The Dark Side of

the Universe

Lecture 26

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 !

The Dark Side of

the Universe

Lecture 26

Most of the Universe is Dark

You, me, Earth,

Planets, stars

Example 1: Gravity by Central Mass (Sun) determines orbital speed of planets.

How do we know the Universe is Mostly Dark ?

Solar System:“Keplerian orbits”

Earth

Jupiter

Mercury

Orbital Speed = G M / Distance

Venus

Mars

Galaxies:“Flat rotation curve”

How do we know the Universe is Mostly Dark ?

Gravity by Central Mass (galaxy) determines orbital speed of stars.

Orbital Speed = G M / Distance

Galaxies:“Flat rotation curve”

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!

Example 2: Gravitational Bending of Light by Mass

How do we know the Universe is Mostly Dark ?

Light Source

Telescope

How do we know the Universe is Mostly Dark ?

Four images of the same galaxy

How do we know the Universe is Mostly Dark ?

Four images of the same galaxy

How do we know the Universe is Mostly Dark ?

What can dark matter be?

Not enough mass: even Jupiter weighs only

0.1% of the Sun

Planets?

What can dark matter be?

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.

Black Holes?

What can dark matter be?

Antimatter and matter annihilate each other into flashes of easily detectable gamma rays

Anti-matter?

Antimatter and matter have identical mass but opposite quantum numbers (electric charge, spin…)

What can dark matter be?Top candidate:

Elementary Particles:

Yet to be discovered !

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Dark Energy

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Supernova Explosionsas

Standard Candles

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.

Type Ia supernovae, which give off a standard amount of light.

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Example 4: Distant supernovae

After

Before

How do we know the Universe is Dark?

Example 4: Distant supernovae are standard candles

How do we know the Universe is Dark?

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23

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Most of the Universe is Dark

You, me, Earth,

Planets, stars

CMB: very cold (-270.275 C, -454.495 F) and nearly uniform relic radiation left over from the hot big bang

Example 3: Cosmic Microwave Background

How do we know the Universe is Dark?

(1965)

If you had microwave eyes:

Example 3: Cosmic Microwave Background

How do we know the Universe is Dark?

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(1992)

(2010)

White points:WMAP (2010)7-year data

Red curve: Theoretical prediction for a universe made of73% dark energy, 23% dark matter, 4% atoms

How do we know the Universe is Dark?

Accelerating Expansion of the Universe:Dark Energy

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Concordance region:

73% dark energy23% dark matter

4% atoms

How do we know

the Universe is Dark?

Kowalski et al (2008)

Dark Energy

Dark Matter

Supernovae probe

CMB probes