A new wa parametrization

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A new w(a) parametrization
W(a)-1?2f(a/aeq)

• Zhiqi Huang
• Co-Supervisors Lev Kofman Dick Bond

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Dark Energy Equation of State (EOS)
Scale factor a1/(1z)
Observationally SNe, BAO , WL, LSS, CMB
(Albrecht et al White Paper) Assuming constant
w, we roughly know w-1.0 0.2

• Theoretical Models --
• Cosmological Constant (w-1)
• Quintessence (-1w1)
• Phantom field (w-1)
• Tychyon fields (-1 w 0)
• K-essence ( no prior on w)

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Standard parametrization
Constraint equation
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Quintessence Model
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Field Equation
Friedmann Equations
W -cos 2?
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Slow roll conditions
? const
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Solution two-parameter parametrization.
the field exits scaling regime at aaex ? -V/V
averaged at low redshift
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Early-Exit Scenario

• Definition slow-roll conditions hold at 0ltzlt10.

Information from scaling regime is erased by
Hubble friction?aex term vanishes at low redshift
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One-parameter parametrization
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Include the phantom field

• f -gt if ? ?2lt 0

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Effective Constraint Equation

• Define w0wa1, wa-dw/daa1

FITTING
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Measuring ?2 (SNeCMBWLLSS)
Modified cosmomc with WL and time-varying w models
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Conclusion

• For general slow-rolling scenario one needs two
  parameters (aex,?)to describe w.
• In early-exit scenario, the information stored in
  aex is erased by Hubble friction, w can be
  described by a single parameter ?2.
• With the simplest one-parameter parametrization,
  phantom (?2 lt0), cosmological constant (?20),
  and quintessence (?2 gt0) models are all
  consistent with current observations.