1	Chapter 16 Dark Matter, Dark Energy, and the Fate of the Universe Chapter Outline Unseen Influences Evidence for Dark Matter Structure Formation The Universe’s Fate: Fire or Ice?
2	Dark Matter in Our Galaxy Observation: galactic rotation curve does not decline in Keplerian fashion far from center of Galaxy Actually grows larger to edge of Galaxy Consequence: more mass controlling motion of stars than can be account for in terms of visible matter Stars, gas, and dust Assumption: about 90% of mass of Galaxy is “dark matter” Matter that does not emitted radiation anywhere across the electromagnetic spectrum No type of common (baryonic) matter has such a property
3	Dark Matter Distribution in Spiral Galaxies Motion of stars and companion dwarf galaxies suggest that the gravitational force they experience is 10 times the mass of visible matter in our Galaxy.
4	Measuring Rotation in a Spiral Galaxy Doppler shifts for 21 cm line of neutral hydrogen A: 21 cm line is blue shifted indicating matter is approaching us B: 21 cm line is not Doppler shifted indicating matter is either moving across line of sight or stationary C: 21 cm line is red shifted indicating matter is receding from us
5	Weighing Galaxies Because orbital speeds of gas clouds tell us the amount of mass contained with their orbit, flat rotation curves imply that a great deal of matter lies far from the galactic center. Mass of a spiral galaxy with gas clouds orbiting at 200 km/s at a distance of 150,000 ly from its center must be at least 5x10¹¹ solar masses.
6	Mass-to-Light Ratio For our Galaxy, it takes 6 solar masses of stellar matter to produce 1 solar luminosity of radiation at the Sun’s location; thus, the Galaxy’s mass-to-light ratio is 6 within the Sun’s orbit. Indicates that large numbers of faint stars exist that contribute very little luminosity – red dwarfs Mass-to-light ratios for central portions of elliptical galaxies are larger, such as 10 – even more red dwarfs Studies of mass-to-light ratios extending to large distances from the galactic center yield values as high as 50 suggesting lots of “dark matter.” But this does not mean lots of red dwarf stars.
7	Dark Matter in Clusters 1933, Fritz Zwicky (1898-1974) noted there is not enough mass in form of galaxies to bind galaxies gravitationally into a cluster Attempts to estimate mass of an entire cluster of galaxies have led to conflicting figures Luminous mass of all galaxies in cluster amounts to only 3-5% of mass needed to provide gravitational stability Dynamical mass, found by analyzing observed radial velocity differences, is many times greater than luminous mass, obtained from light emitted
8	Intracluster Medium
9	Gravitational Lensing
10	Gravitational Lensing by Abell 2218
11	Dark Matter – Ordinary or Extraordinary In what form might this dark matter be? Intergalactic baryonic (protons and neutrons) matter lying between galaxies Cool atomic hydrogen Should emit 21-cm photons, but none observed Molecular hydrogen Should emit ultraviolet light detectable with orbiting observatories, but none observed Hot gas Should emit X-ray photons, but no 21-cm radiation Dozens of clusters are powerful X-ray sources, but estimates are that mass of hot gas is a few times mass of all cluster's galaxies Therefore, hot intergalactic gas is not a sufficient amount of mass
12	Dark Matter (cont’d) Baryonic matter associated with the galaxies in the cluster Sub-luminous galaxies Would have to be millions to billions of them for each bright galaxy Very unlikely that improving technology would not detect them Extended halos of galaxies Massive compact halo objects (MACHOS) known for our Galaxy Large central galaxies of cluster should have rapidly cannibalized other galaxies earlier in life of cluster; hence clusters ought to appear very different from way they do now Evidence exists that dark matter does occupy extended halos in galaxies Black holes Insufficient number of intense X-ray sources exist to believe that black holes constitute the dark matter
13	Massive Compact Halo Objects Red dwarfs, brown dwarfs, and Jupiter-size objects left over from formation of our Galaxy. Gravitational lensing could be used to find such objects and estimate their numbers. Number of lensing events found, but not enough to account for gravitational effect of dark matter.
