Gravitation And the Universe from large Scale-Structures: The GAUSS mission concept
Alain Blanchard, Aubourg ?ric, Philippe Brax, Francisco J. Castender, Sandrine Codis, Stéphanie Escoffier, Fabien Dournac, Agnès Ferté, Fabio Finelli, Pablo Fosalba, Emmanuel Gangler, A Gontcho Satya Gontcho, Adam Hawken, Stéphane Ili?, Jean-Paul Kneib, Martin Kunz, Guilhem Lavaux, Olivier Le Fèvre, Julien Lesgourgues, Yannick Mellier, Jérémy Neveu, Yann Rasera, Cécile Renault, Marina Ricci, Ziad Sakr, Norma G. Sanchez, Isaac Tutusaus, Safir Yahia-Cherif
EExperimental Astronomy manuscript No. (will be inserted by the editor)
Gravitation And the Universe from largeScale-Structures
The GAUSS mission conceptMapping the cosmic web up to the reionization era
Alain Blanchard · Aubourg ´Eric · Philippe Brax · Francisco J. Castender · Sandrine Codis · St´ephanie Escoffier · Fabien Dournac · Agn`es Fert´e · FabioFinelli · Pablo Fosalba · EmmanuelGangler · A Gontcho Satya Gontcho · Adam Hawken · St´ephane Ili´c · Jean-Paul Kneib · Martin Kunz · Guilhem Lavaux · Olivier Le F`evre · Julien Lesgourgues · Yannick Mellier · J´er´emy Neveu · Yann Rasera · C´ecileRenault · Marina Ricci · Ziad Sakr · Norma G. Sanchez · Isaac Tutusaus · Safir Yahia-Cherif
Received: date / Accepted: dateAlain BlanchardIRAP, CNRS, Toulouse, FranceE-mail: [email protected]´Eric AubourgAPC, CNRS, Paris, FrancePhilippe BraxIPhT, CEA, Saclay, FranceFrancisco J. CastenderICE-CSIC, IEEC, Barcelona, SpainSandrine CodisIAP, CNRS, Paris, FranceSt´ephanie EscoffierCPPM, CNRS, Marseille, FranceFabien DournacIRAP, CNRS, Toulouse, France a r X i v : . [ a s t r o - ph . C O ] F e b Alain Blanchard et al.Agn`es Fert´eJPL, Pasadena, USAFabio FinelliINAF, Bologne, ItalyPablo FosalbaICE-CSIC, IEEC, Barcelona, SpainEmmanuel GanglerLPC, CNRS, Clermont-Ferrand, FranceA Gontcho Satya GontchoUniversity of Rochester, USAAdam HawkenCPPM, CNRS, Marseille, FranceSt´ephane Ili´cLERMA, CNRS, Paris, FranceJean-Paul KneibEPFL, Lausanne, SwitzerlandMartin KunzUniversity of Geneva, SwitzerlandGuilhem LavauxIAP, CNRS, Paris, FranceOlivier Le F`evreLAM, CNRS, Marseille, FranceJulien LesgourguesTTK, RWTH Aachen University, GermanyYannick MellierIAP, CNRS, Paris, FranceJ´er´emy NeveuLAL, CNRS, Orsay, FranceYann RaseraLUTH, CNRS, Meudon, FranceC´ecile RenaultLPSC, CNRS, Grenoble, Franceravitation And the Universe from large Scale-Structures 3
Abstract
Today, thanks in particular to the results of the ESA Planck mis-sion, the concordance cosmological model appears to be the most robust todescribe the evolution and content of the Universe from its early to late times.It summarizes the evolution of matter, made mainly of dark matter, from theprimordial fluctuations generated by inflation around 10 − second after theBig-Bang to galaxies and clusters of galaxies, 13.8 billion years later, and theevolution of the expansion of space, with a relative slowdown in the matter-dominated era and, since a few billion years, an acceleration powered by darkenergy. But we are far from knowing the pillars of this model which are infla-tion, dark matter and dark energy. Comprehending these fundamental ques-tions requires a detailed mapping of our observable Universe over the wholeof cosmic time. The relic radiation provides the starting point and galaxiesdraw the cosmic web. JAXA’s LiteBIRD mission will map the beginning ofour Universe with a crucial test for inflation (its primordial gravity waves),and the ESA Euclid mission will map the most recent half part, crucial fordark energy. The mission concept, described in this White Paper, GAUSS,aims at being a mission to fully map the cosmic web up to the reionizationera, linking early and late evolution, to tackle and disentangle the crucial de-generacies persisting after the Euclid era between dark matter and inflationproperties, dark energy, structure growth and gravitation at large scale. Keywords
Cosmology · Dark Energy · Early Universe physics
The questions regarding very high energy physics (far beyond the energies ac-cessible to terrestrial accelerators), and the origin of the acceleration of theexpansion of the Universe are very important and timely questions of bothfundamental physics and cosmology. Cosmological and astronomical observa-tions will provide the essential information, if not all the information needed
