Cosmology from the Cosmic Microwave Background

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Cosmology from the Cosmic Microwave Background

• Katy Lancaster

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About Me..
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About Me..

• Postdoc in the Astrophysics group at Bristol
  working with Professor Mark Birkinshaw, world
  expert in our field
• Various projects, OCRA, AMiBA
• Previously PhD in Cambridge, working on the VSA
• MSci in Bristol (many moons ago!)

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Talk Structure

• Some key concepts in astrophysics, and what we
  spend our time doing!
• The point of all this what are we trying to
  achieve in the field of Cosmology?
• The Cosmic Microwave Background (relic radiation
  from the Big Bang)
• Galaxy clusters and the Sunyaev Zeldovich effect

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Before we go any further.some things you need
to know.
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STAR / PLANET
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GALAXY
You are here!
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GALAXY CLUSTER
THESE OBJECTS ARE THE FOCUS OF MY CURRENT WORK
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The Cosmic Web
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Photon a PARTICLE of light. Remember this,
or please ask if you forget!
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• Astrophysics That branch of astronomy which
  treats of the physical or chemical properties of
  the celestial bodies. Hence astrophysicist, a
  student of astronomical physics.

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Topics in Astrophysics..

• Solar System planets, the sun
• Stars stellar composition, stellar evolution,
  star formation, supernovae, extra-solar planets
• Galaxies structure, properties, stellar
  velocities (dark matter), formation, evolution,
  clustering
• Active galaxies mechanisms, power sources (black
  holes)
• High-energy phenomena Gamma ray bursts
• Galaxy clusters galaxy properties, gas
  properties, lensing (dark matter), super
  clustering.
• Large scale structure, structure formation
  theories
• Cosmology properties of the Universe as a
  whole, formation (the Big Bang), fate??

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• Cosmology The science or theory of the
  Universe as an ordered whole, and of the general
  laws which govern it. Also, a particular account
  or system of the universe and its laws.

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What do you astronomers actually DO?
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• Obtain data
• Go to telescope
• Download from archive

• Process data
• Work out what it tells us!
• Publish in journal

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In practice, need 2 approaches

• OBSERVATIONAL
• Observe celestial bodies (stars, galaxies etc) at
  various wavelengths
• Fit theoretical models to data to choose the most
  appropriate

• THEORETICAL
• Simulate celestial bodies (stellar evolution,
  galaxy formation etc)
• Create models of possible physical processes

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My Work

• COSMOLOGY from
• The Cosmic Microwave Background Radiation (CMB)
• The interaction of the CMB with Galaxy Clusters
  via the Sunyaev Zeldovich Effect
• OBSERVATIONAL - ie obtaining data, data
  processing, extracting science
• Tenerife, Poland, Hawaii, Taiwan..

Very hot topics in Astrophysics at the moment!
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Onto the specifics What are we trying to
achieve in Cosmology today?
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Hubble 1929 The Universe is expanding
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Zwicky 1933 Galaxy clusters contain Dark Matter
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1998 Supernovae suggest Universe is accelerating
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Big questions in cosmology

• Will the Universe expand forever?
• Depends on the mean density
• We can constrain this using the CMB
• What is the Universe made from?
• Normal stuff plus Dark Matter
• What is Dark Matter? Particle physicists working
  on it!
• Why does it appear to be accelerating?
• It is being pushed by Dark Energy
• We can constrain this using the CMB

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But what on earth is it??
The Cosmic Microwave Background is central to our
cosmological understanding
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Penzias and Wilson, 1965

• Observing the galaxy, detected annoying level of
  static in all directions
• Pigeon poo? Aliens??
• No!
• At the same time, Dicke at Princeton predicted
  the existence of relic radiation from the big
  bang, ie the CMB
• Nobel Prize, 1978

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The sky is BRIGHT at radio frequencies.If we
observe the sky with a radio telescope, inbetween
the stars and galaxies, it is NOT DARK.
Visualising the CMB..
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But where does it come from? It all started
with
The Big Bang
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IN THE BEGINNING.
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COSMIC SOUP
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COSMIC SOUP
PROTON
NEUTRON
ELECTRON
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The Big Bang

