Physical Cosmology 10 problem sheets (7 assessed questions = 10% of grade) Mid-term exam (10% of grade) Final exam (80% of grade) Exam will be harder than previous years! What is cosmology? Cosmology is the study of the whole Universe: How it formed, how it evolves with time, what its basic properties and constituents are, and how it is structured on the largest distance scales. Cosmologists study the Universe through a combination of astronomical observations, mathematical theories, and computer simulations. The field has only been recognised as a scientific discipline for about 100 years – before then it was the domain of philosophers and theologists! This module is about the fundamental facts and findings of cosmology, and the physical theories that we have developed to explain them. [video] Expansion rate: 67.36 ± 0.54 km/s/Mpc Age of Universe: 13.797 ± 0.023 Gyr 1908 Henrietta Swan Leavitt (astronomer and “computer”) discovered a relation between the pulsation period and absolute luminosity of Cepheid variable stars. This made it possible to measure the distance to very remote astronomical objects. 1913 Vesto Slipher (astronomer) first measured the radial velocities of “spiral nebulae”. It was not yet known that they were in fact galaxies separate from our own. By 1917 he had found that most spiral nebulae seemed to be moving away from Earth at quite high velocities. 1916 Albert Einstein published his General Theory of Relativity, including the Field Equations that connect the geometry of space-time to its matter/energy content. 1917 Willem de Sitter (astronomer) found the first cosmological solution to the Field Equations: An empty `vacuum' universe that seemed to expand. 1922 Alexander Friedmann (a meteorologist) derived the Friedmann equations from Einstein's Field Equations. These showed how the Universe expands or contracts in the presence of a perfectly homogeneous fluid. Einstein himself reviewed Friedmann's calculations, but didn't realise their significance! 1924 Edwin Hubble (astronomer) observed Cepheid variables in “spiral nebulae”. He concluded that the nebulae were far too distant to be part of the Milky Way; they were galaxies separate from our own. 1927 Georges Lemaître (Catholic priest and astronomer) derived a mathematical relation to explain how galaxies would seem to be travelling away from us in an expanding universe. His work was also not widely appreciated. 1929 Hubble plotted radial velocity data compiled by Milton Humason (his assistant) and Vesto Slipher against the distances to galaxies he had measured using Cepheid variables... 1929 He found that the further the galaxies were from us, the faster they seemed to be moving away. This was soon interpreted as evidence that the Universe is expanding. .