Announcements HW #5 Due Wed, Dec. 10th. Planetarium/Observing: the clock is ticking! Don t forget to fill out your Planetarium/ Observing impression online. NOTE: Planetarium: Large dome you sit inside. Observatory: Telescope you look through.
Final Exam Thursday, Dec. 18th, 2:45 4:45pm, in this room (MH 1005)! Cumulative exam covering the entire semester. 100 questions, no buy-back, but 10 bonus extra-credit questions will be included. In class review the last day of classes (Dec. 10th).
Last Time Stars in galaxies orbit too fast given the amount of luminous matter in them. Galaxies in clusters move too fast, and the hot x-ray gas in clusters is too hot to be explained by the mass in the galaxies and the gas. Solution : Dark Matter!
Last Time All objects with mass bend space, causing light to bend as it passes near them. Giant clusters of galaxies can act as lenses, and make distorted and/or multiple images of galaxies behind them. The amount of bending is determined by the mass... again, the gas and the galaxies is not nearly enough mass (50 ) to account for the degree of bending. Solution : Dark Matter!
Last Time: Dark Matter In general, the mass in the universe is not where the light is! Dark Matter is only inferred, but recently, evidence is much stronger. Very likely nonbaryonic (i.e. not ordinary matter), but we don t know!
Last Time: Dark Matter Dark Matter controls the formation of galaxies and larger structures in the universe. Provides most of the gravity in the universe! Ordinary matter simply follows it around.
Last Time: Dark Energy Galaxies are expanding from each other, but normally they should be slowing down, just as a ball tossed in the air is slowed by the Earth s gravity. In 1998, we learned that the expansion of the universe is not slowing down, but is accelerating instead! We know very little about the mysterious force causing the universe to accelerate in it s expansion. we call it Dark Energy.
The Beginning of Time
Question Why is the sky dark at night? Imagine looking through a forest of trees. The larger the forest, the fewer the gaps. Small Wood Large Forest
Olber s Paradox If the forest is large enough, there are no gaps. The universe is the same way. If the universe were infinite, every direction on the sky would land on the surface of a star somewhere.
Olber s Paradox If the forest is large enough, there are no gaps. The universe is the same way. If the universe were infinite, every direction on the sky would land on the surface of a star somewhere.
Olber s Paradox If the forest is large enough, there are no gaps. The universe is the same way. If the universe were infinite, every direction on the sky would land on the surface of a star somewhere.
Olber s Paradox If the universe were unchanging and infinite, the sky would be uniformly bright! The Big Bang solves this problem neatly: the universe has changed over time, and was once tiny. The observable universe we can therefore see is finite. Hence, the night sky is dark.
The Big Bang 1s Now If the Universe is expanding, then it must have been denser, and thus hotter, in the past.
The Early Universe In the first few seconds, the temperature was so high, photons and matter could transform into each other. The early Universe was filled with radiation, matter, and anti-matter.
Four Forces of Nature Electromagnetism: Creates and controls light, magnets, etc. Gravity: Holds planets in orbit around stars, etc. Strong Force: holds protons and neutrons together in an atom. Weak Force: Drives radioactivity
Four Forces of Nature At higher temperatures, electromagnetism and the weak force become one force called the electroweak force (1970s). At even higher temperatures, do the strong force and electroweak force become one? Now Theories called the Grand Unified Theories (GUT) predict they do.
Planck Era 10-43 sec We have no physics to understand the Universe before 10-43 sec. We need quantum gravity (string theory)?
GUT Era 10-43 10-38 sec GUT: Grand Unified Theory Three of the four known forces of nature were combined!
Electroweak Era 10-38 -10-10 sec Electroweak forces separates into weak force and electromagnetic force. We have direct evidence that these forces unite into one.
Particle Era 10-10 0.001 sec Temperature falls, matter and energy can no longer transform into each other. Left with protons, neutron, electrons, neutrinos (and maybe WIMPs). Matter outnumbered anti-matter. We don t know why.
Era of Nucleosynthesis 0.001 sec 3 min Protons and neutrons combine to form Hydrogen and Helium. We are left with 75% H and 25% helium in the universe. No other baryonic matter.
Era of Nuclei 3 min 380,000 years H and He nuclei, electrons, and photons all move together. Just as in the sun s center, photons cannot move far before hitting an electron.
Era of Atoms 380,000 years 10 9 years At 3000K, H and Helium atoms are formed (electrons combine) Photons in the bath of matter and radiation now move freely. We see these very photons today as the Cosmic Microwave Background.
Era of Galaxies 10 9 years present Areas of slightly higher density in the Universe start the formation of galaxies, and galaxy clusters.
