and Hubble's law

• Observations show that distant galaxies are all moving away from us: their light spectra show redshift.
• Hubble's law shows that the further away a galaxy is, the faster it is receding ($v \propto d$).
• This strongly suggests that at some point in the past, all galaxies must have been at the same point, consistent with an initial explosion.

2. Cosmic Microwave Background Radiation (CMBR)

• The CMBR provides evidence that the Universe had a hot beginning because:
• Big Bang theory predicts the existence of uniform black-body radiationRadiation emitted by a perfect absorber/emitter, with a continuous spectrum whose peak wavelength depends on temperature via Wien's law. that peaks in the microwave region. This is exactly what is observed.
• CMBR is isotropic: it can be detected coming from all directions equally, which indicates the Universe was initially much smaller and more uniform.
• The CMBR wavelength has been redshifted by the expansion of the Universe. It was originally extremely high-energy gamma radiation emitted about 300,000 years after the Big Bang, when the Universe cooled enough for matter and radiation to decouple. The expansion has stretched these wavelengths into the microwave region.
• Crucially, CMBR can be interpreted as the radiation left over from the Big Bang itself.

3. Relative abundance of hydrogen and helium

• At the time of the Big Bang, the Universe was extremely hot. Initially it was too hot for even protons and neutrons to exist as separate particles.
• As it cooled, protons and neutrons formed freely. However, free neutrons are unstable and decay into protons, so the proton-to-neutron ratio quickly rose from 1:1 to about 7:1.
• For a brief window, the temperature was high enough for hydrogen nuclei to fuse into helium nuclei (Big Bang nucleosynthesis). Once the Universe cooled further, fusion stopped.
• During fusion, 2 neutrons combine with 2 protons to form a helium-4 nucleus. Out of every 16 nucleons (14 protons, 2 neutrons), 4 form helium and 12 remain as hydrogen. This gives a hydrogen-to-helium ratio of approximately 3:1 by mass.
• This predicted ratio is consistent with observations of the Universe's oldest objects: roughly 73% hydrogen, 25% helium, and 2% everything else.