Cosmic Visions Dark Energy: Small Projects Portfolio
Kyle Dawson, Josh Frieman, Katrin Heitmann, Bhuvnesh Jain, Steve Kahn, Rachel Mandelbaum, Saul Perlmutter, Anže Slosar
aa r X i v : . [ a s t r o - ph . C O ] F e b Cosmic Visions Dark Energy: Small Projects Portfolio
Cosmic Visions Dark Energy Panel: Kyle Dawson, Josh Frieman, Katrin Heitmann,Bhuvnesh Jain, Steve Kahn, Rachel Mandelbaum, Saul Perlmutter, Anže SlosarFebruary 21, 2018
Executive Summary
The 2014 P5 Report identified understanding cosmic acceleration as one of the key science drivers for high-energy physics in the coming decade. With the Large Synoptic Survey Telescope (LSST) and the DarkEnergy Spectroscopic Instrument (DESI) beginning operations soon, we are entering an exciting phaseduring which we expect an order of magnitude improvement in constraints on dark energy and the physicsof the accelerating Universe. This is a key moment for a matching Small Projects portfolio that can (1)greatly enhance the science reach of these flagship projects, (2) have immediate scientific impact, and (3)lay the groundwork for the next stages of the Cosmic Frontier Dark Energy program. In this White Paper,we outline a balanced portfolio that can accomplish these goals through a combination of observational,experimental, and theory and simulation efforts. The portfolio includes: • Observations that leverage existing facilities to expand the dark energy science reach of LSSTand DESI. Complementary data from spectroscopic, space-based, and multi-wavelength facilities willprovide insights into LSST and DESI data, thereby reducing both statistical and systematic uncertain-ties compared to their baseline programs. This program of observations will need to be complementedby support for members of the Cosmic Frontier community to integrate analysis of these other sampleswith the analysis of LSST and DESI to enhance the constraints on dark energy, inflation, and neutrinomasses; • Experimental R&D to address the key technical challenges the community faces when lookingforward to the next generation of dark energy experiments; • Theory and simulation development that will allow us to explore and extract cosmological infor-mation from small scales and develop novel probes from dark energy surveys. Improved modeling ofsurvey observables on moderately nonlinear scales will pay direct dividends in providing stronger cos-mological constraints. Theoretical exploration of the highly nonlinear regime, and more sophisticatedmodeling of astrophysical effects, unlocks the small-scale cosmological information content of LSSTand DESI. Models in these regimes can potentially yield constraining power that vastly exceeds thestandard projections for constraints on dark energy, inflation, and neutrino masses. The theoreticalexploration and substantial program of simulations of structure formation that this will require willalso enable us to explore the unknown in terms of new interactions, fields, and physical principlesguiding cosmic expansion.The Small Projects portfolio described below follows the general themes laid out in two earlier CosmicVisions White Papers [DHH + + and the LBNL Cosmic Visions Workshop: DarkEnergy in Nov. 2017 . During these workshops, multiple small projects were identified as part of thethree overarching themes outlined above. As shown in Appendix A, the required support for most of theseprojects is in the $1M-$3M regime, leading to a modest overall investment in the $10M range. The supportof several, carefully-chosen, small-scale efforts will have a greater impact than supporting a single larger one,since they provide complementary paths for attacking the complex problem posed by dark energy. Theseprojects can be brought online rapidly, on a timescale that best supports LSST and DESI. Now is thereforethe time to build such a portfolio of research, R&D, and observations in the Cosmic Frontier community,in order to enhance and extend the existing projects and lay the groundwork for the future. http://kicp-workshops.uchicago.edu/FutureSurveys/ http://cvde2017.lbl.gov/ The report is based on contributions from: Tom Abel, Zeeshan Ahmed, Greg Aldering, Sahar Allam, Lori Allen, David
Motivation
The discovery of late-time cosmic acceleration has profound ramifications for fundamental physics, as recog-nized by the 2011 Nobel Prize in Physics. Currently, the three main contenders to explain this phenomenonare (i) a cosmological constant Λ , which can alternately be described as a dominant stress-energy compo-nent with a constant equation of state parameter, w = − , (ii) a dynamical dark energy component witha different and typically time-varying equation of state parameter, w ( a ) = − , and (iii) a modification ofEinstein’s theory of General Relativity on cosmological scales. Lacking a compelling theoretical model forcosmic acceleration at the current time, our field is driven by observations that aim in part to test thecosmological constant plus cold dark matter model ( Λ CDM) and to find hints of new physics beyond it.Our primary means of gaining information about the underlying mechanism of cosmic acceleration arethe cosmic expansion history and the evolution of cosmic structure. The propagation and deflection of lightprovides a complementary means to test gravitation on cosmological scales. The dark energy community hasdeveloped a comprehensive program of imaging and spectroscopic surveys designed to measure expansionhistory, structure growth, and light deflection through multiple observables such as gravitational lensing,galaxy clusters, the large-scale distribution of galaxies, and type Ia supernovae. The program follows thestaged approach described by the Dark Energy Task Force (DETF, [ABC + +