Marina RicciLAPP, CNRS, Annecy, FranceZiad SakrUSJ, Beirut, LebanonNorma G. SanchezLERMA, CNRS, Paris, FranceIsaac TutusausICE-CSIC, IEEC, Barcelona, SpainSafir Yahia-CherifIRAP, CNRS, Toulouse, France Alain Blanchard et al. to answer these questions. The study of the distribution of matter at very largescales, allowed by large surveys of galaxies, complemented by the observationof the properties of the cosmic microwave background (CMB) radiation, arethe primary probes of this new physics, which remains to be discovered andunderstood.The most direct and simplest explanation of the origin of dark energy whichis compatible with all the available data is the cosmological constant. However,it needs fine-tuning and as a vacuum energy suffers from the so called ”cosmo-logical constant problem”, i.e. the theoretical vacuum energy value estimatedfrom quantum particle physics is about 10 times the observed value [40].Such a huge difference between the two values could physically correspond totwo different vacuum states of the universe, namely at two different epochs ofits evolution: one being the classical very large universe today, the other beingthe very early quantum cosmological vacuum. The low value of Λ or vacuumenergy density today corresponds to a large-scale low-energy diluted universeessentially dominated by voids and supervoids as the set of large-scale obser-vations concordantly and independently shows. On the other hand, the highquantum value of Λ could correspond to the high energy and highly dense,small-scale very early quantum vacuum.A different proposal for the origin of the present acceleration supposesa modification of the General Relativity theory (GR) of gravity by break-ing one of the assumptions of Lovelock’s theorem [26], which states that theonly possible equations of motion which contain only second derivatives in4-dimensional space-time from a scalar Lagrangian density satisfying Lorentzinvariance are the Einstein field equations. One common modification is toinclude a scalar field coupled to the curvature tensor terms in these equationsthrough the Horndeski Lagrangian [16]. This includes all possible introductionsof scalar, vector, and tensor degrees of freedom. Among these theories, mostof the additional tensor degrees of freedom have been significantly constrainedby the discovery of a gravitational wave source and its optical counterpart[24]. Moreover, recent analyses of Integrated Sachs-Wolfe effect (ISW)-galaxycross-correlation data [32] have also disfavored other derivative couplings ofthe cubic order as well as beyond Horndeski [25] higher order additional theo-ries. Theories yet to be excluded are those with linear scalar self-interactionsand minimal/derivative couplings to gravity, such as dark energy models basedon a scalar field with a potential (quintessence) [31] or with a non canonicalkinetic term (k-essence) [3]. Other modifications of gravity are still also viable,modifying the remaining assumptions such as models with extra dimensions(e.g. Kaluza-Klein type [20, 21]), or those that relax the Lorentz invariance(e.g. Horava-Lifshitz gravity [21]). But these can all be distinguished from eachother by either a different evolution of the background expansion and/or a dif-ferent cosmic growth history. Some of these models of modified gravity couldbe used to incorporate both late time acceleration and inflation at early times.All these models could be tested with CMB cross-correlation observations, ravitation And the Universe from large Scale-Structures 5 however the latter probe, unlike the distribution of large scale structure, onlygrowth and expansion at the time of recombination. Next generation CMB ex-periments exploring high multipole modes at small scales will need to accountfor the foreground distribution of large-scale structure.At the moment, no favoured model of dark energy seems to emerge im-plying that an effective description of this phenomenon using a finite andwell-chosen set of parameters should be employed.Among these phenomenological tools, the measure of the expansion H ( z ),and growth rate of structure will provide probes for dark energy over cosmictimes and will indirectly probe very high energy cosmic origins. Moreover, thedistribution of matter, traced by galaxies and dark matter through the effectsof gravitational lensing and cross correlations between the former and latterdistribution, offers an even more powerful tool to better understand the char-acteristics of cosmic expansion and the growth rate of structure. The reasoncomes from the fact that the lensing distribution measurements are sensitiveto the sum of the two gravitational potentials Φ + Ψ [4] while galaxy clus-tering, being non-relativistic, is sensitive to the Newtonian potential Ψ only.Within GR, the two are equal, while in modified gravity models there can be ashift called the slip parameter. Therefore, the