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Dark Energy

Ay 21, 2010

Measuring the Acceleration of the Expanding Universe

•  for a few decades, was one of the most-pursued cosmological parameters, known as “q0”, or the “deceleration parameter”. –  note the implicit assumption about the “sign” of

the acceleration! •  ideally, one uses a “standard candle” and notes

change in the relationship between distance and redshift over very large distances (i.e., the variation of the “Hubble constant” over time)

•  For universe with cosm. constant, q0=Ωm,0/2- ΩΛ,0

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Type Ia Supernovae: Best-Bet Standard Candles

Type Ia Supernovae

•  Current best understanding of Type Ia’s is that they result from runaway fusion in a degenerate C-O white dwarf.

•  Believed to occur only when an evolved stellar companion dumps mass onto the WD that causes it to exceed the 1.4 Msun “Chandrasekhar mass”, causing collapse of the WD and central densities at which C-fusion can proceed very rapidly: kaboom.

•  The “light curves” of Type Ia supernovae are remarkably uniform, making them incredibly useful objects for measuring distances to remote galaxies…and, their discovery can be “planned”!

•  Since the progenitors can be old stars, they can occur even in galaxies with no current star formation and no massive stars

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• In reality, Sne Ia are only “kinda sorta” standard candles

• You can improve their usefulness by noting that the variable in peak luminosity and rise/decay time are correlated with their absolute brightness

• this is purely empirical, not well-understood WHY

• Once these “corrections” are applied, they are actually quite good, where the SNe distances can be checked using other distance measures.

• You must assume that the same rules apply for more distant supernovae...

SN Cosmology Project

1998 • The SNe (from 2 independent groups) at z~0.3-0.8 indicate that SNe are fainter at a given z than would be expected under assumption of constant rate of expansion (or slow deceleration)

• All comes down to a difference of ~25% in apparent brightness

• What could go wrong?

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• What could go wrong?

• The SNe are getting faint, and accurate measures of their apparent brightness are very difficult.

• We are talking about SNe that went off ~7 billion years ago. Are they the same as those used to calibrate everything locally?? • What about dust?

• great lengths have been taken to track down possible problems

• none seem to be “show-stoppers”

• best test: finding the expected “deceleration” era

high redshift SNe spectra (mostly from Keck)

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• Note that for the favored model, the deviation from an “empty universe” should reverse sign just beyond a redshift of z~1!

• Such behavior would be unexpected from all of the other possible systematic errors (e.g., dust, etc.)

• Why is this expected? • as you go to higher redshift, the matter becomes more and more important relative to any constant term, and beyond a certain redshift, Λ is unimportant dynamically (IF it is constant!)

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2004

journal version

• Popular version

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The “w” Parameter • Recall the equation of state relating pressure and energy density: P=wε, where ε is energy density and w is a dimensionless number.

• For cosmological constant, w=-1.0 • Does the equation of state evolve with redshift (i.e., is it really a cosmological constant, or something more complicated)? • Requires precision measures of q0 (standard candle) or of curvature (standard rod) as a function of redshift...

2007 journal version

Riess et al 2007

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Cosmic Background “Power Spectrum”

Detailed results say that (e.g.) the universe is spatially flat and the matter density is such that Ωm=0.27!

The angular position of the “first peak” is very sensitive to the sum of Ωm+ ΩΛ

“Cosmic Concordance”

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“Cosmic Concordance?”

“Best fit” from SN data alone has ΩΛ~1.2, Ωm~0.6

Carnegie SN Project (2010)

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What is “Dark Energy” •  We don’t know. We are told to think of it as “negative

pressure”, or the energy associated with the “vacuum”. •  What do we know about how it behaves, so far?

–  “vacuum energy density”=constant, or “quintessence” •  related to whatever process caused “inflation”?

–  it looks like it is either constant or slowly varying –  The “amount” seems fairly well established by the

combination of CBR+SNe observations –  many, many plans to try to measure its possible evolution,

using SNe, and other possible standard rods/candles. –  it matters most at relatively modest redshifts, where

the transition from matter domination to DE domination of the dynamics occurs...

universe begins to accelerate here

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Some aggravating issues... Why are the matter density and dark energy density so similar now??

(What is so special about the “present”??

Some aggravating issues...

Known physics would predict that the natural value of the “cosmological constant”, if it exists, is 120 orders of magnitude larger!

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Some food for thought... • The Universe has entered an accelerating phase and will likely continue that forever. The future of the universe is rather cold and lonely.

• The “Big Jerk” happened right about the same time that the sun and solar system formed (z~0.5)

• If ΩΛ were only a few times larger, the universe would have looked very different!

• If ΩΛ were significantly larger no structure would ever have formed

• How many chances were there to make a universe that looks just like the one we have?

Sobering thoughts... • The stuff we observe directly accounts for only about 4-5% of the total “content” of the universe

• dark matter is firmly established and it fills in another ~23% or so. (but we don’t know what it is)

• “dark energy” is the rest. We’ve only known it’s there for a few years (will it go away?) and yet it makes up the rest of the 73%. We definitely don’t understand it within the context of current physics. There is a limited observational handle on measuring whether or not it evolves.

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“Baryon Acoustic Oscillations” •  There is a small “bump” expected in the fluctuation power

spectrum on scales corresponding to the same oscillations present in the CMB power spectrum

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Eisenstein et al 2005

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