15] on the 4-meter Blancotelescope in Chile to conduct a 5-year, multi-band imaging survey unprecedented in its combination of depthand breadth. DES is designed to employ all the dark energy observables mentioned above, along with cross-correlation with the Cosmic Microwave Background (CMB), and has reported cosmological results fromits first year (Y1) of data using weak lensing and galaxy clustering measurements [DAA + + +
13] concluded in 2014; its successor, eBOSS[DKP + . < z < . , providing a view of cosmic expansion over two-thirds of the history of theUniverse.The early results from the Stage III experiments show broad consistency with the Λ CDM paradigm sofar. Combining the DES Y1 results with Planck CMB measurements, SN Ia data, and BAO measurementsincluding BOSS yields w = − . +0 . − . in the context of the w CDM model with constant dark energyequation of state parameter [DAA + w = − . ± . [AAB + Alonso, Marcelo Alvarez, Adam J Anderson, Jim Annis, Reza Ansari, Rafael Arcos-Olalla, Charles Baltay, Darcy Barron, KeithBechtol, Andrew Benson, Jonathan Blazek, Lindsey Bleem, Sebastian Bocquet, Adam Bolton, Kyle Boone, Andrew Bradshaw,Elizabeth Buckley-Geer, Philip Bull, Kevin Bundy, Robert Cahn, Robert Caldwell, Peter Capak, Chris Carilli, EmanueleCastorina, Tzu-Ching Chang, Swapan Chattopadhyay, Xuelei Chen, Asantha Cooray, Neal Dalal, Marc Davis, Kyle Dawson,Will Dawson, Darren DePoy, Roland de Putter, Marcellinus Demarteau, Tom Diehl, Samantha Dixon, Scott Dodelson, OlivierDoré, Alex Drlica-Wagner, Juan Estrada, Giulio Fabbian, Simone Ferraro, David Finley, Brenna Flaugher, Simon Foreman,Wendy Freedman, Josh Frieman, Josef Frisch, Enrique Gaztanaga, Mandeep Gill, Fred Gilman, Danny Goldstein, DanielGreen, Ravi Gupta, Gaston R Gutierrez, Salman Habib, ChangHoon Hahn, Patrick Hall, Andrew Hearin, Katrin Heitmann,Jacqueline Hewitt, Christopher Hirata, Renee Hlozek, Shirley Ho, Steve Holland, Daniel Holz, Klaus Honscheid, DraganHuterer, Bhuvnesh Jain, Daniel Jacobs, Stephanie Juneau, Elise Jennings, Garrett K. Keating, Robert Kehoe, Stephen Kent,Gourav Khullar, Alex Kim, David Kirkby, Eve Kovacs, Ely Kovetz, Elisabeth Krause, Richard Kriske, Richard Kron, SteveKuhlmann, James E Lasker, Alexie Leauthaud, Ricky Leavell, Khee-Gan Lee, Boris Leistedt, Michael Levi, Ting Li, AdamA Lidz, Huan Lin, Eric Linder, Jessica Lu, Zarija Lukic, Niall F MacCrann, Rachel Mandelbaum, Alessandro Manzotti,Jennifer Marshall, Phil Marshall, Paul Martini, Patrick McDonald, Peter Melchior, Joel Meyers, Kavilan Moodley, Miguel FMorales, Pavel Motloch, Li Nan, Laura Newburgh, Jeffrey Newman, Peder Norberg, Paul O’Connor, Nikhil Padmanabhan,Samuel Passaglia, Jeff Peterson, Jonathan Pober, Jason Poh, Kara Ponder, Anthony R. Pullen, Matt Radovan, Alexandra SRahlin, Marco Raveri, Paul Ricker, Constance Rockosi, Natalie Roe, Aaron Roodman, Ashley Ross, Aditya Rotti, Khaled Said,David Schlegel, Marcel M Schmittfull, Michael Schneider, Daniel Scolnic, Douglas Scott, Uros Seljak, Hee-Jong Seo, RichardShaw, Christopher Sheehy, Nora Shipp, David Silva, Sukhdeep Singh, Zachary Slepian, Anze Slosar, Marcelle Soares-Santos,Tony Spadafora, Paul Stankus, Albert Stebbins, Michael Strauss, Christopher Stubbs, Meng Su, Amy Tang, Tommaso Treu,Mark Trodden, Douglas Tucker, Tony Tyson, Amol Upadhye, Anja van der Linden, Mohammadjavad Vakili, Ricardo Vilalta,Francisco Villaescusa-Navarro, Benjamin Wandelt, Xin Wang, Risa Wechsler, Martin White, Michael Wilson, FriedwardtWinterberg, Kimmy Wu, Christophe Yeche, Jun Zhang, Yuanyuan Zhang, Idit Zehavi, Zheng Zheng α forest sample from BOSS (mean redshift z =2 . ) [BBG +
17, dLB +
17] yield constraints on the distance scale that lie . σ from the predictions of theflat Λ CDM model best fit by the Planck data [PAA + + + H = 67 . ± . km/s/Mpc, in tension with ‘direct’ distance-scalemeasurements that do not assume Λ CDM. For example, measurements using Cepheid variables and nearbyType Ia supernovae imply a value H = 73 . ± . km/s/Mpc [RMH + H = 71 . +2 . − . km/s/Mpc [SBC +
17, BCS + (Ω m , σ ) plane, though the significance of this tension depends on the survey ([KVJ + σ tension between Planck and KiDS, while the DES Y1 results [DAA + Λ CDM, unknown systematic errors in one or more of themeasurements, or a statistical fluctuation. More measurements with increasing precision and accuracy arerequired to better understand the origins of these discrepancies.The Stage III results highlight one of the key tasks for the dark energy community as we head towardStage IV and beyond – to continue to develop robust analysis, observational, and theoretical programs toensure that we can disentangle systematic effects from potential new physics. LSST and DESI are currentlybeing constructed as Stage IV dark energy experiments. LSST will be a dedicated, large-aperture telescopethat will obtain deep, multi-band images of nearly half the sky to great depth over the course of a decade-longobserving campaign, with first light around 2020. Like DES, LSST will constrain the nature of dark energyvia weak and strong gravitational lensing, cluster counts, type