combination of the two observ-ables probes the relation between the two gravitational potentials. Moreover,the precise estimation of weak lensing and galaxy clustering statistics willhelp disentangle degenerate effects from the aforementioned extensions to thecurrent concordance model (or Λ -CDM, named from its main components,dark energy and cold dark matter), i.e., the slip, growth, and expansion ofthe universe, through the determination of the E g factor [27] which encapsu-lates three related observables: the Hubble parameter, the galaxy-velocity andgalaxy-lensing cross spectrum.This distribution of matter is also a diagnostic test of the physics of theearly universe: thus the presence of massive neutrinos could be highlighted fora range of masses inaccessible to experiments in accelerators and potentiallybetter constrained than from CMB experiments of the same calibre [14]. Thisis due to the fact that the dominant effect of massive neutrinos is the sup-pression of the growth of structure on relatively small scales, while neutrinomasses of a few tenths of eV are still relativistic at the time of photon de-coupling and affect the CMB at the background level. There are two effectsthat massive neutrinos could have on CMB anisotropies [15, 23]: one throughthe early ISW, which is roughly ten times smaller than the depletion in thesmall-scale matter power spectrum; and the other through the late ISW effectwhich is difficult to measure due to cosmic variance. Moreover, the fact thatthe suppression of the growth by massive neutrinos is time-dependent leads toincreasingly better constraints from the extension of the redshift coverage oflarge scale structure surveys. Alain Blanchard et al.
The distribution of galaxies and large-scale structures is also a diagnostictool of the physics of inflation. In these scenarios this distribution is indeedgenerated via the fluctuations of the metric produced during a high energyphase transition in the early universe, perhaps at energies on the order of 10 GeV. A favoured path of testing inflation would be the detection of B modesof the polarization of the cosmological background. The amplitude of thesemodes, from the tensor to scalar ratio r , is mandated to be non zero what-ever the inflation model, with the most recent CMB data having provided r < . − .
07 [36]. Besides just improving such constraints, a detection, ifany, would require - and justify - a dedicated mission.The possible presence of a very weak non-gaussian signal is another predic-tion of inflation, for which a minimal level is expected and would be accessibleto surveys that cover a large enough volume of the Universe. Untill now, thedifferent sets of robust and independent cosmological data clearly favor min-imal inflationary models for which the amount of non-gaussianity indicatedby the parameter f NL is very small. e.g., the most recent data provides aconstraint of f NL < f NL > n s , r, f NL ) and adiabaticity, indicate f NL to be of the order 10 − , more pre-cisely f NL ∼ (1 /N e ) ∼ . N e being the inflation number of e-folds [6, 10].Overall, the direction in which both the combined robust observational dataand predictive theory are pointing is towards a very small amount of primor-dial non-gaussianity.Within the horizon at 2035, it is important to keep in mind that in the pri-mordial phases of the Universe, besides inflation and its GUT energy scale attime scales of 10 − sec, there is room for higher energy scales at earlier timeswhich are of the order of the Planck fundamental scale 10 GeV at 10 − secand beyond, i.e., the so-called trans-Planckian regime. This quantum phaseand its late imprints is a targeted field of study in quantum unification the-ories, gravitation and cosmology. The understanding of dark energy, whetherwithin or outside of a standard concordance model description, (namely thecosmological vacuum energy, cosmological constant, or dynamical dark ener-gies for instance), is at the center of these studies. Deviations from the clas-sical Λ -CDM model description if measured in the expansion rate and/or thegrowth of structures would thus impact both the cosmological community andthe wider context of fundamental physics, including string theory. Different probes are used to study primordial inflation, cosmic accelerationand the growth of structures. We propose to make precise measurements of ravitation And the Universe from large Scale-Structures 7 the cosmic web, up to unprecedented redshift, with a high density of sources,to encompass at once matter and space evolutions.The design of the GAUSS mission concept is optimized for the