This document discusses the history of discoveries related to dark matter and dark energy. It describes how observations over the past 100 years have shown that normal matter only accounts for about 4% of the matter in the universe. The other 96% is believed to be dark matter and dark energy. Evidence from galaxy rotations, gravitational lensing, and galaxy cluster dynamics provides strong evidence for the existence of dark matter, which is believed to be some non-baryonic, non-luminous particle that interacts only through gravity and the weak force.

1. Dark Matter
& Dark Energy
Dr. Bryan J. Higgs
28 November, 2012

2. Dark Matter & Dark Energy
Over the past 35 years or so, cosmologists’
and physicists' understanding of the universe
has been turned on its head.
It is now generally accepted in the scientific
community that ‘normal matter’ — the matter
that we experience in our everyday lives, and
that scientists have been studying since the
time of the ancient Greeks — comprises only
about 4% of the matter in the universe.
So, what is the other 96%?
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3. Goals of the Talk
» To describe some of the evidence (and history)
for why scientists believe that Dark Matter and
Dark Energy exist.
» To describe what scientists have proposed to
explain these observations.
» To describe the implications for the beginning
and the end of the universe.
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4. History & Background
First, we need to look at some background
on the history and observations that led us
to this point.
Cast your minds back about 100 years...
(Yup, buggy whip time!)
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5. Einstein's Theory of General Relativity
In 1916, Albert Einstein published
his Theory of General Relativity.
It provided a unified description of
gravity as a geometric property of
space and time.
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6. Assumptions
After the introduction of General Relativity a
number of scientists, including Einstein,
tried to apply the new theory to the universe
as a whole.
This required an assumption about how the
matter in the universe was distributed.
The simplest assumption to make is that if
you viewed the contents of the universe
with “sufficiently poor vision”, it would
appear roughly the same everywhere and
in every direction.
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7. The Cosmological Principle
That is, they assumed that the matter in
the universe is:
– Homogeneous
and
– Isotropic
when averaged over very large scales.
This is called the Cosmological
Principle.
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8. The Static Universe Model
One hundred years ago, astronomers
thought:
The universe was unchanging through time.
The stars of our galaxy (the Milky Way) made
up the whole universe
The galaxy was nearly motionless
Physicists trying to create a model for
the universe had to match these
"facts".
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9. Einstein's Theory of Gravity
Einstein created his model of
the universe, based on his
General Theory of Relativity,
using these assumptions.
He came up with his famous
Field Equations.
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10. Einstein's Field Equations
The Einstein Field Equations are a set of 10
equations in Albert Einstein's general theory
of relativity which describe the fundamental
interaction of gravitation as a result of space-
time being curved by matter and energy.
The expression on the left of the = sign represents
Note, in particular, the the curvature of space-time.
second term on the left, The expression on the right of the = sign represents
the one that includes the the matter/energy content of space-time.
famous Λ (Greek capital
letter lambda)...
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11. The Cosmological Constant
Λ is the famous Cosmological Constant.
It is equivalent to an energy density in
otherwise empty space (the vacuum).
It was originally proposed by Einstein as a
modification of his original theory to achieve
a stationary universe, to match what he
thought was the known situation.
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12. The Fate of the Universe
There are many possible solutions to
Einstein's Field Equations, and each
solution implies a possible ultimate
fate of the universe.
Alexander Friedman proposed a
number of such solutions in 1922, as
did the Belgian Jesuit priest Georges
Lemaître in 1927.
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13. Fate depends on Density
Essentially, the various models of the evolution of the
universe depend on whether or not there is enough
mass in the universe to cause it, through gravitational
attraction, to contract unto itself (the “Big Crunch”).
So how much mass is there in the universe?
How do we weigh the universe?
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14. The Density Parameter
The density parameter, Ω, is defined as
the ratio of the actual (i.e. observed) mass
density, ρ , of the universe to the critical
density, ρcrit , of the universe.
To date, the critical density is estimated to
be approximately five atoms (of hydrogen)
per cubic meter. Not so much!
So what's the significance of the
critical density?
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15. The Shape of the Universe
The Friedmann–Lemaître–Robertson
–Walker (FLRW) model has become
the most accepted theoretical model of
the universe. It is sometimes called the
Standard Model of modern cosmology.
This model describes a curvature (often
referred to as geometry) of the space-
time of the universe.
The curvature of space depends on the
value of Ω, the density parameter.
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16. Closed Universe
If Ω > 1 (i.e., the density is above the
critical density), the geometry of space
is closed like the surface of a sphere.
In a closed universe, gravity eventually
stops the expansion of the universe,
after which it starts to contract until all
matter in the universe collapses to a
point, a final singularity termed the
"Big Crunch" – maybe!
