Today: Start Ch. 18: Cosmology. Homework # 5 due next Wed. (HW #6 is online)

Transcription

1 Today: Start Ch. 18: Cosmology Homework # 5 due next Wed. (HW #6 is online)

2 Dark Matter! A rotation curve is a graph of how fast a something is rotating, as a function of distance from the center.! We can make a rotation curve for anything that s rotating. The outside of a merry-go-round rotates faster than the inside.

3 Rotation Curves! We can graph orbital speed vs. distance for our Solar System.! This Rotation curve tells us about the mass inside the orbit. The farther from the Sun, the slower things are rotating. Rotation speed falls off with distance This implies that most of the mass in the Solar System is inside the Sun

4 This Spiral galaxy is about 400 Million l.y. away. Around its center we see 4 bright sources. A Galaxy. How fast is it rotating?

5 Dark Matter Vera Rubin ( ) was the first person to measure the Rotation Curve of a galaxy In the 1970 s she measured the Andromeda galaxy s rotation She discovered something very unusual.. The rotation speed did not fall off with distance; instead it was flat Vera Rubin Andromeda Galaxy & rotation curve data

6 ! Many galaxies have been found with flat rotation curves! This implies the galaxy s mass is not concentrated in the center.

7 Dark Matter! Galaxies are rotating so fast they should fly apart!! But they aren t flying apart, so they must be held together by gravity from a very large mass.! However, we can t see this mass!! This indicates that up to 90% of the mass of the universe is dark matter!! The dark matter is probably distributed throughout the halo.! If so much of the universe is made of dark matter, then what is it??

8 What is Dark Matter? Some Possibilities:! A huge number of brown dwarfs (unlikely)! Dead stars: old white dwarfs & neutron stars! Small black holes.! Invisible particles, like neutrinos.! We don t really know.! The nature of dark matter is one of the great unsolved mysteries of astronomy

9 Gravitational Lensing! A galaxy s image can be be distorted by mass! Space is curved by mass. (Einstein)! More mass will cause more distortion.! Sometimes a whole cluster of galaxies acts as a lens, distorting the image of what s behind it.! This lets us measure the mass of the galaxy cluster.

10 This Spiral galaxy is about 400 Million l.y. away. Around its center we see 4 bright sources.

11 These four objects are really all one object! (A quasar 8 billion light years away!)

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14 A galaxy cluster lensing another cluster

15 Dark Matter is Everywhere!! Grav. lensing observations also measure the amount of mass.! This mass cannot be explained by the stars and gas alone.! This is further proof that galaxies contain a large amount of dark matter. We now realize that every galaxy is embedded in a halo of dark matter. Our Galaxy & Halo What we see is only the tip of a massive iceberg.

16 Chapter : 18 Cosmology (Sec. 18:5,18:12 optional)

17 Cosmology κόσμος " Cosmology attempts to answer big questions: " What was the Origin of the Universe? " What is Fate of the Universe? " Is the Universe infinite or limited? " The Universe ( Cosmos ) means everything: all matter, all energy, even all of space. " There is no edge or wall to the universe. " A wall divides inside from outside, but there is nothing outside the universe.

18 The Expanding Universe Space itself is a significant part of the universe. (In the theory of relativity, space can be curved) In the universe, space is expanding, carrying the galaxies along! And redshift-ing light from the galaxies. demo

19 The Age of the Universe! The universe is expanding now.! It must have been smaller in the past! Some time in the distant past, everything in the universe would converge to a single point.! How far back must we go?! If we can determine how fast the universe is expanding, we can determine its age.! The Hubble Constant, H o measures the rate of expansion.

20 Hubble s Law: Distant galaxies move away fastest Velocity (v) is proportional to Distance (d): v = H o d The Hubble Constant was measured after decades of observation: H 0 = 70 km/s/mpc Velocity (km/s) Distance (Mpc) This allows us to calculate the age of the universe!

21 The Age of the Universe! Imagine a galaxy at a distance d, moving away at speed v! At some time in the past (t) it was in the same place as our galaxy.! This time is the age of the universe.! t = d / v! But from Hubble s law we know that: v = H o x d! So the age of the universe is:! t = d / (H o x d) = 1/H o! H o =70 km/s/mpc! t = 1/H o = about 14 Billion Years