1. Ben Wandelt Flatiron Institute
Cosmology
Ben Wandelt Flatiron Institute
Institut d’Astrophysique, Paris (IAP)
Lagrange Institute, Paris (ILP)
Sorbonne University

2. Dark energy and SPHEREx++ (or: beyond BAO with SPHEREx)
BAO requires large volume to be effective
High-density, high resolution, all-sky coverage at low z is unique to SPHEREx
Cosmic acceleration (“dark energy”) is known to dominate at low z – so an interesting science target for SPHEREx
But.. low-z structure is non-linear
What to do?

3. The large scale structure challenge
Problem: to access high-res, low-z information we need to deal with non-linearity and "bias"
Possible approaches:
Avoid: Observe at high redshift before density contrast became non-linear (CMB, 21cm, Ly-alpha?)
Adapt: Focus on less complicated parts of the Universe, e.g. those that retain more memory of the initial conditions: cosmic voids
Attack: Physical & statistical forward model of the survey, bias, etc. (perturbative or non-linear)
Benjamin Wandelt

4. The power of cosmic voids
Biggest "objects" in the Universe – fill most of the volume!
Simpler dynamics and evolution than high density regions
The first regions in the universe that are dominated by dark energy; most sensitive to modifications of General Relativity
Sensitive to small scale DM structure
A free, additional observational probe in current and future surveys: ~104 voids in Euclid!
Contrast high at low-z
Several active groups and a rapidly growing body of work
Google “VIDE bitbucket”
Benjamin Wandelt

5. Matter, galaxies, voids in simulation
Hamaus et al. 2014

6. A universal profile for voids
Scaling properties of voids allow reduction from 4 to 2 params
Subsampled DM sims
NL velocity profile matches this NL profile!
Hamaus, Sutter & Wandelt PRL 2014, arxiv:

7. Void-galaxy correlation function in redshift space
Benjamin Wandelt
See month of June on 2017 APS calender!

8. Joint measurement of growth of structure and of expansion geometry
Using BOSS data. AP measurement is 4 times tighter than RSD from SDSS DR12 galaxy clustering analysis!
(Gil-Marin et al. arXiv: )
Preliminary Euclid forecasts => 30 times higher Figure of merit than standard BAO
Hamaus et al. arXiv:
Benjamin Wandelt

9. Joint constraint on growth f/b and on matter density
for wDE = -1
3457 voids from
SDSS
Hamaus et al. arXiv:
Benjamin Wandelt

10. SPHEREx voids probe low-z universe
Preliminary
Old specs,
Using
dz~0.003
only
230,000 voids!
Alice Pisani
Complementarity: expect LH contours to be rotated wrt Euclid, WFIRST

11. SPHEREx will get higher number density of voids at low z; sensitive to cosmic acceleration
Preliminary
Old specs
Using
dz~0.003
only
230,000 voids!
Alice Pisani
Complementarity: expect LH contours to be rotated wrt Euclid, WFIRST

12. Non-Gaussianity with SphereX and Euclid

13. SPHEREx non-Gaussianity
K<<k
This is challenging: need to know selection function very well on large scales; expect large angle selection systematics to be the bottleneck
Potential large angle systematics for SPHEREx:
Confusion could lead to large angle systematics due to star-galaxy separation in galactic coordinates
Zodiacal emission could lead to large angle systematics in ecliptic coordinates S S S

14. SPHEREx non-Gaussianity
A potential solution: cross-correlations
K<<k
Take combinations EUCLID/SPHEREx combinations so the short scale variations due to selection systematics do not couple to long scale fluctuations
Another strategy to explore: Using a Euclid-selected SPHEREx sample X Y Z

15. Conclusions
SPHEREx’s unique strength lies in “low”-z high density pseudo-spectroscopic mapping
This is well-matched to void cosmology
Worth exploring Euclid/SPHEREx cross-correlation and cross-calibration especially for science that is sensitive to selection systematics
Need to explore these issues in simulations!