Chapter 22 Dark Matter Dark Energy and the
- Slides: 55
Chapter 22 Dark Matter, Dark Energy, and the Fate of the Universe © 2010 Pearson Education, Inc.
22. 1 Unseen Influences in the Cosmos Our goals for learning: • What do we mean by dark matter and dark energy? © 2010 Pearson Education, Inc.
What do we mean by dark matter and dark energy? © 2010 Pearson Education, Inc.
Unseen Influences Dark Matter: An undetected form of mass that emits little or no light, but whose existence we infer from its gravitational influence Dark Energy: An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate © 2010 Pearson Education, Inc.
Contents of Universe • “Ordinary” matter: ~ 4. 4% – Ordinary matter inside stars: ~ 0. 6% – Ordinary matter outside stars: ~ 3. 8% • Dark matter: • Dark energy © 2010 Pearson Education, Inc. ~ 23% ~ 73%
22. 2 Evidence for Dark Matter Our goals for learning: • What is the evidence for dark matter in galaxies? • What is the evidence for dark matter in clusters of galaxies? • Does dark matter really exist? • What might dark matter be made of? © 2010 Pearson Education, Inc.
What is the evidence for dark matter in galaxies? © 2010 Pearson Education, Inc.
We measure the mass of the solar system using the orbits of planets: • orbital period • average distance For circles: • orbital velocity • orbital radius © 2010 Pearson Education, Inc.
Rotation curve A plot of orbital velocity versus orbital radius The solar system’s rotation curve declines because the Sun has almost all the mass. © 2010 Pearson Education, Inc.
Who has the largest orbital velocity? A, B, or C? © 2010 Pearson Education, Inc.
Who has the largest orbital velocity? A, B, or C? Answer: C © 2010 Pearson Education, Inc.
The rotation curve of a merry-goround rises with radius. © 2010 Pearson Education, Inc.
The rotation curve of the Milky Way stays flat with distance. Mass must be more spread out than in the solar system. © 2010 Pearson Education, Inc.
Mass in the Milky Way is spread out over a larger region than its stars. Most of the Milky Way’s mass seems to be dark matter! © 2010 Pearson Education, Inc.
Mass within the Sun’s orbit: 1. 0 1011 MSun Total mass: ~1012 MSun © 2010 Pearson Education, Inc.
The visible portion of a galaxy lies deep in the heart of a large halo of dark matter. © 2010 Pearson Education, Inc.
We can measure the rotation curves of other spiral galaxies using the Doppler shift of the 21 -cm line of atomic hydrogen. © 2010 Pearson Education, Inc.
Spiral galaxies all tend to have flat rotation curves, indicating large amounts of dark matter. © 2010 Pearson Education, Inc.
Broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting. These galaxies also have dark matter. © 2010 Pearson Education, Inc.
What is the evidence for dark matter in clusters of galaxies? © 2010 Pearson Education, Inc.
We can measure the velocities of galaxies in a cluster from their Doppler shifts. © 2010 Pearson Education, Inc.
The mass we find from galaxy motions in a cluster is about 50 times larger than the mass in stars! © 2010 Pearson Education, Inc.
Clusters contain large amounts of X ray-emitting hot gas. Temperature of hot gas (particle motions) tells us cluster mass: 85% dark matter 13% hot gas 2% stars © 2010 Pearson Education, Inc.
Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster’s mass. © 2010 Pearson Education, Inc.
© 2010 Pearson Education, Inc.
All three methods of measuring cluster mass indicate similar amounts of dark matter in galaxy clusters. © 2010 Pearson Education, Inc.
Does dark matter really exist? Insert TCP 6 e Figure 22. 11 unannotated © 2010 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. © 2010 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. © 2010 Pearson Education, Inc.
Some observations of the universe are very difficult to explain without dark matter. © 2010 Pearson Education, Inc.
What might dark matter be made of? © 2010 Pearson Education, Inc.
How dark is dark matter? … not as bright as a star. © 2010 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 © 2010 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 © 2010 Pearson Education, Inc. The best bet
MACHOs occasionally make other stars appear brighter through lensing… … but there are not enough lensing events to explain all the dark matter. © 2010 Pearson Education, Inc.
Why Believe in WIMPs? • There’s not enough ordinary matter. • WIMPs could be left over from Big Bang. • Models involving WIMPs explain how galaxy formation works. © 2010 Pearson Education, Inc.
22. 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? © 2010 Pearson Education, Inc.
What is the role of dark matter in galaxy formation? © 2010 Pearson Education, Inc.
Gravity of dark matter is what caused protogalactic clouds to contract early in time. © 2010 Pearson Education, Inc.
Insert TCP 6 e Figure 22. 2 © 2010 Pearson Education, Inc. WIMPs can’t collapse to the center because they don’t radiate away their orbital energy.
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. © 2010 Pearson Education, Inc.
What are the largest structures in the universe? © 2010 Pearson Education, Inc.
Maps of galaxy positions reveal extremely large structures: superclusters and voids. © 2010 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 gravity of dark matter pulls mass into denser regions—the universe grows lumpier with time. © 2010 Pearson Education, Inc.
Models show that gravity of dark matter pulls mass into denser regions—universe grows lumpier with time. © 2010 Pearson Education, Inc.
Structures in galaxy maps look very similar to the ones found in models in which dark matter is WIMPs. © 2010 Pearson Education, Inc.
22. 4 The Fate of the Universe Our goals for learning: • Will the universe continue expanding forever? • Is the expansion of the universe accelerating? © 2010 Pearson Education, Inc.
Will the universe continue expanding forever? © 2010 Pearson Education, Inc.
The fate of the universe depends on the amount of dark matter. © 2010 Pearson Education, Inc.
Since the amount of dark matter is ~25% of the critical density, we expect the expansion of the universe to overcome its gravitational pull. © 2010 Pearson Education, Inc.
In fact, the expansion appears to be speeding up! Dark energy? © 2010 Pearson Education, Inc.
Estimated age depends on the amount of both dark matter and dark energy. © 2010 Pearson Education, Inc.
Is the expansion of the universe accelerating? © 2010 Pearson Education, Inc.
Insert TCP 6 e Figure 20. 14 The brightness of distant white dwarf supernovae tells us how much the universe has expanded since they exploded. © 2010 Pearson Education, Inc.
An accelerating universe best fits the supernova data. © 2010 Pearson Education, Inc.
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