THE DARK UNIVERSE
The Copernican Revolution continues…
Caty Pilachowski, Mini-University 2010
DARK
MATTERS
• “Extraordinary claims require extraordinary
evidence.” (Carl Sagan)
• “Extraordinary claims require extraordinary
proof.” (Marcello Truzzi)
• “The weight of evidence for an extraordinary
claim must be proportioned to its
strangeness” (Laplace)
• “A wise man, therefore, proportions his
belief to the evidence” (David Hume)
Journey into the
Dark Universe
 Foundation Concepts
 Key Observations
 Explaining Our Universe
 Concordance Model
 History of the Universe
The Universe
We See:
Earth and
Moon
Physical Sizes
Earth’s Diameter:
Moon’s Diameter:
Moon’s Distance:
Distance from Sun:
13000 km
¼ Earth’s Diameter
400,000 km
150,000,000 km
8 light minutes
The Universe
We See:
The Sun
Physical Size
Diameter:
Distance:
1,400,000 km
(about 100 x the diameter of Earth)
about 100 x the Sun’s diameter
The
Universe
We See:
The
Nearest
Stars
The closest star to our Sun is Proxima
Centauri, about 4 light years distant
(that’s about 40 trillion km)
The Universe We See:
The Milky Way Galaxy
100,000 LY
Our Milky Way galaxy
contains two hundred
billion stars
The Sun is about
26,000 light years
from the center
The Universe We See:
The Local Galaxy Group
Our Milky Way
Galaxy is part of a
cluster of about 3
dozen galaxies
Andromeda is 3 million LY distant
The Universe We See:
The Local Supercluster
Our Local Group of
galaxies is part of a
larger Supercluster
of galaxy groups
Virgo
Supercluster
Virgo is 60M LY distant
Galaxies and clusters
of galaxies collect
into vast streams,
sheets, and walls of
galaxies.
The
Universe
We See
The Visible
Universe
On the
largest
scales, the
universe
seems to be
more or less
uniform
The Universe
we see is
made of
hydrogen and
helium
90% hydrogen atoms
10% helium atoms
Less than 1%
everything else
(and everything
else is made in stars!)
everything
else
The Universe we see far away appears
younger than the Universe nearby
Astronomers
can see into
the past
“Lookback time”
 Foundation Concepts
Key Observations
THE DARK UNIVERSE
Missing Mass
Expansion
Acceleration
 Explaining our Universe
 Concordance Model
 History of the Universe
The Case of the Missing Mass
• In the 1930s, astronomer Fritz Zwicky
noticed that galaxies in clusters were moving
at speeds to fast to hold the cluster
together
• In the 1950s, astronomer Vera Rubin
found that galaxies were spinning too
fast to hold together
Galaxies and galaxy clusters both behave as if
more mass is present than we can see…
Galaxy Rotation
Mass within
Sun’s orbit:
~1011 MSun
Total mass:
~1012 MSun
What’s the PROBLEM???
The orbits of stars suggest that galaxies contain
several times more mass that we can find in stars, gas
and dust
MISSING MASS!
Dark matter is the material believed to
account for the discrepancy between the mass of a
galaxy as found from the orbits of stars and the mass
observed in the form of gas and dust
The visible
portion of
a galaxy
lies deep
in the
heart of a
large halo
of dark
matter
Evidence for Dark Matter
Rotation of
galaxies
Velocities of
stars in dwarf
galaxies
Galaxy
interactions
Velocities
of galaxies
in clusters
Hot gas in
galaxy
clusters
Collisions of
galaxy
clusters
Gravitational lensing
Velocity Dispersions
in Dwarf Galaxies
 Count the stars
 Add up the light
 Look for any gas
 Add up the mass
Velocity Dispersions in
Dwarf Galaxies
 From spectra and the Doppler shift
 Measure the velocity dispersion
 Determine the total mass
astro-ph/0704126
Calculated for a sample of 194
stars with 32-33 stars per bin
M/L Ratios for MW Dwarfs
Galaxy
MV
L
Radius
Total
mass
(mag)
(106 LSun)
(pc)
(106 MSun)
M/L
Gas Fraction
Sculptor
-11.1
2.15
110
6.4
0.004
Phoenix
-10.1
0.90
310
33
0.006
Fornax
-13.2
15.5
460
68
<0.001
Carina
-9.3
0.43
210
13
<0.001
Leo I
-11.9
4.79
215
22
<0.001
Sextans
-9.5
0.50
335
19
<0.001
Leo II
-9.6
0.58
160
9.7
<0.001
Ursa Minor
-8.9
0.29
200
23
<0.002
Draco
-8.8
0.26
180
22
84
<0.001
Galaxy interactions require
more mass than we can see
Computer
simulation
The real
thing
Antennae Galaxy (HST)
Evidence for dark matter
in clusters of galaxies
We can measure the
velocities of galaxies
in a cluster from
their Doppler shifts
The mass we find
from galaxy motions
in a cluster is about
50 times larger than
the mass in stars!
