What do we know about Dark Matter and Dark Energy

1
What do we know about Dark Matter and Dark Energy?

• CAS Meeting, 14 January 2006
• Dr. Uwe Trittmann
• Otterbein College

2
Starting Point

• Before we can say anything about the dark side,
  we have to answer the following questions
• What is bright matter?
• What do we know about bright matter?

3
Bright Matter

• All normal or bright matter can be seen in
  some way
• Stars emit light, or other forms of
  electromagnetic radiation
• All macroscopic matter emits EM radiation
  characteristic for its temperature
• Microscopic matter (particles) interact via the
  Standard Model forces and can be detected this way

4
The Structure of Matter
Atom Nucleus and Electrons
Nucleus Protons and Neutrons (Nucleons)
Nucleon 3 Quarks
? 10-10m ?
? 10-14m ?
?10-15m?
5
Elementary Particles

• All ordinary nuclear matter is made out of
  quarks
• Up-Quark Down-Quark
• (charge 2/3)
  (charge -1/3)
• In particular
• Proton uud ? charge 1
• Nucleons
• Neutron
  udd ? charge 0

(composite particles)
6
The Forces of the Standard Model

• Force (wave)
• Gravity couples to mass
• Electromagnetic force
• couples to charge
• Weak force
• responsible for radioactive decay
• Strong force
• couples to quarks

• Carrier (particle)
• graviton (?)
• photon
• W, W-, Z0
• 8 gluons

massless carriers ? long ranged
massive carriers ? short ranged
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The particles of the Standard Model
Matter particles have half-integer spin (fermions)
Force carriers have integer spin (bosons)
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Conclusion

• We know a lot about the structure of matter!
• We know a lot about the forces between matter
  particles
• We know al lot about the theory that describes
  all of this (the Standard Model)
• ? Great News !

9
Pie in the Sky Content of the Universe
5
Dark Energy Dark Matter SM Matter
25
70

• ?We know almost everything about almost nothing!

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What is the dark stuff?

• Dark Matter is the stuff we know nothing about
  (but we have some ideas)

Dark Energy is the stuff we have absolutely
no idea about
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Conclusion

• If we dont know anything about it, it is boring,
  and there is nothing to talk about.
• ? End of lecture!

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Alternate Conclusion

• If we dont know anything about it, it is
  interesting because there is a lot to be
  discovered, learned, explored,
• ? beginning of lecture!

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So what do we know? Is it real?

• It is real in the sense that it has specific
  properties
• The universe as a whole and its parts behave
  differently when different amounts of the dark
  stuff is in it
• Lets have a look!

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First evidence for dark matter The missing mass
problem

• Showed up when measuring rotation curves of
  galaxies

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The Mass of the Galaxy

• Can be determined using Keplers 3rd Law
• Solar System the orbital velocities of planets
  determined by mass of Sun
• Galaxy orbital velocities of stars are
  determined by total mass of the galaxy contained
  within that stars orbit
• Two key results
• large mass contained in a very small volume at
  center of our Galaxy
• Much of the mass of the Galaxy is not observed
• consists neither of stars, nor of gas or dust
• extends far beyond visible part of our galaxy
  (dark halo)

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Properties of Dark Matter

• Dark Matter is dark at all wavelengths, not just
  visible light
• We cant see it (cant detect it)
• Only effect is has it acts gravitationally like
  an additional mass
• Found in galaxies, galaxies clusters, large scale
  structure of the universe
• Necessary to explain structure formation in the
  universe at large scales

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What is Dark Matter?

• More precise What does Dark matter consist of?
• Brown dwarfs?
• Black dwarfs?
• Black holes?
• Neutrinos?
• Other exotic subatomic particles?

