Cosmology

1
Cosmology

• Arny, Chapter 11

2
Introduction

• Cosmology is the study of the structure and
  evolution of the Universe as a whole
• How big is the Universe?
• What shape is it?
• How old is it?
• How did it form?
• What will happen in the future?
• Do such questions seem futile to answer or even
  arrogant to ask It seems to be part of the
  human heritage to seek the answers
• What we seem to know now
• The Universe is expanding and is filled with a
  very low-energy background radiation
• The radiation and expansion imply the Universe
  began some 15 billion years ago
• The Universe began as a hot, dense, violent burst
  of matter and energy called the Big Bang

3
Observations of the Universe

• Introduction
• In the early years of the 20th century,
  astronomers envisioned the Universe as a static
  place with only the Milky Way and a few
  companions
• It was not until the 1920s that astronomers
  realized the Universe was filled with other
  galaxies millions of light-year apart and that
  the Universe was expanding
• Distribution of Galaxies
• No matter which way you look (ignoring the zone
  of avoidance), you see about the same number of
  galaxies
• The galaxies are not spread smoothly, but clump
  into groups
• This smooth clumping implies a similar
  distribution for the whole Universe (contrast
  this with the skys Milky Way implying a
  disc-shaped galaxy)

4
Observations of the Universe

• Motion of Galaxies
• In general, a galaxy obeys the Hubble law speed
  of recession is proportional to the galaxys
  distance, the proportionality given by the Hubble
  constant
• The motion away is due to the expansion of space
  itself not like bomb fragments going through
  the air, but like buttons attached to an
  expanding balloon
• Age of the Universe
• Running the Universes expansion backward implies
  all mass becomes confined into a very small
  volume, what was once called the Primeval Atom
• Assuming galaxies have always moved with the
  velocities they now have, the Hubble Law gives
  age for Universe of 15 billion years with H 65
  km/s/Mpc
• More sophisticated models taking into account the
  effects of gravity do not change the age value
  much

5
Observations of the Universe

• Age Discrepancy
• A 15-billion-year-old Universe makes some
  astronomers uneasy since the error bars on this
  age are large enough to make it appear that the
  oldest stars and globular clusters could be older
  than the Universe
• Recent parallax measurements by the Hipparchos
  satellite have shown the Cepheid variables are
  about 10 farther out than previous thought which
  implies that galaxies are farther out, H is
  smaller, and stars are younger
• This new information resolves the earlier age
  discrepancy future work remains to strengthen
  this conclusion

6
Age of the Universe
Calculations indicate an age of about 15 billion
years for the age of the universe. No objects
older than this have been observed. E.G. the age
of the earth is about 4.5 billion years The
oldest stars are about 10 billion years.
7
Beginning of Big Crunch ?????????
Time
Now (15 billion years after B.B.)
Dawn of civilization
8
Observations of the Universe

• The Cosmic Horizon
• The age of the Universe limits the distance we
  can see since the speed of light is finite
• In a static Universe, this distance is directly
  determined from its age and the speed of light
• The maximum distance one can see (in principal,
  but not necessarily in practice) is called the
  cosmic horizon
• The space within the horizon is called the
  visible (or observable) Universe there may very
  well be more to the Universe beyond
• Size of the Universe
• The distance to the cosmic horizon gives a rough
  measure of the radius of the Universe
• For a static 15 billion-year-old Universe, this
  radius is 15 billion light-years

9
Observations of the Universe

• Are we at the Center of the Universe
• The recession of distant galaxies often leads to
  the misconception the Milky Way is the Universes
  center
• However, because space is expanding, no matter
  where you are located, galaxies will move away
  from you there is no preferred center
• This lack of a preferred location is called the
  cosmological principle
• Olbers Paradox
• In 1823, Heinrich Olbers offered Olbers Paradox
  If the Universe extends forever and has existed
  forever, the night sky should be bright but of
  course it isnt
• Olbers reasoned that no matter which direction
  you looked in the sky a stars light should be
  seen
• Resolution Finite age and speed of light means
  only a finite volume of starlight is available
  the night sky is dark

10
What is the Center of the Universe

• The universe has no preferred center of
  expansion.
• We are not the center of the universe nor is
  anything else.
• Every observer, no matter where in the universe
  will see the universe expanding in the same way.