Ia supernovae, and large-scale structure.Beginning in Fall 2019, DESI [DAA + + • The reduction of both statistical and systematic uncertainties for LSST and DESI beyond the currentbaseline to enhance cosmological constraints on dark energy, inflation, and neutrinos. Since theinception of the LSST and DESI concepts, the most important sources of systematic uncertaintieshave been better characterized, and we can now design a small program to help address these sourcesin a targeted way. Small-scale investments in calibration and efforts to lower the systematics floor dueto known effects through specific, targeted observations could have a large payoff. • The exploration of new probes and reduction of systematic uncertainties by cross-correlation betweenLSST and DESI and other multi-wavelength surveys. In this case, our proposed program does notinvolve acquiring new data. Rather, we propose to invest in an effort to develop methodologies tobring together data from DESI or LSST with external datasets for improved systematics control and/orbetter cosmological constraining power. This can also include efforts to ensure complementarity insurvey strategy between these and other surveys (such as WFIRST) in a way that would enhance thescientific gains from both, resulting in a ‘whole is greater than the sum of its parts’ outcome. • The exploration of small scales beyond the current DESI and LSST baselines by enabling a compre-hensive modeling and simulation program. It has been suggested for some time that if the effects ofgravitational collapse and baryonic matter can be encapsulated by a combination of empirical andtheoretical modeling, then extending probes of large-scale structure well into the nonlinear regime cansignificantly improve dark energy constraints, even when self-calibrating a reasonably large numberof “nuisance parameters” [HTBJ06, ZSD + +
14, ZWJ +
14, JJKT15]. We see no fundamental obstacle prohibiting the practicalrealization of the large science returns offered by probing cosmology in the nonlinear regime. The mostsignificant impediment is probably people-hours: bringing these methods to scientific maturity prior to2he arrival of Stage IV data will require a meaningful investment in personnel at the theory-simulationinterface beyond current levels. • The investigation of novel probes. The small scale modeling effort above refers primarily to two-pointcorrelations on nonlinear scales. However, there is a dazzling variety of astrophysical observationson small scales, and a number of these have been identified as offering sharp tests of fundamentalphysics beyond w CDM (i.e., beyond smooth dark energy models). Galaxy surveys, including DESIand LSST, will provide a wealth of data and opportunity for such tests, dubbed “novel probes”. Thephenomenology ranges from supermassive black holes to galaxy dynamics and galaxy cluster profiles.The physics ranges from modified gravity to dark matter interactions and the detection of stringtheory-motivated scalar fields. Starting with the Snowmass report of 2013, a number of communityreports have highlighted this opportunity [JJT +
13, DHH + • Preparing for next-generation experiments. What we learn in the next ten years from DESI and LSSTwill inspire the next generation of Stage V experiments. If LSST and DESI do find evidence for physicsbeyond a cosmological constant, we will need to pursue a new observational program that leverages allmodes so far unexplored to resolve those exciting results. If the Stage IV experiments find consistencywith a cosmological constant, we will have to test this finding at even higher level of precision and fullytest the assumptions of General Relativity in the cosmological model. The simplicity of the backgroundcosmology in the scenario of a cosmological constant would also enable Stage V experiments to offersharper tests of physics such as the nature of dark matter, neutrinos, inflation, and dark radiationcandidates. Well-understood advantages and limitations of the most advanced technologies used inStage IV gives us foresight to begin development today toward the technologies that will be neededfor a Stage V program. In parallel with advancing hardware technologies, we need to advance ourtheoretical understanding of the origin of the cosmological constant and develop new probes that canfind compelling alternative explanations that can be explored with better measurements. As a whole,we need to advance our technologies to enable the most comprehensive Stage V observations andenhance our theory, modeling, and simulation capabilities to inform the design of the best possibleprograms.These enhancements to LSST/DESI science and the potential for larger-scale, dedicated dark energyprograms motivate our Small Projects portfolio. We describe the potential to integrate complementaryobservations into DESI/LSST core projects in Section 2. We describe new technology developments thatwill enable Stage V dark energy experiments in Section 3. Finally, we present a plan for advancing theoryand simulations in Section 4. In total, these three complementary efforts fit well within a cohesive SmallProjects portfolio dedicated to dark energy science. Our Section 2 is perhaps most open to new ideas since itencompasses a large number of very different approaches. Sections 3 and 4 are somewhat more prescriptiveas they reflect a stronger consensus on priorities in the two workshops.