Coma Cluster of Galaxies
Visible Light
X-Ray Light
HOT GAS IN GALAXY CLUSTERS
Clusters contain X-ray emitting hot gas
Temperature of hot gas tells us cluster mass
7 x more gas than stars, but not enough!
Gravitational
Lensing
 Light from a distant galaxy bends around a massive
object (such as cluster of galaxies) between the
distant galaxy and the observer
 Gravitational lensing is predicted by Einstein's theory
of general relativity
General
Relativity
 Gravity bends the
paths of light rays
 Gravity curves space,
so mass acts as a lens
 The paths of all objects, whether or not they have
mass, are curved if they pass near a massive body
 Prediction confirmed in the 1919 solar eclipse
Discovering
Gravitational
Lenses
 Mysterious arcs
discovered in 1986
 Cluster Abell 370 (left)
 Cluster C12244 (right)
The clusters and the
arcs are at very
different distances
The arcs are
highly distorted,
very distant
galaxies
Gravitational Lenses Produce
Multiple Images
source and
lens aligned
source and
lens not
aligned
If the source, the lens, and the observer lie in
a straight line, the source will appear as a ring
around the lens
If the lens is off-center, multiple, distorted
images will be seen
Cluster of Galaxies Cl0024+16
The reddish
objects are
galaxies in the
lensing cluster
The bluish objects
are multiple images
of a much more
distant galaxy
Reconstruct the
distant galaxy
from individual
pieces of the arc
Arcs let us map the
distribution of dark
matter in clusters of
galaxies
The Bottom Line…
The visible matter
does not provide
enough gravity to
produce the
gravitational lenses we
see from galaxies and
galaxy clusters
Dark matter must be
present to account for
what we observe
cluster center
A cluster of galaxies consists
of three components
1. 2% Stars
2. 13% Hot Gas
3. 85% Dark Matter
The galaxies we see are only 2% of the mass
1E 0657-56 –
The Bullet Cluster
Direct observation of Dark Matter
What’s going on with
Cluster 1E 0657-56?
• TWO clusters of galaxies collide
The gas interacts,
the dark matter and
galaxies don’t
The galaxies and dark matter pass
through unimpeded, but the hot gas
is separated from the clusters
The Bullet Cluster
Direct observation of
False Color:
Blue = DM
Red= Hot Gas
White = Galaxies
Dark Matter
All methods of measuring cluster mass
indicate similar amounts of dark matter
Clusters of
Galaxies
Coma
Centaurus
Gravity
holds
clusters
together
Gotta have
Dark Matter!
Perseus
Hercules
Dark Matter makes
galaxies grow
Galaxy formation is
dominated by the
gravitational pull of
dark
matter
4.3 Mpc
or 14 million LY
Small galaxies form
first, grow, and
merge to form
larger galaxies
The two objects
approaching at the
end will merge in
about a billion years
Many of the small
galaxies become
satellites orbiting
larger galaxies
Galaxies Grow through Mergers
Intergalactic
gas
Galaxy
building blocks
observed with
Hubble
Clumps
concentrated
by dark
matter
lead to
galaxies
Simulation
The cosmic web of dark matter, gas,
and galaxies in a young universe
The real thing
Dark Matter
 The universe contains matter we cannot see
 Dark matter interacts with normal matter
through gravity
 Dark matter does NOT interact with light
the way the normal matter does
 The Universe contains 5 or 6 times MORE
dark matter than normal matter
All galaxies are embedded in clouds of dark
matter
What is DARK MATTER?
