Discerning Dark Energy

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Discerning Dark Energy with Type Ia Supernovae
Brian P. Schmidt
The Research School of Astronomy and
Astrophysics Mount Stromlo Observatory
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Our Reference (Standard) Model
Isotropic and Homogenous Universe Robertson-Walker
line element
General Relativity Friedmann Equation
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Luminosity Distance
for a monochromatic source (defined as
inverse-square law)
the flux an observer sees of an object at
redshift z
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Type Ia Supernovae
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Spectra of SN Ia show intermediate mass elements
on the outside 1-2 M? and a total mass in Iron
of about 0.6 M? in the centre.
Ca S Si i
Fe
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SN 1572 - Tychos SNR
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• Model Version 1
• White Dwarf approaches Chandrasehkhar mass (1.38
  M?) by accreting material from a binary companion
• Radius of star drops rapidly, leading to the
  ignition of Carbon in stars core,
• and eventually, a thermal runaway...

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• A few oustanding issues...
• Our knowledge of the physics of
  quasi-relativistic turbulent deflagrations and
  detonations, despite 50 years of Cold War
  research, is still not able to make a Ia explode
  (without cheating)
• Where are the systems that become SN Ias?
• This basic model predicts the secondary donor
  star should remain intact and be visible...
• Ruiz-Lapuente et al. (2004) Claim a discovery in
  the Tychos SN of 1572 ... but I am refuting this
  discovery.

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• Alternative models include, white-dwarf -
  white-dwarf mergers where total mass exceeds 1.38
  M?
• But physics suggests these go straight to Neutron
  Stars
• Also progenitors seem to be missing
• Seems Very heterogenous compared to observations

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SN Ia are not all the same! A Useful Way of
Parameterizing SNe Ia is by the Shape of their
Light Curve
Phillips (1993) Hamuy et al. (1996)
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Proof is really in the puddingSN Ia,
empirically, work.
many methods dm15, MLCS, stretch, BATM, SALT,
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The Standard Model 1995

Universe is Made up of normal matter
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51 citations versus 6...
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High-Z SN Ia History
Zwickys SN Search from 1930s-1960s giving
Kowals Hubble Diagram in 1968 Ib/Ic SN
Contamination realised in 1984/5 1st distant SN
discovered in 1988 by a Danish team (z0.3)
7 SNe discovered in 1994 by Perlmutter et al. at
z 0.4
Calan/Tololo Survey of 29 Nearby SNe Ia completed
in 1994
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4 April
28 April
SN
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Arguing this isnt evolution
Type Ia Supernovae occur in galaxies with no
recent star formation, but preferentially in
Galaxies which are currently rapidly forming
stars. Indicates Progenitors span a wide range
of ages 0.5-10Gyr SN Ia in old galaxies rise and
fall faster, on average, than their siblings in
younger galaxies SN Ia properties do change
based on age! SN Ia distances in young (metal
poor) and old galaxies (metal rich) are
consistent within 2 of each other in the 4-8
Gyr back to z1, changes in galaxy populations
are less than what we see locally - hence we
expect evolution to be less than 2 in DL.
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Although Saul, Adam and Brian have won prizes...
Saul Perlmutter
Adam Riess
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Bruno Leibundgut
It really was a team effort...
Nick Suntzeff
Saurabh Jha
Armin Rest
John Tonry
Chris Smith
Brian Barris
Peter Garnavich
Adam Riess
Stephen Holland
Jason Spyromilio
Chris Stubbs
Gajus Miknaitis
Mark Phillips Mario Hamuy Jose Maza Bob
Schommer Ron Gilliland
Weidong Li
Alex Filippenko
Brian Schmidt
Alejandro Clocchiatti
Tom Matheson
Bob Kirshner

• Pete Challis

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6 More Years of Work 150 Supernova, 250,000
redshifts and a year in space...
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So What is the Dark Energy?

• One possibility is that the Universe is permeated
  by an energy density, constant in time and
  uniform in space.
• Such a cosmological constant (Lambda ?) was
  originally postulated by Einstein, but later
  rejected when the expansion of the Universe was
  first detected.
• If dark energy is due to a cosmological constant,
  its ratio of pressure to energy density (its
  equation of state) is w P/? -1 at all times.
• A model with falsifiable predictions!

