Observational Cosmology: 1'Observational Parameters

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Observational Cosmology 1.Observational
Parameters
The Universe is not made of Atoms it is made of
Stories     Muriel Rukeyser (1913-1980) poet.
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1.1 A Brief History of Observational Cosmology

• Modern Observational Cosmology Timeline

• The state of play at the beginning of the 20th
  Century - Kapetyn universe.
• The Heliocentric model of the Solar System well
  established
• The Milky Way, is an isolated disk shaped island
  of stars
• Other Galaxies e.g. M31 Spiral Nebulae
  observed but are they inside or outside the Milky
  Way?

• 1912? Slipher - Measures a doppler shift
  (redshift) in the spectra of the nebulae (36/41
  receding)

• c.1912? Leavitt - Discovers Period-Luminosity
  relation for Cepheid variable stars

• c.1918? Shapley - Measures distance to LMC
  using Cepheid variable stars

• c.1926? Hubble - Measures distance to M31, M33
• discovers the proportionality between velocity
  and distance ? Hubbles law and the expanding
  Universe

• c.1964? Penzias Wilson - Discover the 2.73K
  microwave background radiation
• Confirms Theoretical Big Bang model

• c.1983? IRAS - Maps the entire infrared sky

• c.1985? Geller, Huchra, de Lapparent, Maddox,
  Efstathiou et al. CfA/APM Redshift Surveys
• map large Scale Structures on order of 20-100Mpc

• c.1987-91? Gregory, Condon - Greenbank Radio
  Surveys emphasis Isotropy of the Universe

• c.1992? COBE - Discovery of the 1/105
  anisotropy in the Cosmic Microwave Background

• c.1995-2000? HST - HST Key Project ? value of
  the Hubble Constant

• c.2003? WMAP - Dark Energy is the major
  constituant of the Universe

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1.2 Cosmological Parameters

• The Search for 2 (or more) numbers Allan
  Sandage 1970

Evolution of the Universe is described by the
Friedmann Equations
We would like to know the Scale Factor R
Expand Scale factor R(t) as Taylor Series around
the present time to
Ho and qo are observable mathematical parameters
(no physics!!)
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1.2 Cosmological Parameters

• The Hubble Parameter, H

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1.2 Cosmological Parameters

• The Deceleration Parameter, q

• for L 0 ? W 2q
• for k 0 ? 3W 2(q1)

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1.2 Cosmological Parameters

• The Deceleration Parameter, q

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1.2 Cosmological Parameters

• The Density Parameter, W (a close relation of qo)

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1.2 Cosmological Parameters

• The Density Parameter, W (a close relation of qo)

Light baryons (0.5) Dark baryons
(3.5) Radiation (lt0.005)
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1.2 Cosmological Parameters

• Dark Energy (a Cosmological Constant), L

The Energy Density of the Universe is dominated
by a dark energy - but what is it ??

• Vacuum Energy ?
• Particle/antiparticle pairs continually created
  and annihilated ? Vacuum Energy Density ?
  CONSTANT
• Pervades all of space at a constant value for
  all time
• Prediction from Quantum Mechanics rL1095kg
  m-3 ? 120 orders of magnitude too high !

• Quintessence - The Fifth Element ?
• Rolling homogeneous scalar field behaving like a
  decaying cosmological constant (i.e. NOT CONSTANT
  )
• Tracker Fields ? Quintessence Field will attain
  the same present value independent of initial
  conditions
• (like a marble spiraling down the plughole)
• Eventually attain the true vacuum energy (energy
  zero point)

It is somewhat strange that at this epoch this
dark energy is small but gt0 such that WL ? Wm
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1.2 Cosmological Parameters

• Dark Energy (a Cosmological Constant), L

• Cosmological w -1 , Stiff Fluid,
• Pervades all of space at a constant value for all
  time - our fate already decided
• Quintessence w lt -1/3 , Soft Fluid,
• Depends on properties of quintessence field
  (which can change with time)

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1.2 Cosmological Parameters

• The Cosmological Parameters

From Acceleration Equation (from Friedmann
Fluid eqns)
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1.3 The Age of the Universe

• Olbers Paradox

WHY IS THE SKY SO DARK ?
Heinrich Olbers 1826 (Thomas Digges 1576)
Consider an infinitely large and old Universe
populated by stars of number density, n, average
luminosity, L
The Sky should be infinitely bright !!!
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1.3 The Age of the Universe

• Olbers Paradox

WHY IS THE SKY SO DARK ?
Heinrich Olbers 1826 (Thomas Digges 1576)
The Sky should be infinitely bright !!!
Possible Solutions to Olbers Paradox ?????

