Cleveland Museum of Natural History, 9 December 2010The Driving Role of Gravity in CosmologyStacy McGaughUniversity of MarylandWorld Map of Hecataeus of Miletus (c. 500 BC)Here there be dragons!Ancient Cosmology: A Flat EarthThe ancient Egyptians conceived the sky as a roof placed over the world supported by columns placed at the four cardinal points. The Earth was a flat rectangle, longer from north to south, whose surface bulges slightly and having the Nile as its center. On the south there was a river in the sky supported by mountains and on this river the sun god made his daily trip (this river was wide enough to allow the sun to vary its path as it is seen to do). The stars were suspended from the heavens by strong cables, but no apparent explanation was given for their movements.Nuit, the goddess of the night, was in a tight embrace with her husband Sibû, the earth god. Then one day, the god Shû grabed her and elevated her to [become] the sky despite the protests and painful squirming of Sibû. But Shû has no sympathy for him and freezes Sibû even as he is thrashing about. And so he remains to this day, his twisted pose generating the irregularities we see on the Earth's surface. Nuit is supported by her arms and legs which become the columns holding the sky. Nuit - the skySibû - the earthShûAncient EgyptianCreation MythUPDOWNUPDOWNGreek Map in 3DIncan CosmologyChinese myth:The world rests on the backof a great turtle.And what does the turtle stand on?Why, it’s turtles all the way down!well, maybe the earth is roundand the center of creation.Geocentric Cosmology© 2007 Pearson Education Inc., publishing as Pearson Addison-WesleyGeocentricPtolemaicEarth at centerHeliocentricCopernicanSun at centerCompeting Cosmologies© 2007 Pearson Education Inc., publishing as Pearson Addison-WesleyThe most sophisticated geocentric model was that of Ptolemy (A.D. 100–170) — the Ptolemaic model: Ptolemy• Sufficiently accurate to remain in use for 1,500 years • i.e., predicted correct positions of planets for many centuries• Arabic translation of Ptolemy’s work named Almagest (“the greatest compilation”) Geocentric CosmologyGeocentric CosmologyHeliocentric Cosmology© 2007 Pearson Education Inc., publishing as Pearson Addison-WesleyCopernicus (1473–1543):• He proposed the Sun-centered model (published 1543).• He used the model to determine the layout of the solar system (planetary distances in AU).But . . .• The model was no more accurate than Ptolemaic model in predicting planetary positions, because it still used perfect circles.Heliocentric Cosmology© 2007 Pearson Education Inc., publishing as Pearson Addison-WesleyGeocentricPtolemaicEarth at centerHeliocentricCopernicanSun at centerThe sun is the source of light in both modelsCompeting CosmologiesRetrograde MotionNeeds epicycles Consequence of LappingInferiority of Mercury & VenusInterior to Earth’s OrbitMust tie to sunPredicts- No parallax- Venus: crescent phase only- Parallax- Venus: all phases© 2007 Pearson Education Inc., publishing as Pearson Addison-WesleyJohannes Kepler(1571–1630) ! “If I had believed that we could ignore these eight minutes [of arc], I would have patched up my hypothesis accordingly. But, since it was not permissible to ignore, those eight minutes pointed the road to a complete reformation in astronomy.”© 2007 Pearson Education Inc., publishing as Pearson Addison-WesleySir Isaac Newton (1642–1727)Formulated the Universal Law of GravityEverything happens ... as if the force between two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.(1662 – 1742)Richard BentleyBentley-Newton correspondenceBentley: would not a finite assemblage of stars collapse from their mutual gravity?Newton: if the matter was evenly diffused through an infinite space, it would never convene into one mass.Bentley: can such a system remain stable?Newton: such an assemblage, even if infinite, is like an array of needles standing upright on their points, ready to fall one way or another.Newton: this frame of things could not always subsist without divine power to conserve it.God actively intervenesto keep things in order.But what if the universe isn’t static?If the universe is expanding, Newton’s gravity gives the right equation of motion just by considering a point on the surface of a sphere that feels the gravity of a uniform density within that sphere (Milne & McCrea).© 2007 Pearson Education Inc., publishing as Pearson Addison-WesleyThe Cosmological Principle• The Universe is– Homogeneous– IsotropicA philosophical assertion that there should be nothing specialabout where we are, so the universe should look much the same to an distant alien observer as to us.© 2007 Pearson Education Inc., publishing as Pearson Addison-WesleyThe Perfect Cosmological Principle• The universe looks the same from everywhere at all times.This is a logical extension of the Cosmological Principlein time as well as space. Trouble is, it is not true.An Expanding Universe?Rμν - ½gμν = 8πGTμν A homogenous, isotropic universeevolving according to Einstein’s field equation must either expand or contract. It can not be static.Or a static one?Einstein’s greatest blunder?Rμν - ½gμν = 8πGTμν + ΛgμνEinstein’s intention was to keep theuniverse static. But it does expand!XEinstein’s geometrical theory of gravity forms the basis of modern cosmology.The expansion history and the geometry of the universe depend on its mass density.FLATCLOSEDOPENlow density -expands foreverhigh density -eventually re-collapsesGravity makes all the differenceCLOSEDFLATOPENNot only does the universe expand,but this expansion is accelerating!Need “Dark Energy” to do that!FLATCLOSEDOPENACCELERATINGA mathematical blunder:Einstein’s cosmological constant Λ makes the expansion accelerate!In addition to Dark Energy, there islots of evidence for Dark Matter.SpiralGalaxyRotationCurveGalaxy ClusterLarge Scale StructureWhat is the Dark Matter?Baryonic Dark MatterHot Dark MatterCold Dark MatterNormal things: " very faint stars, brown dwarfs" other hard-to-see objects (planets, gas)neutrinos - got mass, but not enoughSome new fundamental particle" doesn’t interact with light, so quite invisible. Two big motivations:1) total mass outweighs normal mass from BBN2) needed to grow cosmic structureXX✔gravitating mass >>