# Presentation: 321572

## Observational Probes of Dark Energy

- Timothy McKay
- University of Michigan
- Department of Physics

- Observational cosmology:
- parameters (H0,0) =>
- evolution (a(t), g(z,k))

- For the future: from parameter
- measurement => testing models

**Notes:**
My goal in this brief presentation is to give an overview of some observational probes of dark energy. To do this, I’ll try to rephrase the question in a general way. 
Since Hubble’s discovery of cosmic expansion, we have known that the universe has a history, evolving from a dense, hot, and nearly uniform plasma to the rich and rarified structure we see today. Most remarkable, cosmic history is more than a rearrangement of matter. The space-time which the matter occupies is itself dynamic. 
For half a century, the goal of observational cosmology was to determine parameters of the present day universe: especially the current matter density and expansion rate. During the past decade, these goals have been tentatively achieved. In just the last five years, a suite of new tools has emerged which enable us to be substantially more ambitious. It is now possible to seriously probe the evolution of the universe, to observe the global expansion history and to measure the growth of fluctuations in the spacetime. We are finally able to go beyond constraining parameters of our model for cosmic expansion, and to have some hope of actually testing that model.

## Precision cosmology

- Tools of observational cosmology have become increasingly precise
- Large, well defined, and accurately observed surveys provide samples of SNe, galaxy clusters, galaxy redshifts, quasars, Ly- absorption lines, gravitational lenses, etc.
**Statistical precision is a burden**
- More careful comparison of theory to observables is required to turn precision into accuracy
- “Dark Energy” will play a key role: anomalies in the global evolution of spacetime.

**=> Determining the expansion history**

**Notes:**
The tools of observational cosmology have become increasingly precise. Large samples of objects, each precisely observed, are available for many of the classical cosmological probes. With this precision comes a burden, the requirement that we understand these probes, and their relation to theoretical predictions, to an unprecedented degree. As we stand on the brink of an era of precision cosmology, we must lay the groundwork for an accurate cosmology.
In this development, the dark energy problem will play a key role. “Dark Energy” is really just a name we use to describe anomalies in the global evolution of  spacetime. Since these anomalies have features consistent with an energy density, we label them dark energy. But the real mission of ‘dark energy’ probes is more basic; to determine the expansion history of the universe. 

## Current Supernova Results

- dL(z) measurements, made using type Ia SNe, provide spectacular Hubble diagrams
- These indicate an expansion rate increasing with time
- Shorthand: consistent with ~0.7

- SNe Cosmology Project

**Notes:**
Let’s quickly review where we are. Measurements of the redshift evolution of the luminosity distance, made using type Ia SNe as standard candles, provide spectacular Hubble diagrams. They indicate an expansion rate which is increasing in time. As a shorthand, let’s note that these measurements are consistent with a model having cosmological constant ~0.7.

## Current CMB and mass census constraints

- Measurements of the first CMBR Doppler peak find total=1
- Many measurements of clusters, baryon fractions, etc. find matter~0.3
- Combined, these independently  suggest the existence of dark energy

**Notes:**
There is an important alternative approach. An array of new measurements of the CMB Doppler peak suggest a spatially flat universe. An older set of measurements, developing since the early 1990s, find that the current matter density is only a third of critical. Combined, these measurements independently suggest the existence of dark energy.

## Combined constraints

- Convincing confirmation of anomalies in the expansion history by independent methods.
- We might be able to do this!
- Ignorance is large: cosmic expansion is more complex than we expected but now observationally accessible

**Notes:**
What’s most important about this? The spectacular confirmation of anomalies in the expansion by completely independent methods mostly just gives confidence that trying to do this is not insane. It looks like it’s now possible to probe beyond local cosmological parameters and really study the evolution of the spacetime.

We still know almost nothing about this. But we do have this tantalizing evidence that the expansion history of the universe is both surprisingly complex and observationally accessible.