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Classification of Dark Matter

• Classify the possibilities
• Hot Dark Matter
• Warm Dark Matter
• Cold Dark Matter
• Baryonic Dark Matter

You could have come up with this, huh?!
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Hot Dark Matter

• Fast, relativistic matter
• Example neutrino
• Pro
• interact very weakly, hard to detect ? dark!
• Con
• Existing boundaries limit contribution to missing
  mass
• Hot Dark matter cannot explain how galaxies
  formed
• Microwave background (WMAP) indicates that
  mastter clumped early on
• Hot dark matter does not clump (its simply too
  fast)

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Baryonic Dark Matter

• Normal matter
• Brown Dwarfs
• Dense regions of heavy elements
• MACHOs massive compact halo objects
• Big Bang nucleosynthesis limits contribution

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Cold Dark Matter

• Slow, non-relativistic particles
• Most attractive possibility
• Large masses (BH, etc) ruled out by grav. lensing
  data
• Major candidates
• Axions
• Sterile neutrinos
• SIMPs (strongly interacting massive particles)
• WIMPs (weakly ), e.g. neutralinos
• All of the above are exotic, i.e. outside the
  SM

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Alternatives

• Maybe missing mass, etc. can be explained by
  something else?
• Incomplete understanding of gravitation
• Modified Newtonian Dynamics (MOND)
• Nonsymmetric gravity
• General relativity

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The silent majority Dark Energy
70
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Aside Standard Cosmology

• Based on Einsteins theory of Gravity, aka
  General Relativity
• Assumes isotropic, homogeneous universe
• This smeared out mass property is approximately
  valid if we average over large distances in the
  universe

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General Relativity ?! Thats easy!

Rµ? -1/2 gµ? R 8pG/c4 Tµ?
OK, fine, but what does that mean?

• (Actually, it took Prof. Einstein 10 years to
  come up with that!)

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The Idea behind General Relativity

• In modern physics, we view space and time as a
  whole, we call it four-dimensional space-time.
• Space-time is warped by the presence of masses
  like the sun, so Mass tells space how to bend
• Objects (like planets) travel in straight lines
  through this curved space (we see this as
  orbits), so
• Space tells matter how to move

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Still too complicated?

• Here is a picture Sun
• Planets orbit

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Effects of General Relativity

• Bending of starlight by the Sun's gravitational
  field (and other gravitational lensing effects)

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What General Relativity tells us

• The more mass there is in the universe, the more
  braking of expansion there is
• So the game is
• Mass vs. Expansion
• And we can even calculate who wins!

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The size of the Universe depends on time!
Expansion wins!
Its a tie!
Mass wins!
Time
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The Universe expands!

• Where was the origin of the expansion?
• ?Everywhere!
• Every galaxy sees the others receding from it
  there is no center

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Big Bang

• The universe expands now, so looking back in time
  it actually shrinks until?
• ?Big Bang model The universe is born out of a
  hot dense medium
• 13.7 billion years ago.

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The Fate of the Universe determined by a single
number!

• Critical density is the density required to just
  barely stop the expansion
• Well use ?0 actual density/critical density
• ?0 1 means its a tie
• ?0 gt 1 means the universe will recollapse (Big
  Crunch) ? Mass wins!
• ?0 lt 1 means gravity not strong enough to halt
  the expansion ? Expansion wins!
• And the number is ?0 1 (probably)

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The Shape of the Universe

• In the basic scenario there is a simple relation
  between the density and the shape of space-time
• Density Curvature 2-D example Universe
  Time Space
• ?0gt1 positive sphere closed,
  bound finite
• ?01 zero (flat) plane open, marginal
  infinite
• ?0lt1 negative saddle open, unbound
  infinite

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Expansion of the Universe

• Either it grows forever
• Or it comes to a standstill
• Or it falls back and collapses (Big crunch)
• In any case Expansion slows down!