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Why Now? If Earth born at z1.45 instead of
z0.45, we couldnt have made measurement.
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So What is the Dark Energy?

• A dynamical fluid, not previously known to
  physics.
• In this case the equation of state of the fluid
  would not be constant, but would vary with time.
• Different theories of dynamical dark energy cause
  produce a different evolution of the equation of
  state
• Unfortunately none of these theories is
  particularly constrained, and most can spend much
  of their time looking like a Cosmological
  Constant.

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So What is the Dark Energy?

• Alternatively, accelerating expansion of the
  Universe might mean that ...
• General Relativity needs to be modified?
• standard cosmological model is incorrect
  (Homogeneity and Isotropy wrong?)
• But hard to get these modifications in and not
  conflict with the Cosmic Microwave Background
  Measurements, and those of Large Scale Structure
• .

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Stolen from Karl Glazebrook
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Dark Energy Task Force(Rocky et al.)

• We need to determine as well as possible whether
  the accelerating expansion is consistent with
  being due to a cosmological constant.
• Accepted currency of experiments is constraining
  power to measure w(a)w0w'(a).
• This maybe the currency of choice in comparing
  experiments, but it doesnt mean we should use it
  as the method to report our results!
• (i.e. if a model actually predicts behaviour,
  then compare the predictions of the model in the
  natural space of the observations)
• Exchange Rate 1 unit of w'(a) 100,000,000

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Key Methods of the Future to Constrain Dark Energy

• Supernova (SN) surveys use Type Ia supernovae as
  standard candles to determine the luminosity
  distance vs. redshift relation. The SN technique
  is sensitive to dark energy through its effect on
  this relation.
• Baryon Acoustic Oscillations (BAO) are observed
  in large-scale surveys of the spatial
  distribution of galaxies. The BAO technique is
  sensitive to dark energy through its effect on
  the angular-diameter distance vs. redshift
  relation and through its effect on the time
  evolution of the expansion rate.
• Galaxy Cluster (CL) surveys measure the spatial
  density and distribution of galaxy clusters. The
  CL technique is sensitive to dark energy through
  its effect on a combination of the
  angular-diameter distance vs. redshift relation,
  the time evolution of the expansion rate, and the
  growth rate of structure.
• Weak Lensing (WL) surveys measure the distortion
  of background images due to the bending of light
  as it passes by galaxies or clusters of galaxies.
  The WL technique is sensitive to dark energy
  through its effect on the angular distance vs.
  redshift relation and the growth rate of
  structure.

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My own Summary of the Various Methods

• BAO Very Systematically Clean (very little can
  go wrong!), but the least powerful. not enough
  galaxies to pin down w(z).
• SN The most powerful to use now, but how do we
  know SN Ia properties do not subtly change with
  z.
• Clustering Difficult to know observed growth of
  structure isnt tainted by observational biases.
• Weak Lensing Potentially the most powerful, but
  a LONG ways from proving that it can deliver
  systematic free measurements of sheer and
  phot-zs,

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Current Results on w

• Supernova measurements of DL from z0 to z1.5
  (Nearby, SCP, High-Z, CFHTLS, Essence, Higher-Z)
• BAO (CMB constraint of acoustic scale at z1089)
  measurement of 4 by SDSS at ltz0.35gt
• ?M measurement of 0.27 0.03 via 2dFSDSS
• WMAP LSS HST Key Project combined constraints

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SNLS Austier et al.
Mark Sullivan of the SNLS survey will present
their results this afternoon. These include
results on luminosity distance, limiting
evolution, and looking at the host galaxies of
the objects that explode.
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Essence
Michael Wood-Vasey et al. ApJ in Press
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Flat Universe w-1.05.13 (0.13 mag sys)
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All SN Ia (caveat emptor!!)new SN experiments
exists for a reason...they try to control
systematic errors
w(z)w0wa(z) average (w) has to be near -1,
but large range of values allowed
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Higher-Z
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Hubble has found 50 new Supernovae Half beyond
the reach of the ground
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How you treat w(z) Matters!
4th order linear
Eisensteins w(z)Self-similar in redshift
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Supernovae measureLuminosity Distance!
At this point, The SN discovered via the Essence
Program, SNLS, Higher-Z, and in fact, all other
programs are consistent with A simple
Cosmological Constant Model. Since Dark Energy
Models right now are not full developed. Lets not
fool ourselves fitting a parameterised model
unless we must! DL is consistent to 3-5 from z0
to z1.3 with a flat Universe dominated by a
Cosmological Constant. http//www.ctio.noao.edu/w
project/wresults
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Dark Energy looks like ?