The unifying idea in this section is that there are multiple complementary datasets that can enhance LSSTand/or DESI by reducing some of the limiting systematic uncertainties in their dark energy analyses or byenabling certain analyses that would not otherwise happen with LSST or DESI alone. While LSST andDESI can carry out their baseline dark energy analysis without these external datasets, having the externaldata indisputably enhances and makes more robust these surveys by providing more room in the systematicerror budget for other unanticipated issues that may arise and by enhancing the survey capabilities in otherways.DOE-supported researchers are focused on the high-level science goals of the core DOE programs andare not typically supported to supplement those studies with additional data samples. Increasing supportand flexibility for PIs to bring postdoctoral researchers into their groups specifically to leverage other (non-DOE) facilities has the potential to greatly enhance LSST/DESI. The projects are divided into those that3nvolve supplementary datasets that would need to be acquired through some means (Subsection 2.1) andthose that will enable better cross-survey coordination or some other synergy with already-planned surveys(Subsection 2.2). The typical scale of the projects is < $1M. All projects in the category of Complementary Measurement Efforts will advance the P5 science driversby integrating new data that add value to the LSST/DESI samples. These new data will not provideindependent dark energy results but will rather allow new or refined measurements with these Stage IVexperiments. Programs should be given preference for improving the calibration of Stage IV data, increasingthe dimensionality of Stage IV data for its measurements in weak lensing, galaxy clusters, galaxy clustering,or type Ia SNe, or enabling new cosmological probes with Stage IV data that would not otherwise bepossible. Below are several compelling examples of programs involving LSST or DESI along with someother (non-survey) data: • Photometric calibration:
The Type Ia supernova technique requires that both high- and low-redshift SNe be on the same flux system. Only point-like sources can precisely follow the sametelescope optical path as the supernovae. Therefore standard stars are needed whose flux calibrationas a function of wavelength is on a physical system to an accuracy much better than 1%. Currentphotometric systems have high internal consistency, but their wavelength-dependent calibration isonly accurate to a few percent. A program to establish the “absolute-relative” flux calibration byreferencing standard stars to NIST-traceable light sources is needed. While LSST will attempt toprovide an internally-consistent photometry system, there is broad agreement that it will not beable to establish the “absolute-relative” flux calibration system. In particular, to calibrate out LSSTfilter shifts across the LSST field of view or over the 10-year LSST survey, this calibration needs tobe obtained spectrophotometrically. Moreover, to fully exploit the synergy of LSST and WFIRST,this calibration needs to be extended to NIR wavelengths. Despite the technical challenges, theinstrumentation and an associated telescope to carry out this program would have a modest cost,perhaps under $500K. • Peculiar velocity studies:
Measurements of peculiar velocities can be used as a probe of structuregrowth. Measurements of supernova distances can be used to map out the peculiar velocity field (e.g.,see proof of concept in [FKK + ∼ H attainableusing gravitational wave optical counterparts. • Narrow-band or offset broad-band imaging:
Well-calibrated photometric-redshift estimates arecritical for LSST dark energy science. Narrow-band imaging or imaging with offset broad bandscan lead to significant improvements in photometric redshift estimation [BMA + • Ground-based Spectroscopy:
Several ground-based spectroscopic facilities that would complementLSST and DESI are now in the construction or design phase. The Subaru Prime FocuS Spectrograph[TTS +
16] will begin operations in 2019. Looking further ahead, a fiber-based Wide-Field Optical4pectrograph is being studied for the Thirty Meter Telescope, the Maunakea Spectroscopic Explorer[BAB +
16] is planned as an 11.25m aperture wide field spectroscopic survey telescope, and the GiantMagellan Telescope is envisioned as a facility that will span 320–25000 nm with a collecting areaequivalent to a 24.5-meter telescope. Depending on the goal of the observations, in some cases theseground-based spectroscopic facilities have advantages over DESI such as better sensitivity to highredshift, faint galaxies and potential for targeted observations. As a result, data from these powerfulinstruments could establish a spectroscopic sample for training LSST photometric redshifts [NAA + Here we describe efforts that will pave the way for enabling cross-survey science, as described for example inRefs. [RNA +
17, JSB + • WFIRST + LSST : The fact that WFIRST will have spectroscopy as well as imaging at NIR wave-lengths leads to a number of interesting cross-survey synergy opportunities. Taking advantage of theseopportunities requires work on optimal strategies for doing so, and significant preparatory develop-ment and testing of actual robust pipelines, since any work that coordinates with space-based effortsmust be reviewed and certified well in advance for such collaborations to be permitted. There are afew different aspects to develop: – Supernova imaging and spectroscopy : The LSST survey will, over 10 years, provide a fewhundred thousand supernovae from the wide survey, and another few tens of thousands fromthe deep-drilling fields. WFIRST NIR imaging and spectroscopic follow-up with the integral-field channel for LSST SN discoveries at z < . , while they are still active, can provide a keysample of ∼ z < . SNe with LSST will also significantly enhance the statistical reach of the combinedLSST-WFIRST effort, since at these lower redshifts the larger LSST imager field-of-view is amore efficient discovery tool.) – Spectroscopy for photometric redshift training and/or calibration : If WFIRST cancarry out NIR spectroscopy in parallel mode during the high-latitude imaging survey, it shouldbe possible to build up a training sample with up to galaxies in regions of color-space where5round-based spectroscopy is very challenging. Ideally this would use methods similar to thoseemployed by the C3R2 survey [MCS +
15, MSC +
