Ian M George PHYS 105 (2005 Spring) 2005may02 Announcements Final homework assignment: “Fate of the Universe” tutorial Strict deadline, due Weds May 11 5pm As noted on Final Exam30% Homework30% Quizzes plus Telescope Night20% Mid-term Exam20% A > 85% B 70-84% C 55-69% D 45-54% F < 45%

Ian M George PHYS 105 (2005 Spring) 2005may02 Final Exam Monday May pm LH1 Monday May pm LH1 -there will be plenty of spare time Covers chapters As noted on

Ian M George PHYS 105 (2005 Spring) 2005may02 An Aside: April press release, ESO scientists using the Very Large Telescope facility in Chile The planet, 2M1207b, is ~x5 the size of Jupiter orbiting at a distance nearly twice as far as Neptune is from our Sun. Star and planet are more than 200 light-years away near the southern constellation of Hydra. Star which is brown dwarf Planet 'seen' around distant sun An artist's impression of the planet around the brown dwarf

Ian M George PHYS 105 (2005 Spring) 2005may02 DM Recap last time DM is a generic term for any matter that is: not radiating or reflecting light, but is exerting grav influence not radiating or reflecting light, but is exerting grav influence some of it (tiny bit) is due to MACHOs, some of it (tiny bit) is due to MACHOs, tiny bit due to neutrinos, tiny bit due to neutrinos, but vast majority thought to be WIMPs... but vast majority thought to be WIMPs...

Ian M George PHYS 105 (2005 Spring) 2005may02 Large Scale Structure of Universe On scales of 10 8 ly: galaxies distributed in gigantic chains & sheets surrounding great voids. Chains+sheets from initial regions of density enhancement Chains+sheets from initial regions of density enhancement Voids from initial regions of density depletion Voids from initial regions of density depletion On scales of several x 10 9 ly: galaxies appear evenly distributed. slice of the Universe out to 7 x 10 8 lyslice of the Universe out to 4 x 10 9 ly

Ian M George PHYS 105 (2005 Spring) 2005may02 Simulation: Different Times simulation courtesy of Prof. Nickolay Gnedin (Univ. Colorado) Movie. Click to play. brown color represents neutral Hydrogen

Ian M George PHYS 105 (2005 Spring) 2005may02 The Critical Density So gravitational attraction between galaxies can overcome expansion of Universe in localized regions How strong must gravity be to stop the entire Universe from expanding?!? - it depends on the total mass density of the Universe mass density required for this gravitational pull to equal the kinetic energy of the Universe as the “critical density” mass < critical density, Universe will expand forever mass < critical density, Universe will expand forever mass > critical density, Universe will stop expanding and then contract mass > critical density, Universe will stop expanding and then contract

Ian M George PHYS 105 (2005 Spring) 2005may02 The Critical Density To calculate the current kinetic energy of the Universe need to knows its speed this can be obtained from value of H o this can be obtained from value of H o (see Hubble’s Law v= H o d) this being known, critical density is 10 –29 g / cm 3 (v.small density!) this being known, critical density is 10 –29 g / cm 3 (v.small density!) All luminous matter we observe All luminous matter we observe accounts for < 1% of critical density (!!!!!) for dark matter to stop Universal expansion, for dark matter to stop Universal expansion, average M/L of Universe would have to be 1,000 M  / L  … a few times greater than clusters This line of argument suggests Universe will expand forever

Ian M George PHYS 105 (2005 Spring) 2005may02 Mass Density & Expansion

Ian M George PHYS 105 (2005 Spring) 2005may02 Is Gravity alone ? Recent observations of white dwarf supernovae in very distant galaxies have yielded unexpected results. (remember, white dwarf supernovae make very good standard candles) these supernovae are apparently fainter than predicted for their redshifts! for their redshifts! At a given cosmological redshift galaxies should be closer to us (i.e. shorter lookback time) galaxies should be closer to us (i.e. shorter lookback time) …for greater Universal mass densities …for greater Universal mass densities these supernova are farther back in time than even the these supernova are farther back in time than even the models for an ever-expanding (coasting) Universe predict This implies that the Universal expansion is accelerating!

Ian M George PHYS 105 (2005 Spring) 2005may02 “Dark Energy” ?!? must be an as yet unknown force which repels galaxies !!!! ( aka “dark energy”)

Ian M George PHYS 105 (2005 Spring) 2005may02 Mass Density and Dark Energy

Ian M George PHYS 105 (2005 Spring) 2005may02 4 Models for the Future 1) Recollapsing Universe: expansion will halt & reverse 2) Critical Universe: will not collapse, but expands more slowly with time 3) Coasting Universe: will expand forever with little slowdown 4) Accelerating Universe: expansion will accelerate with time currently favored

