Chapter 16 Dark Matter Dark Energy and the Fate of the Universe Copyright 2009 Pearson Education Inc What is the evidence for dark matter in clusters of galaxies Copyright 2009 Pearson Education Inc We can measure the velocities of galaxies in a cluster from their Doppler shifts Copyright 2009 Pearson Education Inc The mass we find from galaxy motions in a cluster is about 50 times larger than the mass in stars Copyright 2009 Pearson Education Inc Clusters contain large amounts of X ray emitting hot gas The temperature of hot gas particle motions tells us cluster mass 85 dark matter 13 hot gas 2 stars Copyright 2009 Pearson Education Inc Gravitational lensing the bending of light rays by gravity can also tell us a cluster s mass Copyright 2009 Pearson Education Inc A gravitational lens distorts our view of things behind it Gravitational Lensing Illustrated Copyright 2009 Pearson Education Inc All three methods of measuring cluster mass indicate similar amounts of dark matter Copyright 2009 Pearson Education Inc Thought Question What kind of measurement does not tell us the mass of a cluster of galaxies A B C D Measuring velocities of cluster galaxies Measuring total mass of the cluster s stars Measuring temperature of its hot gas Measuring distorted images of background galaxies Copyright 2009 Pearson Education Inc Thought Question What kind of measurement does not tell us the mass of a cluster of galaxies A B C D Measuring velocities of cluster galaxies Measuring total mass of the cluster s stars Measuring temperature of its hot gas Measuring distorted images of background galaxies Copyright 2009 Pearson Education Inc Does dark matter really exist Copyright 2009 Pearson Education Inc Our Options 1 Dark matter really exists and we are observing the effects of its gravitational attraction 2 Something is wrong with our understanding of gravity causing us to mistakenly infer the existence of dark matter Copyright 2009 Pearson Education Inc Our Options 1 Dark matter really exists and we are observing the effects of its gravitational attraction 2 Something is wrong with our understanding of gravity causing us to mistakenly infer the existence of dark matter Because gravity is so well tested most astronomers prefer option 1 Copyright 2009 Pearson Education Inc What might dark matter be made of Copyright 2009 Pearson Education Inc How dark is it Copyright 2009 Pearson Education Inc How dark is it not as bright as a star Copyright 2009 Pearson Education Inc Two Basic Options Ordinary Dark Matter MACHOS Massive Compact Halo Objects dead or failed stars in halos of galaxies Extraordinary Dark Matter WIMPS Weakly Interacting Massive Particles mysterious neutrino like particles Copyright 2009 Pearson Education Inc Two Basic Options Ordinary Dark Matter MACHOS Massive Compact Halo Objects dead or failed stars in halos of galaxies Extraordinary Dark Matter WIMPS Weakly Interacting Massive Particles mysterious neutrino like particles Copyright 2009 Pearson Education Inc The Best Bet MACHOs occasionally make other stars appear brighter through lensing Copyright 2009 Pearson Education Inc MACHOs occasionally make other stars appear brighter through lensing but there are not enough lensing events to explain all the dark matter Copyright 2009 Pearson Education Inc Why believe in WIMPs There s not enough ordinary matter WIMPs could be left over from the Big Bang Models involving WIMPs explain how galaxy formation works Copyright 2009 Pearson Education Inc What have we learned What is the evidence for dark matter in galaxies Rotation curves of galaxies are flat indicating that most of their matter lies outside their visible regions What is the evidence for dark matter in clusters of galaxies Masses measured from galaxy motions temperature of hot gas and gravitational lensing all indicate that the vast majority of matter in clusters is dark Copyright 2009 Pearson Education Inc What have we learned Does dark matter really exist Either dark matter exists or our understanding of our gravity must be revised What might dark matter be made of There does not seem to be enough normal baryonic matter to account for all the dark matter so most astronomers suspect that dark matter is made of nonbaryonic particles that have not yet been discovered Copyright 2009 Pearson Education Inc 16 3 Structure Formation Our goals for learning What is the role of dark matter in galaxy formation What are the largest structures in the universe Copyright 2009 Pearson Education Inc What is the role of dark matter in galaxy formation Copyright 2009 Pearson Education Inc Gravity of dark matter is what caused protogalactic clouds to contract early in time Copyright 2009 Pearson Education Inc WIMPs can t contract to the center because they don t radiate away their orbital energy Copyright 2009 Pearson Education Inc Dark matter is still pulling things together After correcting for Hubble s law we can see that galaxies are flowing toward the densest regions of space Copyright 2009 Pearson Education Inc What are the largest structures in the universe Copyright 2009 Pearson Education Inc Maps of galaxy positions reveal extremely large structures superclusters and voids Copyright 2009 Pearson Education Inc Time in billions of years 0 5 2 2 5 9 8 6 13 7 13 35 70 93 140 Size of expanding box in millions of light years Models show that the gravity of dark matter pulls mass into denser regions the universe grows lumpier with time Copyright 2009 Pearson Education Inc Models show that gravity of dark matter pulls mass into denser regions the universe grows lumpier with time Large Scale Structure of the Universe Copyright 2009 Pearson Education Inc Structures in galaxy maps look very similar to the ones found in models in which dark matter is WIMPs Copyright 2009 Pearson Education Inc What have we learned What is the role of dark matter in galaxy formation The gravity of dark matter seems to be what draws gas together into protogalactic clouds initiating the process of galaxy formation What are the largest structures in the universe Galaxies appear to be distributed in gigantic chains and sheets that surround great voids Copyright 2009 Pearson Education Inc 16 4 The Fate of the Universe Our goals for learning Will the universe continue expanding forever Is the expansion of the universe accelerating Copyright 2009 Pearson Education Inc Will the universe continue expanding forever Copyright 2009 Pearson Education Inc Does the universe have
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