## Measuring the global spacetime

- Measuring the expansion history, the expansion rate as a function of time, amounts to testing the redshift evolution of the effective density:

- Most directly from cosmological distance probes:

**Notes:**
I’ll talk today about measuring the expansion history to first and second order. First, there is the global expansion, the evolution of the overall scale factor. This is perhaps best expressed by the way it couples to the energy content of the universe.
Measurements of cosmic distances, luminosity distances or angular diameter distances, provide the most direct probe of this expansion history.

## Measuring fluctuations in the spacetime 

- In addition to the global expansion, we can study linear perturbations to the metric, the evolution of the growth factor.
- The whole suite of structure formation tools: Large scale structure, galaxy clusters, weak lensing etc.

**Notes:**
In addition to studying the global expansion, we can study linear perturbations to the metric, encoded as the evolution of the cosmic growth factor. The whole suite of structure formation tools, galaxy redshift surveys, studies of galaxy clusters, weak lensing measurements etc. can be brought to bear here.

## Constraining the evolution of eff

- Most observations of classical cosmology...
- Distance probes:
- CMB acoustic peaks
- Type Ia Supernovae
- SZ + X-ray observations of clusters
- Strong lensing statistics
- Ly- forest cross-correlations
- Alcock-Paczynski test
- Galaxy counts (volume element)
**SNe standard candle experiments as an example**

**Notes:**
Let’s talk first about constraining the global expansion history. When we talk about constraining dark energy, we’re talking about the program of what used to be called classical cosmological tests. There are many such tests, including the locations of CMB acoustic peaks, standard candle luminosity distance measures, SZ and X-ray angular diameter distance measurements of galaxy clusters, strong lensing constraints, Ly-alpha forest cross-correlation measurements, Alcock-Paczynski effect tests, and measurements of the evolution of the volume element.

All are possible, all will be further enabled by new data. I will single out the SNe standard candle measurements here as an example. In many ways it is the most mature.

## Observational Probes 2: g(z,k)

- Probes of the growth of structure:
- CMBR
- Weak lensing (esp. with tomography)
- Galaxy clusters
- Ly- forest (at high z)
- Galaxy redshift surveys (z < 1)
**Issues facing galaxy cluster studies**

**Notes:**
There are also many probes of the growth of structure in the universe. Again, microwave background experiments have a role to play. Weak lensing, especially with tomographic reconstruction, will allow a direct probe of matter clustering. Evolution in the number density of galaxy clusters is a sensitive probe of cosmology. Lyman-alpha forest studies at high redshift, and galaxy redshift surveys at low redshift round out the list.

I will take galaxy cluster surveys as an example, and describe the kinds of issues these measurements face if they’re going to provide really accurate cosmological constraints.

## What are the limitations?

- Criteria for comparison:
- How closely do the observables relate to theory?
- True standard candle => dL is great
- Abell richness => mass is poor
- How precisely can each observable, in practice and in principle, be measured?
- SZ decrement from high-z clusters is great
- Ly- forest at low redshift is very hard
- Cosmic variance, projection effect noise in lensing....
- How mature is each method? To what extent has the list of possible limitations been faced and overcome?

**Notes:**
When we consider this laundry list of tools, we need some criteria for comparison. I think the important factors include the following. 
How closely do the observable quantities relate to the theoretically calculable ones? If you have a true standard candle, your ability to probe dL(z) is great. If you label galaxy clusters by Abell richnesses your understanding of mass is very loose.
How precisely can the observables, in practice and in principle, be observed? Here the ability to observe the SZ decrement to very high redshift is an example of a great plus, and the disappearance of the Ly- forest into the ultraviolet at low redshift exemplifies a problem. The ‘in principle’ limitations are things like cosmic variance and the noise introduced into lensing measurements by projection.
Finally, we need to ask honestly about how mature each method is. To what extent has the list of possible limitations been faced and overcome?

