Chapter 26: Cosmology: The Origin and Evolution of the UniverseCosmology is the science concerned with the structure and evolution of the universe as a whole.-How large is the Universe? What is its structure? How long has it existed, and how has it changed over time? 26-1: History of Cosmology-Newton’s Static Universe – Newton concluded we must be living amid a static, infinite expanse of stars, and the universe will exist forever without changing majorly.-Olbers’ Paradox– (Heinrich Olbers, 1800s) If space goes on forever, with stars scattered throughout, then no matter where you look in the sky, you’d eventually see a star, and thus even at night the whole sky should be lit up. Olbers’ paradoxis that the night sky is actually dark.-Einstein added the cosmological constant(Λ) to the equations of general relativity in order to make them agree with the popular idea that the universe is static. The CC was supposed to represent a pressure that tends to make the universe expand, thus balancing against gravitation. Einstein called the CC his “biggest blunder” because it was entirely made up but he was actually onto something (dark energy/expansion).26-2: Expansion of the Universe / Hubble Law / Cosmological Redshift-Hubble is usually credited with discovering our universe is expanding-Hubble law– greater distance = greater redshift. That is, remote galaxies are moving away from us with speeds proportional to their distances.ov = H0dv = recessional velocityH0= Hubble constant (73 km/s/Mpc, or ~ 70 km/s/Mpc)d = distance of galaxy-Expanding Universe: Important things to rememberoGalaxies are NOT moving away from each other through empty space… space itselfis expanding, carrying the galaxies along with it.oRedshift of light from distant galaxies is NOT a Doppler shift… the wavelength of photons expand as space expands (cosmological redshift).oA cluster of galaxies will NOT spread out infinitely as universe expands… clusters stay together at some point because of gravitation.-Cosmological redshift– NOT the same as Doppler shift. Caused by the expansion of space.oz = (λ – λ0) / λ0z = redshiftλ = the wavelength we observe, λ0= the unshifted wavelength of a photonolookback time– how far into the past you are looking when you see an object (bc of speed of light).
-Cosmological principle – the assumption that the universe is homogeneous and isotropic (we assume this!) on the largest scales.
- Spring '09
- General Relativity, Big Bang, Redshift, Physical cosmology, Cosmic microwave background radiation