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17. Open Universe
If Ω < 1 (i.e., the density is below the
critical density), the geometry of space
is open – negatively curved like the
surface of a saddle.
An open universe expands forever, with
gravity barely slowing the rate of
expansion. The ultimate fate of an open
universe is universal heat death, the
"Big Freeze".
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18. Flat Universe
If Ω = 1 (i.e., the density is equal to the
critical density), the geometry of space
is flat – like a plane surface.
A flat universe expands forever but at a
continually decelerating rate. The
ultimate fate of the universe is the
same as an open universe – a “Big
Freeze”.
(Note that we are talking about space-time,
so the shapes at left are merely analogies in
lower dimensions.)
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19. A Primeval "Cosmic Egg"?
In 1927, Georges Lemaître published a
model of the universe suggesting that the
universe might have originated when a
primeval "cosmic egg" exploded in
spectacular fireworks, creating an
expanding universe.
Published in an obscure journal, it wasn't
taken seriously at the time. But now, his
contribution is highly valued.
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20. Discovery of Galactic Redshifts
In 1912, Vesto Slipher was the first to
observe the shift of spectral lines of
galaxies, making him the discoverer of
galactic redshifts.
Redshifts are analogous to the Doppler
effect – think racing cars or trains passing
you at speed.
An observed redshift due to the Doppler
effect occurs whenever a light source
moves away from an observer.
Conversely, light sources moving towards
an observer are blueshifted.
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21. More on Redshifts
You will often see a “z value” quoted as
a measure of a redshift.
λobsv is the observed wavelength of a spectral line
λemit is the emission wavelength of that line
If z > 0, there is a redshift
If z < 0, there is a blueshift
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22. Hubble's Discovery
In 1928, Edwin Hubble
found that the further the
distance to a nebula, the
greater the receding velocity
of that nebula.
He used Cepheid variable
stars as “standard candles”
to estimate their distance,
and measured their redshifts
to estimate their velocity.
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23. Galactic Redshifts
Here are some
examples of how
spectral lines are
shifted in stars and
galaxies.
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24. Einstein's “Biggest Blunder”?
Evidence mounted that the universe was
not static, but expanding.
This was consistent with the original
Einstein model; Einstein could have
predicted it, but had assumed the static
universe was a given.
Einstein later remarked that the
introduction of the cosmological constant
was the biggest blunder of his life.
But was it? Wait a little while...
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25. “Big Bang” or Steady State?
There were two primary explanations put
forth for the expansion of the universe:
» Lemaître's “Big Bang” theory, advocated
and developed by George Gamow.
» A Steady State model, proposed in 1948
by Hermann Bondi, Thomas Gold, and Fred
Hoyle, in which new matter would be
created as the galaxies moved away from
each other. In this model, the universe is
roughly the same at any point in time.
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26. Zwicky's Discovery
In 1933, Bulgarian-born Swiss
physicist Fritz Zwicky, while
investigating the Coma cluster of
galaxies, stumbled upon a major
discrepancy between theory and
observation.
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27. “Missing Mass?”
By studying the rotation of a galaxies
within the Coma Cluster, Zwicky estimated
that the visible mass of those galaxies was
400 times less than the mass needed to
explain their rotational motion.
But Zwicky, while ahead of his time, was a
pugnacious character, disliked by many of
his colleagues, so his ideas were often not
taken seriously.
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28. Vera Rubin's Discovery
In the late 1960s and early 1970s, Vera Rubin
measured the velocities at which galaxies
rotate, using a telescope at the Kitt Peak
Observatory in Arizona,
She used a sensitive spectrometer to determine
the spectrum of light coming from the stars in
different parts of spiral galaxies.
She discovered something unexpected:
The stars far from the centers of galaxies, in the
sparsely populated outer regions, were moving
just as fast as those closer to the galaxy's center.
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29. Galactic Rotation
View
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30. Zwicky was Right!
This was odd, because the visible mass of a galaxy does not
have enough gravity to hold such rapidly moving stars in orbit.
It followed that there had to be a tremendous amount of unseen
matter in the outer regions of galaxies where the visible stars are
relatively few.
Rubin and her colleague Kent Ford went on to study some sixty
spiral galaxies and always found the same thing.
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31. Explanation: “Dark Matter”
Rubin's observations and calculations showed that most
galaxies must contain about ten times as much “dark”
mass as can be accounted for by the visible stars.
Eventually other astronomers began to corroborate her
work and it soon became well-established that most
galaxies were in fact dominated by "dark matter":
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32. Why is it called “Dark” Matter?