Big Bang Model The big bang model is consistent with many things about the universe, but it has predicted two: The Cosmic Microwave Background The initial abundance of H and He.
The Cosmic Microwave Background In the Era of Nuclei, photons could not move freely. In the Era of Atoms, photons stream through the Universe unimpeded. We see this radiation left over from the big bang as the Cosmic Microwave Background.
The Cosmic Microwave Background The CMB was discovered by Penzias and Wilson in 1965 (accidentally).
The Cosmic Microwave Background At the beginning of the Era of Atoms, the leftover radiation had a temperature of 3,000 K. Its thermal spectrum peaked in optical/infrared.
The Cosmic Microwave Background As the Universe expands and cools, the wavelengths of the photons become stretched to longer and longer wavelengths. The peak of the leftover thermal radiation is now at about 1 mm, in the microwave region.
The Cosmic Microwave Background The COsmic Background Explorer (COBE) mission was launched in the 1990s. It detected the CMB and it was found that it corresponded to a thermal spectrum of 2.73 K. The temperature of space is 2.73 above absolute zero!
The Cosmic Microwave Background The Wilkinson Microwave Anisotropy satellite (WMAP) mapped the CMB in all directions... and found a very uniform map.
The Cosmic Microwave Background The Wilkinson Microwave Anisotropy satellite (WMAP) mapped the CMB in all directions... and found a very uniform map. But it isn t perfectly uniform. Slight variations in temperature occur: 1 part in 100,000! There reveal the density enhancements that led to structure (galaxies, star, planets, life) in the universe!
WMAP s View
Structure Forming
The Light Elements Hydrogen and Helium are the only elements produced in the quantity by the big bang. Today we observe that everywhere in the universe about 25% of baryonic matter is Helium.
Particle Era 10-10 0.001 sec Matter couldn t convert into radiation, but protons could convert into neutrons. There were equal numbers of protons and neutrons.
Era of Nucleosynthesis 0.001 sec 3 min As temperature drops, the reaction converting neutrons into protons and visa versa began favoring protons. Why?
Neutron-Proton Reactions Neutrons have slightly more mass than protons. (E=mc 2 ). Neutrons Protons + energy Protons + energy Neutrons The temperature got low enough that there was not enough energy to convert protons to neutrons, but neutrons keep becoming protons.
Neutron-Proton Reactions Theoretical calculations predict 7 protons for every neutron once fusion began. The mass ratio after fusion is therefore 12 to 4 or 75% H and 25% He!
Inflation Questions Big Bang doesn t answer: Where does structure in the universe come from? Why is the universe so uniform? Why is the density of the universe so close to the critical value, which just (barely) halts expansion? We believe an incredible expansion occurred in the very early universe, in which it expanded 10 30 in less than 10-36 s!
Where does the structure in the Universe come from? Quantum fluctuations exist in a vacuum but are very small. Inflation expanded these fluctuations and they became the seeds for structure formation.
Why is the Universe So uniform? How can regions on opposite sides of the universe have very nearly the same temperature? Easy to explain with inflation!
Why is the Universe So uniform? How can regions on opposite sides of the universe have very nearly the same temperature? Easy to explain with inflation!
By measuring the CMB, we now know very well the composition of the universe: 4% Atoms (everything we know) 23% dark matter 73% something else!
Critical Density The matter in the universe is close to 25% of the critical density needed to eventually halt collapse. With Dark Energy included, the mass-energy density of the universe is precisely critical. If the universe were ever slightly overdense or under-dense, it would have since moved very far from critical density. The universe had to be exquisitely balanced to avoid this, but inflation naturally predicts this.
Fate of the Universe, revisited Time and space began with the big bang. What will happen in the end? Dark energy will continue to drive accelerated expansion. Galaxies will get further and further apart. So far we won t be able to see them (further than the limits of our observable universe). As all the gas in galaxies is turned into stars, star formation will stop.
Fate of the Universe, revisited Only stellar remnants remain: brown dwarfs, white dwarfs, neutron stars, black holes. Collisions between these remnants will eventually send stars out of the galaxies. Supermassive (but dead) black holes will remain. If Earth survives, it will likely be a cold dark rock in the infinite void of space. Eventually (10 40 years), even the protons will decay.
The Lesson? Enjoy your time in the universe; it won t last forever.
Course evaluations Now! Please give me your honest feedback. Thanks! HW #5 due Wed. Final Dec. 18th, review on Wed. Last chance observing/planetarium Fri. (Sat. Planetarium as well). Don t forget to fill out your impressions online!