1、Astro 10-Lecture 14:Large Scale Structure and CosmologyHow big is the universe?What does the universe look like on the largest scales?How did the universe get to be the way it is now?Extra Credit AssignmentWrite a 5 to 10 page paper going into detail on a single aspect of astronomy (currents events,
2、 a type of object, etc.). Be concise. Dont pick too broad a subject. Will be accepted up until the final. Worth up to 5% of your grade If you are more than 5% away from a boundary, I wont grade it. Plagiarism earns a course grade of F, regardless.Structure in the UniverseStars Star Clusters (100s to
3、 Millions of Stars) Galaxies (Millions to Several Hundred Billion Stars) Galaxy Clusters (10s to 1000s of Galaxies)Are there any larger structures in the universe?Superclusters, Walls, and Voids.Galaxies and Clusters appear to be clustered into huge walls with voids between them.But on the largest s
4、cales the universe begins to look smoothConcepTestWhy does it look like there are fewer galaxies far away from us?Which brings us to cosmology.Has the universe always been here?Did it always have stars and galaxies?Will it always be here?Start with something even more basic.Why is the sky dark at ni
5、ght?(Olbers Paradox)In a forest, no matter which way you look, you see a treePossible Reasons.Is dust blocking the light?No. The dust would be heated by the star light until it was as hot as a star.Is the light red shifted out of the visible?The energy would still be there to heat up the dust (and t
6、he earth).Possible Reasons.A. Perhaps the universe is finiteB. Perhaps the universe is young enough that light from distant stars hasnt reached us?C. Perhaps the universe is different far away from us?Searching for an answerRemember Hubbles Law?v=Hod The farther something is, the faster it is moving
7、 away from usEverything was closer together in the past.We can work backward to find out when distant galaxies were in the same place we were. (In other words the universe has a finite age)How old is the universe?v=Hod How long would it take a galaxy traveling at velocity v to go distance d?d/v=1/Ho
8、If Ho=70 km/s/Mpc1/Ho = 1 Mpc/(70 pc/Myr) = 14 billion years“The Observable Universe” Since the universe isnt infinitely old, the part that we can see isnt infinitely largeThis doesnt mean the universe is finite. It just means there might be parts of the universe we cant see.If the universe is 14 bi
9、llion years old, we cant see anything farther away than 14 billion light years.“Lookback time”Also recall that when we look at something 2 billion light-years away were looking at something as it was 2 billion years ago.If we look far enough we should be able to see galaxies forming.Assumptions in C
10、osmologyHomogeneity: Matter (and energy) are evenly distributed on the largest scalesIsotropy: The universe looks the same in all directionsUniversality: The physical laws that govern the universe are the same everywhere (and everywhen).The Cosmological Principle:An observer anywhere in the universe
11、 sees approximately the same thing. No place is special. No edge or center.ConcepTest T/F: We are at the center of the observable universe.Basic Concepts of CosmologyThe Hubble Flow: The universe is expanding. Galaxies are receding from us due to this expansion. Gravity is working to slow this expan
12、sion. Is there enough gravity to eventually stop the expansion?Basic Concepts: The Big-Bang TheoryIf you could run the universe back in time, eventually all the galaxies would merge into a hot, high-density plasma. So, its logical to assume that the universe started as a hot, high-density plasma.Wel
13、l talk about other evidence for this later.More Basic Concepts of CosmologyDimensions:A zero dimensional object: A one dimensional object:A two dimensional object:The geometry of the universe We perceive space as 3 dimensional Relativity says time is a 4th dimension Gravity due to energy (and mass)
14、can curve space Some theories of physics require that space have 11 or more dimensionsThe geometry of the universe The universe has no boundary or edge Either it is infinite, or curved through a 4th (or 5th) dimension to meet itself.A one-dimensional object curved through a 2nd dimension to meet its
15、elf An infinite one-dimensional objectMass and Energy determine the curvature of the universe The possibilities:1. The total energy is negative The universe is curved back in upon itself (CLOSED) The universe will stop expanding and collapse back on itselfMass and Energy determine the curvature of t
16、he universe The possibilities:2. The total energy is positive: The universe is curved in such a way that it doesnt curve in on itself like a hyperbola or parabola. (OPEN) The universe will expand foreverMass and Energy determine the curvature of the universe The possibilities:3. The total energy is
17、zero The universe is flat The universe will just barely expand forever (it will stop after an infinite amount of time.How do we tell what the geometry of the universe is?Things that are geometrically true in a flat space are not necessarily true in a curved space.Determining the Geometry of Space-Ti
18、me A BCACBFlat SpaceA+B+C=180oClosed SpaceA+B+C180oOpen SpaceA+B+CArea/Radius2 These effects are too small to measureMoreover the curvature of space near earth is dominated by the earths gravity.Same is true for volume Flat Space: Volume/Radius3=4/3Closed Space: Volume/Radius34/3If you assume that t
19、he density of galaxies is constant, you can measure this by counting galaxies.Result: Nearby, at least, space is close to flat.For large distances, its tough to avoid selection effects.How else can we measure the geometry of Space-Time? General Relativity says that curvature of space is caused by gr
20、avity. Or is it that gravity is caused by the curvature of space?It doesnt matter which causes which, just that in the end, mass and energy are the cause of curved space.Determining the Geometry of Space-Time Now all that we need to do is to measure the total amount of matter and energy in the universe in order to determine the curvature. If we define density , such that =1 would result in a flat universe, then what is for the universe?
