AST 111 1st Edition Lecture 36 Outline of Last Lecture I. Cosmic (microwave) Background Radiation [CMB] continueda. COBEb. WMAPII. Theoretical Cosmologya. Cosmological Principalb. Perfect Cosmological Principali. Steady-State Theoryii. String TheoryOutline of Current Lecture I. The Big Banga. Chronologyb. Comments on the Big Bang TheoryII. The Forces of NatureIII. The Big Bang Theory (plus inflation)a. Cosmic Horizon ProblemIV. The Future of the UniverseCurrent Lecture The Big Bango Fred Hoyle, strong advocate for the steady-state theory, provided the name for the theory (Big Bang).These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.o It is based on the logical consequences of the discovery of the expanding universe, the Cosmological Principal, and the assumption that physical processes operate according to the General Theory of Relativity.o The Big Bang Chronology:Era Time Temperature (˚K) Remarks? 0 ? “Big Bang”; initialexpansionSupergravity10−43sec1032Planck Time; onlyradiation existedInflation10−32sec1027Rapid expansion;1050timesbiggerParticle 1 sec1010Protons, quarks,electrons,radiationNucleo-synthesis 180 → 300 sec108Temperature lowenough for fusionH. Helium wasmade.Radiation/Matter4 × 105yr 3000 Universe becametransparent toradiationMatter109yr 100 Galaxies formedCurrent 13.8 BY 2.7 Nowo Comments on the Big Bang Theory As is the case for all scientific theories, it is the current best explanation of a phenomenon. It must be predictive in content. Observational results must be compared to its predictions and these can be used to disprove the theory. The Forces of Natureo The Strong and Weak nuclear forces, electromagnetism and gravity were merged as one single force under the extreme conditions of the early universe.o Scientists have long been trying to find ways to “unify” the forces and they have had some success: “The Electro-Weak force”.o The “Grand-Unified Theories” or GUTS. If a theory can be found explaining all forces acting as one, it would be the “Theory of Everything”. Big Bang Theory (Plus Inflation)o Inflation had to be added to the Big Bang theory to explain the “Uniformity” of the CMB and the “cosmic horizon” problem. o Uniformity means: Isotropy: On a very large scale, the universe looks the same in all directions. Homogeneity: If we take a very large number of samples anywhere in the universe, you get about the same contents.o Cosmic Horizon Problem If the universe is 13.8 BY old, then two superclusters in opposite directions are ~ 28 BLY apart, but are observed to have similar properties. They must have formed close together and came to have similar properties. Then a sudden drastic expansion (inflation) of space occurred briefly, separating them tremendously and then normal expansion resumed. The inflation era allowed space to balloon up by a factor of ~ 1050 in a fraction of a second. This easily was faster than the speed of light. The Future of the Universeo Assuming the Big Bang theory is the best possible explanation, the future depends on which of three possible versions we live in: Closed: the universe stops expanding after a finite time. Then a contracting phase occurs. (Big Crunch. Oscillation?) Flat: the universe coasts to a halt after infinite time. A unique version of the Big Bang. Open: the universe still continues to expand strongly after infinite time. Things get dark and cold everywhere.o Which version is it? Measure the mass of all things seen in the universe and divide by the volume. If the average density is… ˃ critical density → closed universe = critical density → flat universe ˂ critical density → open universe (currently favored by observations)o Relativistic Evolving Big Bang Models of the UniverseAssumes H =75 km/ sec per MpcCritical density 2 ×10−29gm/ cm3Version Age (BY) Curv. AverageDensityExtentClosed t ˂ 9 + ˃ criticaldensityFiniteFlat t = 14 O = criticaldensityInfiniteOpen 9 ˂ t ˂ 14 - ˂
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