Recap of Last Time….

1. Recap of Last Time…. Convenient to define Mass-to-Light ratio (M/L) and use Solar units, so for Sun M/L= 1 M/L Can measure M/L as function of distance from galaxy center using 21cm rotation curves (spirals) or stellar velocities (ellipticals) Find M/L >> 1M/L and increasing with distance from galaxy center Large fraction of the matter is not emitting as stars generically called “Dark Matter” [M/L > 50 M/L for some galaxies !!] 2005may02

2. Recap of Last Time (II) … • Also find M/L >> 1 M/L for Clusters of Galaxies… • from measurements of • 1) speeds & positions of galaxies within cluster 2) temperature & distribution of hot gas between galaxies (optical) (X-ray) Coma cluster of galaxies • Start today talking about • 3) M/L from observations of gravitational lensing • by clusters of galaxies 2005may02

3. Gravitational Lensing by Clusters 2005may02

4. Einstein’s Theory of Relativity: massive objects distort spacetime. a massive cluster bends path of light which approaches it (like a lens) blue arcs are lensedimages of a galaxy which is behind foreground cluster angle at which light is bent depends on mass by analyzing lensed images, can calculate cluster mass All previous methods for finding mass depended on Newton’s Law of Gravity this method uses more sophisticated theory of gravity Gravitational Lenses 2005may02

5. Massive Object passing thru Balt.. 2005may02

6. Dark Matter in Clusters The cluster masses which are measured by all three of these independent methods agree: 1)speeds & positions of galaxies within cluster 2) temperature & distribution of hot gas between galaxies 3) observations of gravitational lensing by clusters of galaxies For most galaxy clusters M/L > 100 M/ L So galaxy clusters contain far more mass in dark matter than in stars 2005may02

7. 22.4Dark Matter: Ordinary or Not?! • What do we mean when we ask whether dark matter is “ordinary” or “extraordinary” matter? • What are MACHOs, & can they account for dark matter? • What are WIMPs, & can they account for dark matter? • Why can’t neutrinos account for dark matter in galaxies? Learning Goals: 2005may02

8. What is Dark Matter Made of? Dark matter couldbe protons, neutrons, & electrons. i.e. “ordinary” matter: same stuff we’re made of • if so, then only thing unusual about dark matter is that it is dim However, some/all dark matter could be made of particles which we have yet to discover... i.e. some kind of new “extraordinary” matter Physicists call “ordinary” matter baryonic matter (protons & neutrons are called baryons) call “extraordinary” matter nonbaryonic matter. 2005may02

9. Idea: Galactic halo contains baryonic matter which is dark: low-mass M dwarfs, brown dwarfs, and Jovian-sized planets they are too faint to be seen at large distances Known as “MAssive Compact Halo Objects” or MACHOs Can detect them if they pass in front of a star where they… gravitationally lens the star’s light star gets much brighter for a few days to weeks So can measure MACHO’s mass However these events occur to only one in a million stars per year. must monitor huge numbers of stars An Ordinary Matter Candidate • Recent Results: # MACHOs detected so far does not account for • Milky Way’s dark matter 2005may02

10. Extraordinary Matter Candidates Already studied a “nonbaryonic” form of matter: ….the neutrino…detected from Sun neutrinos interact with other particles through only 2 of the “4” forces: I.e. viaGravity and “the weak force” (hence they are “weakly interacting”) Recent Results: neutrino can not account for dark matter masses are too low & speeds so high, they will escape gravitational pull of a galaxy… … but could there be other similar, but more massive particles ? 2005may02

11. A Universe full of WIMPs ? Idea: What if there are massive weakly interacting particles?!? i.e. “Weakly Interacting Massive Particles” or WIMPs these particles are theoretical … not yet been discovered but are predicted by models of particle physics are being searched for in particle accelerators WIMPs would • have to be massive enough to exert gravitational influence • emit no electromagnetic radiation (light) (or be bound to any charged matter which could emit light) • not collapse with a galaxy’s disk as weakly interacting particles • remain gravitationally bound in galaxy halos Currently: WIMPs are the favored explanation fordark matter 2005may02