Can’t see it, taste it, touch it, smell it…
We can only detect it by gravity
We don’t know!
Detecting Dark Matter is one of
the most active areas of high
energy physics, and a reason to
build large accelerators.
So, What Could It Be?
Dark Matter Candidates:
Black holes
Low-mass objects like loose
planets
Elementary particles
What about WIMPS??
• “Weakly Interacting Massive Particles”
– As yet undiscovered elementary particles
• High energy particle theories suggest
such elementary particles exist
 WIMPS are a plausible, but not firm,
consequence of several theories in
particle physics
What about Dark Energy?
First, the expansion of the Universe!
Slipher*, Hubble, and Einstein
*V. M. Slipher is an IU alumnus!
Hubble found that the spectra of more
distant galaxies are shifted toward the red –
the further the galaxy, the larger the shift
Distance (Mpc)
1000
800
600
400
Distance - Velocity Relation
200
0
0
10000
20000
30000
40000
Velocity (km/sec)
50000
60000
70000
Hubble’s
Law
More distant galaxies are moving away
from us at greater speed
THE UNIVERSE IS EXPANDING
Will the Universe Keep
Expanding Or Re-Collapse????
It depends on the density of the Universe
The critical density in the current epoch is 10-29
g/cm3, about one hydrogen atom per cubic meter
About 25 times more than the observed mass of
stars and gas
Will dark matter recollapse the Universe?
There isn’t nearly enough to re-collapse the Universe
But the universe isn’t even slowing down….
The more we learn, the
stranger it gets…
The speed of a ball tossed up in the air slows
down because of gravity
Observations at the end of the 20th century
established that the Universe is not just
coasting, or slowing down because of its own
gravitational pull, but actually speeding up.
Supernova 1998ba
We observe Type Ia
supernovae (exploding
dead stars) to measure
the distances of
extremely distant
galaxies
This tells us how fast
the Universe expanded
when it was younger
The speeds of very distant galaxies tell us the
Universe is expanding faster today than in the past
WhyType Ia
Supernovae?
All Type Ia supernovae are due to implosion of
dead stars
They all have the same amount of fuel to burn
and produce the same luminosity
They therefore act like “standard candles”
Distant Type Ia Supernovae
are too faint…
Distant supernovae
are further away
than predicted by
Hubble’s Law
The expansion of
the Universe was
slower in the past
The Universe is speeding up!
The universe is expanding faster today
than it did in early times
This expansion cannot be caused by
ordinary matter or dark matter
The acceleration suggests the possibility of
a new type of repulsive force (anti-gravity)
that acts on very large scales
DARK ENERGY
The universe is NOT what it seems…
A little history….
 When Einstein developed the theory of general
relativity, astronomers thought the Universe was
“static” – neither expanding nor contracting
 Einstein included a repulsive force called the
cosmological constant to balance gravity (anti-gravity!)
 The cosmological constant was rejected when we
learned the Universe was expanding
 The cosmological constant is now needed to explain why
the Universe is speeding up
What is Dark Energy?
A “fifth force?” …in addition to
strong force (holds nuclei together)
weak force (interactions of electrons)
electrical/magnetic force (holds atoms
together)
gravity (works over large distances)
Something else?
Another big question!
THE DARK UNIVERSE
Foundation Concepts
Key Observations
Explaining our Universe
Concordance Model
History of the Universe
The Big Bang
& The Dark
Universe
Origin of
Structure
Inflation
• The Universe began
in a very hot, very
dense, very compact
state
• All of our visible
universe was
contained in a very
small volume
• Space expanded to
its present volume
• The Universe
continues to expand
today – and dark
energy is increasing
the rate of
expansion
Evidence for
the Big Bang
The expansion of space
• The abundance of helium (and other light
isotopes)
• Cosmic microwave background radiation
Structure in
Our Universe
We see galaxies
organized in a fluffy,
sponge-like web in
space
Galaxies are grouped in
clusters, sheets, and
filaments that
surround great voids
Seeing Structure in the Hubble Deep Field
Studying the
locations of
galaxies in the
HDF shows the
structure of the
Universe
Why is
DARK MATTER important?