combina-tion of probes called “3 × Alain Blanchard et al. galaxies. The main derived statistics is then the 2-point correlation functionof the ellipticities in redshift bins. This shear statistic is directly linked tothe matter power spectrum. Measuring shear in tomography is therefore agreat probe of the growth of structure and used in current and forthcomingcosmological surveys to constrain Λ -CDM model parameters and test beyond Λ -CDM models. The magnification modifies size and magnitude of galaxies be-cause it changes size while conserving surface brightness. It also changes thenumber density of galaxies as some background galaxies become detectablethanks to the flux enhancement. WL measurements using magnification havebeen done, e.g. with DES [12], through these different effects. Magnificationand shear probe the same underlying dark matter distribution but are proneto different systematics, so they will help constrain cosmology better.3 × × × ravitation And the Universe from large Scale-Structures 9 strong lensing could also be powerful probes. Dedicated studies of these probescan be made from the same raw data. In addition to providing meaningfulastrophysical results, by providing an internal check on the coherence of allcosmological probes in the same survey, these studies will permit us to con-trol systematic effects. The use of type Ia supernovae would require a specificobservational strategy as light curves are mandatory. This probe may sufferfrom many astrophysical systematic effects at very high redshift and will belargely exploited at low and intermediate redshift by the Rubin Observatory(previously known as the LSST) in conjunction with spectra measured by theATLAS project [39]. Therefore the GAUSS mission concept has not been en-visioned to include this probe.Note that the other quantities, E g especially designed to separate modifiedgravity from expansion and growth behaviors (cf. section I), and f NL aimingto follow the primordial fluctuations properties will be highly relevant to aGAUSS-like survey. These quantities are emerging for galaxy surveys [13], [7],[8] and no precise predictions regarding their constraints are presented in thisWhite Paper. E g is an observable aimed at being a model independent gravitational con-sistency check [42]. It can be performed by using the same set of galaxies thatserve to trace non-relativistic gravitationally-driven motion, and as foregroundlenses for probing the relativistic deflection of light from background sources.In this way, it could be ascertained whether the relative amplitude of thesetwo effects, driven by the same underlying density perturbations traced by thelenses, assuming an expansion for a given set of cosmological parameters, isconsistent with the prediction of GR. It is carried out by cross-correlating lensgalaxies to both the surrounding velocity field using redshift spectral distor-tion and to the shear of background galaxies using galaxy-galaxy lensing [5].While galaxy-galaxy lensing, galaxy clustering, and galaxy redshift distortionsare strongly sensitive to the galaxy bias and to the amplitude of the matterperturbations, however, the combination of these quantities in E g is such thatboth nuisance parameters cancel out. Also, CMB lensing has been proposed asa more robust tracer of the lensing field for constructing E g at higher redshiftsand at large scales while avoiding intrinsic alignments. Few constraints havebeen put on E g using these two methods and on several datasets for sparseredshift ranges [30] with different results as to whether GR is preserved ordiscrepancies found. The goal of our future mission will be to use new, sig-nificantly deeper, spectroscopic and imaging survey datasets to extend thesetests to higher source densities and more distant redshifts and to deliver thebest dataset to constrain this ultimate test of gravity.In addition to the cosmological probes, GAUSS will provide a legacy cat-alog to the community containing a significant fraction of the galaxies of ourobservable universe, about ten times more galaxies than the output of theimminent projects which are DESI, Vera Rubin observatory and Euclid, in spectrometry or in photometry. This catalog will be a unique database formulti-wavelength, multi-messenger studies. Different tracers of matter havedifferent biases, which means different relation between the tracer and theunderlying dark matter, and their combination can help lowering the cosmicvariance, e.g. [2]. So the GAUSS catalog will have to be correlated with ex-ternal probes, like weak lensing of CMB or neutral hydrogen intensity