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CU-Boulder ASTR 1120 - The Expansion of the Universe

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• What is the Cosmological Principle & what does it imply?• What does the expansion of the Universe imply about the beginning of the Universe?• Will the Universe expand forever?The Expansion of the Universe1. The Universe is homogeneoushomogeneous ⇒ the same throughout2. The Universe is isotropicisotropic ⇒the same in every directionThe Cosmological PrincipleVanilla ice cream with chocolate chips isA)homogeneous.B)isotropic.Concept QuestionVanilla ice cream with chocolate chips isA)homogeneous (only on scales much greater than the average distance between chips).B)isotropic.Concept QuestionTests1. The Universe is homogeneous on scales larger than 100 Mpc (the size of the largest superclusters)2. The Universe is isotropic: galaxy surveys, cosmic microwave background radiationFig. 22-14Large Scale Structure1. The Universe is homogeneous2. The Universe is isotropicImplications1.The Universe has no edge2.The Universe has no centerExample: The 2-d surface of a balloonThe Cosmological PrincipleDoes the expansion of the Universe imply that we are at the center?v = H0dA)YesB)NoQuestion to PonderDoes the expansion of the Universe imply that we are at the center?v = H0dA)YesB)No!Balloon DemoQuestion to Ponder1.A denser (hotter) past ⇒ The Big Bang2.14 billion year age3.Space itself is expanding (Big Bang Model)4.We must reinterpret the cosmological redshift:Photons get stretched out by the expansion of space!Balloon DemoWhat does the expansion of the Universe imply?Are galaxies, planets, and people expanding?A)YesB)NoConcept QuestionAre galaxies, planets, and people expanding?A)YesB)No—we are bound together by gravity and the electromagnetic force, respectivelyConcept QuestionFig. 22-17Big BangBig Crunch Cold, Sparse UniverseThe Possible Fates of the UniverseFig. 22-18The Hubble Parameter appears to be increasing!Expansion rate = slope= ∆dist/∆timeThe Acceleration of the UniverseThe energy causing the increasing expansionWe don’t know what it is, except that it is apparently an energy associated with space itself. “Dark Energy”Critical Density = ρcdensity required to “close” the Universeρc= 10-26kg m-3(6 hydrogen atoms m-3!)Ω= ρ/ρc Ω = 1 ⇒ Universe closedThe Content of the UniverseCritical Density = ρcdensity required to “close” the Universeρc= 10-26kg m-3(6 hydrogen atoms m-3!)Ω= ρ/ρc Ω = 1 ⇒ Universe closedΩBM= 0.04 Baryonic matterΩDM= 0.29 Dark matterΩΛ= 0.67 Dark energyThe Content of the


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CU-Boulder ASTR 1120 - The Expansion of the Universe

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