One way to test the theory of inflation is to measure the imprint of inflationary ripples on the large-scale structure (LSS) of galaxies. SPHEREx will probe the statistical distribution of inflationary ripples by measuring the large-scale 3D distribution of galaxies. It will achieve this by measuring galaxy redshifts over a large cosmological volume at low redshifts.
The fNL parameter characterizes non-Gaussianity in the distribution of inflationary fluctuations. In particular, multi-field inflationary models generally predict a high level of non-Gaussianity (|fNL| > 1) while single-field models generally predict low levels of non-Gaussianity (|fNL| < 10-2). SPHEREx will achieve a sensitivity of at least fNL=1 (2 σ), allowing it to distinguish between these two classes of models (see Fig 1).
SPHEREx will reduce uncertainty in fNL by a factor of more than 10 over current values. SPHEREx can put better constraints on fNL than can a CMB measurement since the large-volume 3D survey accesses many more modes than a 2D CMB measurement (see Fig 2).
SPHEREx will determine redshifts for hundreds of millions of galaxies by fitting measured spectra to a library of galaxy templates (see Fig 3). SPHEREx's infared range allows it to exploit the nearly universal 1.6um rest frame bump in these fits. The distribution of galaxies extending to moderate redshift (in SPHEREx's spectral range) covers an enormous effective volume (see Fig 2).
SPHEREx utilizes the power spectrum and bispectrum, the Fourier transforms of the 2- and 3-point correlation functions respectively.
SPHEREx will classify galaxies according to redshift accuracy, categorizing 490 million galaxies at < 10% accuracy and 16 million galaxies at 0.3% accuracy. The low accuracy sample drives the power spectrum fNL sensitivity, and the high accuracy sample drives the bispectrum as well as key inflationary parameters, i.e. index of spectrum (ns) for departure from scale invariance, its running ( αs), and its departure from geometric flatness ( Ωk). SPHEREx's projected constraints on these parameters are summarized in Fig 1.
SPHEREx will complement planned Euclid and WFIRST spectroscopic surveys, which focus on higher redshifts to probe the equation of state of dark energy. SPHEREx's lower redshift survey will allow its measurement of inflationary parameters to be largely independent. This full suite of inflationary observables, combined with CMB polarization measurements, will give a complete picture of inflation.
The SPHEREx observing strategy naturally generates a wide+deep map at each ecliptic pole (the southern map will be shifted to avoid the LMC). Both maps will enable unique measurements of spatial fluctuations in the extragalactic background light (EBL) which will lead to new insights into the origin and history of galaxy formation. Specifically, SPHEREx will probe signals from the intra-halo light (IHL) and from the epoch of reionization (EOR) to minimum levels in the EBL.
Surveys which focus only on detection of individual galaxies discard important information available in the diffuse light from faint emission sources, such as intra-halo light (IHL) and dwarf galaxies. Fluctuations in EBL trace these faint sources. The amplitude of the linear clustering signal, proportional to the total photon emission, exceeds that expected from large-scale clustering of known galaxy populations, suggesting EBL is a fruitful signal for probing yet-undiscovered features of the origin and history of galaxy formation.
Diffuse IHL emissions at redshifts of z < 1 arise from stars disassociated from their parent galaxies. During a collision of galaxies, some stars are stripped from their parent galaxies by dynamical friction and form extended stellar halos, some extending out to 300 kpc (Tal & van Dokkum 2011). The substantial IR fluctuations from the IHL are thought to be responsible for the corresponding IR fluctations in the EBL. SPHEREx's sensitive multi-band fluctuations will probe the history of starts producing the IHL.
The Epoch of Reionization (EOR) marks the end of the Universe's dark ages, in which the first collapsed objects produced energetic UV photons that reionized the surrounding hydrogen gas. Estimates of the UV luminosity function at z > 6 suggest the majority of UV intensity driving reionization was due to dwarf galaxies.
EOR fluctions in the EBL trace exactly these dwarf galaxies. By extrapolating the HST-observed luminosity funciton of z > 7 faint galaxies, one can put a lower bound on the EOR component of the EBL. SPHEREx has the sensitivity in multiple spectral bands to probe for the EOR component's distinctive spectral features using information in auto- and cross-correlations.
Water Ice and Biogenic Molecules
SPHEREx will be a game changer in resolving long-standing questions about the amount and evolution of key biogenic molecules (H2O, CO, CO2, and CH3OH) throughout all phases of star and planetary formation.
Molecular Clouds to Protoplanetary Disks
Molecular clouds contain the gas and compounds that give rise to protoplanetary disks and, ultimately, to planets. While ices within these molecular clouds are a repository for important elements, they are also sites of active chemistry. For example, hydrogenation and oxygenation reactions occur within these ices, as does the production of complex organic molecules resulting from the interaction of the ices with radiation.
However, the evolution of molecular clouds to protoplanetary disks is largely unknown, hampered primarily by the lack of spectra available for Galactic molecular clouds and protoplanetary disks. SPHEREx will remedy this by increasing by 100-fold the number of ice spectra available of molecular clouds, young stellar objects, and protoplanetary disks. Armed with the SPHEREx data, it will be possible to understand, in a statistically significant way, how ice content correlates with, among other factors, cloud density, internal temperature, presence or absence of embedded sources, external UV and X-ray radiation, elemental abundances (e.g., C/O ratio), gas-phase composition, and cosmic-ray ionization rate.
Synergy with other missions