maps,among others, for consistency checks on one hand and to get still more robustand tighter constraints on cosmology on the other hand. From the observational point of view, although the origin of dark energy isunknown, its presence is effective in the recent universe (at redshift smallerthan 5) and dominant in the very recent universe (at redshift smaller thantypically 0.5). The presence of dark energy modifies the expansion rate, thegrowth rate of structure in the linear regime but also leads to subtle differencesin the dynamics in the mildly nonlinear regime of structure formation. Givenour lack of understanding on the origin of the cosmic acceleration, it is vitalto obtain measurements of observational quantities that are sensitive to itspresence and properties: the history of the expansion rate H ( z ), the angulardistance D A ( z ), the growth rate of cosmic structure f g ( z ), and diagnostics ofgravity in the mildly nonlinear regime, like the E g quantity.Several ground and space experiments are already scheduled for these typesof studies that will deliver their ultimate constraints at the 2030-2035 horizon.A metric of their performance for intercomparison has been proposed someyears ago through the figure of merit (FoM), that is the inverse of the areaof the constraint contours in the two parameters dark energy model Cheval-lier–Polarski–Linder (CPL). Although the full performance of a project is notreduced to a single number, the FoM is still a very useful number for compari-son. Here we used as much as possible the FoM computed in a flat cosmologicalmodel. We have performed a forecast study and establish a number of resultsthat serve as a guideline for the present White Paper: individual probes likeWL (from shear measurement) and the 3D power spectrum (from spectroscopicsamples) when combined between them provide a FoM much higher than thatobtained on each probe individually, i.e. one plus one is (much) more than two.For instance, we have considered a fiducial survey with 30 gal/arcmin for thephotometric/WL part, and 1 gal/arcmin for the spectroscopic part coveringthe redshift range 0 . − ravitation And the Universe from large Scale-Structures 11 and XC boosts the FoM above 1000. This is already above foreseen surveys(like Euclid or Rubin observatory) for which FoM is anticipated to be around500, but might correspond to what the combination of these various surveyscan provide by 2035. This also illustrates that the critical ingredient for theultimate constraint on dark energy properties resides in the cross-correlationbetween the WL probe and the photometric sample GCp. The role of crosscorrelation between GCs and other probes has not yet been investigated indetail but preliminary investigation that we performed shows that an increaseof 50% on the FoM is realistic. This additional gain is however not taken intoaccount in the following.Let us now examine what improvement can be expected. In order to gainover existing or foreseen surveys, the main avenue is to increase the total num-ber of objects that are observed. The gain anticipated compared to our fiducialsurvey is presented in Figure 1 in which the FoM is computed when the densityof objects is increased by some factor compared to the fiducial one. The Rubinobservatory targets a density number of 40 galaxies/arcmin , targeting a FoMof 600 (in flat models) similar to the 30 galaxies/arcmin Euclid photomet-ric survey targeting a FoM of 400 (in non-flat models using the combinationwith Euclid spectroscopic data). For the spectroscopic sample, Euclid targetsaround 50 million galaxies, a number comparable to what is anticipated fromthe DESI sample (with noticeably different selection rules allowing DESI toachieve a higher FoM ∼
150 for the spectroscopic probe alone). For our fiducialmodel, the spectroscopic sample will be around 700 milions spectra. Improv-ing the density of galaxies in the spectroscopic sample allows us to improvethe FoM by taking into account more small-scale data for the spectroscopicsample, a strategy of less interest at fainter density.As shown in Figure 1, the final FoM, resulting from the combination ofthe main probes, increases by a factor of more than ten relative to foreseensurveys if the number of targets in both the photometric and the spectroscopicsamples are increased by a factor of 10. Increasing the number of targets bya further factor of ten would however not produce the same gain, instead thegain would be closer to a factor 3. Thus, this is a good indication that thereis a significant gain to be achieved by increasing the number of targets by10, but that