The Invisible Universe Dark Matter and Dark Energy

1
The Invisible UniverseDark Matter and Dark Energy

• Ian Morison

2
News first visible image of a planet orbiting a
star

• The Search for Other Worlds
• 22nd January 2009

3
Additional Lecture

• God and the Universe
• 1 pm on the 1st December 2008 next Monday
• St Georges Hanover Square

4
A thought experiment
5

• V sqrt(2GME / r)
• V escape velocity
• G Universal Constant of Gravitation
• ME Mass of Earth
• r Radius of Earth
• So we can find the Mass of the Earth
• ME v2 r / 2 G

6
Evidence for Dark Matter

• 1) Galaxy Cluster Dynamics
• Why dont clusters of galaxies spread out?

7
Fritz Zwicky
8
(No Transcript)
9
(No Transcript)
10
Evidence for Dark Matter

• 2) The Rotation Curves of Galaxies
• Why do stars rotate round the galaxy as fast as
  they do?

11
A problem with Stars and Gas in Galaxies

• It was first pointed out by Jan Oort in 1932 that
  stars towards to outer parts of the Milky Way
  were moving more quickly than would be expected.

12

• In 1959, hydrogen-line observations of two
  spiral galaxies made by Louise Volders showed
  that they were rotating faster than one should
  expect from their mass.
• I have re-created these observations using a
  small radio telescope at Jodrell Bank and will
  describe them later in the talk.

13
The rotation problem was revived again and made
unforgettable by Vera Rubin in the 1970s.
14
Vera Rubin
15
Orion

• The Orion Nebula in the Sword of Orion
• Called an HII region

16
(No Transcript)
17
H-a Spectral Line
18
Eta Carinae Nebula
19
The H-a Spectral Line
20
HII regions in M33
21
Rotation Curve of HII regions
22
(No Transcript)
23
Rotation Curves
24
(No Transcript)
25
MOND

• Modified Newtonian Dynamics
• We only know that Newtons laws are accurate when
  the gravitational acceleration is large.
• It is possible to modify Newtons Law and get a
  very good fit to galaxy rotation curves.
• BUT MOND finds it difficult to account for other
  observations so we will ignore!

26
Evidence for Dark Matter

• 3) Weighing a galaxy

27
M33 Doppler
28
M33 Spectrum

• The H-line spectrum is centred at a velocity
  of 175 km/sec. This tells us that the Galaxy as a
  whole is coming towards us at a speed of 175
  km/sec. ( - values towards us!)

29

• The spectrum has a width of 200 km/sec. This
  tells us that the galaxy is rotating. The outer
  parts on one side are coming towards us at
  100km/sec, the other away from us at 100
  km/sec.

30
As the galaxy is inclined to us the actual
rotation speed is greater.

• Though the galaxy is presumably circular, its
  dimensions on a photographic plate are 71 x 45
  arc minutes.
• This implies that it is inclined to our line of
  sight at an angle of arcsin (45 / 71) 39
  degrees.
• The true rotational velocity of the outer parts
  of the galaxy about its centre should thus be
  close to 100 / sin(39) km/sec 158 km/sec.

31
The size of M33

• From an image of the galaxy and its distance we
  can calculate its radius.
• M33 is 73 arc minutes across.
• It lies at a distance of 2.36 x 1022 m.
• 73 arc minutes is 73/ (60 x 57.3 ) 2.1 x 10-2
  radians
• Radius of M33 is thus 0.5 x 2.1 x 10-2 x 2.36 x
  1022 m.
• 2.47 x 1020 m.

32
We can calculate Mass!

• The gravitational force on a star at this
  distance to overcome centripetal acceleration
• G M m / r 2 m v2 / r
• ( M mass of Galaxy, m mass of star, r
  distance of star from centre
• v velocity of star around centre)

33
Mass of M33

• This gives M r v2/G
• 2.47 x 1020 x (1.58 x 105)2/ 6.67 x
  10 11 kg
• 9 x 1040 kg
• 3.66 x 1040 / 2 x 1030 solar masses
• 45,000 million solar masses.
• 45 billion solar masses

34
BUT

• From the light we see from the galaxy and
  assuming it has a similar makeup to our own
  galaxy we can estimate the mass of normal matter
  from the MASS to LIGHT ratio.
• This gives a mass of 8 Billion Solar masses
• This implies that there is 5 times more mass
  that is invisible to us!

