Final review: Milky Way Galaxies Active galaxies Cosmology: –The future of the universe –The beginning of the universe Test schedule (in LL203) –8 am class:

Final review:• Milky Way• Galaxies• Active galaxies• Cosmology:

– The future of the universe– The beginning of the

universe

• Test schedule (in LL203)– 8 am class: Wednesday,

4/29, 7-9– 9 am class: Tuesday, 4/28,

9:30-11:30

Our place in the galaxy: Early views

The solution: Globular clusters and variable stars

Observing the Milky Way

The sky at 21 centimeters

Rotation curves

Dark matter

What is dark matter?

• We don’t know. This is actually one of the most important unanswered questions in modern astronomy.

• A few ideas:– MACHOS– neutrinos– WIMPS

Sagittarius A*

Types of galaxies

Distance ladder

The Hubble law

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Hubble law :

v = H0d

v = recessional velocity of galaxy

H0 = Hubble constant

d = distance to galaxy

The Coma cluster

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Structure in nearby universe

Galaxy interactions and spiral arms

• Close encounters between galaxies provide another way of forming spiral arms.– A simulation is found on the

text website.

• Some astronomers argue that the spiral arms in the Milky Way are due to interactions with the Large Magellanic Cloud.

Galaxy formation

Quasars

• These objects were called quasi-stellar radio sources which was soon shortened to quasars.– Soon many starlike objects with large redshifts

were discovered that emitted no radio waves.– Called “radio-quiet” quasars and comprise 90%

of all known quasars.

• All quasars have large redshifts meaning they are very distant.

Quasar distribution

• There are no quasars with small redshifts.– This means there are

no nearby quasars.– Nearest is about 800

million ly from Earth.

• Quasars were common in the distant past, but there a none in present-day universe.

The expanding universe

Observations of the CMB

Density of the universe

• A flat universe is a special case with a specific density.– Call this density the critical density or c.

• Spherical: 0> c, • Flat: 0= c, • Hyperbolic: 0<c,

• Alternatively, we define the curvature of the universe by the ratio of the combined average mass density to the critical density. c

Results of curvature measurements

• We find that 0=1.0 with an uncertainty of 2%.– This says the universe is flat. (0= c)

• Unfortunately, m is measured to be only about 24% of the critical density.– Radiation density is insignificant.

• Radiation, matter and dark matter acount for 24% of the total density of the universe. What accounts for the rest?– Must be some form of energy we cannot detect gravitationally or

electromagnetically.– Dark energy!

Actual measurements

The inflationary model

• Theory suggests that the universe experienced a brief period of inflation shortly after the Planck time.– Planck time: First 1.35x10-43 s

of the lifetime of the universe. Before this time the laws of physics as we know them didn’t apply.

• During inflation the universe expanded by a factor of 1050 in about 10-32 s!

Uncertainty principle for mass and time

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Δm × Δt =h

2πc 2

Δm = uncertainty in mass

Δt = uncertainty in time

h = Planck's constant

c = speed of light

Inflation: From virtual to real particles

Nucleosynthesis

• At t=3 min the universe was cool enough for protons to combine to form helium.– Same process as in the center of a

star.

• Lithium (3 protons) and beryllium (4 protons) also formed this way.

• At t=15 min the universe was too cool for this to happen and no further nucleosynthesis occurred until stars formed.

Population III stars

• Population II stars can’t be the oldest stars in the universe.– The original stars were

Population III stars.– These stars had masses

from 30 to 1000 M.– The death of these stars

provided matter incorporated into next generation of stars.