Astronomy 114 Lecture35:TheBigBang Martin D. Weinberg weinberg@astro.umass.edu UMass/Astronomy Department A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 1/18
Announcements PS#8 due Monday! Final Exam: Friday, 5/18 at 10:30 AM in Hasbrouck Lab, Rm. 124 A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 2/18
Announcements PS#8 due Monday! Final Exam: Friday, 5/18 at 10:30 AM in Hasbrouck Lab, Rm. 124 Today: Cosmology, Chap. 28 A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 2/18
Cosmology: Olbers Paradox Why is the sky dark at night? A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 3/18
Cosmology: Olbers Paradox Why is the sky dark at night? 19 th century cosmology asserted that the Universe is: 1. Infinite in extent 2. Homogenously filled with matter 3. Infinitely old A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 3/18
Cosmology: Olbers Paradox Why is the sky dark at night? 19 th century cosmology asserted that the Universe is: 1. Infinite in extent 2. Homogenously filled with matter 3. Infinitely old Infinite Universe: all lines of sight intersect a star No 1/r 2 dilution of brightness Night sky should be bright as a star Similar to looking through a forest: Every line of sight intersects a tree trunk Can not see through forest A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 3/18
Possible solutions to theparadox 1. Universe is not infinite in size 2. Universe has few stars far away 3. Universe has finite age A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 4/18
Possible solutions to theparadox 1. Universe is not infinite in size 2. Universe has few stars far away 3. Universe has finite age Obscuring dust does not help! Would absorb and re-emit the radiation Universe would reach thermodynamic equillibrium A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 4/18
Possible solutions to theparadox 1. Universe is not infinite in size 2. Universe has few stars far away 3. Universe has finite age Obscuring dust does not help! Would absorb and re-emit the radiation Universe would reach thermodynamic equillibrium Hubble Law implies: Universe has finite age A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 4/18
Implications of the Hubble Law All galaxies are receding from each other A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 5/18
Implications of the Hubble Law All galaxies are receding from each other Strange but Consistent with Special & General Relativity Consistent with thecopernican philosophy that rejects a cosmology with the Earth at thecenter A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 5/18
Isotropy and Homogeneity Hubble observed galaxies in many directions Found same number of faint galaxies in all directions Universe is (approximately) isotropic! A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 6/18
Isotropy and Homogeneity Hubble observed galaxies in many directions Found same number of faint galaxies in all directions Universe is (approximately) isotropic! Counted galaxies as a function of brightness, b Found 8 times more galaxies with b/4 Same galaxy at b/4 is twice as distant Volume is 8 times larger! Universe is (approximately) homogeneous! A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 6/18
The Big Bang Theory (1/3) Explains nearly all cosmological observations Hubble s Law: v = H o d We observe the Universe to be expanding Galaxies that are farther away from us are moving away from us faster than those nearby No center of the expansion Infinite raisin bread Expanding balloon All observers see recession A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 7/18
The Big Bang Theory (2/3) A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 8/18
The Big Bang Theory (2/3) Density of Universe decreases as the Universe expands Define scale factor, R, defining the distance between two stationary points Run the expansion backward in time... Density and Temperature increase as R decreases When R = 0, density =, temperature = A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 8/18
The Big Bang Theory (3/3) The Big Bang is not an explosion! The expansion of the Universe is not an expansion of matter into space, but rather an expansion of space itself The Big Bang did not happen at some point in space, but rather everywhere in space at the same time A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 9/18
Why doesn t the Solar System expand? The galaxies recede because they started out unaffected by each others gravity Once expanding, they continue to expand unless something stops them Planets orbit because they are gravitionally bound to the Sun Force of gravity by the Sun stops any expansion (as does force of gravity by the Galaxy, Local Group, etc.) Not just that it only expands slightly it really doesn t expand at all True for other forces as well: electromagnetic, strong, weak A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 10/18
The Age of the Universe (1/3) Distant galaxies imply looking back in time The redshift of the galaxy determines this time, which we call thelookback time Extrapolate R back to zero to determine the age of the Universe! Requires Einstein s general theory of relativity A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 11/18
The Age of the Universe (1/3) But: Distant galaxies imply looking back in time The redshift of the galaxy determines this time, which we call thelookback time Extrapolate R back to zero to determine the age of the Universe! Requires Einstein s general theory of relativity Assume expansion rate is constant Simple estimate to age of the Universe using Hubble s constant A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 11/18
The Age of the Universe (2/3) We know that d = vt Use Hubble s Law: v = H o d This implies that the age of the Universe is t o = 1/H o (Hubble time) A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 12/18
The Age of the Universe (3/3) If H o = 50 km/s/mpc then: t o = 1 50 ( ) Mpc km/s 10 6 ( pc Mpc ( km = 2 10 2 s 1Mpc 3.086 10 13 pc ) ( ) 1 = 6.17 10 17 1yr s km 3.16 10 7 s = 1.96 10 10 yr = 19.6 billion years ) A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 13/18
The Age of the Universe (3/3) If H o = 50 km/s/mpc then: t o = 19.6 billion years If H o = 70 km/s/mpc then: t o = 1 70 ( ) Mpc km/s 10 6 ( pc Mpc ( km = 1.42 10 2 s 1Mpc 3.086 10 13 pc ) ( ) 1 = 4.38 10 17 1yr s km 3.16 10 7 s = 1.39 10 10 yr = 13.9 billion years ) A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 13/18
The Age of the Universe (3/3) If H o = 50 km/s/mpc then: t o = 19.6 billion years If H o = 70 km/s/mpc then: t o = 13.9 billion years If H o = 100 km/s/mpc then: t o = 9.8 billion years A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 13/18
The Age of the Universe (3/3) If H o = 50 km/s/mpc then: t o = 19.6 billion years If H o = 70 km/s/mpc then: t o = 13.9 billion years If H o = 100 km/s/mpc then: t o = 9.8 billion years Because of gravity, the expansion was faster in the past Universe must be younger than the Hubble Time Globular clusters? A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 13/18
Lookback time and the particle horizon Observer can receive no message from particles whose lookback time is larger than the age of the Universe Calledparticlehorizon Particle horizon changes with time A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 14/18
Cosmic Microwave Background: Prediction Observed helium abundance too large to be the result of supernovae Extrapolate backwards in time At some point: Universe as hot as star center Fuse H to He! Prediction: Cosmic Microwave Background Radiation from this epoch will shift to larger wavelength as Universe expands Black body temperature radiation will decrease A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 15/18
Cosmic Microwave Background: Discovery CMB = Cosmic Microwave Background Engineers designed first microwave satellite uplink Signal independent of pointing antenna Assumed additional noise in their receivers that they could not understand Discovered relic cosmic radiation: T=2.73 K A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 16/18
COBE satellite Cosmic Background Explorer Demonstrated that microwave background was a precise blackbody! A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 17/18
COBE satellite Cosmic Background Explorer Demonstrated that microwave background was a precise blackbody! Temperature profile close to isotropic but not quite A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 17/18
Big Bang frame of reference The CMB determined the frame of reference for the Universe Smooth temperature anisotropy is a Doppler shift The Sun is moving w.r.t. the frame at rest in the CMB A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 18/18
Big Bang frame of reference The CMB determined the frame of reference for the Universe Smooth temperature anisotropy is a Doppler shift The Sun is moving w.r.t. the frame at rest in the CMB Component from Sun s motion about Milky Way Component from Milky Way s in the Local Group Component from the Local Group s motion toward the Virgo Cluster A114: Lecture 35 09 May 2005 Read: Ch. 28,29 Astronomy 114 18/18