35
Evidence for Dark Matter

• 4) Gas Entrapment
• X-Ray observations show trapped gas in galaxy
  clusters

36
NGC 4555
37

• NASA's Chandra X-ray Observatory that revealed
  that NGC4555 is embedded in a cloud of 10
  million-degree Celsius gas. The hot gas cloud has
  a diameter of about 400,000 light years, roughly
  twice that of the visible galaxy, in the right
  image.

38
NGC 2300
39
Evidence for Dark Matter

• 5) Gravitational Lensing and the distortion of
  distant galaxies

40
Abell Cluster 2218
41
(No Transcript)
42
Weak lensing
43
(No Transcript)
44
Looking back 6 billion years
45
Evidence for Dark Matter

• 6) Dark Galaxies?

46
H-Line Spectrum
47
Nothing Seen?
48
Evidence for Dark Matter

• 7) Formation of the Galaxies

49
WMAP
50

• The Universe 400,000 years after its origin

51

• Until atoms formed 400,000 years after the big
  bang, normal matter and light were interacting
  forming a very smooth mix.
• Galaxies would then take 8 billion years to
  form as, very slowly, the gas became lumpy.
• But galaxies formed in 1 billion years! How?

52

• Dark Matter could start clumping immediately
  after the big bang and so form gravitational
  wells in which the normal matter could fall when
  atoms finally formed.
• So gave galaxy formation a head start.

53
(No Transcript)
54
What is Dark Matter?

• As yet we do not know.
• Axions, WIMPs, Neutralinos and neutrinos are all
  candidates.
• Searches are actively taking place.
  

• A high resolution image
• of Dark Matter

55
Hot Dark Matter or Cold Dark Matter?

• Hot very light particles moving close to the
  speed of light
• Cold massive particles moving relatively slowly

56
Neutrinos

• Vast numbers in the Universe.
• Long thought to have no mass and to travel at the
  speed of light.
• Now proven to have a small mass.

57
Sudbury Mine experiment has confirmed that
neutrinos must have mass
58
Neutrino Mass

• Best current estimate is that the average
  neutrino mass is less than 1 millionth of the
  mass of the electron.
• The total mass of neutrinos in the visible
  Universe would then only be a very small part of
  the Dark Matter.
• Two much Hot Dark Matter makes it very difficult
  to form galaxies to form so this is somewhat of a
  relief!

59
What might Cold Dark Matter be? (1)

• One possibility is a light neutral axion
  whose existence was predicted by the Peccei-Quinn
  theory in 1977.
• 10 trillion in every cubic centimetre!
• Theoretically, axions can change to and
  from photons in the presence of a very strong
  magnetic field.

60
Passing light through a wall!
61
Lawrence Livermore

• Converting axions into microwave photons

62
What might Cold Dark Matter be? (2)

• Extensions to the Standard Model of Particle
  Physics Supersymmetry - indicate that WIMPS
  (Weakly Interacting Massive Particles) might be a
  major constituent of CDM.
• A leading candidate is the neutralino the
  lightest neutral supersymmetric particle.

63
How can we detect WIMPS?

• Billions of WIMPS could be passing through you
  each second.
• Occasionally they will interact with the nucleus
  of an atom making the nucleus recoil.
• We can design instruments that, in principle
  detect these recoils.

64
The Boulby Mine

• To reduce the unwanted background noise' from
  cosmic rays, UK experiments are being carried out
  in caverns in salt 1100 metres below ground at
  the bottom of Europe's deepest mine, at Boulby,
  North Yorkshire.

65
1100m below ground level

• At this depth, collisions in the rock have
  stopped all but one in a million of the cosmic
  rays.
• Meanwhile, of the thousand million WIMPs a second
  passing a square meter only about three would
  collide in the rock on their way down - and they
  are only slowed down a little, not stopped.

