Introduction
On January 2, 2026, NASA’s Jet Propulsion Laboratory announced that the SPHEREx (Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer) observatory has successfully completed its first all-sky spectroscopic infrared survey, marking a transformative milestone in observational astronomy. Unlike conventional imaging surveys that capture broadband photometry, SPHEREx simultaneously maps the entire celestial sphere in 102 distinct near-infrared wavelength channels spanning 0.75 to 5.0 micrometers, effectively creating a comprehensive “rainbow map” of the cosmos [1]. This unprecedented dataset enables investigations spanning cosmic inflation theory, galaxy evolution during the epoch of reionization, and the distribution of biogenic molecules in star-forming regions throughout the Milky Way. The completion of this first survey represents the culmination of over a decade of mission development and validates the revolutionary approach to wide-field spectroscopy at the foundation of SPHEREx’s design.
The Technology of Linear Variable Filters
SPHEREx’s observational capabilities derive from its innovative implementation of Linear Variable Filters (LVFs), which enable simultaneous spectroscopic measurements across wide fields of view without the throughput penalties of traditional dispersive or interference filter systems. An LVF is a specialized optical component in which the wavelength of peak transmission varies continuously along one spatial dimension of the filter substrate. SPHEREx employs six separate LVF arrays covering different wavelength ranges, with spectral resolution (lambda/delta-lambda) varying from approximately 40 at shorter wavelengths to 130 at longer wavelengths [2].
The optical design positions these LVFs in the focal plane after a 20 cm diameter telescope, allowing each detector pixel to sample a unique combination of sky position and wavelength. By executing a specific scanning pattern over the course of the mission, SPHEREx builds up complete spectral energy distributions for every point on the sky. This approach achieves spectroscopic coverage that would require hundreds of individual observations with conventional filter-wheel systems, fundamentally changing the economics of all-sky spectroscopic surveys.
The detector arrays consist of HgCdTe photodiodes optimized for the 0.75-5.0 micrometer wavelength range, with two arrays covering 0.75-2.42 micrometers and two arrays covering 2.42-5.0 micrometers. Operating at approximately 80 K to minimize thermal noise, the detectors achieve read noise levels below 20 electrons and dark current rates below 10 electrons per second per pixel. The combination of low noise performance and the multiplexed spectroscopic capability enables SPHEREx to detect sources several orders of magnitude fainter than previous all-sky infrared surveys such as WISE (Wide-field Infrared Survey Explorer).
Mapping the Large-Scale Structure of the Universe
One of SPHEREx’s primary science objectives addresses fundamental questions about cosmic inflation, the hypothesized period of exponential expansion that occurred in the first 10-32 seconds after the Big Bang. Inflation theory predicts that quantum fluctuations during this epoch were stretched to cosmological scales, seeding the density perturbations that eventually grew into galaxies and galaxy clusters. These primordial fluctuations imprint characteristic statistical patterns in the three-dimensional distribution of galaxies that can be measured through large-scale structure surveys [3].
SPHEREx will map the positions and redshifts of over 450 million galaxies across cosmic time, with particular emphasis on detecting emission from ionized hydrogen (H-alpha) and doubly ionized oxygen ([OIII]) at redshifts between 0.5 and 3.5. By measuring the power spectrum of galaxy clustering on scales exceeding 100 Mpc, SPHEREx can constrain the amplitude and shape of primordial non-Gaussianity, a key prediction that distinguishes between competing inflation models. Current constraints on the non-Gaussianity parameter f_NL have uncertainties of approximately plus-or-minus 5, while SPHEREx aims to reduce this uncertainty to approximately plus-or-minus 1, potentially ruling out broad classes of inflation models.
The all-sky spectroscopic approach provides critical advantages over traditional redshift surveys that target pre-selected galaxies. By capturing complete spectral information for all sources above the detection threshold, SPHEREx eliminates selection biases that can systematically affect large-scale structure measurements. The survey’s wavelength coverage specifically targets emission lines that trace star formation activity, providing direct constraints on the cosmic star formation history and its relationship to dark matter halo assembly.
Probing the Epoch of Reionization
The epoch of reionization, spanning redshifts from approximately z = 6 to z = 15 (corresponding to 150-500 million years after the Big Bang), marks the period when the first stars and galaxies ionized the neutral hydrogen pervading the early universe. Understanding this transition requires measuring both the timing and topology of reionization, which reflect the properties of the first luminous sources and the clumpiness of the intergalactic medium. SPHEREx contributes to this investigation by measuring fluctuations in the extragalactic background light (EBL) at near-infrared wavelengths [4].
The integrated emission from all galaxies throughout cosmic history creates a diffuse background that varies with wavelength and angular scale. At wavelengths between 1 and 5 micrometers, this EBL receives contributions from galaxies at redshifts extending to the epoch of reionization, with emission redshifted from rest-frame ultraviolet and optical wavelengths. By measuring angular power spectra of EBL fluctuations, SPHEREx can detect the signature of galaxy clustering during reionization, even for galaxies too faint to detect individually.
