Transcript Lecture 26 The Dark Side of the Universe Announcements • Homework Read Ch. 24: Life in the Universe Mastering Astronomy: Chapter 24 Due Friday, Dec 6, at.

Lecture 26
The Dark Side
of
the Universe
Announcements
• Homework
Read Ch. 24: Life in the Universe
Mastering Astronomy: Chapter 24
Due Friday, Dec 6, at 6pm
• Today’s lecture covers Ch. 22:
All you need to know is in the lecture.
• Winter Break: Read Chapter 22 for fun.
Final Exam:
Thursday, Dec 19, 11:30-2:30
RSF Field House: Main Floor Gym
• Multiple Choice
• All Chapters: 1-5, 9-14, 24 & All Lectures.
Emphasis on Chapters 12, 13, 14, 24
• Review Sessions:
Tue and Thu (Dec 10 & 12) at 5-6:30
in 2040 VLSB
Come with questions !
Lecture 26
The Dark Side
of
the Universe
Most of the
Universe is Dark
You, me,
Earth,
Planets, stars
How do we know the
Universe is Mostly Dark ?
Example 1: Gravity by Central Mass (Sun)
determines orbital speed of planets.
Mercury
Venus
Earth
GM(< r)
V =
G M / Distancer
2
Orbital Speed =
Mars
Jupiter
Solar System:
“Keplerian orbits”
How do we know the
Universe is Mostly Dark ?
Gravity by Central Mass (galaxy)
determines orbital speed of stars.
Orbital Speed =
Galaxies:
“Flat rotation curve”
G M / Distance
How do we know the
Universe is Mostly Dark ?
Gravity by Central Mass (galaxy)
determines orbital speed of stars.
Orbital Speed =
G M / Distance
Speed doesn’t fall !
Mass, M, must be
increasing outward!
Galaxies:
“Flat rotation curve”
How do we know the
Universe is Mostly Dark ?
Example 2: Gravitational Bending of Light by Mass
Light Source
Telescope
How do we know the
Universe is Mostly Dark ?
How do we know the
Universe is Mostly Dark ?
Four images of the
same galaxy
Four images of the
same galaxy
How do we know the
Universe is Mostly Dark ?
What can dark matter be?
Planets?
Not enough mass: even Jupiter weighs only
0.1% of the Sun
What can dark matter be?
Black Holes?
Graveyards of massive stars. But massive stars
are rare and made of normal atoms.
Once formed, black holes suck in normal matter.
So you can’t hide much dark matter in it.
What can dark matter be?
Anti-matter?
Antimatter and matter have identical mass but opposite
quantum numbers (electric charge, spin…)
Antimatter and matter annihilate each other into flashes
of easily detectable gamma rays
What can dark matter be?
Top candidate:
Elementary Particles:
Yet to be discovered !
17
Dark Energy
18
Supernova Explosions
as
Standard Candles
Type Ia supernovae, which give off a standard amount of light.
The distance to the galaxy that contains the supernova: Compare how bright they know the explosion should be
with how bright the explosion appears. Using the inverse square law of light, they can
compute the distance to the supernova and thus to the supernova's home galaxy.
How do we know the Universe is Dark?
Example 4: Distant supernovae
After
Before
20
How do we know the Universe is Dark?
Example 4: Distant supernovae
are standard candles
22
23
24
Most of the
Universe is Dark
You, me,
Earth,
Planets, stars
How do we know the Universe is Dark?
Example 3: Cosmic Microwave Background
CMB: very cold (-270.275 C, -454.495 F) and nearly
uniform relic radiation left over from the hot big bang
How do we know the Universe is Dark?
Example 3: Cosmic Microwave Background
If you had microwave eyes:
(1965)
(1992)
28
(2010)
How do we know the Universe is Dark?
White points:
WMAP (2010)
7-year data
Red curve: Theoretical prediction for a universe made of
73% dark energy, 23% dark matter, 4% atoms
Accelerating Expansion of the Universe:
Dark Energy
30
How do we know
the Universe is
Dark?
Kowalski et al (2008)
Supernovae probe
Dark Energy
CMB probes
Concordance region:
73% dark energy
23% dark matter
4% atoms
Dark Matter