66
(No Transcript)
67
(No Transcript)
68
Nyobium Detectors
69
The WIMP wind

• In June, the Earths motion round the Sun (29.6
  km/sec) is in the same direction as the Suns
  motions through space (232 km/sec) giving a
  higher WIMP flux.
• In December the two velocities are subtracted
  giving a lower WIMP flux.
• The difference is 7

70
PAMELA

• Payload for Antimatter/Matter Exploration and
  Light-nuclei Astrophysics

71

• PAMELA is looking for annihilations of Dark
  Matter particles called Neutralinos that produce
  antiprotons and positrons.

72

• It has detected an excess of high energy
  positrons coming from the galactic centre.
• This could be evidence of dark matter
  interactions there and, as such, is possibly the
  first detection of the presence of dark matter
  apart from its gravitational effects.

73

• This excess, the authors say, may constitute
  the first indirect evidence of dark-matter
  particle annihilations although they add that
  there could yet be other explanations, such as
  the presence of a nearby pulsar

74
An even bigger problem.

• There is not enough Normal Matter and Dark Matter
  
• By a factor of 4!

75
Where is all the mass/energy required to give the
observed density of the universe?

• The WMAP observations enable us to calculate the
  Density of the Universe.
• We believe that normal matter (also called
  Baryonic Matter as it is made up of baryons
  protons and neutrons) only makes up 4
• Dark Matter adds a further 23

76
Only 4 is Normal Matter

• 1 is visible as stars and excited gas
• 3 is invisible
• Gas, dust, black holes, etc.
• 23 is Dark Matter
• Axions, Neutralinos, Neutrinos etc.

77

• But that only makes up 27 of the total
• What can make up the rest?

78
A solution to the missing mass/energy problem?

• DARK ENERGY?!
• Do we have any evidence for it? Yes.

79
The Hubble Plot

• A linear plot of recession speed against distance
  would show that the universe is expanding at a
  constant rate.

80
(No Transcript)
81
Standard Candles

• Let us assume that we have observed an object of
  a certain type in a galaxy whose distance we
  know.
• Suppose we observe what we believe to be the same
  type of object in a more distant galaxy and its
  apparent brightness is 10000 times less.
• From the Inverse Square Law we can deduce that it
  is 100 times further away.

82
A Supernova

• Supernovae are the brightest individual objects
  in the Universe and so may be observed at very
  great distances.

83
Type 1a Supernova

• We believe that one type of supernova has a peak
  brightness which is the same for all such
  supernova explosions.
• This is Type 1a - associated with white dwarfs

84
White Dwarfs

• The remnant of a star like the Sun whose core has
  is that 1.44 solar masses this is know as the
  Chandraseckhar Limit

85
Type 1a Supernovae
86
(No Transcript)
87
(No Transcript)
88
They have a characteristic Light Curve
89
Type 1a

• Decay of Ni-56 followed by the decay of Co-56

90
Observations of Distant Type Ia Supernovae
91
A supernova in M51
92
(No Transcript)
93
(No Transcript)
94
(No Transcript)
95
(No Transcript)
96
CosmologicalTime Dilation!

• The more distant supernova appear to have longer
  decay curves.

97
(No Transcript)
98
Not was expected!
99
An Accelerating Universe?

• In contrast to the standard Big Bang Models -
  all of which have a rate of expansion that is
  reducing with time these observations indicate
  that the rate of expansion is now increasing.
• Could this be the L term in Einsteins equations
  making itself felt?
• We suspect so.

100
The Cosmological Constant, L

• The L term in Einsteins equations of General
  Relativity was added to prevent the collapse of a
  static universe.
• It represents a form of antigravity caused by the
  pressure of space.
• Not originally needed as the universe was
  actually expanding.
• Could now be causing the expansion to accelerate

101
We have a consistent model

• 4 Normal Matter
• 23 Dark Matter
• 73 Dark Energy

102

• Age of Universe 13.7 Billion Years
• The pressure produced by the Dark Energy is now
  making the expansion rate of the Universe
  INCREASE with time.

103
The size of the Universe over time.
104
(No Transcript)
105
The Far Future

• As the clusters of Galaxies move further apart
  carried ever faster by the expansion of the space
  between them there will be less and less for
  astronomers to see.
• In 100 billion years there will be nothing else
  to see!

106

• This is the best time in the life of the
  universe to study astronomy!