The formation
of structure
and of galaxies
requires the
extra mass
provided by
dark matter
Without dark matter,
the Universe as we
know it would not exist
Dark Matter
Dominates the
Structure of the
Universe
Center for Cosmological Physics,
University of Chicago
http://cosmicweb.uchicago.edu/index.html
 The formation of clusters and filaments in a universe
filled with cold dark matter
 The box is 140 million light years on a side
 Simulation begins when the Universe is less than 1% of
its current age and matter is uniformly distributed
 Small fluctuations grow to large structures
 Structures formed quickly
Explaining the Origin of Structure
• The simple Big
Bang model gives
a Universe with
no structure
• To explain why
the Universe has
structure, we
need “inflation”
What is Inflation?
• In the first 10-38 to 10-36
seconds, regions of the
Universe expanded from
the size of an atomic
nucleus to the size of the
Solar System
• The rate of expansion
then slowed
• Prior to inflation, matter
on opposite sides of our
Universe was in contact
• Quantum fluctuations prior
to inflation became the
seeds of structure
Inflation can
make all the
structure by
stretching tiny
quantum ripples
to enormous size
These ripples in
density then
become the
seeds for all
structures in
the Universe
THE DARK UNIVERSE
Foundation Concepts
Key Observations
Explaining our Universe
Concordance Model
History of the Universe
The Modern
Cosmologists
View of the
Universe
The Globular Cluster Messier 3
Basic Constraint!
• The Universe should not be younger than
the objects in it
• The oldest stars are 13 billion years old
• The Universe must be at least 13 billion
years old
Mass
• Mass alters the expansion of the Universe
through its gravitational effect on space
•
• With enough mass in the Universe, gravity
can halt the expansion and cause the
Universe to recollapse on itself
• Dark matter and regular matter together
provide just 1/3 the amount of mass
needed to stop the Universe from
expanding forever
The Concordance Model
• Three factors control the fate of the
Universe:
– the current expansion rate
– the amount of mass in the universe
– the acceleration factor of the universe
• The Universe is 13.7 billion years old
• The Universe will expand forever, at
a faster and faster rate
THE DARK UNIVERSE
Foundation Concepts
Key Observations
Explaining our Universe
Concordance Model
History of the Universe*
*Thanks to Fred Adams, U Michigan
The History…
The Universe evolved quickly after the
Big Bang
Stars and galaxies formed within 300 Myr
Gravity vs. Dark Energy
Dark energy was
insignificant at early
times, otherwise stars
and galaxies could not
have formed
Gravity dominated when
galaxies were close
together
As the Universe
expanded, galaxies grew
further apart, dark
energy began to
dominate gravity
The Future
DEGENERATE ERA – 10 trillion trillion trillion years after the Big Bang
•
•
•
•
Planets detach from stars
Stars and planets evaporate from galaxies
Most ordinary matter in the universe is locked up in degenerate stellar remnants
Eventually, even the protons themselves decay
BLACK-HOLE ERA - 10,000 trillion trillion trillion trillion trillion trillion trillion
trillion years after the Big Bang
• The only large objects remaining are black holes, and even they evaporate
THE FINAL, DARK ERA –
Only photons, neutrinos, electrons and positrons remain, wandering through a universe bigger than
the mind can conceive.
Occasionally, electrons and positrons meet and form "atoms" larger than the visible universe is
today.
From here into the infinite future, the universe remains cold, dark and empty.
Final Thoughts
THE DARK UNIVERSE
The possibility exists that our “bubble
universe” is only one of many universes that
could have formed, Each with its own,
different version of the Concordance Model
Other universes could have very different
physical conditions and we will never see
them – they may be on different “branes” or
in other dimensions that we cannot measure
From the Universe to the Multiverse
Not only do we
not occupy a
preferred place
in our Universe,
we may not
occupy any
preferred
universe in the
Multiverse!
Origin of the CMB – the thermal
radiation of the first atoms
Isotropic microwave
radiation
Testing
Inflation
Why is the Cosmic Microwave Background
Radiation almost perfectly isotropic?
