No Structure on Largest Scales (Galaxies distributed fairly uniformly)‏ Surprising given structure on smaller scales Cosmological Principle: Universe is homogeneous and isotropic Homogeneity => universe has no edge Isotropy => universe has no center “The cosmological principle has some very far-reaching implications. For example, it implies that there can be no edge to the universe because that would violate the assumption of homogeneity. And there can be no center because then the universe would not look the same in all directions from any noncentral point, violating the assumption of isotropy. This is the familiar Copernican principle expanded to truly cosmic proportions—not only are we not central to the universe, but no one can be central, because the universe has no center!”

Olbers` Paradox If the universe is homogeneous, isotropic, infinite, and unchanging, then the night sky should be as bright as the sun. Why isn't it?

The universe is expanding => Assumption that universe is unchanging is incorrect. => Doesn’t matter whether universe is finite or infinite, we only see a finite part. => Light from objects greater than 14 billion light years away has not had time to reach us.

Cosmological Scales How do we know that the universe is expanding?

Almost all galaxies exhibit show redshifted spectral lines => they are receding from us! Size of universe is not static, nor shrinking due to influence of gravity. Universe is expanding.

Redshift or recessional velocity is proportional to distance: the farther away a galaxy is, the faster it is receding. V = H 0 x D (Hubble's Law)‏ velocity (km / sec) Distance (Mpc)‏ Hubble's Constant (km / sec / Mpc)‏ Or graphically... Current estimate: H 0 = km/sec/Mpc 1 pc ~ 3 Light Years

Hubble Expansion Analogy 1 Mpc6 Mpc12 Mpc 2 Mpc12 Mpc24 Mpc Accuracy increases with distance. The farther out, the faster the expansion. Works for any chosen point of reference. As the ruler cools, all points converge. 1 pc ~ 3 Light Years

Question The spectrum of an AGN is _____.  A) mainly due to starlight  B) very dim  C) non-blackbody  D) highly stable

Question Hubble's law tells us that the ______.  A) closer a galaxy is, the faster it moves away  B) farther a galaxy is, the faster it moves away  C) farther a galaxy is, the slower it moves away  D) farther a galaxy is, the larger it is

Question Isotropy implies that the universe has no _____.  A) center  B) curvature  C) horizon  D) edge

Dark Matter “Upward of 90 percent of the matter in the universe is dark” Two types: 1) normal matter that we can't “see” 2) exotic subatomic particles that are still a mystery How do we even know it's there? Gravitational Effects! (Animation)‏ This matter is not just dark in the visible portion of the spectrum—it is undetected at any electromagnetic wavelength.

Recall the Cosmological Principle: the universe is homogeneous (the same everywhere -> no edge) and isotropic (the same in all directions -> no center). But NOT unchanging, the Universe is EXPANDING.

Hubble's Law imply that at some time in the past all galaxies in the universe lay on top of each other! Big Bang: Beginning of the Universe Expansion of Universe is Expansion of Space Itself Therefore, can have expanding Universe AND Cosmological Principle. time = velocity/distance = distance/(Ho * distance) = 1/Ho So, if Ho ~ 70 km/sec/Mpc -> Universe is 14 Billion yrs old

The Big Bang Primordial explosion creating our universe – all matter and space, “beginning” of time. Hot and dense conditions => like a “fireball” Evidence: Specific prediction for initial amounts of hydrogen and helium Cosmic Microwave Background Radiation & 2006 Nobel Prize in Physics

Primordial Nucleosynthesis  Creation of base level of helium (~25%) in the early universe. Supports big bang model!  Stopped when temperature and density became too low for fusion to continue

The Formation Of Atoms Early universe hotter and opaque to radiation Universe expanded and cooled Electrons & nuclei combined => neutral atoms Epoch of Decoupling Universe becomes transparent to EM radiation. Period during which nuclei and electrons combined to form atoms

The Epoch of Decoupling After atoms of hydrogen and helium formed, only certain wavelengths of radiation— those corresponding to the spectral lines of those atoms, could interact with matter. The universe became nearly transparent to radiation. Universe continued to expand and radiation cooled becoming CMB

The Fate of the Universe What property determines the ultimate fate of the universe?

Affect of Density on the Universe