In the near future, a novel cone-shaped spacecraft is set to embark on a mission to explore the heavens in unprecedented ways. Named SPHEREx (which stands for Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer), this NASA initiative aims to deliver an exceptional all-sky three-dimensional mapping of hundreds of millions of galaxies across our universe. One of the significant mysteries the mission seeks to unravel is the origin of our universe. SPHEREx aims to uncover fresh insights in the pursuit of understanding cosmic inflation, a widely studied hypothesis suggesting that our nascent universe expanded an astonishing trillion-trillion times in a fraction of a second—far shorter than it takes to snap one’s fingers. Following that initial surge, the universe kept expanding but at a more leisurely rate.
As astonishing as inflation appears, the theory, introduced by physicist Alan Guth and peers during the late 1970s and early 1980s, persists robustly, having made several precise forecasts regarding the characteristics of our universe. Presently, the crucial inquiry most cosmologists ponder is not if inflation happened but how.
“If inflation is indeed the correct theory, which we believe it is, then what initiated it?” inquires Jamie Bock, the lead investigator of SPHEREx and the Marvin L. Goldberger Professor of Physics at Caltech and senior research scientist at the Jet Propulsion Laboratory. “To draw a parallel, consider the theory of evolution. It took scientists considerable time to comprehend the mechanisms that elucidate how evolution operates via genetic mutations, well after Darwin laid the foundational theory. Similarly, inflation aptly describes our universe, but we find ourselves grappling to grasp how it originated. Our strategy with SPHEREx is to systematically collect data to evaluate models of inflation.”
Bock has been engaged with SPHEREx since 2012, two years prior to his team’s initial proposal of the mission concept to NASA. The space agency endorsed SPHEREx for launch in 2019 and now, six years later, the finished spacecraft is set to be launched in late February aboard a SpaceX Falcon 9 rocket from Vandenberg Space Force Base in central California. The mission is supervised by JPL, which is directed by Caltech on behalf of NASA. Multiple teams collaborated to construct SPHEREx, including scholars from Caltech’s campus and JPL, BAE Systems, and the Korea Astronomy and Space Science Institute. Caltech’s IPAC astronomy department will manage the mission’s data center, processing the information which will subsequently be made publicly accessible through the NASA IPAC Infrared Science Archive.
SPHEREx’s three main objectives include probing the origins of water and organic compounds in planetary systems, investigating the history of galaxy formation, and discerning the mechanisms underlying cosmic inflation—the “bang” in the big bang that initiated our universe’s expansion. The mission will scan the sky four times over a two-year span, gathering intricate spectral data for every point, or pixel, within the expanse. For each point, it will capture infrared light in a spectrum of 102 hues, with wavelengths ranging from 0.75 to 5 microns.
“We will survey everything visible in the sky and record a spectrum for every pixel regardless of what it is—comets in our solar system, planets, stars, galaxies. We anticipate that our dataset will enhance our comprehensive understanding of the cosmos: Whatever celestial object you fancy, we will analyze its spectrum,” states Olivier Doré, the project scientist for SPHEREx at JPL.
Since spectral data indicates the distance to galaxies, this mission will enable the creation of the first all-sky three-dimensional map of the vast cosmic web that saturates our observable universe. “It’s akin to mapping uncharted territory or a new world,” Doré remarks.
This three-dimensional mapping will grant scientists the capability to investigate the distribution, or clumpiness, of galaxies—a quality that subtly varies from one inflation model to another.
“I cannot think of a more profound inquiry: observing the initial fractions of a second of existence,” states Phillip Korngut, the mission’s instrument scientist at Caltech. “The clumpiness of galaxy locations links to quantum fluctuations in the primordial universe when it was unimaginably small and hot. We are making precise evaluations of galaxy density differences and will subsequently mathematically relate that back to early universe events.”
Expanding Universe
Researchers proposed the cosmic inflation theory to elucidate certain attributes of our universe that were challenging to comprehend using the big bang theory alone. For example, inflation clarifies why our universe appears so flat. Albert Einstein’s general theory of relativity introduced the idea that mass can warp space. Our Sun distorts the space around it, resulting in Earth essentially revolving around the Sun. However, it appears that the universe’s overall shape remains flat as a pancake.
“The dramatic expansion of the universe eliminated any curvature that existed,” explains Bock, who participated in the balloon-borne experiment known as BOOMERANG (Balloon Observations of Millimetric Extragalactic Radiation and Geophysics) that, in 2000, presented compelling evidence for this flatness. That telescope soared high above Earth to map a slice of the Cosmic Microwave Background (CMB)—ancient light from a period when our universe was merely 380,000 years old. Leveraging this data, the researchers effectively constructed a massive triangle across space, demonstrating that the angles summed up to 180 degrees—a signature indicative of flat space (If one were to draw a giant triangle across Earth, the angles would total more than 180 degrees due to its positive curvature.)