Ian M George PHYS 105 (2005 Spring) 2005may02 4 Models for the Future

Ian M George PHYS 105 (2005 Spring) 2005may02 What have we learned? Does dark matter really exist? Does dark matter really exist? Yes, inferred from its gravitational effects on visible matter evidence for its existence is overwhelming evidence for its existence is overwhelming — if we correctly understand theory of gravity (can never be 100% sure of course) How does the distribution of dark matter compare to the distribution of luminous matter in spiral galaxies? How does the distribution of dark matter compare to the distribution of luminous matter in spiral galaxies? luminous matter is concentrated in the disk dark matter is distributed throughout the spherical halo & beyond How do we determine the distribution of mass in distant galaxies? How do we determine the distribution of mass in distant galaxies? spiral galaxy: rotation curve - flat at large distances from center: ellipticals: orbital speeds of its stars at different distances from center, (measured from broadening of spectral lines)

Ian M George PHYS 105 (2005 Spring) 2005may02 How does a galaxy’s M/L ratio tell us how much dark matter it contains? How does a galaxy’s M/L ratio tell us how much dark matter it contains? What have we learned about galaxies from their mass-to-light ratios? What have we learned about galaxies from their mass-to-light ratios? M/L ratio gives how many solar masses of matter galaxy contains for each solar luminosity of light output. can estimate M/L ratio if galaxy were made only of stars, any excess mass must be dark matter Find: indeed M/L of galaxies is much higher than stars-only estimate Describe 3 (independent) ways to measure total mass of a cluster of galaxies. Describe 3 (independent) ways to measure total mass of a cluster of galaxies. (1) orbital speeds & positions of the galaxies (2) temperature & distribution of its hot, intracluster medium (3) distortion of more distant galaxies by gravitational lensing What have we learned?

Ian M George PHYS 105 (2005 Spring) 2005may02 What have we learned about dark matter in galaxy clusters? What have we learned about dark matter in galaxy clusters? All three methods agree: All three methods agree: large amounts of dark matter in clusters What do we mean when we ask whether dark matter is ordinary or extraordinary matter? What do we mean when we ask whether dark matter is ordinary or extraordinary matter? Ordinary matter is made from protons, neutrons, and electrons; referred to as “baryonic” matter since protons & neutrons are both baryons. But baryonic matter does not account for all the dark matter Possible that most dark matter is made of “nonbaryonic” particles (yet to be discovered). i.e. form of matter very different from that encountered in daily life (so “extraordinary”) What have we learned?

Ian M George PHYS 105 (2005 Spring) 2005may02 What are MACHOs, and can they account for dark matter? What are MACHOs, and can they account for dark matter? MAssive Compact Halo Objects ‘ordinary objects’… (dim stars, brown dwarfs, planet-size bodies) … populating galactic halo without being visible to our telescopes MACHOs exist (seen via gravitational lensing) but probably not in large enough numbers to account for all dark matter What are WIMPs, and can they account for dark matter? What are WIMPs, and can they account for dark matter? Weakly Interacting Massive Particles undiscovered particles of extraordinary (nonbaryonic) matter undiscovered particles of extraordinary (nonbaryonic) matter do not interact with light leading candidate for dark matter Why can’t neutrinos account for dark matter in galaxies? Why can’t neutrinos account for dark matter in galaxies? are weakly interacting particles but too light & travel too fast to be gravitationally bound in galaxies. What have we learned?

Ian M George PHYS 105 (2005 Spring) 2005may02 How does structure appear to be growing in the universe? How does structure appear to be growing in the universe? grew from regions of slightly enhanced density in early universe. Gravity in these higher density regions drew matter together to form galaxies, drew those galaxies together to form clusters, currently superclusters are just beginning to form. What does the universe look like on very large scales? What does the universe look like on very large scales? Galaxies distributed in gigantic chains & sheets surrounding voids. Galaxies distributed in gigantic chains & sheets surrounding voids. What is the critical density? What is the critical density? average matter density universe must have in order for strength of gravity to be enough to someday halt the expansion of universe (assuming today’s expansion rate). Although may be dark matter unaccounted for, appears that overall matter density is only about 30% of critical density. What have we learned?

Ian M George PHYS 105 (2005 Spring) 2005may02 Describe the four general models for the future expansion of the universe. Which model is currently favored? Describe the four general models for the future expansion of the universe. Which model is currently favored? (1) Recollapsing universe: expansion will someday halt and reverse. (2) Critical universe: universe will never collapse but will expand more and more slowly with time. (3) Coasting universe: universe will continue to expand forever, with little change in rate of expansion. (4) Accelerating universe: expansion of universe will accelerate with time. Recent observations favor the accelerating universe. Do we know what might be causing the universe to accelerate? Do we know what might be causing the universe to accelerate? No!!! although people give names to the mysterious force that could be causing acceleration (“dark energy”, “quintessence”, or “cosmological constant”) — no one (yet) really knows what it is !!!! — no one (yet) really knows what it is !!!! What have we learned?