A35 Lectures 24-25 - 24 The Big Bang Really Happened 3 Pillars of Big Bang 1 Cosmic Expansion 2 The Cosmic Microwave Background 3 Helium Synthesis in

# A35 Lectures 24-25 - 24 The Big Bang Really Happened 3...

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24 The Big Bang Really Happened 3 Pillars of Big Bang 1. Cosmic Expansion 2. The Cosmic Microwave Background 3. Helium Synthesis in the Hot Big Bang I. Cosmic Expansion The Universe is homogenous and isotropic. GRAPH -Y axis: velocity [km/s] -X-axis: distance [mega parsecs] -Slope: has units of km/s / Mpc -slope is something related to time because distances cancel out. -Equation that describes line: V = H D {b=0} -V [km/s] = H [km/s/Mpc] x D [Mpc} -H is the Hubble Constant --> tells us what fraction of the distance the Universe stretches out at each second Computing the Cosmic Age -V = H x D that’s the speed of recession -t = Distance/Speed = D/(H x D) -t = 1/H -Need to get the units right (km/sec/Mpc) --> 1/sec -1 Mpc = 3 x 10^22 m -t in seconds -1 year is ~ 3 x 10^7 sec -See Endnote 2 to Chapter 6 in The Expanding Universe for calculation process -From expansion: t = 14 Giga years for a Hubble constant of 70 (H/70 [km/s/Mpc]) Other ages we have talked about: -Radioactivities in the solar system ~ 5 Gyr -Ages of the globular cluster stars ~ 12 Gyr -Ages of the oldest white dwarfs in the Milky Way disk ~ 10 Gyr -1 Giga year = 1 billion years Historic Cosmic Events -Oldest stars in the galaxy ~ 13 Gyr ago -Oldest stars in the galaxy’s disk ~ 10 Gyr -Sun formed ~ 5 Gyr ago -First multi-celled life on Earth ~ 1 Gyr ago -End of the Dinosaurs ~ 65 x 10^6 years ago -First Hominids ~ 2 x 10^6 years ago -Beginning of civilization ~ 10,000 years ago -Harvard College Founded ~ 350 years ago -Starting in 1920, the Hubble Constant value has decreased substantially. In 1929, scientists believed the Hubble Constant to be 500 km/sec, t ~ 2 Gyr. This value for the constant conflicts with the ages of elements, stars, and other astronomical objects. -The value we’ve come to now is very far from the value in the 1920’s and the estimate of the uncertainty was always wrong, i.e. high precision, low accuracy. Even in 1929, people knew that that the age of certain elements was greater than 2 Gyr due to geological aging. -Looking at the Hubble Diagram for Type Ia supernovae, we can use the redshift to measure the distance, and thus find out which galaxies are near and which are far away. -Visible Matter is lumpy on scales up to about 100 Mpc. Diagram shows us that universe is not homogenous (namely in spread of galaxies) within 100 Mpc. It is not the same everywhere. There are places of high and low density. There are about 2 million galaxies that have been measured. A theoretical calculation of dark matter can be compared with the observed clumping of matter.

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• Spring '09
• RobertKirshner
• Big Bang