1Announcements• A few slots are stillopen in the class;see prof. to sign up• Web-page computer hasbeen down; o.k. now• Turn in your studentcontract• Don’t forget to do theOWL tutorial and registeryour PRS clicker• PRS clickers sold out, sowe’ll start that on Thurs.Astro.101Sept. 9, 2008Your clicker should look like this:The Age of the UniverseIf the entire age of the Universe were 1 calendar year, then1 month would be equivalent to roughly 1 billion years• Jan. 1: The Big Bang• Feb: Our galaxy formed• Sept. 3: Earth formed• Sept. 22: Earliest life on Earth• Dec. 26: Rise of dinosaurs• Dec. 30: Dinosaurs disappear• 25 seconds before midnight: firstagriculture• 11 seconds before midnight: pyramidsbuilt• 1 second before midnight: Kepler andGalileo obtain first correctunderstanding of the Solar SystemThe implications of the finite speed of lightLight moves rapidly (!), but it does not move infinitely fast.• If we observe a galaxy that is 11billion light years away, then the lightstarted moving towards us 11 billionyears ago and is only arriving at oureyes right now.• What happened to that galaxy 1000years later? We won’t know until 3008• However, we can compare it to agalaxy that is only, say, 5 billion lightyears away to study galaxy evolution.• Our Solar System formed roughly 5billion years ago, and we similarlystudy how the old Solar Systemcompares to the current Solar System.• For example, comets are leftover junkfrom when the planets first formed,i.e., effectively fossilsThe Funky Discoveries of EdwinHubble and Albert EinsteinThere are many galaxies in the universe external toour Milky Way. They exhibit two odd behaviors:1. The vast majority of galaxies are movingaway from us.2. The more distant the galaxy, the faster itis moving away from us!PRS question. True or false: this meansthat there was a huge explosion, and sinceall galaxies are moving away, we are closeto the center of the explosion.1. TRUE 2. FALSE(Survey Question: Automatic Full Credit)In the 1920s, Edwin Hubble observed the following:2There are at least 100,000,000,000 galaxies inthe universe. They exhibit two odd behaviors:1. The vast majority of galaxies are movingaway from us.2. The more distant the galaxy, the faster itis moving away from us!PRS question. True or false: this meansthat there was a huge explosion, and sinceall galaxies are moving away, we are closeto the center of the explosion.1. TRUE 2. FALSEConsider the idea of a hugecosmic explosion:• Suppose that objects movemore rapidly close to theexplosion source and slowdown as they move awayA B C• Galaxy A moves morerapidly than B, so itovertakes B. Likewise, Bovertakes C.• If we live in Galaxy B, Awould appear to bemoving toward us, and Cwould be moving towardus too.• In the center, all galaxies wouldbe moving away, but moredistant ones would move moreslowlyConsider the idea of a hugecosmic explosion:• We should see evidence ofthe explosion in the spatialdistribution of the galaxies.A B C• This idea predicts: nogalaxies near the center;many galaxies in a shellaround the periphery. Thisis not observed; we seecomparable numbers ofgalaxies in all directions.• Placing ourselves in the center violates the “Copernicanprinciple” — we are likely to be a random place, not thespecial location of the very center (also known as the“cosmological principle”)An Einstein-style “thoughtexperiment”• Suppose that the universe is araisin cake with equally spacedraisins.• As the cake bakes, it expands atthe same rate in all directions.• Let’s measure the distancesbetween the raisins at thebeginning, and then again oncehour later.• Suppose that we live on a raisinthat we (fondly) call the “localraisin”.The funkiest bit of all: the expansion of theUniverse!• All of the raisins appearto be moving away fromus.• The farther away theyare, the faster they aremoving.– Just like raisins in araisin cake, galaxies allmove apart from eachother as the “dough”(space itself) expands.Raisin Number Distance before baking Distance after baking Speed 1 1 cm 3 cm 2 cm/hr 2 2 cm 6 cm 4 cm/hr 3 3 cm 9 cm 6 cm/hrSpeed = Distance Time The “Hubble Law” and the “Age” of the UniverseEdwin Hubble noticed that galaxyvelocities all follow a simple relation:Velocity = Constant x DistanceThe “Hubble Constant”But we also know thatVelocity = DistanceTimeFor a particular galaxyVelocity = Constant x Distance = DistanceTimeThereforeHubble Constant = 1TimeThis is the timethat has passedsince the expansionstarted!!!This can be measured!!3Einstein’s “GeneralTheory of Relativity”• Mass affects space itself.• Sufficiently massive objects curve thefabric of space.• Space can move. Near a black hole, spaceitself is falling into the black hole.• If space can move, the universe can becontracting or expanding.• Einstein thought that this was ridiculousand fudged the theory to make it go away.“My greatest blunder”The Observable UniverseSeveral lines of evidence indicate that the universe isroughly 14 billion years old:- The Hubble Law- The ages of stars This means that the greatest distance we can observe is14 billion light years. This is the limit of the observable universe. A comment about our origins…The most distant stuffThat we can observeContains only hydrogenand helium. No carbon,nitrogen, oxygen, etc.Nearby stuff, however, isContains significant amountsOf carbon, oxygen, etc.Your ancestors may surprise you• In the early history of the Universe, there was nothing buthydrogen and helium.• All other elements were manufactured deep in the cores ofstars by nuclear fusion reactions, or in nuclear reactionsthat occur when a massive star ends its life in a violentexplosion called a supernova.• Seek to be one with the cosmos? You already are!Gravitational lensing.The force of gravity affectslight as well as mass. Gravitychanges the path light followsjust like an optical lens!4Another funky Einstein thing: Gravitational lensing• This is a gravitational lens.• Einstein’s Theory of Relativity states that theforce of gravity affects light as well as mass.• a massive galaxy cluster will bend the path oflight which approaches it (like a lens)• the blue arcs are the lensed images of a galaxywhich is behind the clusterGravitational lensingcomes in various flavorsBelow: arcs dueto gravitational lensingLeft: four images of a