Transcript Spheroid Dominates Hubble’s galaxy classes Disk Dominates Spiral galaxies are often found in groups of galaxies (up to a few dozen galaxies)

Spheroid
Dominates
Hubble’s galaxy classes
Disk
Dominates
Spiral
galaxies are
often found
in groups of
galaxies
(up to a few
dozen
galaxies)
Elliptical
galaxies are
much more
common in
huge clusters
of galaxies
(hundreds to
thousands of
galaxies)
Maps of galaxy positions reveal extremely large
structures: superclusters and voids
Time in billions of years
0.5
2.2
5.9
8.6
13.7
13
35
70
93
140
Size of expanding box in millions of lt-yrs
Models show that gravity of dark matter pulls mass into
denser regions – universe grows lumpier with time
Structures in galaxy maps look very similar to the ones
found in models in which dark matter is WIMPs
Cepheid
variable stars
are very
luminous
Cepheid variable stars with longer periods have greater
luminosities
White-dwarf
supernovae
can also be
used as
standard
candles
Step 5
Apparent
brightness of
white-dwarf
supernova tells
us the distance
to its galaxy
(up to 10
billion lightyears)
We measure galaxy distances using
a chain of interdependent
techniques
Hubble settled the debate by measuring the distance to the
Andromeda Galaxy using Cepheid variables as standard
candles
The spectral features of virtually all galaxies are
redshifted  They’re all moving away from us
Hubble’s Law:
velocity = H0 x distance
Distances of
farthest
galaxies are
measured
from
redshifts
One example of something that expands but has no center
or edge is the surface of a balloon
Cosmological Principle
The universe looks about the same no matter
where you are within it
• Matter is evenly distributed on very large scales
in the universe
• No center & no edges
• Not proved but consistent with all observations to
date
Expansion stretches photon wavelengths causing a
cosmological redshift directly related to lookback time
Fate of
universe
depends
on the
amount
of dark
matter
Lots of
dark matter
Critical
density of
matter
Not enough
dark matter
Brightness of distant white-dwarf supernovae tells us how
much universe has expanded since they exploded
Accelerating universe is best fit to supernova data
old
older
oldest
Estimated age depends on both dark matter and dark energy
The early
universe must
have been
extremely hot
and dense
Four known forces
in universe:
Strong Force
Electromagnetism
Weak Force
Gravity
Primary Evidence
1) We have detected the leftover radiation
from the Big Bang.
2) The Big Bang theory correctly predicts the
abundance of helium and other light
elements.
The cosmic
microwave
background –
the radiation left
over from the
Big Bang – was
detected by
Penzias &
Wilson in 1965
Background radiation from Big Bang has been freely
streaming across universe since atoms formed at
temperature ~ 3,000 K: visible/IR
Background has perfect
thermal radiation
spectrum at temperature
2.73 K
Expansion of universe has redshifted thermal
radiation from that time to ~1000 times longer
wavelength: microwaves
WMAP gives us detailed baby pictures of structure in
the universe
Abundances of
other light
elements agree
with Big Bang
model having
4.4% normal
matter – more
evidence for
WIMPS!
Mysteries Needing Explanation
1) Where does structure come from?
2) Why is the overall distribution of matter so
uniform?
3) Why is the density of the universe so close
to the critical density?
An early episode of rapid inflation can solve
all three mysteries?
Density = Critical
Density > Critical
Density < Critical
Overall
geometry of the
universe is
closely related
to total density
of matter &
energy
Inflation of
universe flattens
overall
geometry like
the inflation of a
balloon, causing
overall density
of matter plus
energy to be
very close to
critical density
Patterns of structure observed by WMAP show us the
“seeds” of universe
Observed patterns of structure in universe agree (so far)
with the “seeds” that inflation would produce
Sound waves :
red/blue = high/low
gas & light pressure
Many waves of
different sizes,
Directions & phases,
all “superposed”
Water waves :
high/low level of
water surface
“Seeds” Inferred from CMB
• Overall geometry is flat
– Total mass+energy has critical density
• Ordinary matter ~ 4.4% of total
• Total matter is ~ 27% of total
– Dark matter is ~ 23% of total
– Dark energy is ~ 73% of total
• Age of 13.7 billion years
In excellent agreement with observations of present-day universe
and models involving inflation and WIMPs!
Finite, But Without Edge?
2-dimensional analogy:
Surface of a sphere:
Surface is finite, but has no edge.
An ant has no sense of the third
dimension, to him there’s no
center - All points are equal.
Any point on the surface can be
defined as the center of a
coordinate system.
Expanding Space
Analogy:
A loaf of raisin bread where the
dough is rising and expanding,
taking the raisins with it.
The Age of the Universe
Knowing the current rate of expansion of the
universe, estimate the time for galaxies to move to
where they are today:
Time = distance / velocity
velocity = (Hubble constant) * distance
T ≈ d/v = 1/H ~ 14 billion years
379,000 years old: First light escapes; Universe
already has structure (light still arriving today)
Early fluctuations become denser
condensations of matter
First stars form after 200 million years
Galaxies (groups of billions of stars) and
galaxy clusters form, according to the floorplan
laid out at 379,000 years
The Universe today: lots of stars and galaxies!
Olbers’ Paradox
If universe were
1) infinite
2) unchanging
3) everywhere
the same
Then, stars would
cover the night sky
Olbers’ Paradox
If universe were
1) infinite
2) unchanging
3) everywhere
the same
Then, stars would
cover the night sky
Night sky is
dark because
the universe
changes with
time
As we look
out in space,
we can look
back to a
time when
there were no
stars