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Science of the Dark Energy Survey

Science of the Dark Energy Survey. Josh Frieman Fermilab and the University of Chicago Astronomy 41100 Lecture 2, Oct. 15, 2010. DES Collaboration Meeting. Go to: http://astro.fnal.gov/desfall2010/Home.html Science Working group meetings on Tuesday. Plenary sessions Wed-Fri.

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Science of the Dark Energy Survey

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  1. Science of the Dark Energy Survey Josh Frieman Fermilab and the University of Chicago Astronomy 41100 Lecture 2, Oct. 15, 2010

  2. DES Collaboration Meeting Go to: http://astro.fnal.gov/desfall2010/Home.html Science Working group meetings on Tuesday. Plenary sessions Wed-Fri.

  3. Cosmological Constant as Dark Energy Einstein: Zel’dovich and Lemaitre:

  4. Cosmological Constant as Dark Energy Quantum zero-point fluctuations: virtual particles continuously fluctuate into and out of the vacuum (via the Uncertainty principle). Vacuum energy density in Quantum Field Theory: Theory: Data: Pauli Cosmological Constant Problem

  5. Dark Energy: Alternatives to Λ The smoothness of the Universe and the large-scale structure of galaxies can be neatly explained if there was a much earlier epoch of cosmic acceleration that occurred a tiny fraction of a second after the Big Bang: Primordial Inflation Inflation ended, so it was not driven by the cosmological constant. This is a caution against theoretical prejudice for Λ as the cause of current acceleration (i.e., as the identity of dark energy).

  6. Light Scalar Fields as Dark Energy Perhaps the Universe is not yet in its ground state. The `true’ vacuum energy (Λ) could be zero (for reasons yet unknown). Transient vacuum energy can exist if there is a field that takes a cosmologically long time to reach its ground state. This was the reasoning behind inflation. For this reasoning to apply now, we must postulate the existence of an extremely light scalar field, since the dynamical evolution of such a field is governed by JF, Hill, Stebbins, Waga 1995

  7. Scalar Field as Dark Energy(inspired by inflation) • Dark Energy could be due to a very light scalar field j, slowly evolving in a potential, V(j): • Density & pressure: • Slow roll: V(j) j

  8. Scalar Field Dark Energy aka quintessence General features: meff< 3H0 ~ 10-33 eV (w < 0) (Potential > Kinetic Energy) V ~ m22 ~ crit ~ 10-10 eV4  ~ 1028 eV ~ MPlanck V(j) (10–3 eV)4 j 1028 eV Ultra-light particle: Dark Energy hardly clusters, nearly smooth Equation of state: usually, w > 1 and evolves in time Hierarchy problem: Why m/ ~ 1061? Weak coupling: Quartic self-coupling  < 10122

  9. The Coincidence Problem Why do we live at the `special’ epoch when the dark energy density is comparable to the matter energy density? matter ~ a-3 DE~ a-3(1+w) a(t) Today

  10. Scalar Field Models & Coincidence `Dynamics’ models (Freezing models) `Mass scale’ models (Thawing models) V V e.g., e– or –n   MPl Runaway potentials DE/matter ratio constant (Tracker Solution) Pseudo-Nambu Goldstone Boson Low mass protected by symmetry (Cf. axion)JF, Hill, Stebbins, Waga V() = M4[1+cos(/f)] f ~ MPlanck M ~ 0.001 eV ~ m Ratra & Peebles; Caldwell,etal

  11. PNGB Models • Tilted Mexican hat: M4 • Spontaneous symmetry breaking at scale f • Explicit breaking at scale M • Hierarchy protected by symmetry f Frieman, Hill, Stebbins, Waga 2005

  12. Dynamical Evolution of Freezing vs. Thawing Models Caldwell & Linder Measuring w and its evolution can potentially distinguish between physical models for acceleration 12

  13. Runaway (Tracker) Potentials Typically  >> Mplanck today. Must prevent terms of the form V() ~ n+4 / Mplanckn up to large n What symmetry prevents them?

  14. Perturbations of Scalar Field Dark Energy If it evolves in time, it must also vary in space. h = synchronous gauge metric perturbation Fluctuations distinguish this from a smooth “x-matter” or  Coble, Dodelson, Frieman 1997 Caldwell et al, 1998

  15. Dark Energy Interactions Couplings to visible particles must be small Carroll 1998 Frieman & Gradwohl 1990, 1992 Couplings cause long-range forces Example: scalar field coupled to massive neutrino Attractive force between lumps of 

  16. What about w < 1? The Big Rip • H(t) and a(t) increase with time and diverge in finite time • e.g, for w=-1.1, tsing~100 Gyr • Scalar Field Models: • need to violate null Energy condtion+ p > 0: • for example: • L = ()2 V • Controlling instability requires cutoff at low mass scale • Modified Gravity models • apparently can achieve effective w< –1 without • violating null Energy Condition Caldwell, etal Hoffman, etal

  17. Modified Gravity & Extra Dimensions • 4-dimensional brane in 5-d Minkowski space • Matter lives on the brane • At large distances, gravity can leak offbraneinto • the bulk, infinite 5th dimension Dvali, Gabadadze, Porrati • Acceleration without vacuum energy on the brane, • driven by brane curvature term • Action given by: • Consistency problems: ghosts, strong coupling

  18. Modified Gravity • Weak-field limit: • Consider static source on the brane: • Solution: • In GR, 1/3 would be ½ • Characteristic cross-over scale: • For modes with p<<1/rc: • Gravity leaks off the brane: longer wavelength gravitons free to • propagate into the bulk • Intermediate scales: scalar-tensor theory