• Not really an explosion
• Universe expanded rapidly as a whole and is still
  expanding today as a result of the Big Bang
  (Hubble)
• Matter was created in the form of tiny particles
  (protons, neutrons, electrons)
• Too hot for normal stuff to form (eg atoms,
  molecules)
• Photons scatter off charged particles like a
  fog (Thomson scattering)

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300,000 years later

• Universe much cooler, atoms start to form..
• Hydrogen, Helium, normal stuff

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Much cooled, atoms form, photons released
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Universe now neutral, Photons escape
These photons, viewed today, form the Cosmic
Microwave Background Radiation
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Summary Formation of the CMB

• The Universe started with the Big Bang
• It was initially hot, dense and ionised
• Photons were continually scattered from charged
  particles until.
• .temperature decreased and atoms formed (neutral
  particles)
• Photons (light) escaped and became able to
  stream freely through the Universe.
• Observe the same photons today, much cooled, as
  the Cosmic Microwave Background

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An important aside formation of structures
At the same time as all this was going on,
structures were starting to form out of the
cosmic soup
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GRAVITY!
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Back to the CMB..
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The CMB today

• Can observe the CMB today, 13.7 billion years
  after the Big Bang
• Radiation is much cooled 2.73 K (-270.42C)
• Conclusive evidence for the Big Bang theory -
  proves Universe was once in thermal equilibrium
• So..... what does it look like?

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• Observe blank sky with a radio telescope.
• Rather than darkness, see Uniform, high-energy
  glow
• Turn up the resolution......

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• Tiny temperature differences (microK)
• When the CMB photons escaped, structures were
  starting to form
• These structures have now become galaxies
• The structure formation processes have affected
  the CMB and we see the imprint as hot and
  cold spots
• Very difficult to measure!

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What does the CMB tell us?

• Measure the strength of the temperature
  differences on different scales, eg COBE 1992

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A plethora of other experiments followed this
up.until.
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What does the CMB tell us?

• Measure the strength of the temperature
  differences on different scales, eg WMAP 2003

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What does the CMB tell us?

• In practice, we need information from a wide
  range of resolutions, or scales
• Measure the strength of the temperature
  differences on different scales
• Low resolution (eg COBE)
• Higher resolution (eg WMAP)
• Theorists come up with a model (function, like
  ymx c but more complex!) including all of the
  physics of CMB/structure formation
• Observers fit the model to real observations of
  the CMB (like drawing a line of best fit),
  tweaking the values of each parameter

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What does this tell us?

• The function on the previous slide is complex and
  involves many terms including
• Density of Universe in ORDINARY MATTER
• Density of Universe in DARK MATTER
• Density of Universe in DARK ENERGY
• (The sum is the total density, and governs the
  fate of the Universe as discussed earlier).
• We can constrain some of the big questions in
  cosmology by observing the CMB

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Current best model

• The Universe appears to be flat (critical)
• Will just expand forever
• But measurements suggest that only 30 of this
  density can come from matter
• Contributions from ordinary and dark matter
• This points towards the existence of something
  else which we call Dark Energy
• Dark energy is believed to be pushing the
  Universe outwards, i.e. accelerating the
  expansion

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What next for CMB research?

• New satellite, Planck, launch date 2008?
• Set to solve all the mysteries..allegedly!
• This, and some ground based experiments are
  trying to measure CMB polarisation (difficult!)
• Another route look for secondary features in
  the CMB (ie those that have occurred since the
  Big Bang)

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Before we move onQuick CMB revision.

• The CMB is light originating from the Big
  BangWe can see it coming from all
  directionsThe sky glows at radio frequencies

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More recent imprints on the CMB

• Lets forget the tiny temperature fluctuations
  for now!
• Majority of CMB photons have travelled through
  the Universe unimpeded
• But some have interacted with ionised material on
  the way
• Main contributor Galaxy clusters

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• Rich Clusters - congregations of hundreds or even
  thousands of galaxies
• See cluster galaxies and lensing arcs in the
  optical
• But only around 5 of a clusters mass is in
  galaxies (Most of the mass is in Dark Matter)
• But a sizeable fraction is found in hot gas......