12. 22.5Structure Formation • How does “structure” appear to be growing in the Universe? • What does Universe look like on very large scales? Learning Goals: 2005may02

13. At “close” range, gravitational attraction can overcome Hubble expansion. we see this in a galaxy’s “peculiar velocity” Structure: “Peculiar” Velocities silly name! • although Universe as a whole expands, • galaxies close to one another • attract one another • peculiar velocity is a galaxy’s deviation from the Hubble Law • currently can measure it for galaxies out to 3 x 108 ly 2005may02

14. Peculiar Velocities of Galaxies see Fig 22.13 2005may02

15. Idea: Universal structure began with slight enhancements in density of matter in very early Universe. these regions collapsed into protogalactic clouds to form galaxies individual galaxies fell in towards one another to form clusters individual clusters are now congregating to form superclusters These “collapses” against Universal expansion are facilitated by dark matter. more dark matter than “normal” (baryonic) matter … so dark matter content will dominate the gravitational attraction & (hence) dynamics The Growth of Structure OK, so what does the universe actually look like?.. 2005may02

16. Large Scale Structure of Universe On scales of 108 ly: galaxies distributed in gigantic chains& sheets surrounding great voids. • Chains+sheets from initial regions of density enhancement • Voids from initial regions of density depletion On scales of several x 109 ly:galaxies appear evenly distributed. slice of the Universe out to 7 x 108 ly slice of the Universe out to 4 x 109 ly 2005may02

17. Simulation: Different Scales 2005may02

18. Clustering in the USA 2005may02

19. Simulation: Different Times brown color represents neutral Hydrogen Movie. Click to play. simulation courtesy of Prof. Nickolay Gnedin (Univ. Colorado) 2005may02

20. 22.6The Universe’s Fate • What is the critical density? • What are the 4 general models for future expansion of the Universe? Which model is currently favored? • Do we know what might be causing the universe to accelerate? Learning goals: 2005may02

21. The Critical Density So gravitational attraction between galaxies can overcome expansion of Universe in localized regions How strong must gravity be to stop theentire 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 stop expanding and then contract 2005may02

22. The Critical Density To calculate the current kinetic energy of the Universe need to knows its speed this can be obtained from value of Ho (see Hubble’s Law) • this being known, critical density is 10–29 g / cm3(v.small density!) • All luminous matter we observe accounts for < 1% of critical density (!!!!!) • 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 2005may02

23. Mass Density & Expansion 2005may02

24. Recent observations of white dwarf supernovae in very distant galaxies have yielded unexpected results. (remember, white dwarf supernovae make very good standard candles) Is Gravity alone ? these supernovae are apparently fainter than predicted for their redshifts! At a given cosmological redshift • galaxies should be closer to us (i.e. shorter lookback time) • …for greater Universal mass densities • 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! 2005may02

25. “Dark Energy” ?!? • must be an as yet unknown force which repels galaxies !!!! • (aka “dark energy”) 2005may02

26. Mass Density and Dark Energy 2005may02

27. 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 2005may02

28. 4 Models for the Future 2005may02

29. What have we learned? • Does dark matter really exist? Yes, inferred from its gravitational effects on visible matter evidence for its existence is overwhelming —ifwe 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? 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? 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) 2005may02

30. What have we learned? • 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? 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. (1) orbital speeds & positions of the galaxies (2) temperature & distribution of its hot, intracluster medium (3) distortion of more distant galaxies by gravitational lensing 2005may02

31. What have we learned? • What have we learned about dark matter in galaxy clusters? • 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? 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”) 2005may02

32. What have we learned? • 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? Weakly Interacting Massive Particles 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? are weakly interacting particles but too light & travel too fast to be gravitationally bound in galaxies. 2005may02

33. What have we learned? • How does structure appear to be growing in the universe? grew from regions ofslightly 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? Galaxies distributed in gigantic chains & sheets surrounding voids. • 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. 2005may02

34. What have we learned? • 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? 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!!!! 2005may02