17] to ensure optimal target selection that makesthe most of the available time to fill in the photometric redshift training sample.In order to ensure feasibility of these programs, work on their design needs to start well in advance ofthe launch of WFIRST. • LSST and DESI + CMB S4 : There are a number of areas of scientific gain from the combina-tion of CMB S4 and LSST [SS17] or DESI (within the constraints imposed by limited area overlap).These include use of CMB lensing cross-correlation with LSST to provide an external calibration con-straint on the combination of shear and photometric redshift uncertainties [SKE +
17] and to constrainstructure growth to higher redshifts, given the shape of the CMB lensing kernel. The combination ofLSST, DESI, and CMB-S4 is even more powerful, given that including redshift-space distortions inthe measurement provides a way to distinguish between dark energy and modified gravity [ZLBD07].This work will require cross-correlation measurements to far smaller scales than the DESI BAO mea-surements, which connects to the proposed work on modeling in Section 4.An additional area for discussion in the community is the topic of joint pixel processing betweenWFIRST, Euclid, and LSST (c.f. [RNA + Much of the recent rapid progress in cosmology can be attributed to new experiments enabled by instrumen-tation technologies developed over the last decade. These efforts followed a shared philosophy with the latestP5 report to pursue technology development in a balanced mix that addresses short-term, immediate needand long-term R&D. Likewise, the R&D efforts that made possible the Stage III and Stage IV dark energyexperiments were pursued through partnerships between national laboratories, universities, and the privatesector. Examples include CCDs, fiber positioners, and CCD electronics and data acquisition hardware.Dark energy experiments depend critically on thick, high resistivity CCDs that provide very high quan-tum efficiency in the near-infrared. CCD development activities for dark energy science were performed atthe Lawrence Berkeley National Laboratory (LBNL) in partnership with DALSA Semiconductor . Overa ten-year period, these deep-depletion CCDs advanced from an early prototype to a device that can bemanufactured in bulk with few cosmetic defects and nearly optimal sensitivity across the wavelength range , < λ < , Å[HBD + + +
12, SAB + + + , the community has identified three specific technical improvements that canlead to significant enhancements of LSST or another order of magnitude advancement in spectroscopic sur-vey power. We describe an experimental program to tackle these three technological hurdles. We estimatethe costs for each of these three R&D efforts to range from $1M to $2M: • Ground Layer Adaptive Optics : Ground-based imaging and spectroscopic surveys suffer degradedresolution due to turbulence in the ground layer and in upper layers of the atmosphere. By correctingfor this turbulence in real time, adaptive optic systems can yield substantially improved angularresolution and significant improvements in the signal-to-noise ratio. For example, an improvementfrom seeing of 0.7 arcseconds to 0.5 arcseconds will yield a factor of two improvement in signal-to-noise for the faintest objects in any imaging program.Multi-Conjugate Adaptive Optics systems (MCAO) use multiple natural or laser guide stars withseveral deformable mirrors. The GeMS instrument at Gemini South [DAB +
14] achieves diffraction-limited imaging across a field of view of more than 1 arcminute. However, these fields are too smallfor cosmological surveys and the required large number of galaxies.Ground Layer Adaptive Optics (GLAO) offers an alternative to those systems that try to reach the fulldiffraction limit of the telescope. In GLAO, wavefront sensors assess common ground-layer turbulenceover a much larger field of view using bright guide stars as a reference. One such GLAO system hasbeen tested on the University of Hawai’i 2.2-meter telescope on Maunakea, Hawai’i. The “imaka”GLAO pathfinder system has been shown to produce images with FWHM of 0.33 arcseconds in thevisible and near infrared over a 0.33 degree field of view. Equally important to the improved resolutionis the temporal uniformity of resolution recorded by the imaka system. The RMS in the FWHM fromexposure to exposure is reduced relative to the non-corrected seeing, but further testing needs to bedone to fully quantify the effect and dependence on observing site.The imaka system offers proof of concept to the capabilities of GLAO systems to improve image qualityover a large field of view. LSST, DESI, and future cosmology surveys all push for fields of view anorder of magnitude larger than the 0.33 degree diameter system being tested now in Hawai’i. Thereis only one vendor working on the technology for large GLAO mirrors , so DOE support would spurfurther advancement in the field.A development effort between 2020 and 2025 could (1) demonstrate the science gains from suchsystems, (2) characterize the trade-offs between telescope aperture size, field of view, and GLAOimprovements, and (3) further develop the technology for wide-field GLAO suitable for DOE science.If shown to be feasible, such a system could potentially be employed through an updated secondarymirror at LSST. Such a deployment, even at the midway point in the survey, would significantlyenhance the reach of LSST toward faint sources at high redshift that are needed to optimize lensingmeasurements. An implementation of GLAO on the Mayall primary mirror for an extension of DESIor in a future spectroscopic facility would equally benefit from such a system; studies of diffraction-limited data from the Hubble Space Telescope indicate that improvements to 0.6 arcsecond FWHMwill optimize sensitivity to high redshift galaxies in a fiber-fed spectrograph. • Germanium CCDs : Silicon CCDs have matured for optical bands covering , < λ < , Å,but the effective band-gap around 1 eV limits their effectiveness at redder wavelengths. The primaryspectroscopic feature used to determine redshift in galaxy surveys is due to forbidden transitions insingly-ionized oxygen ([OII]). These [OII] emission lines occur at 3727 Å in the galaxy restframe,causing the signal to appear beyond the 10,000 Å cutoff in a silicon detector for galaxies at redshifts http://cvde2017.lbl.gov/; http://kicp-workshops.uchicago.edu/FutureSurveys/ > . . Enormous, relatively unexplored volumes will still be available at these higher redshifts evenafter