Cosmology: Understanding the Origin, Structure, and Evolution of the Universe - Prof. Phil, Study notes of Astronomy

Download Cosmology: Understanding the Origin, Structure, and Evolution of the Universe - Prof. Phil and more Study notes Astronomy in PDF only on Docsity! Cosmology Cosmology: study of the origin, structure and evolution of the Universe What would we like a theory of cosmology to explain? • `Origin’ of the Universe (what was the Universe like at much earlier epochs?) • Fate of the Universe • How did the structure seen in today’s Universe form? • How did the elements form? • + … We will discuss the hot big bang model for understanding some of these problems. ASTR 3830: Spring 2004 Elements of the hot big bang model Model rests on a number of fundamental assumptions and observations: 1. Universe is homogenous and isotropic When averaged over sufficiently large volumes, the Universe is isotropic (same in all directions) and homogenous (other observers at different locations see the same general picture of the Universe at the same time). Described as the cosmological principle - evidently makes the mathematical description much easier as there are no special places or directions. ASTR 3830: Spring 2004 2. Universe is expanding The mean distance l between conserved particles is increasing with time at the rate: † dl dt = H0l Constant of proportionality is time-dependent, the present value is Hubble’s constant H0. Best observational estimates of H0 are: † H0 = 71± 4 kms -1 Mpc-1 (this is value quoted by the WMAP team from observations of the microwave background - HST Key Project result from measurements of Cepheids in external galaxies is identical within the errors). ASTR 3830: Spring 2004 Can define a Hubble length: c / H0 ~ 4000 Mpc at which this expression for the recession velocity extrapolates to the speed of light - more detailed relativistic treatment is needed for distances of this order. Can also define a Hubble time: 1 / H0 ~ 1010 years …this is to order of magnitude the age of the Universe. ASTR 3830: Spring 2004 3. Universe expanded from a hot dense state Implied by a naïve extrapolation of the expansion back to much earlier times. Direct evidence from the existence and spectrum of the microwave background, which is very well fit by a blackbody spectrum: ASTR 3830: Spring 2004