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• X-rays - see hot gas
• via Bremstrahlung
• 10-30 of total mass

• Chandra Image of the Coma cluster

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Cluster Gas

• Gas stripped from galaxies and sucked in from
  outside
• Trapped in huge gravitational potential
• Hot, dense and energetic
• Ionised (charged) - may interact with incident
  radiation (such as the CMB)
• Accurately represents the characteristics of the
  whole Universe
• Clusters are Cosmic Laboratories

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Sunyaev and Zeldovich, 1969

• Postulated that the CMB could interact with the
  gas in galaxy clusters
• The Sunyaev Zeldovich (SZ) Effect

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What is it, exactly?

• Low energy CMB photon collides with high energy
  cluster electron
• Photon receives energy boost
• Net effect shift CMB to higher frequencies in
  the direction of a cluster

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What is it, simply?

• Cluster makes partial shadow in the CMB

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What is so interesting?

• Its INDEPENDENT of the DISTANCE of the cluster
  responsible
• The strength of the shadow tells us about the
  characteristics of the CLUSTER GAS
• Mirrors UNIVERSAL CHARACTERISTICS

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What does it look like?
VSA image (from earlier!)
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Exciting new Science!

• In most branches of Astronomy, it is difficult to
  observe very distant objects
• The SZ effect is distance-independent, so in
  theory we can observe ALL clusters in existence
• Current hot topic surveying the sky using radio
  telescopes to find new clusters via the SZ effect

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To study Cosmology via clusters, we need lots of
themA large, sensible sample of objects is
usually called a catalogue
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SZ Cluster surveys

• Cluster catalogues to date have been derived from
  X-ray observations
• Severe limitations since the X-ray signal falls
  off quickly with increasing distance
• SZ surveys will enable us to generate catalogues
  of ALL clusters in existence (with a few
  caveats!)
• Cluster evolution
• Study how cluster properties change as a function
  of distance (and hence cluster age)
• Evolution of the Universe
• Study how the number of clusters per unit volume
  changes with distance cosmology

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My Work

• I previously worked with the Very Small Array,
  looking at both the CMB and the SZ effect
• I am now involved with two new SZ experiments,
  OCRA and AMiBA
• We are
• Studying known clusters
• Performing surveys to find new ones

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OCRA

• Prototype detector on Torun telescope, Poland
• 32m dish
• Various receivers, ours works at 30GHz
• Suffers from atmospheric contamination
• Most useful observations are made in the winter

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OCRA

• We recently published results from 4 well-known
  clusters
• Now observing larger sample, should be able to
  derive more science from this
• Future array receiver, blind surveys
• Excellent imaging instrument

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AMiBA

• Taiwanese project, based in Hawaii
• 90 GHz Interferometer
• Hexapod mount
• Testing observations with 7 60cm dishes
• High significance detections of well-know
  clusters, will be published soon!

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AMiBA

• Ultimately 19 dishes, 1.2m diameter?
• Potential problems with the platform flexing
• Also problems with ground emission
• Very powerful survey instrument
• Also polarisation in the CMB

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Challenges.

• The SZ effect is TINY
• Galaxy clusters contain galaxies (!), which may
  emit radio waves and drown the SZ signal
• Require further information, or observations at
  multiple frequencies.
• Radio galaxies are less bright at higher
  frequencies, but higher frequency observations
  suffer from atmospheric contamination
• Remember the fluctuations in the CMB itself?
  They can also contaminate!
• Go to higher resolution

Can overcome most problems but its not easy!
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Summary

• The Big Bang left behind radiation which we can
  observe at radio frequencies today
• The Cosmic Microwave Background
• The CMB has imprints upon it caused by the
  formation of the structures we see today (eg
  galaxies)
• The CMB tells us much about the Universe as a
  whole
• Galaxy clusters may create shadows in the CMB
• The Sunyaev Zeldovich Effect
• The SZ effect is distance-independent so very
  useful for cluster physics and also Cosmology

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ANY QUESTIONS?