DESI is completed. Many physical models (such as early dark energy) are best explored bycontrasting the expansion history and growth rate at these high redshifts with measurements of thesame in the local Universe. Detectors with sensitivity at wavelengths longer than 1 micron will extendgalaxy surveys to these higher redshifts.While infrared InGaAs and HgCdTe CMOS detectors have been used in ground- and space-basedobservatories, these detectors are expensive, require substantial cooling, and suffer from low yield inthe fabrication process. An alternative to CMOS detectors has recently been identified through workperformed at MIT Lincoln Laboratory. Germanium CCDs can be processed with the same tools usedto build silicon imaging devices, show promise for read noise and sensitivity comparable to that ofsilicon detectors, and offer a high quantum efficiency to wavelengths as red as 1.4 microns when cooledto 77 K. This increase in wavelength coverage will allow a spectroscopic identification of [OII] emissionlines to z = 2 . , a factor of two increase in volume over what is accessible in the DESI galaxy sample.Fabrication of germanium CCDs faces several challenges that need to be addressed before these devicescan be integrated onto large focal planes. Several processes in doping, etching, and film deposition aresimilar to those in silicon CCD fabrication, and may be compatible with DALSA’s capabilities. How-ever, water solubility and low-temperature limitations result in the need for changes in gate-electrodetechnologies. In addition, there is only one wafer vendor in germanium and further investigation isrequired to ensure that purity requirements can be met at scale production on large wafers. Finally,germanium is higher density than silicon and requires a full assessment of handling and packagingtechniques. We estimate that five-years of effort are required to develop a manufacturing pipeline thatcan produce the first packaged germanium CCD’s for dark energy surveys. • Fiber Positioner Systems at 5 mm Pitch : The BOSS and eBOSS surveys have collectively sam-pled the spectra of more than three million objects, each requiring an individual fiber optic supportedby a custom aluminum plate. These fibers have been manually placed by dedicated technicians on analmost daily basis since 2009. The process of plugging typically takes about 30 minutes for each ofthe ∼ fields that comprise the BOSS and eBOSS cosmology samples. The massive overhead ofhuman effort inspired the investment toward new robotic fiber positioners for future experiments evenbefore BOSS started observations.The fiber positioners for DESI consist of 5000 individual robots supported by a 812 mm diameteraspheric focal plane at a 10.4 mm pitch between neighboring units. Each of these robots is driventhrough two rotational axes by independent, brushless 4mm diameter DC gearmotors from Namiki .The size of these motors, fasteners, and mounting interfaces places a hard limit of roughly 10 mmpitch between units. The assembly is well underway, but is complicated by the tight spacing betweenpositioners, need for manually applied glue joints, splicing of fibers, and large number of individualparts (675,194 in total).Following DESI and LSST, the potential exists to make major advances in constraining the cosmo-logical model from a massive spectroscopic program. With spectroscopy of 40,000 galaxies per squaredegree, a dedicated facility could obtain a clustering sample that contains almost all of the cosmolog-ical information to redshifts z < . . A project of such scope is another order of magnitude increasein capability over DESI and requires a comparable scaling of fibers in the focal plane. Given thecurrent limitations of optics to roughly 1.2 meter in diameter, new fiber positioner technologies willlikely be required to populate the focal plane at a sufficiently high density. A technology that allowsfiber spacing at a 5 mm pitch over a 1.2 meter focal plane would allow simultaneous spectroscopy from50,000 fibers.An increase in multiplexing from DESI’s 5000 fibers should be developed either as an upgrade ofthe DESI focal plane (with a spacing of 5 mm or less) or an optical/mechanical solution on othertelescopes. It may be possible to improve the two axis DESI positioners by using smaller motors,press-fit joints instead of glue, alternatives to splicing, and new approaches to handling a one tonfocal plane assembly. On the other hand, the “Tilting Spine” technology has been used in opticalspectrographs and was selected as the fiber positioner technology for the 4MOST facility at the ESOVISTA telescope [BFB + Simulations play a central role in modern cosmology in many ways – they allow us to build virtual universesin which we can explore new physics beyond Λ CDM, investigate systematic uncertainties and associatedmitigation strategies, and develop and test new probes. Predictions from simulations are crucial ingredientsin obtaining cosmological constraints from the observations. Developing efficient simulation codes and toolsto convert the simulations into well-validated, synthetic skies entails major efforts that require input fromtheorists and observers alike. The problems posed by the complexity of such efforts can only be addressedthrough a strong lab-university partnership. For example, SLAC and Argonne National Laboratory incollaboration with university partners and other Labs have built strong simulation and modeling programsover the years. These efforts contribute to all DOE-supported dark energy experiments.The importance of cosmological simulations and computing was highlighted in the P5 report underEnabling R&D and Computing and in the more general Recommendation 29 that addresses computing andsimulation needs of the HEP community. The Decadal Survey 2010 also made explicit recommendations forstrong support of such a program. Given the importance and the exciting scientific opportunities openedup by comprehensive modeling and simulation efforts, the third component of our Small Projects portfoliodescribes four concrete areas with potentially high impact.The first area concerns the exploration of the deeply nonlinear regime of galaxy clustering. Such aneffort would open up new opportunities to extract cosmological information from available observationsthat otherwise will be lost. Carefully modeling the galaxy-halo connection on these scales is the key tounlock this information. Next, we will describe an effort that would allow us to study and develop novelprobes that go beyond the current Λ CDM paradigm. A third area of potentially great impact would be acoherent effort in building a multi-wavelength virtual observatory. Such an effort would allow us to takefull advantage of the “Bridging Surveys and Wavelengths” program, described in Section 2.2. Finally, theprogram could support building a new infrastructure to share simulations and tools among different surveysand therefore lower the overall computational burden for the Cosmic Frontier. We describe in more detailbelow such a simulation and theory program. The first three efforts listed could be carried out at a cost of$1M-$2M each, while the infrastructure program is expected to lie in the $2M-$3M range. • Unlocking Small Scales:
The scientific potential for extracting cosmological information fromsmall scales has been appreciated for a long time: “The nonlinear domain appears to be a gold mineof cosmological information, but one whose riches may prove extremely difficult to extract” [Teg97].Recently, Krause and Eifler [KE17] carried out a realistic FoM forecast in the present-day context.They showed that by extending a cross-correlation analysis of LSST galaxy clustering and lensinginto the highly nonlinear regime, FoM gains by a factor of 2-4 can be realized. During the 20 yearsbetween these two publications, the resolution in simulations has increased by orders of magnitude, thestatistics (via larger volume simulations) has been improved, and the available observational data setsfor validating our modeling approaches on small scales have been vastly enhanced. The time is ripe totherefore attempt to gain access to the cosmological information on small scales. We stress that thiseffort only requires more sophisticated modeling and simulation of the standard set of observationsalready planned by Stage IV dark energy missions such as LSST, DESI, and WFIRST. Therefore,with only a small investment in simulation and modeling efforts, an exciting opportunity exists toenhance the scientific potential of upcoming surveys.Although the labor involved in extracting cosmological constraints from the nonlinear regime is sub-stantial, a future roadmap can be constructed by melding recent work on detailed modeling of the9alaxy-halo connection (see, e.g., Refs. [Bec15], [Coh17], [Bul17], [MNW17]) with a large suite of cos-mological simulations, both in the gravity-only and hydrodynamics arenas. The roadmap has threecomponents that need to be addressed to bring such an effort to maturity. First, it will be essential toexploit leadership-class computing facilities for the simulation and modeling approaches, the latter ofwhich needs a dedicated program. Second, a major calibration and validation effort will be required.Such an effort includes: (i) calibrating the galaxy population models with enough flexibility to fit alarge compilation of observational data; (ii) validating the models against traditional semi-analyticmodels and hydrodynamical simulations, which will be treated as mock datasets whose underlyingcosmology one attempts to recover; (iii) iterating on this procedure using different datasets, refiningthe model as needed by a range of cosmological data vectors. The third component of such an effortinvolves using the tools and methodology to forecast the constraining power of nonlinear cosmologi-cal observables, to identify cosmological data vectors that can be robustly predicted with the fewestnuisance parameters, and to build likelihood emulators that will generate cosmological constraints. • Going Beyond w CDM:
In Section 1, we motivated the enhancement of a novel probes program topursue physics beyond w CDM. The first part of the enhancement relates to data analysis, here wefocus on the second and third: generation of mock data to test analysis codes, and connecting newideas to observational tests. There is both an opportunity and a challenge in the different elementsof new physics that can operate at small scales. For example, dark matter interactions and gravitycan both impact the profiles and dynamics of galaxy clusters. Therefore, in carrying out gravity tests,we must be careful to have robust models of dark matter that extend beyond the simplest CDMcandidate – that is the challenge. The opportunity is that by disentangling the precise range of scalesand phenomenology we can jointly test for dark matter interactions and gravity.The new physics must be modeled either through full simulations or approximate treatments. Asignificant opportunity lies in adapting dark energy simulations by approximating the effects of newphysics via analytical models. This effort would not require expensive new computations, but ratherexpertise in topics such as screening effects in modified gravity or dark matter interactions coupledwith numerical efforts. A related opportunity lies in connecting new ideas to observational tests: aprocess that currently can take several years owing to the different communities involved. With LSSTand DESI offering a new generation of tests, a far more rapid translation of theory to tests can beachieved by providing targeted resources. These could fund collaborative efforts between theorists andobservationally oriented cosmologists, with simulations serving as testbeds for new ideas. • A Multi-wavelength Virtual Observatory:
As outlined in Section 2.2, cross-correlations of differ-ent data sets hold a wealth of information and constraining power. In order to take full advantage ofthis opportunity, a comprehensive effort in analysis, modeling, and simulations has to accompany theobservational campaigns. Such an effort would provide “same sky mocks” for the different wavelengthscovered by the surveys with proper correlations between all of the signals that will be measured. Onthe analysis front, an approach has to be developed that utilizes nuisance parameters, bias models,covariances, and systematic models jointly between signals. We will need to investigate the level ofabstraction at which we can deal with foreground separation, time-stream filtering, photometric cali-bration, etc. For this task, simulations will be an essential tool. The ultimate goal is to facilitate jointanalysis of simulated data for validation, calibration, and correlation across multiple probes.Creating a multi-wavelength virtual observatory across all observables, including CMB (projected lens-ing maps, thermal and kinetic SZ), direct broadband emission across all wavelengths, from far infraredvia optical to X-ray emission, and specialized