• Absorption by dust ?
• -Dust would be heated until it emitted at the
  same temperature as the stars ?
• Not all lines of sight intersect a star ?
• - Finite angular size of stars may block a line
  of sight ? Intensity surface brightness of
  stars ?
• Number density and Luminosity not constant
  (nL?constant) ?
• - would require nL to decline faster than 1/r,
  r?? ?
• Universe is not infinitely large ?
• - For a finite universe, the average stellar
  background intensity, I (nL/4p)rmax
• Universe is not infinitely old ?
• - Not all light has reached us, the maximum
  intensity would be I (nL/4p)cto

Primary Resolution to Olbers Paradox - The
Universe is NOT infinitely old The light outside
our horizon has not yet had time to reach
us (Thermodynamic interpretation why is the
Universe so cold ?- Stars have not had time to
heat up Universe)
Olbers Paradox - Evidence for a finite age of
our Universe
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1.3 The Age of the Universe

• How old is our Universe ? - Evidence

• For an expanding (decelerating) Universe, the
  age of the Universe HUBBLE TIME 1/Hoto

• Hubble Time to age by assuming Universe has
  always been expanding at its current rate.
• If Universe was expanding faster in the past -
  to overestimates the age of the universe (to).
• If Universe is expanding faster today than in
  the past - to underestimates true age of Universe
  (to).

• Observational Constraints on to
• Radioactive Dating
• White Dwarf Cooling Age
• Age of Globular Clusters

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1.3 The Age of the Universe

• How old is our Universe ? -
• 1) Radioactive Dating

• Dating from proportions of element isotopes
• Oldest Rocks from Earth, Moon, Meteorites ? t
  4.50.3 Gyr
• Consistent with formation age of the Solar System
  (5Gyr)

Dating of radioactive isotopes in stellar
atmospheres 232Th half life 14Gyr ? too long
(only changes by factor 2 on timescale of
Universe) Use Uranium 238U half life 4.5Gyr
385.957nm
Cayrel et al. (2001) - First measurement of
Stellar Uranium CS31082-0018 lg(U/H)-13.70.14 t
12.5 3 Gyr (Principal uncertainty from
production ratios) N.B. usually the lines are too
weak but CS31082-0018 had unusually weak Fe line
strengths
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1.3 The Age of the Universe

• How old is our Universe ? -
• 2) White Dwarf Cooling Age

• Stars lt8Mo ? White Dwarfs
• Passively cool and dim after formation
• Fainter White Dwarfs ? Older White Dwarfs
• Search for the faintest White Dwarfs

Richer et al. (2002) - HST 8 days exposure ?30th
mag. White dwarfs (109 fainter than faintest
stars with the naked eye) Sharp edge in White
Dwarf LF corresponding to age 11Gyr t 12 - 13
Gyr (uncertainty from White Dwarf core
crystallization)
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1.3 The Age of the Universe

• How old is our Universe ? -
• 3) Age of Globular Clusters

• Globular Clusters
• ? Stars form same time

• These stars evolve
• ? Main Sequence of HR diagram

• Massive stars evolve to giant branch
• Main Sequence Turn off
• at MSTO L?M4
• Main Sequence Lifetime t?M3 ?L-3/4

• Most massive stars evolve fastest
• Turn off Main Sequence faster
• Less luminous Older Cluster
• MSTO point occurs at lower To

• Use RR Lyrae Stars to calculate distance to
  Cluster
• ? From the distance, can calculate the luminosity

Chaboyer (2001) - t 13.5 2 Gyr (Principal
uncertainty from distance scale)
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1.3 The Age of the Universe

• The Age of the Universe (L 0 World Models)

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1.3 The Age of the Universe

• Age of the Universe (L 0, W0 World Model)

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1.3 The Age of the Universe

• Age of the Universe (L 0, W1 World Model)

• The L 0, W1 World Model is the classical
  Einstein De Sitter (EdeS) universe
• For Ho72 ? already inconsistent with age
  estimates
• For Ho50 ? already still O.K.