Another characteristic explained by inflation is the homogeneity of our cosmos. When astronomers examine distant areas of the sky positioned in opposite directions, those regions typically appear similar, even though they are so far apart that light has not had sufficient time to travel from one to the other. How can they exhibit such similarity given their great distance apart? The inflation theory suggests that everything in the universe was once compressed into a minuscule region, and then, as space ballooned exponentially (at rates exceeding the speed of light), areas that were previously in contact became distanced while preserving remnants of their shared history.
Since the 1980s, the fundamental predictions of the inflation theory have been examined and enhanced by both spaceborne and terrestrial investigations like BOOMERANG, which mapped the CMB. While the CMB exhibits a largely uniform appearance, tiny temperature fluctuations, known as anisotropies, manifest as splotches in the background illumination. These fluctuations emerged from minuscule quantum variations that reverberated through matter in our universe as it underwent rapid inflation. Much like wind-sculpted sand dunes, certain regions of space ended up accumulating more matter than their surroundings. The more densely populated areas—which are warmer and occasionally referred to as “hot spots” on the CMB—are theseeds that subsequently formed into stars and galaxies.
Bock has participated in numerous CMB initiatives, a roster that encompasses BOOMERANG; the European Planck satellite, operational from 2009 to 2013; and the BICEP–Keck suite of experiments conducted at the South Pole. Planck employed the same spiderweb bolometer technology created by Bock and his team, which was also utilized for BOOMERANG; BICEP, which commenced operations in 2006, utilizes more sophisticated arrays of sensors.
The objective of the BICEP–Keck partnership is to search for distinctive indications of inflation: curly formations in polarized light termed B-modes. These swirling formations may have been generated as gravitational waves—which are ripples not in matter but in the fabric of space-time—traveled through the expanding universe. The current stage of the partnership, referred to as BICEP Array, features the most sensitive receivers to date, each approximately ten times more potent than previous versions. While the collaboration has yet to observe B-modes, it has established the field’s most stringent upper limits on their intensity.
“These restrictions enhance precision in identifying the accurate theory of inflation and have recently dismissed several otherwise appealing inflation models,” remarks Bock.
A Fresh Perspective on the Cosmic Network
Data from SPHEREx and BICEP–Keck can collectively enlighten us significantly more about inflation than either could individually, offering cosmologists optimism that the mechanisms driving inflation may ultimately be deciphered. However, while the CMB presents an incredibly robust instrument for examining inflation theories associated with polarized light, it does possess limitations concerning the varieties of inflation theories that SPHEREx is investigating.
“The CMB is fundamentally a shell,” Korngut clarifies. “It’s a 2D surface of light. With SPHEREx, we will observe in 3D.”
CMB research measured the patches or hotspots in the background illumination, whereas SPHEREx’s extensive 3D galactic charts will be examining a later phase of development that occurred after the hotspots gravitationally evolved into galaxies.
“It’s not as straightforward to analyze these galaxies over the hotspots, but there is a wealth of additional data,” states Mark Wise, Caltech’s John A. McCone Professor of High Energy Physics, who has formulated theories of inflation (and who is not part of the SPHEREx team). “SPHEREx will provide us another perspective on inflation, and such perspectives are limited. Its data will be invaluable.”
By utilizing SPHEREx’s galaxy maps, researchers will have the capability to investigate an intriguing aspect of numerous inflation theories that has been nearly impossible to tackle until now—specifically, whether the arrangement of minute ripples of matter formed at the time of inflation adheres to a so-called Gaussian distribution. A Gaussian distribution, commonly referred to as the bell curve, is a concept employed in statistics. For instance, if one plotted the heights of hundreds of adult women in the United States, the outcome would conform to a bell shape, with the majority of women measuring around an average height of approximately 5’4″, and fewer individuals being shorter or taller. This represents a Gaussian distribution. Conversely, if one plotted the sizes of all women, including children, a bell shape would not emerge due to the shorter heights of numerous children skewing the results. The outcomes would be non-Gaussian.
Whether the distribution of primordial matter ripples is Gaussian or not carries significant implications for the universe’s earliest moments. Physicists theorize that inflation was driven by a repulsive surge originating from a high-energy field known as the inflaton—in other words, from a singular field. A single field, according to theorists, would typically result in a straightforward, Gaussian distribution. However, more intricate models of inflation involve multiple fields that could interact with one another to generate a non-Gaussian distribution.
“There may be minor-scale variations from one field, let’s say, and then larger ones from another field. Those fluctuations can interact, resulting in the extent of small-scale variation being larger or smaller on broader scales. This effect can create non-Gaussianity,” Bock explains.
These primordial ripples from the big bang continue to be evident in the distribution of galaxies throughout our universe. By assessing the degree of galaxy clustering across the sky, researchers can evaluate complex non-Gaussian inflation models against the simpler Gaussian alternatives.