## At what redshifts should we probe?

- Effect of dark energy becomes apparent at late times
- Expansion passes from decelerating to accelerating
- Effective density asymptotes to vacuum contribution
- DE is apparent at z < 3

- Tegmark: astroph/0101354

**Notes:**
If dark energy is what we think it is, it’s effect becomes apparent only at late times. During this period, the expansion passes from decelerating to accelerating, and eventually the effective density asymptotes to something like the vacuum contribution.

For the most part, we expect dark energy to be apparent at redshifts less than 3.

## Type Ia Supernovae

- Type Ia’s are proven ‘standardizable’ candles
- Stretch factor related to amount on Ni in explosion
- Achievable dispersion in peak luminosity ~10%: measures dL vs. z

**Notes:**
Let’s start by walking through the SNe approach to dark energy measurement. Type Ia SNe are proven ‘standardizable’ candles. When their light curves are adjusted by an observable ‘stretch’ factor, a dispersion of around 10% in peak luminosity is achievable. This allows measurement of the relationship between dL and z.

## Extending the SNe results:A wide variety of concerns

- Evolution of the SNe population
- Drift in mean metallicity, mass, C-O
- Variation in mean SNe physics parameters: distribution and amount of Ni, KE, etc.
- Gravitational lensing magnification

- Dust
- Normal
- Clumpy or ~homogeneous grey
- Galactic extinction
- Observational biases
- Malmquist
- K correction, calibration, and color tems
- Contamination by non-Ia explosions

- SNe observations internally provide ways to check all of these: e.g. **SNAP**

- SNAP material from Saul Perlmutter

**Notes:**
In extending the existing SNe results, we can list a wide variety of concerns.

List....

I want to describe how these issues can be addressed in future SNe projects. To do this, I’ll talk about how the SNAP satellite project intends to address these concerns.

## SNe evolution: all ages are found at every redshift

- Light curves and spectra provide an effective fingerprint

**SN are phenomenologically rich, full of diagnostics**

- Like to like...

**Notes:**
A few general comments...When you think about SNe evolution, you must remember that no supernova ‘knows’ what redshift it lives at. At every redshift, we have SNe drawn from a broad range of environments. As long as we compare similar SNe drawn from different redshifts, this kind of evolution cannot be a problem.

Second, SNe events are phenomenologically rich. Their light curves and spectra provide an effective fingerprint, enabling us to effectively sort them.

## Extensive information for each SNe is essential!

- Restframe B&V to
- z=1.7 using NIR

- Host galaxy morphology from high resolution imaging

- Spectroscopic type Ia ID etc.

**SNAP can provide this kind of data**

**Notes:**
If we’re to take advantage of this rich suite of information, we need extensive, precise information for every SNe we want to use. This information should include:
Host galaxy morphology from high resolution imaging (before during and after the SNe)
Decent spectroscopic information
High signal-to-noise light curves in restframe B and V for SNe at all included redshifts.

The SNAP mission is designed to provide just this kind of data.

## Sort into closely defined classes:Compare like to like only

- Sort into closely defined classes:Compare like to like *only*

**Notes:**
Given a data sheet like this, the SNe can be sorted into closely defined classes. We will then compare like to like SNe only in the construction of Hubble diagrams.

## Allows for variations in true peak brightness between classes

- This is really what ‘stretch factor’ rescaling is already doing.

**Construct a Hubble diagram for each class**

**Notes:**
Doing this allows for variations in peak brightness among the classes. 

## Use distribution of magnitudes about the mean to remove lensing

- Use distribution of magnitudes about the mean* *to remove lensing

- Break Hubble diagram into slices to look at lensing distributions

**Notes:**
Once the collective Hubble diagram is assembled, we can look at the distribution of magnitudes about the mean to determine the effects of gravitational lensing magnification.

## Evolution to high redshift may prove key

- Degeneracies in models are reduced as the redshift range increases.
- Studies at z<1 can tell us *that* dark energy exists, but can’t say much about *what *dark energy is.