Surprise of the year 1998 (Birthday of Dark
Energy) All wrong! It accelerates!
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Enter The Cosmological Constant

• Usually denoted ?0, it represents a uniform
  pressure which either helps or retards the
  expansion (depending on its sign)

• Physical origin of ?0 is unclear
• Einsteins biggest blunder or not !
• Appears to be small but not quite zero!
• Particle Physics biggest failure

37
Effects of the Cosmological Constant

• Introduced by Einstein, not necessary
• Repulsive ? accelerates expansion of universe

Hard to distinguish today
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Triple evidence for Dark Energy

• Supernova data
• Large scale structure of the cosmos
• Microwave background

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Microwave Background Signal from the Big Bang

• Heat from the Big Bang should still be around,
  although red-shifted by the subsequent expansion
• Predicted to be a blackbody spectrum with a
  characteristic temperature of 3Kelvin by George
  Gamow (1948)
• ? Cosmic Microwave Background Radiation (CMB)

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Discovery of Cosmic Microwave Background
Radiation (CMB)

• Penzias and Wilson (1964)
• Tried to debug their horn antenna
• Couldnt get rid of background noise
• ? Signal from Big Bang
• Very, very isotropic (1 part in 100,000)

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CMB Heres how it looks like!
Peak as expected from 3 Kelvin warm object
Shape as expected from black body
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Maybe pigeons?

• Proposed error pigeon crap in antenna
• Real reason a signal from the Big Bang

Pigeon trap
? Horn antenna
43
Latest Results WMAP(Wilkinson Microwave
Anisotropy Probe)

• Measure fluctuations in microwave background
• Expect typical size of fluctuation of one degree
  if universe is flat
• Result
• Universe is flat !

44
Experiment and Theory
Expect accoustic peak at l200 ? There it is!
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Supernova Data

• Type Ia Supernovae are
• standard candles
• Can calculate distance
• from brightness
• Can measure redshift
• General relativity gives us distance as a
• function of redshift for a given universe
• Supernovae are further away than
• expected for any decelerating (standard)
• universe

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Supernova Data
magnitude
redshift
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Redshift Everything is moving away from us!

• Measure spectrum of galaxies and compare to
  laboratory measurement
• lines are shifted towards red
• This is the Doppler effect Red-shifted objects
  are moving away from us

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Example Spectrum of a Quasar
Highly redshifted spectrum
? the quasar is very far away and
keeps going!
Quasar
Lab
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Large Scale Structure of the Cosmos

• Large scale structure of the universe can be
  explained only by models which include Dark
  Matter and Dark Energy

Experiments 2dF GRS, SDSS
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Properties of Dark Energy

• Should be able to explain acceleration of cosmic
  expansion ? acts like a negative pressure
• Must not mess up structure formation or
  nucleosynthesis
• Should not dilute as the universe expands ? will
  be different of content of universe as time
  goes by

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The Pie changes - As time goes by
-11.5
-7.5
¼ size ½
Now
2 size 4
24.5
11.5
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Why does the Pie change?

• Dark energy density stays constant
• Matter density falls of like volume
• Volume grows, mass stays constant
• Big Question why do we live in an era where the
  content is rather democratic?
• Because we are here to observe!
  (Dangerous answer)

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What is Dark Energy?

• We have a few ideas what it could be
• Unfortunately none of these makes fits our job
  description
• Wanted Dark Energy Candidate

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Dark Energy Candidates

• Global Vacuum Energy
• Local Vacuum Energy
• Dynamical Dark Energy
• Modified Gravity

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Global Vacuum Energy

• Cosmological constant
• Constant in space and time
• Same across the universe
• Pro
• Could be explainable from first principles
• Con
• No known explanation yet

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Local Vacuum Energy

• Constant in the observable universe, but
  different in very distant parts of cosmos
• Pro
• Maybe explains why cosmological const. is so
  small here
• Con
• Requires different domains

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Dynamical Dark Energy

• Quintessence
• Slowly varying energy source
• Pro
• Testable
• Can gradually go to zero energy
• Con
• Has not been detected

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Modified Gravity

• Modification of general relativity on large
  scales
• Pro
• Does not need dark energy
• Con
• Hard to modify and still explain existing data

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Threefold Evidence

• Three independent measurements agree
• Universe is flat
• 30 Matter
• 70 dark energy

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Measuring Dark Energy
Dark energy acts like negative pressure, and is
characterized by its equation of state, w
p/? ? We can measure w!
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Conclusion

• Need more ideas
• No problem! Thats what theorists produce every
  day
• Need more data
• Some space missions (Planck, etc) are on the way
• LHC probing SUSY will start operation in 2007