  19. Cosmological Solutons • Modified Friedmann equation: • Early times: H>>1/rc: ordinary behavior, decelerated expansion. • Late times: self-accelerating solution for (-) • For we require • At current epoch, deceleration parameter is • corresponds to weff=-0.8

  20. Growth of Perturbations • Linear perturbations approximately satisfy: • Can change growth factor by ~30% relative to GR • Motivates probing growth of structure in addition to expansion rate

  21. Modified Gravity: f(R) This particular realization excluded by solar system tests, but variants evade them: Chameleon models hide deviations from GR on solar system scales

  22. Bolometric Distance Modulus • Logarithmic measures of luminosity and flux: • Define distance modulus: • For a population of standard candles (fixed M), measurements of  vs. z, the Hubble diagram, constrain cosmological parameters. flux measure redshift from spectra

  23. Distance Modulus • Recall logarithmic measures of luminosity and flux: • Define distance modulus: • For a population of standard candles (fixed M) with known spectra (K) and known extinction (A), measurements of  vs. z, the Hubble diagram, constrain cosmological parameters. denotes passband

  24. K corrections due to redshift SN spectrum Rest-frame B band filter Equivalent restframei band filter at different redshifts (iobs=7000-8500 A)

  25. Absolute vs. Relative Distances • Recall logarithmic measures of luminosity and flux: • If Mi is known, from measurement of mi can infer absolute distance to an object at redshiftz, and thereby determine H0 (for z<<1, dL=cz/H0) • If Mi (and H0) unknown but constant, from measurement of mi can infer distance to object at redshiftz1 relative to object at distance z2: independent of H0 • Use low-redshiftSNe to vertically `anchor’ the Hubble diagram, i.e., to determine

  26. Type Ia Supernovae as Standardizable Candles SN 1994D

  27. Ia SN Spectra ~1 week after maximum light Filippenko 1997 II Ic Ib

  28. Type Ia Supernovae Thermonuclear explosions of Carbon-Oxygen White Dwarfs White Dwarf accretes mass from or merges with a companion star, growing to a critical mass~1.4Msun (Chandrasekhar) After ~1000 years of slow cooking, a violent explosion is triggered at or near the center, and the star is completely incinerated within seconds In the core of the star, light elements are burned in fusion reactions to form Nickel. The radioactive decay of Nickel and Cobalt makes it shine for a couple of months

  29. General properties: Homogeneous class* of events, only small (correlated) variations Rise time: ~ 15 – 20 days Decay time: many months Bright: MB ~ – 19.5 at peak No hydrogen in the spectra Early spectra: Si, Ca, Mg, ...(absorption) Late spectra: Fe, Ni,…(emission) Very high velocities (~10,000 km/s) SN Ia found in all types of galaxies, including ellipticals Progenitor systems must have long lifetimes Type Ia Supernovae *luminosity, color, spectra at max. light

  30. SN Ia Spectral Homogeneity(to lowest order) from SDSS Supernova Survey

  31. Spectral Homogeneity at fixed epoch

  32. SN2004ar z = 0.06 from SDSS galaxy spectrum Galaxy-subtracted Spectrum SN Ia template

  33. How similar to one another?Some real variations:absorption-line shapes at maximumConnections to luminosity?Matheson, etal, CfA sample

  34. Supernova IaSpectral Evolution Late times Early times Hsiao etal

  35. Layered Chemical Structure provides clues to Explosion physics

  36. SN1998bu Type Ia Multi-band Light curve Extremely few light-curves are this well sampled Suntzeff, etal Jha, etal Hernandez, etal

  37. Luminosity m15 15 days Time Empirical Correlation: Brighter SNeIa decline more slowly and are bluer Phillips 1993

  38. SN Ia Peak Luminosity Empirically correlated with Light-Curve Decline Rate Brighter  Slower Use to reduce Peak Luminosity Dispersion Phillips 1993 Peak Luminosity Rate of decline Garnavich, etal

  39. Type Ia SN Peak Brightness as calibrated Standard Candle Peak brightness correlates with decline rate Variety of algorithms for modeling these correlations: corrected dist. modulus After correction, ~ 0.16 mag (~8% distance error) Luminosity Time

  40. Published Light Curves for Nearby Supernovae Low-z SNe: Anchor Hubble diagramTrain Light-curve fittersNeed well-sampled, well-calibrated, multi-band light curves

  41. Low-z Data

  42. Carnegie Supernova Project Nearby Optical+ NIR LCs

  43. Correction for Brightness-Decline relation reduces scatter in nearby SN Ia Hubble Diagram Distance modulus for z<<1: Corrected distance modulus is not a direct observable: estimated from a model for light-curve shape Riessetal 1996

  44. Acceleration Discovery Data:High-z SN Team 10 of 16 shown; transformed to SN rest-frame Riessetal Schmidt etal V B+1

  45. Riess, etal High-z Data (1998)

  46. High-z Supernova Team data (1998)

  47. Likelihood Analysis This assume Goliath etal 2001

  48. Exercise 5 • Carry out a likelihood analysis of using the High-Z Supernova Data of Riess, etal 1998: use Table 10 above for low-z data and the High-z table above for high-zSNe. Assume a fixed Hubble parameter for the first part of this exercise. • 2nd part: repeat the exercise, but marginalizing over H0 with a flat prior, either numerically or using the analytic method of Goliath etal. • Errors: assume intrinsic dispersion of and fit dispersions from the tables and dispersion due to peculiar velocity from Kessler etal (0908.4274), Eqn. 28, with

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