observables such as Lyman- α forest, unresolved 21-cm,damped Lyman- α systems, etc. is a challenging task. To take full advantage of up-coming observationsacross these wavelengths, gravity-only simulations at very high mass resolution and hydrodynamicssimulations including feedback effects and baryon physics have to be carried out. The gravity-onlysimulations would be the bedrock for, e.g., the optical sky catalogs, CMB lensing measurements, andalso allow the modeling of foregrounds relevant for the CMB surveys, such as the cosmic infrared back-ground. The hydrodynamics simulations will be important for, e.g., modeling the Sunyaev-Zel’dovicheffect or cluster cosmology investigations, Lyman- α observables and other small-scale physics. In ad-dition to the simulation efforts, building tools to create synthetic skies are essential. Some of thesetools already exist but given the quality of up-coming observations, will have to be sharpened. Thevalidation of these catalogs – and therefore ensuring their high fidelity – would be another major task.Some of this would be done at the project level, but close collaborations between the surveys wouldbe essential to fully realize the potential of the multi-wavelength mock catalogs.10 Enabling Community Science:
Many of the tools and simulations that the dark energy communityis developing can be applied across different surveys. Large-scale simulations require efficient codesand substantial supercomputing resources. Given the limited resources currently available and giventhe costs of generating these simulations, it is most natural to develop an infrastructure that allows foreasy access to such simulations. The establishment of an access point where tools and simulations canbe easily shared would be extremely beneficial. Such an effort will require investment in people to buildout an infrastructure that is sustainable in the long-term. A recently formed task force is investigatingcommon use of simulations and tools for LSST, WFIRST, Euclid, and DESI and has identified thefollowing important topics for consideration: common infrastructure to share simulation products andtools, a base of numerical simulations to generate synthetic sky maps, tools to generate syntheticsky maps, large-scale simulation campaigns covering different cosmologies and enabling covarianceestimates, investigation of systematic effects via simulations (including baryonic effects), and advancedstatistical methods. While all of these topics are of great importance, such a program would go beyonda Small Projects Portfolio. We strongly believe that the first item, common infrastructure to sharesimulations and tools, would have the most immediate impact and focus on this in the following.An infrastructure for sharing simulations and tools can take on different levels of sophistication,depending on the available resources. The most straightforward implementation would only allow thecommunity to browse the data products and to download the relevant data to their local computingresources. A more comprehensive approach would allow the community to also access computationalresources where the data resides to carry out first level analysis. The results, if of general interest,could then be added to the available data for other users. In addition, generic tools could be madeavailable for the analysis of the available data. Finally, the infrastructure could be completed byenabling the community to contribute tools and datasets. Small-scale efforts that could seed suchan infrastructure already exist. For example, DESCQA, described in Ref. [MKH +
17] provides anenvironment to contribute to the validation of synthetic sky catalogs, or open source codes for analysis,such as Halotools [HCT + Following two community workshops in 2016 and 2017, this document outlines the broad consensus onsmall-scale efforts that can improve the scientific output of DESI and LSST, the two DOE flagship darkenergy experiments that will take place in the next decade. In addition to this White Paper the communityis also preparing two other White Papers outlining roadmaps for longer-term development of the large-scalesurvey science within the DOE HEP: the southern spectroscopic survey roadmap and the 21-cm cosmologyroadmap. Both documents are in preparation and will be released in Spring 2018.Our proposal is informed by significant advances in understanding the dominant sources of systematicerrors and advances in the methods of data analysis in the time since these experiments were conceived.While the experiments will deliver their designed science, small inputs of additional funding will make theresults significantly more robust. Additionally, new avenues of analysis and data combinations will enablenew insight into the nature of dark energy and dark matter. Despite a challenging budgetary environment,the dark energy research is marching forward with vigor. We argue for a balanced but similarly vigorousapproach. The proposed portfolio combines guaranteed science exploring large, theoretically well-understoodscales with more speculative approaches that extract information from smaller scales where the statisticaland systematic errors from measurements are minuscule compared to theory uncertainties.We have organized many ideas into three overarching sets: observations that leverage existing facilities,experimental R&D that will inform design of the next generation of experiments, and a broad program oftheory and simulation development. In Appendix A we have sorted these ideas into a readiness/cost matrix.11
Project Matrix
In the following table, we provide a summary for the possible start dates and rough cost estimates for thedifferent components of our Small Projects Portfolio.Readiness Total Cost<$1M $1M - $3M<2020
Extending DESI/LSST ⋆ :- Photometric calibration instrumentation- Narrow-band or offset broad-band imaging- WFIRST + LSST synergies
Theoretical and Simulation Advances :- Modeling & simulations for small scaleclustering- Modeling & simulations beyond Λ CDM- Multiwavelength Virtual Observatory- Enabling Community Science2020-23
Extending DESI/LSST ⋆ :- Personnel costs for ground-based spec-troscopy- Peculiar velocity studies- LSST and DESI + CMB S4 synergies
New Technology Developments :- Ground layer adaptive optics over 10 deg field of view- Germanium CCDs manufactured at scale- Fiber Positioner Systems at 5 mm pitch ⋆ Less prescriptive category with more scope for new options and ideas.
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