- Eric Linder: LBNL

**Notes:**
In this program, following the evolution of dL vs. z to high redshift may prove key. The degeneracies among dark energy models are reduced as the observed redshift range increases. Studies at z<1 can tell us that dark energy exists, but can’t say much about what dark energy is.

## SNe can achieve real model constraints

- Assume SNAP
- ~2000 SNe to z=0.7 and to z=1.7
- Each observed precisely enough to fill in its datasheet
- Known systematic uncertainties included
- 10% constraints on w, 30% constraints on w'

- Linder 2002: LBNL

**Notes:**
The SNe method can achieve meaningful constraints. Assuming a nominal SNAP mission, identifying 2000 SNe to z=1.7, pretty precise constraints can be placed on dark energy parameters. Perhaps 10% measurements of w, and 30% measurements of w’.

## Galaxy cluster surveys

- Probing growth of linear perturbations by measuring the space density of the largest peaks
- Analytic theory and N-body simulations predict dn/dM as a function of z
- Cosmology comes from *comparison* of observed dn/dM vs. z to theory

- Cluster detection measures something other than mass: observables like SZ decrement, X-ray flux, galaxy v, shear.....
- To approach dn/dM vs. z we need to know:
- 	M(observables,z)
- 	Efficiency(observables, z)
- The mass function is very steep!

## What is a cluster for theorists?

- A large peak in the dark matter density
- Mass defined (for example) as total mass within R200, where mean overdensity is 200 times the critical density => M200

- Springel et al. 2001

## What is a cluster for observers?

- Large peak in matter density
- Dark matter clump (~80% of mass)
- Many luminous galaxies (~2%: 10% of baryons)
- BCG and red sequence
- Additional galaxies
- Diffuse light
- Hot gas (~18%: 90% of baryons)
- Emits X-rays
- Causes SZ decrement in microwave background

- Cluster of galaxies

- SZ decrement

- Carlstrom et al. 2002

## Estimating mass in observers clusters

- Clusters of galaxies: galaxy richness, luminosity, velocity dispersion
- Clusters of hot gas: X-ray flux, temperature,  SZ decrement
- Clusters of projected mass: strong lens geometry, weak lensing shear
- How to find R200  and M200 without loose assumptions...
- Two approaches:
- Learn the astrophysics to understand M=f(observable,z)
- Learn to predict dn/d(observable,z) instead of dn/dM

- Strong lensing

- Weak Lensing

- X-ray Gas

## Analogy to SNe

- For SNe, we want to know luminosity: measure spectrum, stretch, rise time, extinction, peak to tail ratio etc....
- For clusters, we want to know mass: measure SZe, Fx, Tx, gal, lensing, Ngal, etc.

- z = 0.041

## Massive cluster surveys are coming

- 2DF and SDSS 3D surveys (~103 to z~0.15)
- SDSS 2.5D photo-z surveys (~105 to z~0.5)
- SZ surveys: SZA, SPT, AMiBA, etc.
- Lensing surveys from Legacy, LSST, and SNAP

- Joy and Carlstrom: Science

- Fantastic sensitivity to high redshift!

## Cluster surveys: in their childhood

- Clusters make great cosmological probes
- Very detectable
- Evolution is approachable
- Sensitive (exponential) dependence on cosmology
- Clusters are complex: we must understand them better to use them for cosmology

- We need to observe and model clusters in their full richness to test our understanding
- We need to count all clusters:
- absolute efficiency required
- fundamentally a Poisson limited process (cosmic variance)

## Conclusions

- Tremendous new observational prospects
- Optical SNe and lensing surveys on ground and in space
- SZ surveys
- CMB anisotropy and polarization
- Completing these will require serious support and high priority
- Interpreting these observations accurately will require extensive new modeling efforts

## A wish list

- Care in comparisons between observation and theory
- Enhance support for serious new observational programs: no reason to wait
- Coordination of observational programs: independent studies of structure are less helpful
- Coordination between observers and modelers: N-body simulations => ‘observable’ simulations

**Now is the time to study expansion history**