11
Observations of the Universe

• The Cosmic Microwave Background
• The proposed very-dense early Universe implied
  that it must have been very hot, perhaps 10
  trillion K
• It was proposed that as the Universe expanded and
  cooled, the radiation that existed at that early
  time would survive to the present as microwave
  radiation
• This radiation was accidentally discovered by
  Arno Penzias and Robert Wilson in 1965 and has
  since then been referred to as the cosmic
  microwave background (CMB)
• The CMB follows a perfect blackbody spectrum with
  a temperature of 2.726 K (about 3 K, a bit above
  absolute zero)

12
What is cosmic background radiation?
This is background microwave radiation that is
uniform in all directions and indicates a
temperature for the current universe of about 3
degrees Kelvin. This supports the Big Bang theory.
13
What is the cosmological redshift?
Photons from distant galaxies are all redshifted.
This is a color shift in the radiation spectrum
of galaxies. The further away the galaxy the
greater its redshift. This redshift is caused by
the expansion of the universe - further evidence
for a Big Bang. All galaxies are receding from
each other as a result of the Big Bang.
14
Observations of the Universe

• Composition of the Oldest Stars
• Current theory suggests that the early Universe
  consisted of protons, neutrons, and electrons
• The initial hot and dense state allowed nuclear
  reactions to create helium
• Based on estimates of the early Universes
  expansion rate, about 24 of the matte should be
  transformed to helium in good agreement with
  what is observed in old stars in the Milky Way
  and other galaxies
• Similar measurement of deuterium (2H) and lithium
  also support the hot, dense early Universe idea

15
Observations of the Universe

• Conclusions Deduced from the Basic Observations
  of the Universe
• In the early 1950s, a steady state cosmology
  was proposed for the Universe
• New matter forms continually out of nothing to
  create new galaxies in the empty, expanding space
  between older galaxies
• Average density of Universe stays constant, so on
  average the Universe looks the same at all times
• Problems with steady-state model
• Necessitates continuously creating new matter out
  of nothing
• Requires special process to create CMB
• Universe does not look the same in the past
• Big Bang theory represents most viable
  explanation for observed Universe

16
Evolution of the UniverseOpen or Closed

• Introduction
• The Universe is currently expanding, but what of
  its future
• Will it expand forever
• Will it stop expanding and collapse
• Some consequences
• Expanding forever means that as all the stars
  consume their hydrogen, the Universe will become
  black and empty this scenario is the open
  universe
• A Universe that collapses as a Big Crunch might
  lead to another Primeval Atom, leading perhaps to
  the birth of another universe this scenario is
  the closed universe
• The expansion speed of the Universe becomes zero
  when the Universe has reached infinite size -
  this scenario is the flat universe

17
Evolution of the UniverseOpen or Closed

• Introduction (continued)
• The energy content of the Universe depends on
  what type of universe we are in
• An open universe has positive total energy
• A flat universe has zero total energy
• A closed universe has negative total energy
• In principal, if we measure the energy content of
  the Universe, we can tell what type it is
• The energy content of the Universe is the sum of
  its positive kinetic energy of expansion and its
  negative energy of gravitational binding
  (basically its mass content)

18
Evolution of the UniverseOpen or Closed

• The Density of the Universe
• The mass density of the Universe gives an
  equivalent means of determining its total energy
  content and its easier to measure
• To determine if the Universe is open or close,
  compare its density (r) to the critical density
• rc 3H2/(8pG)
• where H is Hubble constant and G is the
  gravitational constant
• If r gt rc, the Universe is closed
• If r lt rc, the Universe is open

19
Evolution of the UniverseOpen or Closed

• The Density of the Universe (continued)
• The critical density is 10-29 g/cm3, about one
  hydrogen atom per cubic meter and this is about
  25 times more than the mass density determined
  from observed stars and gas
• Based on this, the Universe looks open
• But what about dark matter
• Estimated amount is just enough to close the
  Universe
• Is this a coincidence or is there something
  deeper here

20
Evolution of the UniverseOpen or Closed

• A Cosmological Repulsion?
• Another way to ascertain the Universes fate is
  to look at very distant galaxies galaxies in
  the past to see how fast the Universes
  expansion has slowed
• Interestingly, using supernova in very far and
  faint galaxies as distance indicators, it appears
  the Universe is speeding up, not slowing down
• How is this possible
• Einsteins general relativity equations include a
  cosmological constant that represents a repulsive
  force
• When the expansion of the Universe was
  discovered, the cosmological constant was thought
  to be zero
• Latest measurements imply this may not be the
  case
• A Universe that is speeding up implies it is
  older than previously thought helping further
  solve the star age discrepancy

21
The Shape of the Universe

• Einsteins General Theory of Relativity is built
  around the curved space
• Curved space is not easy to visualize, but there
  are two-dimensional models that can help
• Positive curvature - also called closed
  resembles the surface of a sphere, parallel lines
  meet, and triangles have interior angles with a
  sum greater than 180
• Negative curvature - also called open resembles
  the surface of a saddle, parallel lines never
  meet, and triangles have interior angles with a
  sum less than 180
• Flat curvature what people typically think of
  as space, parallel lines do not meet, and
  triangles have interior angles with a sum equal
  to 180
• Curved space was used to explain capture of light
  by black holes and the bending of light in a
  gravitational lens