• As near as we can tell the Universe is expanding
  just as a Cosmological Constant would predict.
• based on luminosity distance between z0 to z1.5
  from SN Ia -
• Angular-size distance (modified) between z0.35
  and z1089 (BAOCMB)
• and power spectrum info from LSSCMB.
• CMB, BAO, LSS, an SN Ia are all Consistent.

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Systematic Errors in SN Ia
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Supernovae-End of the Line?

• Essence (and I believe SNLS) are beginning to
  reach the systematic barrier.
• The value of w(z) that we obtain now varies at
  about the size of our statistical errors on the
  choices we make in analysing the data.
• A Hubble Bubble.maybe present (Jha et al.
  2006)if so, we need to enlarge the nearby sample
  of objects beyond zgt0.04 (KAIT, SDSSII,SkyMapper)
• Extinction Simultaneously trying to fit
  extinction and light curve shape is difficult,
  even with extensive data.
• Essence separates out the effect of SN colour and
  Reddening into two separate vectors via MLCS.
• SNLS uses SALT which uses a single colour to
  account for both.
• How one does these corrections is our largest
  source of uncertainty at present - it changes
  DL(z) by 2 (w0.1)

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Improving Dark Energy Measurements with SN
Iaakahow to ask for more telescope time

• Large sample of SN Ia (nearby and far) in
  non-star forming hosts to limit extinction
  problems.
• 1 in 7 SN Ia discovered is appropriate
• SNLS (150 _at_ ltzgt0.6) by 2009
• SDSS-II (40 ltzgt0.25) by 2009
• SkyMapper (200 ltzgt0.06) by 2012
• and of course increasing our physical
  understanding of SN Ia.

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SkyMapper

• 1.35m telescope with 5.7 sq degree imager (10s
  readout time)
• All Southern Sky Survey (2pi steradians)
• 6 colours 6 epochs each epoch about SDSS
• 1250 sq-degrees continually covered in poor
  seeing will find 100 SN Ia at zlt0.085 per year
• First light, 2nd half this year.

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The Future of Equation of StateMeasurements

• Improving SN Ia measurements becoming very
  difficult.
• BAO will provide interesting (systematic free?)
  measurements in the next few years at z0.7 (AAT)
  z1.2 (Subaru), but will not markedly improve on
  precision of SN Ia measurements. ADEPT or WFMOS
  or SKA will provide 2-3 times better precision
  than currently possible and naturally combine
  with SN Ia/CMB to give D from z0 to z1089
• In the future, the only hope of pushing to an
  order of magnitude greater precision is to build
  hugely expensive surveys. But I am extremely
  dubious of the ability to constrain systematic
  errors (be it SN or Weak Lensing).

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What Exactly is it that we are testing?

• It is proposed that we spend billions of dollars
  to measure the equation of state of the Dark
  Energy over the next decade.
• But what are we going to learn if we find Dark
  Energy continues to looks like ??
• As things stand currently, we are not going to
  rule out anything...almost all theories can look
  like ? to any level we are able to measure.

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Why I think Simon White could be right...

• Astronomy is still ripe with mysteries that are
  not Dark Energy.
• We are proposing to spend a large fraction of the
  Astronomy effort in a problem which I think could
  easily yield a null result.
• So Dark Energy is certainly good for Astronomy to
  dabble in - Cosmological experiments maybe able
  to relatively easily debunk the ? model. But
  huge projects that only measure w seem to be on
  the wrong side of the risk-benefit-cost line - at
  least right now.

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What Dark Energy Needs.

• Theorists, We need testability
• e.g. Extra-dimensionality models an be rejected
  by ?-scale gravity measurements.
• Observers, we need experiments that can make
  progress on measuring w that are either
  relatively cheap, or, if expensive, allow
  astronomy to progress on a wide range of its
  problems.