the gain tends to weaken beyond that and would be extremelychallenging as it would require deeper imaging than the Hubble ultra deep field.The primary gain anticipated from a given telescope is proportional to thesquare of its diameter multiplied by the field of view. However, with a densityof 300 objects per arcmin , for a ground-based telescope, a significant fractionof galaxies in the field will overlap due to the blurring produced by the at-mosphere. This would make the weak lensing analysis extremely complex andchallenging. Thus, only a space mission could provide accurate images of thequality necessary for weak lensing analyses if the number of objects has tobe increased significantly. The limitations of ground-based image quality are Fig. 1
An estimation of the gain compared to a fiducial survey (having 30 gal/arcmin forthe photometric part and 1 gal/arcmin for the spectroscopic part, covering the redshiftrange 0 . − ∼ k max = 1 compared to k max = 0 . k max = 0 . compounded by the further limit on the availability of color information fromthe ground. Broad optical as well infra-red photometry is necessary nowadaysto estimate photometric redshifts and thereby optimally perform weak lensinganalysis. Indeed, contamination by unresolved sources is a dominant limita-tion of weak lensing analysis that exquisite image quality has the potential tocircumvent. In order to reach the foreseen density of objects, it is necessary togo 2.5 magnitudes deeper than the depth of typical working density of Euclidor the Rubin Observatory for photometry and 4.5 magnitudes deeper for thespectroscopic survey. Compared to the Euclid space telescope this needs anincrease in efficiency of around 60.Spectroscopic surveys of galaxies from the ground are limited by telluriclines. Slitless spectroscopy is limited by the systematics intrinsic to this method.In order to achieve a dense high-redshift (in the range 1-4) spectroscopic galaxysample the most efficient approach is infrared-space-based slit spectroscopy inthe range 0.5-5 µ m.For dark energy studies, the final FoM is severely dependant on the com-bination of WL and GC data. A survey focussed on the imaging part, with anexquisite quality will therefore achieve a remarkable progress on the FoM evenwithout the combination of the spectroscopic part. This conclusion is howeverlimited by the fact that we do not have yet a full quantitative investigation ofthe role of the cross-correlation between spectroscopic data and weak lensingdata, and by the fact that on mildly non-linear scales the test of modified grav-ity theories needs information coming from spectroscopic data. Identically, alarge off-axis telescope is a technology that has not yet been implemented inspace. A more classical solution could be adopted. The currently known landscape is summarized in Figure 2. It gathers the mainspace missions from Europe (mainly ESA but also DLR for eROSITA), NASAand Asia (which stands for JAXA and CSA projects). Most of these NASA,JAXA and CSA missions are also ESA Missions of Opportunity (MoOs). Theblack boxes outline missions that have cosmology among their main goals.The unique European mission, Euclid, should end around 2028. The currentproposal concerns the next generation, after Euclid, Spherex and the RomanSpace Telescope (previously known as WFIRST), eventually completed by thespectroscopic ATLAS follow-up.While Euclid should explore up to a few billion galaxies, with photometricredshifts, typically up to a redshift of 2 with spectroscopic measurements fora few percent of them; the Roman Space Telescope should perform a fairlysimilar survey but deeper. In the meantime, Spherex will map the entire sky
Fig. 2
Main Space missions connected to cosmic web dark energy or gravitation at low spatial resolution with a mid-spectral resolution in infrared. The goalof the GAUSS project is to obtain spectroscopic redshifts as well as the shapesof tens of billions of galaxies up to a redshift of 5.Observational cosmology takes place on the ground too. Here we summa-rize some of the main ground-based galaxy surveys. The Dark Energy Spec-troscopic Instrument (DESI) is installed at the focal plane of a 4-meter tele-scope in Arizona to measure the spectra of more than 30 million galaxies andquasars covering 14,000 square degrees with the survey expected to start in2020 [11], now stopped because of the epidemic. The ESO cosmology redshiftsurvey with the 4-meter Multi-Object Spectroscopic Telescope (4MOST) inChile will start in 2022 [33], providing high resolution spectra of 8 million ob-jects covering 15,000 square degrees up to z = 3 .
