CosmologyDistribution of GalaxiesThe Big BangThe Expansion of the UniverseOlber’s Paradox / Size of Observable UniverseCosmic HorizonComposition of the Early UniverseCosmic NucleosynthesisCosmic Microwave BackgroundSlide 10Surface of Last ScatteringThe First Few MomentsBig Bang TimelineGeometry of the UniverseThe Universe is FlatThe Accelerating UniverseCritical DensityDark EnergyThe Fate of the Universe1Cosmology•The study of the structure and evolution of the Universe as a whole.•Seeks to answer questions such as:–How big is the Universe?–What shape is it?–How old is it?–How did it form?–What will happen to it in the future?The Hubble Deep Field2Distribution of Galaxies•Galaxies appear to be distributed throughout the Universe•Galaxies clump together into clusters and superclusters.•Distant galaxies appear to be moving away from us. •We know this from the redshifts in the spectrum of galaxies.A map of one million galaxies3The Big BangA plot of the distance of galaxies versus their recessional velocity.The slope of the line is Hubble’s constant.•Since the Universe is expanding it must have been smaller in the past.•At some point in the past the entire Universe must have been packed into some very small size.•In 1927, Abbé George Lemaitre, a Belgian cosmologist and priest proposed that the Universe began as a “Primeval Atom” and estimated its age.•To estimate the age you need to know the expansion rate – in other words Hubble’s Constant, H.•H is 71 +/- 4 km/sec/Mpc giving an age to the Universe of 13.7 +/- 0.2 Billion years.4The Expansion of the Universe•The motion of galaxies away from us is due to the expansion of the Universe.•Like raisins in a loaf of bread, when baked, all move away from one another, so too are all the galaxies moving apart.•The raisins (galaxies) are not moving. It is the bread (space) itself that carries the raisins (galaxies) away.•Space itself is expanding and simply carrying the galaxies with it.Olber’s Paradox / Size of Observable Universe•Q: If the universe is infinite in extent, why isn’t the night sky uniformly bright in all directions from the light from all galaxies?•Answers–The light from distant (rapidly receding) galaxies is redshifted to wavelengths we can’t see with our eyes–If (since) stars did not exist at the very beginning of the universe, we receive no starlight beyond the distance to the oldest stars (the forest of stars is limited)–The light from galaxies outside our cosmic horizon has not reached us5Cosmic Horizon67Composition of the Early Universe•As the Universe cooled Hydrogen nuclei began to form. •Conditions were still hot enough for fusion reactions to occur forming Helium.•Nearly all elements heavier than Helium however were formed later by stars.•We notice that the oldest stars have much less heavy elements than the Sun. This is further evidence for the Big Bang theory.•After neutral atoms formed the radiation that filled the Universe was allowed to freely propagate.Cosmic Nucleosynthesis•A particle physicist’s proof of the Big Bang•Curves show the relative fraction of primeval elements compared with the proton-neutron density for the present day. •The bottom line is that the universe was 24% Helium after the Big Bang; the remaining 3% (present-day abundance of Helium is 27%) was made in the core of stars.•This has been verified by comparing the amount of Helium in old stars with that of newer stars (made from fusion-processed gas ejected by supernovae). •A prediction of this theory is that we should see a cosmic microwave background in all directions in space 8Cosmic Microwave Background•Universe was so dense initially that any emitted light was absorbed on-the-spot (opaque)•At some point the universe expanded and cooled enough so that the radiation due to the temperature of the matter (recall Wien’s law) could propagate and reach us today•The universe became transparent at about 380,000 years after the Big Bang, when it was about 3000K•Since the universe is expanding that radiation is redshifted. The spectrum of the radiation today has a characteristic temperature of 2.7K (in the microwave region of the electro-magnetic spectrum9Cosmic Microwave Background10Surface of Last Scattering1112The First Few Moments•Initially the Universe was incredibly hot.•Matter and energy were freely transforming into one another through Einstein’s Mass-Energy relation.•The mass formed were in matter anti-matter pairs. Matter and anti-matter destroy one another when they come into contact but just enough matter existed to survive.•This remaining matter is what fills the Universe.Big Bang Timeline13Geometry of the Universe14•Closed/finite: sum of triangle angles is more than 180 degrees•Open/infinite: sum of angles is less than 180 degrees•Flat: sum of angles is 180 degrees•How to measure a large enough triangle in the universe?•Instead look at size of CMB fluctuationsThe Universe is Flat15WMAP observations show fluctuations on a scale corresponding to theoretical model fluctuations for a flat universeThe Accelerating Universe16Observations of distant Type Ia supernovae suggest that the expansion of the universe is accelerating.A consequence of this observation is that the universe may continue expanding indefinitely.Critical Density17Dark Energy1819The Fate of the Universe•The Universe may expand forever (open) or it may collapse back in on itself (closed).•Which occurs depends on the amount of mass in the Universe.•With enough mass, the mutual gravitational attraction will slow the expansion and cause it to collapse. •Astronomers do not see enough mass to cause this collapse to occur.•There might be enough “Dark Matter” however.An HST picture showing a nearbygalaxy and dozens of more distant