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Announcements. Abstract of project due: Feb 9 1st midterm: Feb 12 Review session ?????. Lecture 11 Newton’s Cosmology I: Absolute Space and Time. What do you think about the following questions ?. What happened before the universe existed? What is outside the universe?

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    1. Announcements • Abstract of project due: Feb 9 • 1st midterm: Feb 12 • Review session ????? Astronomy 201 Cosmology - Lecture 11

    2. Lecture 11Newton’s Cosmology I: Absolute Space and Time Astronomy 201 Cosmology - Lecture 11

    3. What do you think about the following questions ? • What happened before the universe existed? • What is outside the universe? • What is the universe like right now ? • How did it get there ? Astronomy 201 Cosmology - Lecture 11

    4. The Physical Universe • Proposition: the universe can be understood in terms of natural laws (here: Newton’s laws) • physical: anything that can be objectively observed (at least in principle) • physical universe: the universe of material objects, of energy, of space, of time Astronomy 201 Cosmology - Lecture 11

    5. The Copernican principle • Man, Earth, Sun, the Galaxy etc. are not special/are not at a special place in the universe. This a priori does not exclude that there may exist special objects/special places. Astronomy 201 Cosmology - Lecture 11

    6. Quiz Quiz Quiz Quiz Quiz Quiz NameSID Date Answer Signature Astronomy 201 Cosmology - Lecture 11

    7. What key ingredient was ignored by Kapteyn and Shapley so they came to opposite conclusions regarding the size of the Milky Way and the location of the Sun ? Astronomy 201 Cosmology - Lecture 11

    8. Answer: both ignored the effect of dust. Astronomy 201 Cosmology - Lecture 11

    9. The anthropic principle • The weak anthropic principle states that the universe must be compatible with our existence. • The strong anthropic principle states that the universe is such as it is because its purpose is to create life. Astronomy 201 Cosmology - Lecture 11

    10. Example • Existence of life (as we know it) strongly depends on the presence of carbon • If the nuclear force (the force that keeps atomic nuclei together) would have been slightly different, no carbon could have been formed. • Why is the nuclear force as strong as it is ?Anthropic principle If it had been different, we wouldn’t ask this question. Astronomy 201 Cosmology - Lecture 11

    11. constant density (homogeneous) boundary A toy universe According to Newton, what is going to happen ? • The model Universe is going to collapse under its own gravity Astronomy 201 Cosmology - Lecture 11

    12. Newton’s Universe • In order to avoid collapse • homogeneous • isotropic • infinite size • no center • infinite in time • has always been • will always be  perfect cosmological principle Astronomy 201 Cosmology - Lecture 11

    13. The cosmological principle • homogeneous: the universe looks the same everywhere on large scales there is no special place (center) • isotropic: the universe looks the same in all directions on the sky •  there is no special direction (axis) Astronomy 201 Cosmology - Lecture 11

    14. The perfect cosmological principle • homogeneous: the universe looks the same everywhere on large scales there is no special place (center) • isotropic: the universe looks the same in all directions on the sky there is no special direction (axis) • unchanging: The universe looks the same at all times  there is no special epoch Astronomy 201 Cosmology - Lecture 11

    15. Homogeneity and Isotropy Copernican Principle  Isotropy Homogeneity + Isotropy around another point  Isotropy Homogeneity + Astronomy 201 Cosmology - Lecture 11

    16. Does the cosmological principle apply to our universe ? The cosmic microwave background radiation (CMB) = afterglow from the big bang. It’s smooth to 1 part in 105  yes, the universe appears to be homogeneous and isotropic Astronomy 201 Cosmology - Lecture 11

    17. Does the strong cosmological principle apply to our universe ? Galaxies today Galaxies 10 billion years ago  no, the universe appears to change with time Astronomy 201 Cosmology - Lecture 11

    18. Announcements • Abstract of project due: Feb 9 • 1st midterm: Feb 12 • Review session ????? Astronomy 201 Cosmology - Lecture 11