The endeavor resembles analyzing population distributions across a country. How closely are individuals grouped in cities versus rural areas? A non-Gaussian characteristic would manifest as denser clusters of galaxies than predicted by simplistic inflation models—or, in the context of our analogy, as more densely populated urban areas.
Nonetheless, it is not solely the intensity of galaxy clustering in a specific region of the sky that is critical. Since the remnants of inflation will be most pronounced on the largest scales, the most reliable insights into inflation arise from mapping extensive volumes of the cosmos. Returning to the city analogy, discovering a non-Gaussian characteristic would be akin to mapping larger and larger segments of Earth and uncovering even more expansive megacities with less populated voids in between.
“The largest scales also provide us a viewpoint into inflation because they haven’t been complicated by other physical processes,” states Bock. “At smaller scales, for instance, the gravitational interactions among galaxies are stronger and can mask the signatures of the primordial universe.”
SPHEREx is exceptionally well-equipped to map these extensive scales as it will operate in space, where the instrument remains unaffected by Earth’s atmosphere and is highly stable, along with observing in infrared light.
“Dust within our galaxy absorbs light and can obscure large scales, but this effect is significantly diminished in the infrared compared to optical,” Bock affirms.
Doré adds: “This highlights the necessity for SPHEREx. We are in pursuit of the unique imprint on the cosmic web that can solely be observed by charting galaxies within a colossal sphere surrounding us. Witnessing the imprints from the universe’s genesis embedded in this structure is astonishing, exquisite, and magical. This embodies the distinctive power of physics.”
The team will additionally examine triangles among galaxies to gauge the clustering of galaxies.
“Squeezed triangles, which connect three galaxies where one end is quite short, are ideal for identifying the interplay between large and small scales emanating from multiple fields,” Bock observes.
Chen Heinrich, a research scientist at Caltech on the SPHEREx team, mentions that the types of quantum-level particle and field interactions they are investigating cannot be replicated in an Earth-based laboratory. “The universe has conducted the experiment for us,” she says. “We can gain insights aboutthe initial phases of our cosmos by examining the cosmic network of galaxies. It’s incredibly fascinating.”
The Largest Map of All
In order to create such an immense 3D celestial map, SPHEREx had to make a compromise between the number of galaxies it can observe and the precision of their calculated distances. The distances of the galaxies are established through a process known as redshift, which occurs when light from the galaxies is elongated to longer wavelengths due to the universe’s expansion.
“One of the advancements for SPHEREx is low-resolution spectroscopy, which enables us to obtain numerous redshifts,” Bock states. “On one hand, the visibility of spectral lines is limited, but lower-resolution spectroscopy allows us to cover more of the sky at a quicker pace. We will observe hundreds of millions of galaxies with lower precision, and tens of millions with greater precision.”
Korngut clarifies that SPHEREx essentially operates in the opposite manner to NASA’s James Webb Space Telescope (JWST), which excels in detailed exploration. “JWST can delve deeply into small segments of sky and investigate galaxies intricately,” he remarks. “For us, galaxies are merely points in the cosmos.”
By comparison, JWST’s observational field, from the standpoint of its NIRCam (Near Infrared Camera) instrument, covers approximately 1 percent of the surface area of the full moon, while SPHEREx’s observational field corresponds to a celestial area of around 200 moons. “The ratio between the solid angle in SPHEREx’s observational field and NIRCam on JWST is 14,000,” Korngut remarks.
A significant hurdle in constructing SPHEREx was designing a thermally stable spacecraft. The craft, which will orbit the Earth, must manage the warmth of our planet in addition to that of the Sun. “The temperature of the detectors ought to remain consistent regardless of the pointing direction,” Korngut notes. The instrument itself, which underwent primary testing at Caltech, is kept at a frigid 45 Kelvin, or minus 228 degrees Celsius. This temperature is preserved by a system of three nested, martini-shaped cones that encompass the entire spacecraft and passively release surplus heat back into space.
Once SPHEREx is launched, it will systematically gather data that will be made public within two months following collection. The mission is set to continue for a total of two years. One year thereafter, the complete set of data analyzed by the scientific team will be disclosed. As with other all-sky endeavors, such as NASA’s WISE, the maps are expected to lead to an abundance of discoveries, both nearby and distant. Astronomers will utilize the wealth of data collected during the mission to investigate comets, asteroids, stars, our Milky Way, other galaxies, and much more. What insights this mission will provide regarding cosmic inflation remains to be uncovered. “With this small telescope, we can explore the grand-scale structure of galaxies and gain understanding of the early universe,” Bock states. “It’s quite astonishing.”
NASA and SpaceX are aiming for late February to launch SPHEREx. For further updates, visit https://www.jpl.nasa.gov/missions/spherex/.