The CMB is isotropic because regions now
on opposite sides of the sky were close
together before inflation pushed them far
apart
Testing
Inflation
 Patterns of structure
observed by WMAP
show the “seeds” of
universe
 Observed patterns of
structure in the
Universe agree (so far)
with the “seeds” that
inflation would produce
WMAP’s observation of the “seeds” of
structure inferred from the CMB confirm
the existence of dark energy
• 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
Recall the Uncertainty Principle
• The uncertainty principle states that you
cannot know both the position x and the
momentum p of a particle more precisely
than Planck’s constant h/2p  “h-bar”
• When dimensions are small, particles must
therefore move in order to satisfy the
uncertainty principle
• This motion creates a “zero point energy”
> 0
Uncertainty Principle
Dx Dp = h/2
The Universe We See
in Microwave Light
In 1965, two engineers at
AT&T’s Bell Labs in New Jersey
were puzzled by a constant
“noise” in a microwave antenna
they were calibrating
Cosmic Microwaves
A uniform, faint
The noise could
not be explained signal from all
by terrestrial or over the sky
extraterrestrial
sources. It comes
from every
direction
1978 Nobel Prize
in Physics to
Penzias and
Wilson
COBE data points
Cosmic Microwaves
The universe was hot and
dense in the distant past
Spectrum of a
2.726 K degree
blackbody
COBE
• The temperature of the Universe is 2.726 K
• The Universe was once much hotter
• The light stretched from visible light to
microwave light as the Universe expanded
WMAP: Wilkinson
Microwave Anisotropy
Probe
WMAP shows the whole sky in microwaves
Tiny temperature differences from place to place are
the beginning of cosmic structure in the Universe
Temperature varies by only millionths of a degree
The microwave light
captured in this
picture is from
380,000 years after
the Big Bang, over 13
billion years ago
A baby
picture of
the
Universe
• Imagine a temperature map of the
Earth in June 1992
• WMAP makes a similar map of the sky
What are we seeing in
the baby picture?
 The microwave
radiation comes from
the time when the
temperature of the
Universe became low
enough for atoms to
form (about 3000K)
 Matter became
transparent, allowing
light to travel great
distances
 It is like seeing the
bottom layer of clouds
on an overcast day.
Background
radiation
from Big
Bang has
been freely
streaming
across
universe
since atoms
formed at
temperature
~ 3,000 K:
visible/IR
On what angular scales do we
see variations in the CMB?
A “Power Spectrum”
How strong are variations on
different angular scales?
Different angular scales probe
different aspects the early Universe
• Astronomers compute
a complex physical
model of conditions
and adjust parameters
to match the observed
curve
• Parameters include an
expansion rate, a
composition, age, etc.
A simple “Big
Bang” doesn’t
explain all the
properties of the
Universe
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?
Explaining the Origin of Structure
• The simple Big
Bang model gives
a Universe with
no structure
• To explain why
the Universe has
structure, we
need “inflation”
What is Inflation?
• A period of
extremely rapid
expansion when the
Universe was very
young.
• 10-38 to 10-36
seconds
• Regions of the
Universe expanded
from the size of
an atomic nucleus
to the size of the
Solar System
Alternative to Dark Matter: MOND Modified Newtonian Dynamics
 For accelerations a less
than a0, reduce gravity
acceleration by the factor
a/a0
a(a/a0) = GM/r2
 This gives flat rotation
curves
 A single value of a0 works
for galaxy rotation curves
 But MOND is untested
experimentally
 MOND can‘t explain DM
in clusters and far out in
halos
MOND can’t explain it all
• While MOND can reproduce galaxy
rotation curves, it is harder to explain
– Galaxy cluster velocity dispersions
– Observations of gravitational lenses
– The Bullet Cluster and the DM ring
• MOND still requires DM to account for
all the observations
• Which is a simpler explanation, DM or
MOND+DM?
Galaxy Formation
Galaxies form when the Universe was
young
Galaxies form where filaments of
dark matter intersect
Filaments provide gas and dark
matter
The gas fuels star formation, while
the galaxy cannibalizes dark matter
and smaller galaxies
Dark matter provides the
gravitational mass necessary
to form galaxies
36 kpc
72 kpc
144 kpc
288 kpc
Summary: Dark Matter Evidence
Many dynamical phenomena cannot be
explained with the observed mass content of
the universe
Problem can be solved with one radical
assumption
85% of all matter is dark matter
initially distributed as ordinary matter
interacts with normal matter only through gravity
Stars, gas are now more concentrated than
dark matter
WMAP’s observation of the
“seeds” of structure inferred
from the CMB confirm the
existence of dark energy
• 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