22
General Relativity the Universe can have only
one of three possible shapes
Shape 1 spherical, closed geometry
k 1
finite, unbound
? l
23
Shape 2 saddle-shaped, open geometry
k -1
? l
infinite, unbound
24
But the Universe has shape 3
Shape 3 flat or Euclidean geometry
k 0
? l
infinite, unbound
(unbound no edge)
25
The Shape of the Universe

• Measuring the curvature of space
• In principle one could directly measure the
  interior angles of a triangle or an equivalent
  geometric arrangement, but to date, practical
  limitations prevent it
• CMB provides another way
• CMB is extremely uniform across the sky except
  for tiny variations in brightness from place to
  place
• The spatial sizes of these variations can be
  predicted based on conditions in the early
  Universe
• Analysis of variations indicate that Universe is
  flat with a non-zero cosmological constant

26
The Origin of the Universe

• Introduction
• The early Universes high temperature and density
  imply that it may have had a very simple
  structure
• Mass and radiation mingled in a manner unlike
  their sharp distinction today
• Radiation is so energetic that it easily
  transforms to mass mass and radiation behaved
  as a single entity
• Radiation, Matter and Antimatter in the Early
  Universe
• Emc2 tells us not only can mass be transformed
  to energy (as with fusion in stars), but that
  energy (in photons) can be transformed into mass
• The creation of mass, however, must come in
  pairs, one of the pair is ordinary matter, the
  other is antimatter
• The antiparticle of the electron is the positron,
  the antiparticle of the proton is the antiproton,
  etc

27
The Origin of the Universe

• History of Matter and radiation in the Early
  Universe
• At one microsecond after the Big Bang
• Temperature 1013 K, hot enough for photons to
  create quarks and antiquarks
• Diameter smaller than Earths orbit
• Universe expands at near speed of light and cools
• Lower temperature no longer produces
  quarks/antiquarks
• Subatomic physics dictates that existing
  quarks/antiquarks annihilate asymmetrically
  leaving an excess of quarks
• Surviving quarks combine into protons, neutrons

28
Radiation and Matter Origin of the Universe
After about 1 microsecond

• Universe was hot and dense
• Radiation converted into protons and antiprotons
  plus quarks and anti-quarks and visa versa
• Small volume, jammed with quarks and antiquarks
  annihilating each other by creating radiation,
  began to expand.
• The cooler radiation was no longer hot enough to
  create quarks and thus most quarks disappeared
  from the universe.

29
Radiation and Matter Origin of the Universe
After about 1 microsecond

• A few quarks survived and combined to make
  protons and neutrons.
• Without this tiny imbalance in quarks (symmetry
  breaking) the universe would be void of matter
  containing only cosmic radiation.
• Space was then filled with ordinary matter and
  radiation and continued to expand.

30
Radiation and Matter Origin of the Universe
After about 5 seconds

• Universe still hot enough to create light
  particles, electrons and anti-electrons.
• Protons and electrons interact to produce
  neutrons
• Universe continues to expand
• Universe cools to a few billion degrees and the
  creation of matter stops.

31
Radiation and Matter Origin of the Universe
About 3 minutes

• Expansion cools the universe to a few hundred
  million degrees.
• Approximately 1/4 of the protons are fused into
  helium.
• All of space is now permeated by helium and
  hydrogen (about 71 H and 27 He).

32
The Origin of the Universe

• History of Matter and radiation in the Early
  Universe (continued)
• At 3 minutes after the Big Bang
• Temperature is a few hundred million degrees
• ¼ of protons fuse into helium
• Next half million years
• Further expansion and cooling
• Electrons begin to bind to protons to make
  hydrogen molecules (this is referred to as the
  recombination era)
• At end of period, photons and matter go their
  separate ways

33
Radiation and Matter Origin of the Universe
About a million years

• Expansion cools the universe to about 3000 K
• Atoms can now form by nuclei capturing electrons
  - recombination.
• Before recombination, the universe behaved as a
  single blend of radiation and matter.
• After recombination, matter and radiation acted
  as separate entities.

34
Radiation and Matter Origin of the Universe
The matter Era

• Since matter and radiation are no longer locked
  together, radiation just expands with the
  universe to become the 2.7 K cosmic radiation of
  today.
• Clouds of matter condense out to form clumps of
  matter, galaxies, etc. Formation of galaxies
  needs dark matter?

