Cosmology. An Analogy 11/28/2010. Cosmology Study of the origin, evolution and future of the Universe

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1 Cosmology Cosmology Study of the origin, evolution and future of the Universe Obler s Paradox If the Universe is infinite why is the sky dark at night? Newtonian Universe The Universe is infinite and unchanging Resolution: the Universe can t be infinite! An Analogy 1

2 Einstein s Universe The Universe cannot be static! It must be either expanding or contracting! He did not believe! Added a Cosmological Constant to his equations to make the Universe static Hubble s Observations Showed the Universe was expanding! Einstein realized he had made a big mistake! The Cosmological Principle Over very large distances the Universe is: 1. Homogeneous: every part of the Universe is the same as every other part 2. Isotropic: the Universe looks the same in every direction 2D Analogy of the Expanding Universe Universe = surface of expanding balloon Galaxies = coins stuck on surface Conclusions: 1. The expansion of the Universe is an expansion of space between galaxies. Galaxies themselves do not expand! 2. The expansion of the Universe is uniform in all directions so all galaxies appear to be moving away from all others 3. The further a galaxy is away from us the more expansion of space there is giving the impression that galaxies are moving away faster 4. The Universe has no physical center or edge that can be detected 2

3 Cosmological Redshifts If the Universe is expanding it must have been smaller in the past! How small? Galaxy redshifts not caused by the motion through space, but the expansion of space itself! The Big Bang A Cosmic Singularity Perhaps the Universe originally existed as a point of infinite density where space, time and matter did not exist! Not an explosion of matter through space but the creation of space and matter itself from a singularity! Who or what caused the Big Bang? We don t know since the Laws of Physics break down at singularities! The Cosmic Fireball Soon after the Big Bang the Universe was very hot and dense, consisting entirely of high energy radiation Much of this radiation was converted into the matter we see today! 3

4 As the Universe expanded and cooled, some radiation remained from the Cosmic Fireball which became redshifted by the expansion of the Universe Horn Antenna at AT&T Labs, Holmdel, NJ Prediction: today it should be seen in the radio or microwave region First detection of the cosmic microwave background radiation (CMBR) The CMBR is almost a perfect blackbody at 3K! Problem: If the CMBR is the remnant radiation from the cosmic fireball that created all matter in the Universe, why is this radiation so smooth when the Universe today is so clumpy? It is very uniform and smooth! The Cosmic Background Explorer (COBE) COBE CMBR Map Searched for temperature variations in the CMBR The temperature variations are less than 20 millionths of a degree! 4

5 Wilson Microwave Anisotropy Probe (WMAP) Wilson Microwave Anisotropy Probe CMBR Map Three Possible Futures for our Universe Future of the Universe Three possible futures Should depend on average density of matter in Universe since matter exerts gravity which should gradually slow the expansion 5

6 critical density, ρ c Minimum density to halt the expansion Compare with observed density, ρ 0 : Ω 0 = ρ 0 / ρ c Ω 0 = density parameter Bound (Recollapsing) Universe ρ 0 > ρ c and Ω 0 > 1 Unbound (Coasting) Universe ρ 0 < ρ c and Ω 0 < 1 Marginally Bound (Critical) Universe ρ 0 = ρ c and Ω 0 = 1 Problem: Cannot directly measure ρ 0! Visible matter alone: ρ 0 = 1 / 20 ρ c But dark matter also contributes to ρ 0! Latest observations of Type I supernovae suggests our Universe is not any of these! 6

7 We live in an accelerating Universe! How can this be? This acceleration phase started about 5 billion years ago, before which it was decelerating There must be a repulsive force counteracting gravity which is pushing galaxies apart! Dark Energy What is it? It appears to be a form of energy associated with spacetime and is present even when space is empty! 7

8 Most of the Universe is Dark Energy! How big is the Universe? Dark energy makes up most of the Universe! The Universe has no physical boundary which can be detected! However, the Universe does have a visible boundary! The Hubble Ultra-Deep Field Image The Cosmic Light Horizon Galaxies seen out to 13.2 billion light years! 8

9 The Universe has an observable boundary! Curvature of the Universe There is a limit to how far back we can see due to the finite speed of light In a marginally bound Universe, the overall curvature of space is flat! In a bound Universe, the overall curvature of space is positive (spherical) In an unbound Universe, the overall curvature of space is negative (hyperbolic) By comparing the sizes of the observed hotspots in the CMBR with the sizes predicted by theory we can determine the overall curvature of space and hence the type of Universe we are in! 9

10 The Boomerang Experiment Measured sizes of hotspots in CMBR The hotspots are of the expected size indicating we live in a Universe with flat space that is approximately 13.7 billion years old The Early Universe Matter and Radiation in the Universe Antimatter Subatomic particles with the same mass but opposite changes to ordinary matter Matter Antimatter Proton, p + Antiproton, p - Electron, e - Positron, e + Neutron, n Antineutron, ñ 10

11 Protons and Neutrons are made of Quarks! How can there be an antineutron? Annihilation and Pair Production Neutrons and Antoneutrons neutron = udd total charge = +2/3-1/3 1/3 = 0 antineutron = ūđđ total charge = -2/3 + 1/3 + 1/3 = 0 Both have a zero charge! The Planck Era The first s after the Big Bang where current laws of physics break down The Hadron and Lepton Eras Formation of protons and electrons via pair production 11

12 The Nuclear Era Formation of Helium via Fusion Universe was initially opaque to radiation Hydrogen and helium formed in a 75% to 25% ratio by mass Era of Recombination The Flatness Problem Why is space flat? Standard Big Bang Theory has no explanation! After neutral atoms form Universe is transparent to radiation photon become CMBR Cosmic Inflation Solution to the Flatness Problem 12

13 Observed temperature fluctuations in the CMBR support inflation! Dark matter was responsible for the formation of the first galaxies 13