Bruce Bassett - Constraining Exotic Cosmologies
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Transcript of Bruce Bassett - Constraining Exotic Cosmologies
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Constraining Exo-c Cosmologies Bruce Basse4
SAAO, AIMS & UCT
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Conclusions
• Ge?ng convincing data constraints is much, much harder for exo-c models than for the “standard model”
• There are few data in cosmology that are truly model-‐independent
• One should be cau-ous and more demanding when evalua-ng claims that exo-c models are ruled out from observa-onal data
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A crude characterisa-on of science is that it is the circle:
Create a (new) theory that fits the exis-ng data. Make new predic-ons.
Take new data
1
2 3 Test the Theory predic-ons Against the new data
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In the beginning there is either…
data and no theory (e.g. astronomy)
or
theory and no data (e.g. quantum gravity)
Specialisa-on in one even when there are both data and theory
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1994, 140 cita-ons
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In prac-ce…
σi di
ti
χ2 ~ 9 di
If the data depends on the theory/model, then they have to be analysed together, self-‐consistently.
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Example 1: Type Ia Supernovae
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SEARCH TEMPLATE DIFFERENCE
g
r
i
SDSS Manual Scan Interface
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Lightcurves
ugr lightcurve fits to templates SDSS SN Survey
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Cons-tu-on 09 ΩM=0.281±0.026 (+BAO)
Evolution of the Hubble Diagram 1996-2009
SCP 96 ΩΜ=0.94±0.31 (flat)
High-‐z / SCP 98 ΩM=0.28±0.10 (flat)
ESSENCE+SNLS 07 ΩM=0.267±0.023 (flat)
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Luminosity Distance
€
dL (z) =(1+ z)H0 −Ωk
sin H0 −Ωkdz'H(z')∫
⎛
⎝ ⎜
⎞
⎠ ⎟
Distance Modulus
€
µ(z) = 5log10dL (z)Mpc
⎛
⎝ ⎜
⎞
⎠ ⎟ + 25 + K
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So…
• SNIa distances seem cosmology independent
• But there is a problem…peculiar veloci-es
• In LTB models, the peculiar veloci-es are generically much larger than in FLRW. This changes the “true” redshii and has to be included in doing any fi?ng.
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ESSENCE+SNLS 07
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Loops -‐ A neat test of LTB?
Mustapha et al, 1998 Clarkson, BB, Liu, 2008
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Example 2: Baryon Acous-c Oscilla-ons
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Bashinsky and Bertschinger, 2001
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White, Eisenstein, Seo
Evolu-on of a Spherical Overdensity
Baryon Density Photon Density Mass Profiles
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Baryon Density Photon Density Mass Profiles
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Baryon Density Photon Density Mass Profiles
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Baryon Density Photon Density Mass Profiles
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Baryon Density Photon Density Mass Profiles
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Baryon Density Photon Density Mass Profiles
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Baryon Acous-c Peak
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Sta-s-cal Standard Rulers
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The power of BAO
Us Tangen-al
Radial
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Assump-ons that go into the BAO
1. Greens func-on (and sound horizon) are isotropic
2. Sound horizon is a known size 3. Know the true dA – z rela-on 4. Galaxy bias effects can be ignored/removed
5. Nonlinear effects and peculiar veloci-es are small and can be calibrated by N-‐body simula-ons
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BAO and Nonlinearity
Crocce et al
Nonlinearity moves the peak to smaller scales and broadens it.
Strongly dependent On amount of nonlinearity And strength of gravity
Linear FLRW Predic-on
Nonlinear FLRW Predic-on
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None of these are true for LTB
Don’t use the DV results blindly!
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A test of the Copernican Principle
• If we compare constraints from the radial and transverse BAO and SNIa, assuming FLRW, we would not expect them to agree if the true model is LTB
• …because the expansion rates in the radial and transverse direc-ons are different.
Clarkson, BB, Liu, 2008
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What about the CMB?
The key ques-on for the
CMB is: “are there coherent acous-c oscilla-ons for a generic
on-‐centre LTB observer?” (cf. ac-ve sources)
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Urres-lla et al, 2008
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Conclusions
• Ge?ng convincing data constraints is much, much harder for exo-c models than for the “standard model”
• There are few data in cosmology that are truly model-‐independent
• One should be cau-ous and more demanding when evalua-ng claims that exo-c models are ruled out from observa-onal data
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