Patrick F. Bassett, NAIS President [email protected] Change Agency Leadership.
Bruce Bassett - Constraining Exotic Cosmologies
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Transcript of Bruce Bassett - Constraining Exotic Cosmologies
Constraining Exo-c Cosmologies Bruce Basse4
SAAO, AIMS & UCT
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
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
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
1994, 140 cita-ons
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.
Example 1: Type Ia Supernovae
SEARCH TEMPLATE DIFFERENCE
g
r
i
SDSS Manual Scan Interface
Lightcurves
ugr lightcurve fits to templates SDSS SN Survey
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)
Luminosity Distance
€
dL (z) =(1+ z)H0 −Ωk
sin H0 −Ωkdz'H(z')∫
⎛
⎝ ⎜
⎞
⎠ ⎟
Distance Modulus
€
µ(z) = 5log10dL (z)Mpc
⎛
⎝ ⎜
⎞
⎠ ⎟ + 25 + K
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.
ESSENCE+SNLS 07
Loops -‐ A neat test of LTB?
Mustapha et al, 1998 Clarkson, BB, Liu, 2008
Example 2: Baryon Acous-c Oscilla-ons
Bashinsky and Bertschinger, 2001
White, Eisenstein, Seo
Evolu-on of a Spherical Overdensity
Baryon Density Photon Density Mass Profiles
Baryon Density Photon Density Mass Profiles
Baryon Density Photon Density Mass Profiles
Baryon Density Photon Density Mass Profiles
Baryon Density Photon Density Mass Profiles
Baryon Density Photon Density Mass Profiles
Baryon Acous-c Peak
Sta-s-cal Standard Rulers
The power of BAO
Us Tangen-al
Radial
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
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
None of these are true for LTB
Don’t use the DV results blindly!
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
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)
Urres-lla et al, 2008
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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