5. The LSST (Large SynopticSurvey Telescope), recently renamed the Vera C. Rubin Observatory, will con-duct a photometric survey, starting at the end of 2022, of ∼ ,
000 squaredegrees with an 8-meter telescope in six bands based in Chile over ten yearswith a cadence suited to detect about 500 supernovae per night [17]. Whilethey have many advantages and will undoubtedly produce high quality science,none of these surveys is able to recover the shapes of galaxies with the sameaccuracy as space-based instruments, owing to the presence of the atmosphere.The entire cosmic web, from the end of the epoch reionization until now,should be ultimately mapped in the visible/infrared domains. It would pro- ravitation And the Universe from large Scale-Structures 15 vide legacy data, thanks to the accuracy and the completeness of the survey,which can be correlated with CMB lensing, Sunyaev-Zeldovich or HI maps forinstance.Cosmological use of probes like WL and GC is more efficient when com-bined with cosmic microwave background (CMB) observations which directlymap the first billion years. Some degeneracies are broken, for instance betweenthe amplitude of primordial energy fluctuations and the optical depth linked tothe reionization period, which currently prevent better constraints on the totalmass of the neutrinos. The GAUSS project is perfectly timed to take advantageof the results of LiteBIRD and the S4 ground-based efforts. The cosmologicalparameters derived by these S4 experiments should be obtained with errorstwo times lower than current experiments and a significantly more precise mapof the CMB lensing, obtained by the large ground-based telescopes, should beavailable for correlation with the huge GAUSS galaxy catalogue.The cosmological landscape will soon contain the Square Kilometre Array(SKA) which will conduct a huge spectroscopic galaxy survey, by detecting the21 cm emission line of neutral hydrogen (HI) from around a billion galaxiesover 3/4 of the sky, out to a redshift of z ∼ The proposed imaging survey should have color images in several bands. Al-though one could envision having single detectors allowing spectral informationper pixel, here we consider a strategy where 8 bands per field are necessary,
Fig. 3
Number of spectroscopic redshifts in surveys as a function of the year, extracted fromthe White Paper proposing MegaMapper, an answer to the Decadal Survey on Astronomyand Astrophysics Astro2020 call [34]. The Euclid space mission and the present GAUSSconcept mission have been added for comparison. with a dichroic that would mean two focal planes with 4 bands each from 0.5 µ m to 5 µ m, those bands being achieved through filters on the detectors. Thisis aimed at avoiding the use of mechanical systems as much as possible.The primary limitation will come from the total number of pixels of thecamera: an appropriate sampling to fully benefit from the space image qualityis to have a pixel size of 0.05 arcsec (although a pixel size of 0.1 arcsec wouldbe acceptable). The field of view would be 2 degrees in size, to get an areaof around 4 square degrees, 8 times wider than Euclid and slightly less thanhalf the field of view of the Rubin Observatory. Assuming a pixel size of 5 µ mand a ten Giga-pixels camera, the physical size will be around 70 cm. Assum-ing half of the field is used by the imaging system with 8k ×
8k detectors, thisneeds 160 detectors in the focal plane. An extra factor of efficiency of 8 isneeded which implies the need to have a 3-4 meter class telescope. In orderto get the highest image quality for the images, an off-axis mirror will be thepreferred solution. Such solutions have been investigated and their advantageshave been quantified [22, 35]. ravitation And the Universe from large Scale-Structures 17
In order to achieve a massive spectroscopic sample we propose to use Dig-ital Micro-mirror Devices as slit selectors [9]. Although this technology hasnot yet been demonstrated in space, it has been used in astronomy [27] andwe anticipate that this or some equivalent technology will allow massive slitspectroscopy on selected targets. A significant fraction (50% as a guideline) ofthe focal plane should be used for this. Sharing the focal plane in this way willavoid having moving mechanical systems for dealing with both spectroscopicand photometric channels. Spectroscopic devices will be located in the outerpart of the field as the spatial resolution is less critical for spectroscopy. 164k ×
4k detectors with 10 µ m pixels can achieve a mutiplex factor greater than20 000. Using slit spectroscopy will suppress the sky background inherent inthe slitless-grism spectroscopy usually used in space missions (HST and Euclidfor example) thus enabling a significant gain in sensitivity. The statistical distribution of matter over a very large volume of the universewill remain the primary tool to investigate the source of the accelerated ex-pansion of the universe as well as the physics of the very early universe. Largephotometric and spectroscopic surveys of galaxies over the same sky area areparticularly efficient for these objectives. A space mission like GAUSS willsurpass by more than an order of magnitude all currently foreseen projects(Euclid, Rubin Observatory, DESI, Roamn Space Telescope, . . . ) thanks toa very deep flux limit (including in the infrared domain up to ∼ µ m), avery high multiplexing capability allowing to map the distribution of a uniquetracer from redshift 0.5 to 5, with limited systematics, and the power of probecombinations and their cross-correlation within a single experiment, a uniqueadvantage of GAUSS over most existing projects (with the exception of Eu-clid). This would allow, for instance, a definitive measurement of the totalmass of neutrinos from a single experiment, and provide major progress inour understanding of Dark Energy and Inflation, two major problems of bothcosmology and fundamental physics. Thus, a mission like GAUSS linking theearly and late phases of cosmic evolution, with their hugely different energyscales, provides unique clues for cosmology, gravitation, and inflation physics,without any equivalent tool for investigating these topics. References
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