Introduction to Cosmology

For Worldview of Modern Science March 23/11

Kent Peacock / Department of Philosophy / U of Lethbridge Amazing Shots from the Hubble Archive Edwin P. Hubble (1889—1953) Home Sweet Home Best Guess View of Galactic Structure Andromeda M74 Typical Barred Spiral Sombrero Galaxy (M104) Sombrero in IR M81 Hubble Deep Field Hubble Ultra Deep Field Quasar 3C 273 Cosmic Microwave Background Radiation Planck Spectrum of CMBR Issues in Modern Cosmology

• “Cosmology” from Greek word cosmos, meaning “string of jewels”! – Einstein: the theoretical basis – Confirmation of external galaxies – Discovery of expansion (Hubble), 1930 – Big Bang vs. Steady State debates of 1950s – CMBR discovered 1960s; confirmed Big Bang – COBE (1990s) revealed detailed structure of CMBR – Discovery of acceleration of expansion; one of major observational surprises in recent science; leads to prediction of mysterious “dark energy”. Olber’s Paradox

• Reveals how much one can work out if one is willing to question the obvious. – Q: Why is the sky dark at night? • Credited to Heinrich Olbers (1758–1840) but it seems that many others arrived at a similar conclusion. • Suppose that the universe was (i) static and (ii) filled with a uniform distribution of stars or galaxies. • Imagine shell of stars of radius R around the Earth. • Each shell has area 4π R2; hence the number of stars in each shell will go up as R2. • The light from each shell hitting Earth decreases in intensity as 1/R2; hence each shell contributes about the same amount of radiation to the light hitting the Earth. • Hence if there are an infinite number of shells, the Earth would be baked! • From this simple reasoning, one must conclude either that the universe is expanding, or that it is finite.