Preliminary analysis of the first SPHEREx all-sky map has already revealed fluctuations consistent with galaxy clustering at redshifts above z = 7, complementing direct galaxy detection efforts by the James Webb Space Telescope. The statistical approach pioneered by SPHEREx provides constraints on the luminosity function of faint galaxies during reionization, addressing the critical question of whether low-mass galaxies or more exotic sources like Pop III stars dominated the production of ionizing photons. These measurements will help resolve ongoing debates about the primary drivers of cosmic reionization.
Interstellar Ices and the Chemistry of Life
Beyond its cosmological objectives, SPHEREx addresses fundamental questions in astrobiology by mapping the distribution of water ice and organic molecules in star-forming regions throughout the Milky Way. Infrared spectroscopy provides the primary method for detecting and characterizing ices in dense molecular clouds, as solid-phase molecules produce broad absorption features at specific wavelengths. Water ice exhibits a characteristic absorption band at 3.0 micrometers, while carbon monoxide ice shows absorption at 4.67 micrometers. More complex organic molecules including methanol and carbon dioxide produce additional features across the SPHEREx wavelength range.
The first SPHEREx all-sky map contains spectroscopic data for over 300 million stars and hundreds of known molecular clouds, enabling statistical studies of ice composition as a function of environment. By measuring ice abundances in molecular clouds at various evolutionary stages, from quiescent dark clouds to active star-forming regions, SPHEREx traces the chemical pathways leading from simple interstellar molecules to the complex organic inventory delivered to planetary systems. This addresses the fundamental question of whether the molecular building blocks of life are synthesized locally in individual planetary systems or represent a common inheritance from the interstellar medium.
Early results from the SPHEREx ice mapping program have identified systematic variations in the ratio of carbon monoxide to water ice between different molecular cloud complexes, suggesting that local physical conditions (temperature, density, radiation field) significantly affect ice composition. These observations provide critical constraints for astrochemical models of ice formation and processing, with direct implications for understanding the organic material incorporated into protoplanetary disks and, ultimately, into planets and comets.
Data Management and Scientific Legacy
The first SPHEREx all-sky survey has generated approximately 1.5 petabytes of raw data, requiring sophisticated pipeline processing to convert detector readouts into calibrated spectrophotometric measurements. The SPHEREx data processing pipeline, developed by IPAC at Caltech, implements flat-field corrections, cosmic ray removal, background subtraction, and wavelength calibration for each of the 102 spectral channels. All processed data products will be released through the NASA/IPAC Infrared Science Archive, enabling the broader astronomical community to exploit this unique dataset for investigations beyond the mission’s primary science objectives.
The planned survey strategy calls for four complete all-sky maps over the mission’s two-year baseline duration, with each subsequent map improving photometric sensitivity and enabling variability studies. By comparing spectrophotometry across multiple epochs, SPHEREx can identify transient phenomena, variable stars, and potentially even detect exoplanet transits in the infrared for bright host stars. The mission’s legacy will extend decades beyond its operational lifetime, as the all-sky spectroscopic dataset provides a definitive reference for source characterization and target selection for future observatories.
Conclusion
The successful completion of SPHEREx’s first all-sky spectroscopic survey represents a landmark achievement in observational astronomy, demonstrating the feasibility and scientific power of wide-field infrared spectroscopy. By simultaneously addressing questions ranging from the physics of cosmic inflation to the chemical origins of life, SPHEREx exemplifies the multi-disciplinary potential of transformative survey missions. The combination of innovative Linear Variable Filter technology, sensitive infrared detectors, and comprehensive survey strategy has produced an unprecedented dataset that will enable discoveries across virtually all areas of astrophysics. As subsequent all-sky maps accumulate over the mission lifetime, SPHEREx will refine its measurements and expand its scientific impact, establishing a new paradigm for spectroscopic surveys that will influence mission design for decades to come.
References
1. Doré, O., et al. “Science Impacts of the SPHEREx All-Sky Optical to Near-Infrared Spectral Survey: Report of a Community Workshop Examining Extragalactic, Galactic, Stellar and Planetary Science” (2016). arXiv:1606.07039. https://arxiv.org/abs/1606.07039
2. Korngut, P. M., et al. “SPHEREx: An All-Sky NIR Spectral Survey” (2018). Proceedings of SPIE, 10698, 106981U. DOI: 10.1117/12.2312860
3. Alvarez, M. A., et al. “Testing Inflation with Large Scale Structure: Connecting Hopes with Reality” (2014). arXiv:1412.4671. https://arxiv.org/abs/1412.4671
4. Cooray, A., et al. “The SPHEREx All-Sky Survey: Probing the History of the Universe with Near-Infrared Background Fluctuations” (2016). Astrophysical Journal, 825(2), 161. https://arxiv.org/abs/1602.05178
5. Pontoppidan, K. M., et al. “The James Webb Space Telescope Mission” (2022). Astrophysical Journal Letters, 936(1), L14. https://arxiv.org/abs/2207.05632