Dark matter cannot be seen
directly with telescopes;
evidently it neither emits nor
absorbs light or other
electromagnetic radiation at
any significant level.
Hence “dark” (as opposed
to luminous) matter.
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33. Evidence for Dark Matter
A gravitational lens is formed when
the light from a very distant, bright
source (such as a quasar) is "bent"
around a massive object (such as a
cluster of galaxies) between the
source object and the observer.
Studies of many cases of lensing by
galaxy clusters show evidence for
large amounts of dark matter.
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34. Gravitational Lensing
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35. The Bullet Cluster
The most direct observational
evidence to date for dark matter is in a
system known as the Bullet Cluster, a
collision between two galaxy clusters.
» X-ray observations show that much of the
baryonic matter (in the form of gas, or
plasma) in the system is concentrated in
The Bullet Cluster: Hubble Space the center of the system.
Telescope image with overlays.
The total projected mass distribution
» However, weak gravitational lensing
reconstructed from strong and weak observations of the same system show that
gravitational lensing is shown in much of the mass resides outside of the
blue, while the X-ray emitting hot
gas observed with the Chandra X- central region of baryonic gas.
ray Observatory is shown in red.
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36. Summary of Evidence
 Observations of the rotational speed of spiral galaxies
 The confinement of hot gas in galaxies and clusters of
galaxies
 The random motions of galaxies in clusters
 The gravitational lensing of background objects, and
 The observed fluctuations in the cosmic microwave
background radiation
All require the presence of additional gravity, which can be
explained by the existence of dark matter.
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37. Dark Matter Candidates
Dark matter candidates are usually categorized as:
Baryonic
Composed of baryons, i.e. protons and
neutrons and combinations thereof.
Non-Baryonic
Hot Dark Matter (HDM)
Particles that have zero or near-zero mass,
and so move relativistically.
Cosmological simulations with
Cold Dark Matter and Warm Cold Dark Matter (CDM)
Dark Matter. Halos selected at
environments which could
Particles sufficiently massive that they
represent the Milky Way, the move at sub-relativistic velocities
Andromeda nebula M31 and
M33.
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38. MACHOs?
One potential baryonic form of dark matter is
MACHOs (MAssive Compact Halo Objects):
A MACHO is a small chunk of normal baryonic
matter, far smaller than a star, which drifts through
interstellar space unassociated with any solar
system.
Recent work has suggested that MACHOs are not
likely to account for the large amounts of dark
matter now known to be present in the universe
RAMBOs (Robust Association
of Massive Baryonic Objects)
have also been postulated.
These are dark clusters of brown
dwarfs or white dwarfs.
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39. Brown Dwarfs?
Stars with below 8% of the Sun's mass are
called brown dwarfs. They are not hot
enough to ignite the nuclear burning that
keeps ordinary stars shining.
Other candidates for dark matter include:
 Cold "planets" moving through
interstellar space, unattached to any
star, could exist in vast numbers
without being detected
 So could comet-like lumps of
frozen hydrogen
 So could black holes.
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40. WIMPs?
One potential non-baryonic form of dark
matter is WIMPs (Weakly Interacting
Massive Particles)
The main theoretical characteristics of a
WIMP are:
Interaction only through the weak
nuclear force and gravity
Large mass compared to standard
particles
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41. Axions?
There are strong reasons for suspecting that dark
matter isn't made of ordinary atoms at all. This
argument is based on an isotope of hydrogen,
deuterium (1 proton + 1 neutron). It turns out that if
dark matter were made from ordinary atoms, then
theory predicts that there should be much less
deuterium in the Universe than we actually observe.
So, dark matter could consist of some form of 'exotic'
particle.
One possibility is the Axion, a hypothetical particle
whose existence would explain what is otherwise a
puzzling feature of quantum chromodynamics (QCD),
the leading theory of strong interactions.
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42. Neutrinos?
Another particle has been regarded as a
candidate for dark matter: the elusive neutrino.
Neutrinos have no electric charge, and hardly
interact at all with ordinary atoms: almost all
neutrinos that hit the Earth go straight through it.
Because neutrinos so greatly outnumber atoms,
they could make up the dominant dark matter,
even if each weighed only a hundred millionth as
much as an atom.
The best evidence for neutrino
masses comes from the Super- Experiments imply a non-zero mass for the
Kamiokande experiment in Japan, neutrino, but one that is too small to account for
which used a huge tank in a former much of the dark matter.
zinc mine.
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43. SuperPartner Particles?