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1.3 The Age of the Universe

• Age of the Universe (L 0, Wgt1 World Models)

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1.3 The Age of the Universe

• Age of the Universe (L 0, Wlt1 World Models)

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1.3 The Age of the Universe

• Age of the Universe (L ? 0World Models)

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1.3 The Age of the Universe

• Age of the Universe (L ? 0)
• (Wm WL1 flat case)

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1.3 The Age of the Universe

• An Age Crisis ?

Age Problem ! Ho72 EdeS ruled out !
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1.4 The Redshift

• The Redshift

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1.4 The Redshift

• The Redshift

• The correct interpretation of the cosmological
  Redshift of galaxies is NOT a Doppler shift.
• Rather, the wave fronts expand with the
  expanding Universe, stretching the photon
  wavelength.

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1.4 The Redshift

• Cosmological Redshift

Cosmological explanation Redshift - natural
consequence of assumption that R(t) is an
increasing function of time The wavefronts expand
with the expanding Universe, stretching the
photon wavelength.
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1.4 The Redshift

• Cosmological Redshift from General Relativity

Start from The Robertson-Walker Metric
ds the interval t time R the scale
factor r,q,f co-moving coordinates k the
curvature
For a photon (null geodesic dS0) traveling
radialy outward (df0 dq0)
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1.4 The Redshift

• Cosmological Redshift from General Relativity

Over single period of wave (blue light) t l/c
1.5x10-15s ? 10-33 Ho-1 ? R(t) ? constant
For an expanding Universe, R(to)gt R(te) ? zgt0 ?
redshift is observed
Emission from more distant objects - traveling
for greater time - more redshifted than nearer
objectes
At a redshift of 3 ? Universe was 1/4 present size
Highest observable redshift at surface of last
scattering, z 1000 ? Universe was 1000th
present size
Note 1z is really more fundamental than z
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1.4 The Redshift

• Cosmological Redshift

lobs(A)
Cosmological explanation Redshift - natural
consequence of assumption that R(t) is an
increasing function of time The wavefronts expand
with the expanding Universe, stretching the
photon wavelength.
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1.4 The Redshift

• Look Back Time

Speed of light is finite ? observations not
instantaneous We cannot help but look back in
time !!

• Nearest star (4.3ly) ? see as it was 4.3 years
  ago
• Sun 7mins
• Andromeda Galaxy 1.5 million years ago

• For Distant galaxies at high redshift, z ,
• observe them at look back times of Gyr,
• when Universe was a fraction of its present age

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1.4 The Redshift

• Look Back Time

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1.4 The Redshift

• Look Back Time

Snapshots at different redshift sample different
look back times
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1.4 The Redshift

• Observable Cosmological Parameters

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1.5 Our Universe

• Observations of the Cosmological Parameters

• Supernova Project
• (Observational Cosmology 2.1 Observational
  Parameters this seminar)
• Hubble Key Project
• (Observational Cosmology 2.4 The Distance
  Scale)
• WMAP Results
• (Observational Cosmology 2.2 The Cosmic
  Background)

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1.5 Our Universe

• Supernova Project Results

• Effect of Cosmological Constant Changes
  relationship between Redshift - time - Distance

• Can we find such a population ?
• QSOs ?
• Supernova (Massive exploding stars) ?
• Supernova sub class - Type 1a o

• Supernova sub class - Type 1a
• After supernova occurs the light gradually
  fades.
• Absolute magnitude at peak of supernovae depends
  on shape of their light decay curve.
• Brighter Supernovae - slower climb and decay of
  light curve.
• Measure absolute magnitude, observed flux ?
  DISTANCE !!!

