Transcript Chapter 14

 A galaxy is a large collection of stars
and gas.
 A galaxy like the Milky Way contains some
10 billion stars.
 There are hundreds of billions of galaxies in
the universe.
 Cosmology is the study of the universe,
including
• Structure: how matter is arranged.
• History: how stars, galaxies, and structure
change.
• Origins: conditions at early times.
• Fate: the ultimate future of the universe.
 Cosmological principle: the physical laws that
apply to our part of the universe apply to all
parts of the universe.
 Fundamental theory that has been tested.
 Homogeneous: generally the same in all
places.
 Isotropic: the same in all directions.
 True on large scales.
 We live in an expanding universe.
• This is an absolutely fundamental idea!
 It follows from two observations:
• Galaxies are moving away from us.
• Their speeds are proportional to their
distances:
Galaxies farther away are moving away more
quickly.
 We’ll discuss each consideration at length.
 We measure speeds with the Doppler shift.
 All galaxies except the nearest have a
redshift: observed wavelength > rest
wavelength.
 Redshifted spectral lines = movement away.
 We define a number z for the redshift.
• The larger z is, the faster the galaxy is moving
away.
 If we measure distances and velocities, we
find Hubble’s law.
• The velocity at which a galaxy is moving away
from us is proportional to the distance of that
galaxy.
• A galaxy twice as far away from us is moving
away twice as fast.
 Hubble constant (H0): constant of
proportionality for this relationship.
 What does this mean when considering
the cosmological principle?
 It might appear that we are in the center of
the universe, with all galaxies moving away.
 This is incorrect! There is no center.
 Simple model: paper clips on a rubber band.
 All observers see the same view.
 All see other galaxies moving away, with the
ones farther away moving more quickly.
 The universe is expanding uniformly.
 The Hubble constant is a fundamental
number: it tells us the age of the universe.
 Create a distance ladder to find distances
using objects with known luminosity.
 Type Ia supernovae are especially good
distance indicators.
 Very luminous standard candles.
 H0 currently: 72 (km/s)/Mpc
 Distant galaxies have a large look-back time.
 Light has a large, but finite speed; it takes
27,000 years for the light from an object
27,000 ly away to reach us.
 Galaxies will be
farther apart in the
future.
 Galaxies were closer
together in the past.
 Hubble time: time
when separation
between galaxies
was zero.
• Estimate of age of
the universe.
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The Hubble time was 13.6 billion years ago.
This moment is called the Big Bang.
Galaxies are not flying away from each other.
Space itself is stretching or expanding.
 The scale factor is
used to describe
the universe’s
expansion.
 A larger scale
factor means the
universe has
grown.
 The expansion does not affect atoms, stars,
or anything else, including laws of physics.
 Everything in the universe was once in a tiny
volume! The Big Bang happened everywhere.
 Redshifts of galaxies
are not due to
Doppler shifts.
 The light is
“stretched out” as it
travels through the
expanding universe.
• Cosmological
redshift.
 More travel = greater
redshift.
 If all matter is in a small volume, it means
conditions were very hot.
 Due to expansion, light redshifted, and
temperatures dropped.
 Prediction: a blackbody spectrum uniformly
redshifted by the expansion of the universe to
a temperature of about 5–10 Kelvin.
 Found in 1965 by
Penzias and Wilson
 Form: a blackbody
spectrum with a
temperature of about
3 K.
 The sky faintly glows
in microwaves.
• Cosmic microwave
background (CMB)
radiation.
 Microwaves are
from when the
universe was hot
and ionized.
 At several hundred
thousand years, the
temperature cooled
so protons and
electrons could form
neutral H atoms.
• Recombination.
 Then, light was no
longer blocked
from its travel by all
of the matter.
 The light could
travel freely, and
cooled
by a factor of about
1,000 to about 2.7
K, as confirmed by
satellite data.
 Before
recombination,
everything would
have been much
hotter and more
dense.
 At high densities,
nuclear reactions
occur.
 Big Bang
nucleosynthesis
produces mainly
hydrogen and
helium.
 Prediction: 24%
of matter should
be helium.
 In fact, this is
observed!
Suppose we observed that there were many more distant
galaxies in the northern half of the sky than in the southern
half. Which statement would be true about the universe?
A.
B.
C.
D.
It is homogeneous and isotropic.
It is homogeneous but not isotropic.
It is isotropic but not homogeneous.
It is neither homogenous nor isotropic.
The value of H0 is about 72 km/sec/Mpc. Suppose it were
twice as big. Compared to our current estimates, what
would the age of the universe be?
A. The same as now.
B. Younger.
C. Older.
Name one fact below that by itself does NOT support our
idea that the Big Bang happened.
A. The existence of the cosmic background radiation.
B. That 24% of matter is helium.
C. That the universe is expanding.