Supersymmetry arises naturally
from the combination of the two
cornerstones of 20th century
physics: quantum mechanics
and relativity. It is the unique
symmetry that relates the two
fundamental kinds of particles:
Bosons, which act as the
carriers of forces
If Supersymmetry is realized in nature, every fermion in
the SM must have a bosonic partner particle and vice
versa. Fermions, which act as the
constituents of matter
No such “superpartner particle” has been observed so
far, and recent LHC experiments have cast doubt on the
theory.
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44. MOND?
In 1983, Mordehai Milgrom, a physicist (another
Bulgarian-born!) at the Weizmann Institute in
Israel, proposed Modified Newtonian dynamics
(MOND), a modification of Newton's law of
gravity, to explain the galaxy rotation problem.
While MOND provides an explanation for the
observed galactic rotations, and has been
extensively examined by many others, it does
not appear to be consistent with other
observations.
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45. So What is Dark Matter?
So, dark matter could be composed of any
number of particles, both known and
exotic:
MACHOs
WIMPs
Massless neutrinos
Axions
Neutralinos
Photinos
Or who knows what else?
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46. How Fast is the Universe Decelerating?
We have known since Hubble that the
universe is expanding.
We know that gravity should cause this
expansion to slow down, depending on
how much matter is present in the
universe.
If we measure this deceleration, we
could determine the fate of the
universe.
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47. Type Ia Supernovas
To do this, we need to find a set of
'standard candles' which can be used
to determine the distance to
extremely remote objects.
It turns out that one class of
supernovae, Type Ia supernovae, can
be used as standard candles.
A supernova results from the violent
Multi-wavelength X-ray /
infrared image of SN 1572 or explosion of a white dwarf star.
Tycho's Nova, the remnant of a
Type Ia supernova
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48. Type Ia Supernova Creation
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49. Two Supernova Teams
In 1998/9, published observations of
Type Ia supernovae by
The High-z Supernova Search Team
The Supernova Cosmology Project
suggested that the expansion of the
universe is actually accelerating – a
Brian Schmidt,
Saul Perlmutter,
total surprise to everyone.
& Adam Riess.
The 2011 Nobel Prize in Physics was
awarded for this work.
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50. A New Paradigm of the Universe
So, it seems from all the
evidence that the
universe's evolution
doesn't fit the original
models!
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51. Corroboration of Results
Since then, these observations
have been corroborated by
several independent sources:
Cosmic microwave background
radiation
Gravitational lensing
Large scale structure of the
cosmos
Improved measurements of
supernovae
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52. Dark Energy
Evidence for Dark Matter and
Dark Energy has accumulated,
and it is now estimated that
only about 4% of the
matter/energy in the universe
is 'ordinary matter'.
In other words, we have no
real clue what the other 96%
consists of!
This is most embarrassing!
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53. So, what is Dark Energy?
Candidates for Dark Energy include:
Einstein's cosmological constant – dark
energy is a property of space itself.
An unidentified energy field, called
“quintessence” – fills space like a fog and
is similar to what drove inflation
None of the above – perhaps it's an illusion
created by incorrect theories.
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54. Cosmological Constant?
Remember that Einstein
thought it was his biggest
blunder?
The Cosmological Constant
has returned, and is the leading
candidate for a Dark Energy
explanation.
Maybe Einstein didn't blunder?
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55. A Slight Problem...
The Cosmological Constant being
nonzero means that the vacuum can
contain energy!
However, when physicists calculate
the vacuum energy using our best
theory, the Standard Model, they
come up with an estimate that is 120
orders of magnitude (10120) too large!
This is even more embarrassing!
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56. Quintessence
The name Quintessence (“fifth
essence”) dates back to the Ancient
Greeks (Earth, Water, Fire, Air and...)
It has been proposed by some to be
a fifth fundamental force.
The main difference between
quintessence and the cosmological
constant is that quintessence can
vary with space and time.
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57. Implications
Cosmologists estimate that the acceleration
began roughly 5 billion years ago.
Before that, it is thought that the expansion
was decelerating, due to the attractive
influence of dark matter and baryons.
The density of dark matter in an expanding
universe decreases more quickly than dark
energy, and eventually the dark energy
dominates.
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58. The “Big Rip”
The Big Rip is a cosmological
hypothesis first published in 2003,
about the ultimate fate of the universe,
based on phantom energy, an
extreme form of quintessence.
It predicts that the matter of the
universe will progressively be torn
apart by the expansion of the universe
at a certain time in the future.
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59. “The End of the Universe is Nigh!”
Don't worry!
It won't happen for
billions of years.
We all have more
immediate worries!
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60. Summary
We have significant evidence for large quantities
of something in the universe we call:
Dark Matter
and
Dark Energy
And we don't really know what either of them are!
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