Chapter 16 Dark Matter, Dark Energy, & The Fate of the Universe

16.2 Evidence for Dark Matter Our Goals for Learning • What is the evidence for dark matter in galaxies and in clusters of galaxies? • Does dark matter really exist? • What might dark matter be made of?

What is the evidence for dark matter in galaxies?

We measure the mass of the solar system using the orbits of planets Orb. Period Avg. Distance Or for circles: Orb. Velocity Orbital Radius

Rotation curve A plot of orbital velocity versus orbital radius Solar system’s rotation curve declines because Sun has almost all the mass

Which person has the largest rotational velocity on this merry-go-round? That is, who moves fastest? A (close to center), B (halfway to edge), C (close to edge), or D (all the same)? Get your vote ready…

Which person moves the fastest on this merry-go-round? A (close to center) B (halfway to edge) C (close to edge) D (all the same) :00

Rotation curve of merry-go-round rises with radius

Rotation curve of Milky Way stays flat with distance Milky Way is not a solid body, but its mass must be more spread out than mass in the solar system (spiral anim.)

Mass in Milky Way is spread out over a larger region than the stars (last anim.)

Mass in Milky Way is spread out over a larger region than the stars Most of the Milky Way’s mass seems to be dark matter!

Mass within Sun’s orbit = 100 billion Suns: 1.0 x 1011 MSun Total mass = 1,000 billion Suns (one trillion): ~1012 MSun

The visible portion of a galaxy lies deep in the heart of a large halo of dark matter

We can measure rotation curves of other spiral galaxies using the Doppler shift of the 21-cm line of atomic H

Is it possible to measure rotation curves for spiral galaxies that we see face-on? Sure. They’re just another spiral galaxy. Yes. In fact, they’re the easiest kind to measure, because you can see the spiral arms move. No. Their rotation doesn’t cause a Doppler shift in our direction. :00

Spiral galaxies all tend to have flat rotation curves indicating large amounts of dark matter

Broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting These galaxies also have dark matter

What would you conclude about a spiral galaxy whose rotational velocity rises steadily with distance beyond the visible part of its disk? Its mass is concentrated at the center It rotates like the solar system It’s especially rich in dark matter It’s just like the Milky Way :00

What do we mean by dark matter and dark energy?

16.1 Unseen Influences in the Cosmos 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 thought to be the source of a repulsive force causing the expansion of the universe to accelerate

Contents of Universe (% of total Energy) “Normal” Matter: ~ 4.4% Normal Matter inside stars: ~ 0.6% Normal Matter outside stars: ~ 3.8% Dark Matter: ~ 25% Dark Energy ~ 71%

Activity 34 part II, pages 118-119

4A: What does a solid-body rotation curve look like? Rises quickly (within 5 kpc), then levels off Straight line from lower left to upper right :00

4B: Which of the curves in Fig 4B: Which of the curves in Fig. 2 does the Milky Way’s rotation curve look like? Point Mass Disk Mass Solid Mass None of the above :00

5A: If solid-body rotation is depicted using arrows on a disk (a la Fig. 3), then… The shortest arrow is at the center, and the longest at the edge The longest arrow is at the center, and the shortest at the edge All the arrows are the same length :00

6B: The Sun is 7. 5 kpc from the center of the Galaxy 6B: The Sun is 7.5 kpc from the center of the Galaxy. The rotational velocity of the Sun around the center of the Galaxy is: 250 km/s 210 km/s 190 km/s None of the above :00

Point Mass Disk Mass Solid Mass 7ABC: Which rotation curve in Fig. 2 is most similar to the real curve in Fig. 6? Point Mass Disk Mass Solid Mass :00

7D: What’s wrong with Xavier’s reasoning? Spiral arms do wind up or unwind Spiral arms are made of gas and dust; it’s the stars that orbit as a solid object The matter in spiral arms doesn’t stay there – the arms are “traffic jams” where different stars and gas clouds keep piling up :00

7F: It appears there is a major component of mass that we cannot see, called… WIMPs MACHOs Dark matter Dark energy Baryonic matter :10