14	Extraordinary Dark Matter Non-baryonic matter possibilities Neutrinos (weakly interacting low-mass particles) Known as the hot dark matter concept because neutrinos are fast-moving or very energetic particles Possess no electrical charge and hence can not emit electromagnetic radiation They never bound together with charged particles in nuclei-like structure so light emitting particles can not reveal their presence Interact with matter only through gravitational force (very small masses) and the weak nuclear force If a part of dark matter, they are a component lying outside galaxies Because of their very small masses, they must exist in much larger numbers than we suspect if they are to constitute all of the dark matter
15	Extraordinary Dark Matter (cont’d) WIMPS (weakly interacting massive particles) Known as the cold dark matter concept because they are massive and slow-moving or low-energy particles Because they are slow-moving, they can collect into galaxies where they attract baryonic matter to produce the light-emitting aspects of a galaxy In the formation of our Galaxy, baryonic matter settled toward the center and flattened into a disk Because WIMPS are weakly interacting, they remained more or less spherically distributed in an extended halo WIMPS possess no electrical charge so that they cannot radiate away their energy. Thus they maintain the original distribution of the protogalactic cloud
16	Scale-Size for Structure Observation shows us that existence covers an extremely large range in scale size From small-scale quantum world of fundamental particles and fields to Large-scale relativistic world of clusters of galaxies 10⁶¹ orders of magnitude
17	Clustering Size and Separation
18	Structure in the Universe Gravitational attraction within galaxies and clusters of galaxies is sufficient to retard the expansion of spacetime partially or completely Local Group moving away from center of Virgo Cluster at 400 km/s slower than predicted by Hubble’s Law Peculiar velocities are velocities of galaxies within clusters of galaxies (local velocity as opposed to Hubble expansion of space) Expansion of space is most important on scale sizes of the great voids lying between superclusters Action of dark matter in establishing structure Gravitational attraction of dark matter is responsible for holding galaxies together Dark matter also responsible for pulling galaxies together in clusters and larger structures, whose scale sizes are several hundreds of millions of light years across
19	Peculiar Velocities of Galaxies Flowing into Superclusters Each arrow shows velocity of a galaxy relative to our Galaxy at center Note galaxies are flowing toward high-density regions Probably superclusters in process of formation
20	Large Scale Structure in the Universe Blank features are immense cosmic voids Regions of space that contain almost no galaxies Easily 300 Mly across and possess volumes of 30 million Mly³ Existence of such realms that are apparently empty has a most profound significance
21	Large Scale Structure in the Universe
22	Large Scale Structure in the Universe
23	Supercomputer Simulation of Structure
24	Cosmological Principle 1933 E. A. Milne proposed that the large-scale distribution of matter was homogeneous based on the observed isotropy of the universe. Assumption consistent with postulates of special relativity. Assumption consistent with Hubble's law of recession. Has become basic postulate of cosmology, i.e., article of faith. Cosmological Principle - Since we observe that the universe is isotropic, all observers anywhere in space should see the universe in its essential features in the same way in all directions. All places are alike at any instant of cosmic time.
25	Friedmann’s Expanding Universes 1922 Alexander Friedmann and Georges Lemaitre, Catholic priest, in 1927 developed large class of model universes. Homogeneous, isotropic, expanding, and all contained matter. Friedmann universes George Gamov, Ralph Alpher, and Robert Herman, 1948, used Friedmann models to predict cosmic microwave background radiation (CMBR)
26	Cosmic Microwave Background Radiation 1965, Arno Penzias and Robert Wilson obtain first definitive evidence for cosmic microwave background radiation.
27	CMBR Temperature Variations
28	CMBR: The Universe When 380,000 Years Old (WMAP) H = present speed of universe’s expansion = 71 km/s/Mpc W_m = present density of matter (baryonic and dark) divided by value necessary to make universe flat = 0.27 W_L = present density of dark energy = 0.73 t₀ = 13.7 billion years t_dec = 379,000 years
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30	Possible Fates of the Universe Critical density (10^-29 g/cm³) Matter density that would cause the galaxies in the universe once infinitely separated to come to rest relative to each other Possible fates (Friedmann Universes) Recollapsing or closed universe - matter density greater than critical value and the expanding universe stops and begins to contract Critical or flat universe – matter density is just sufficient such that galaxies will come to rest relative to each other when infinitely far apart in the infinite future Coasting or open universe - matter density less than critical value and the expanding universe continues to expand forever Accelerating – an additional force (Einstein’s cosmological constant) beside gravity is operating that is either attractive (expansion slows with time) or repulsive (expansion speeds up with time)
31	Distant White Dwarf Supernovae Hubble Space Telescope image of white dwarf supernova in very distant galaxy. Note redness of the galaxy. White dwarf supernovae are good standard candles visible in the most distant parts of the universe. Can be used to measure rate at which gravity is slowing expansion of the universe.
32	Mysterious Acceleration of Spacetime Expansion Studies of white dwarf supernovae in very distant galaxies suggest that the expansion of spacetime in the universe is speeding up (accelerating) rather than the expected slowing down. Quantity responsible for the acceleration is now called dark energy. Acts much the way Einstein’s cosmological constant behaves in the computation of model universes.
33	Possible Expansion Models
34	The Big Picture Measurements of the mass and luminosity of galaxies and galaxy clusters indicate that they contain far more mass in dark matter than in stars. Despite dark matter’s great abundance in the universe, we do not know what it is. Superclusters, walls, and voids much larger than clusters of galaxies extend many millions of light-years across the universe. They probably began as slight enhancements in the density of dark matter and are still forming. Dark matter holds the key to the fate of the universe. There appears to be insufficient dark matter to stop the expansion of space. Discovery of the possible accelerating expansion of space supports a fate of continued expansion infinitely far into the future.