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1.5 Our Universe

• Supernova Project Results

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1.5 Our Universe

• Supernova Project Results

• Imagine yourself in your local TV store in front
  of a wall of TV's.
• Let's say the average TV (monitor) is 500x500
  pixels. Now, stack 4 rows of 8 TVs that's 32 TVs
  which is the display size you need to display 1
  entire image from 1 of our CCD chips. The Mosaic
  Camera on the CFHT telescope has 12 chips.
• For 12 chips, you need to take your set of 4x8
  TVs, place 6 of these sets side by side, place
  another 6 sets more on top of them. That's 384 TV
  monitors.
• 384 TVs 1 entire exposure from our CCD camera.
  
• In 1 night we take 42 exposures.... that's 8064
  TVs worth of pixels.
• We need to search 16128 TVs worth of data to
  find the handful of supernovae.
• We get hungry doing this, and while in La
  Serena, Chile, we recommend getting your pizza
  delivered from Pizza Mania Specialita in pasta e
  pizza, para servirse, llevar y domicilio.

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1.5 Our Universe

• Supernova Project Results

? DL (z0.1) (Einstein De Sitter universe)
95 (Concordance Cosmology Universe) ? DL (z1)
(Einstein De Sitter universe) 75
(Concordance Cosmology Universe)
qolt0 ? Universe is accelerating ?standard candles
of lower brightness
qogt0 ? Universe is decelerating ?standard candles
of higher brightness
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1.5 Our Universe

• Supernova Project Results

A GOOD FIT Ho72kms-1Mpc-1 Wm,o 0.3 WL,o 0.7
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1.5 Our Universe

• SuperNova Acceleration Probe SNAP

• The next generation of supernova detections
• Dedicated dark energy probe
• Discovery 12 years

• Supernova Projects Discovery 80 Supernova
• SNAP discovery rate 2,000 per year
• 1.8- to 2.0-meter telescope,
• 1-square-degree imager,
• 1-square-arcminute near-IR imager,
• 3-channel near-UV-to-near-IR spectrograph.
• Observing strategy monitor a twenty-square-degre
  e region near NEP/SEP,
• Discover and follow supernovae that explode in
  that region.
• Every field visited frequently enough with
  sufficiently long exposures that every supernova
  up to z 1.7 will be discovered within, on
  average, two restframe days of explosion.

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1.5 Our Universe

• SuperNova Acceleration Probe SNAP

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1.5 Our Universe

• The Hubble Key Project

• Discovery of Cepheids with the HST
• Comparison of many distance determination
  methods
• Comparison of systematic errors
• Determination of Ho10

• Cepheids in nearby galaxies within 12 million
  light-years.
• Not yet reached the Hubble flow
• Need Cepheids in galaxies at least 30 million
  light-years away
• Hubble Space Telescope observations of Cepheids
  in M100.
• Calibrate the distance scale

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1.5 Our Universe

• The Hubble
• Key Project

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1.5 Our Universe

• The WMAP Results

Wilkinson Microwave Anisotropy Probe (2001 at L2)
Detailed full-sky map of the oldest light in
Universe. It is a "baby picture" of the
380,000yr old Universe

• Temperature fluctuations imprinted on CMB at
  surface of last scattering

• Open Universe - photons move on faster
  diverging pathes gt angular scale is smaller for
  a given size

• Temperature fluctuations over angular scales in
  CMB correspond to variations in matter/radiation
  density

Scientific American 2002
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1.5 Our Universe

• The WMAP Results

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1.5 Our Universe

• Putting it all together

• Supernovae Data
• ? Ho 72
• ? Accelerating expansion
• WL gt 0

• Hubble Key Project Data
• Ho 72

• CMB Data
• ? Flat universe
• ? WL gt 0

• Combine SN, CMB, LSS
• ? Dark energy WL 0.7

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1.6 Summary

• Summary

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1.6 Summary

• Summary

?
Observational Cosmology 1. Observational
Parameters
Observational Cosmology 2. The Cosmic Background
?