35
The Origin of the Universe

• The Formation of Galaxies
• Considering ages of several galaxies, galaxy
  formation had to start soon after recombination
  era
• Protogalaxies formed from gravitational collapse
  of gas clouds
• Gravity too feeble to create galaxies in time
  scales needed
• Need for dark matter to speed things up
• Dark matter forms clumps around which the
  protogalaxies form
• Areas rich in dark matter clumps form large scale
  galaxy chains and sheets
• Area depleted in a dark matter form voids
• Galaxy formation is still considered unsolved

36
The Inflationary Universe

• What was the state of the Universe before one
  microsecond?
• Universe was even hotter and denser
• Universe was smaller than the size of a proton
• Gravity is no longer a force of attraction, but
  one of repulsion
• This repulsive force creates a violent explosion
  which cosmologists call inflation
• Began about 10-35 seconds and lasted 10-32
  seconds
• Inflationary period ends where the previous Big
  Bang ideas begin

37
The Inflationary Universe

• The inflationary models of the universe mark the
  frontier of our understanding of the cosmos and
  give tentative answers to several unsolved
  mysteries
• Some models suggest creation from nothing
• Others suggest existence of other separate
  universe
• Still others posit that the Universe has 10 or 11
  dimensions
• Finally, these models also try to explain why
  space is so flat, and how all the forces of
  nature relate to one another

38
The Inflationary Universe

• Grand Unified Theories
• Before the start of the inflation period, 3 of
  the 4 fundamental forces (electromagnetic,
  strong, and weak) where joined together in a
  manner described by grand-unified theories
  (GUTs).
• As the Universe inflated, symmetry breaking
  separated the forces releasing energy.
• This energy was then used by the false vacuum (a
  non-zero energy state with negative pressure).
• According to General Relativity, a negative
  pressure manifests itself as a repulsive
  gravitational force (lasting only for the brief
  inflation period).

39
j

• Is the universe open or closed
• Open means that it will expand forever
• Closed means that sometime it will stop
  expanding and will begin to contract - the Big
  Crunch.
• Third possibility is a flat universe. This
  universe just stops expanding at some point and
  remains at that size.

40
THE UNSEEN EFFECT OF DARK MATTER
41
Overview

• Definition
• Current Understanding
• Detection Methods
• Cosmological Impact

42
Definition of Dark Matter
Matter that can be seen by its gravitational
effects, but does not emit light.
Dark Matter
Not Dark Matter
43
Hot or Cold?
Dark matter comes in two forms
Hot Dark Matter (HDM) -very small particles
(neutrinos) -relativistic velocities
-Relativity velocity -to fast to form structures
44
Hot or Cold?
Dark matter comes in two forms
Cold Dark Matter (CDM) -more massive and
slower -able to form smaller structures like
galaxies
45
Baryons vs. Non-Baryons
CDM could be made of two types of matter
Baryons -Strongly interacting fermions
(subatomic particle) -Normal matter
Non-Baryons -Formed during the Big
Bang -Suitable candidate not directly observed
(yet)
46
MACHOs
MAssive Compact Halo Objects

• Brown Dwarfs
• Exist in the halo of galaxies
• Attempts to explain Cold Dark Matter without new
  particles

47
WIMPs
Weakly Interacting Massive Particles

• Undiscovered non-baryonic particle
• Interacts only through the weak and gravitational
  forces
• High mass corresponds to a lower kinetic energy,
  making the particle cold

48
Consensus?

• No WIMPs have been directly observed
• Groups studying MACHOs have not found enough
  objects to account for the missing mass problem

• Cold Dark Matter probably a mixture of both
  baryonic and non-baryonic matter
• We still do not know for sure

49
Looking for WIMPs
Several groups are currently running experiments
to find WIMPs

• Cryogenic Dark Matter Search (CDMS)
• Cryogenically cooled crystals
• DAMA experiment
• Scintillation detectors
• Both detect the collision between a WIMP and
  target nuclei

50
Universal Composition
51
Universal Implications
? Actual Density / Critical Density
52
Universal Overview

• Dark matter slows the universal expansion rate
• Density of dark matter affects the fate of the
  universe
• Low density leads to accelerating expansion
• High density leads to Big Crunch
• Dark matter density affects the universal
  geometry
• Low density leads to open universe
• High density leads to closed universe

53
Universal Overview

• Current measurements indicate a flat universe
  with accelerating expansion
• The existence of dark matter can explain these
  observations
• Detecting dark matter can confirm measurements

54
Review Questions
Why do astronomers believe the universe is
expanding? What is cosmic background radiation
and what is its origin? How was helium formed
from hydrogen in the early universe? What is the
approximate age of the universe? What is dark
matter? What is meant by the big bang and the big
crunch? What is meant by an open and closed
universe?