What Hundreds of Millions of Galaxies Can Teach Us About the Big Bang
Soon, a new cone-shaped spacecraft will launch into space on a mission to chart the skies like never before. Called SPHEREx (for Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer), the NASA mission will provide a unique all-sky 3D map of hundreds of millions of galaxies in our universe. Among several big questions the mission is poised to answer is how our universe came to be. SPHEREx will provide new clues in the quest to understand cosmic inflation, a much-studied theory that states our newborn universe expanded a trillion-trillion-fold in a fraction of a second—much less time than it takes to snap your fingers. After that initial blast, the universe continued to expand albeit at a more leisurely pace.
As mind-bending as inflation is, the theory, which was proposed by physicist Alan Guth and others in the late 1970s and early 1980s, continues to stand the test of time, making several accurate predictions about features in our universe. Now, the pressing question on most cosmologists' minds is not whether inflation occurred but how.
"If inflation is the right theory, and we think it is, then what caused it?" says Jamie Bock, the principal investigator of SPHEREx and the Marvin L. Goldberger Professor of Physics at Caltech and senior research scientist at the Jet Propulsion Laboratory. "As an analogy, think about the theory of evolution. It took people a long time to understand the mechanisms explaining how evolution works through genetic mutation, well after Darwin established the basic theory. In a similar way, inflation successfully describes our universe, but we are struggling to understand how it came about. Our approach with SPHEREx is to methodically gather data to test models of inflation."
Bock has been working on SPHEREx since 2012, two years before he and his team first proposed the mission concept to NASA. The space agency selected SPHEREX for flight in 2019 and now, six years later, the completed spacecraft is scheduled to launch into space at the end of February aboard a SpaceX Falcon 9 rocket from Vandenberg Space Force Base in central California. The mission is managed by JPL, which is managed by Caltech for NASA. Several groups worked together to build SPHEREx, including researchers from Caltech's campus and JPL, BAE Systems, and the Korea Astronomy and Space Science Institute. Caltech's IPAC astronomy is operating the mission's data center, which will process the data, while the data itself will be made publicly available through the NASA IPAC Infrared Science Archive.
SPHEREx's three primary goals are to explore the origins of water and organic molecules in planetary systems, the history of galaxy formation, and the mechanisms behind cosmic inflation—the "bang" in the big bang that set our universe in motion. The mission will map the sky four times over a period of two years, capturing detailed spectral information for every point, or pixel, on the sky. For each point, it will observe infrared light in a rainbow of 102 colors, ranging in wavelength from 0.75 to 5 microns.
"We will look at everything in the sky and get a spectrum for every pixel no matter what is there—comets in our solar system, planets, stars, galaxies. We expect our data set to expand our broad knowledge of the cosmos: Whatever your favorite object on the sky is, we will measure its spectrum," says Olivier Doré, the project scientist for SPHEREx at JPL.
Because spectral information reveals the distance to galaxies, the mission will be able to create the first all-sky 3D map of the sprawling cosmic web of galaxies that pervades our observable universe. "It's like mapping a new territory or world," Doré says.
That 3D map will allow scientists to study the distribution, or clumpiness, of galaxies—a trait that subtly differs from one model of inflation to the next.
"I can't think of a more profound question: studying the first fractions of a seconds of existence," says Phillip Korngut, the mission's instrument scientist at Caltech. "The clumpiness in galaxy positions is tied to quantum fluctuations in the early universe when it was unfathomably tiny and hot. We are making precise measurements of galaxy density variations and then will tie that back mathematically to what happened in the early universe."
Ballooning Universe
Scientists introduced the theory of cosmic inflation to explain certain features of our universe that were hard to make sense of with the big bang theory alone. For instance, inflation explains why our universe is so flat. Albert Einstein's general theory of relativity introduced the fact that space can be curved by masses. Our Sun curves the space around it, causing Earth to essentially roll around the Sun. But it turns out the overall shape of the universe is as flat as a pancake.
"The dramatic inflation of the universe stretched out any curvature that existed," says Bock, who was part of the balloon-borne experiment known as BOOMERANG (Balloon Observations of Millimetric Extragalactic Radiation and Geophysics) that, in 2000, presented the most convincing evidence yet for this flatness. That telescope flew high above Earth to map a patch of the Cosmic Microwave Background (CMB)—ancient light dating back to an epoch when our universe was only 380,000 years old. Using these data, the scientists were able to essentially construct a giant triangle across space and show that the angles added up to 180 degrees—a signature of flat space (If one drew a giant triangle across Earth, the angles would add up to more than 180 degrees due to its positive curvature.)
Another feature explained by inflation is the uniformity of our cosmos. When astronomers look at different patches of sky located in opposite directions, those regions generally look the same, even though they are so far apart that not even light has had time to travel from one side to the other. How can they be so similar when they are that far apart? The theory of inflation says that everything in the universe was once squeezed close together in a very tiny region, and then, when space exponentially inflated (at speeds faster than light travels), areas that were once in contact became separated while retaining features of their shared past.
Since the 1980s, the basic predictions of inflation have been tested and refined by space and ground-based experiments like BOOMERANG, which mapped the CMB. While the CMB is largely uniform in appearance, tiny temperature variations, called anisotropies, appear as splotches in the background light. These variations originated from very tiny quantum fluctuations that rippled through matter in our universe as it drastically inflated. Like wind-carved sand dunes, some regions of space ended up with more matter than others. The denser spots—which are warmer and sometimes referred to as "hot spots" on the CMB—are the seeds that later grew into stars and galaxies.
Bock has worked on several of these CMB projects, a list that includes BOOMERANG; the European Planck satellite, which flew from 2009 to 2013; and the BICEP–Keck series of experiments at the South Pole. Planck used the same spiderweb bolometer technology developed by Bock and colleagues and used on BOOMERANG; BICEP, which first began operating in 2006, uses more advanced arrays of detectors.
The goal of the BICEP–Keck collaboration is to search for telltale signs of inflation: curly patterns in polarized light called B-modes. These swirly patterns may have been produced as gravitational waves—which are ripples not in matter but in space-time itself—washed through the swelling cosmos. The current phase of the collaboration, called BICEP Array, includes the most sensitive receivers yet, each about 10 times more powerful than the earlier generation. Although the collaboration has not detected B-modes, it has set the field's strongest upper limits on their brightness.
"These constraints help narrow in on the correct theory of inflation and have recently ruled out some otherwise attractive models of inflation," Bock says.
A New View of the Cosmic Web
Data from SPHEREx and BICEP–Keck can together teach us a lot more about inflation than either on its own and give cosmologists hope that the processes behind inflation may one day be uncovered. But, while the CMB provides an incredibly powerful tool for probing theories of inflation related to polarized light, it does have its limits with respect to the kinds of inflation theories SPHEREx is testing.
"The CMB is basically a shell," Korngut explains. "It's a 2D surface of light. With SPHEREx, we will see in 3D."
CMB studies measured the splotches or hot spots in the background light, while SPHEREx's large 3D galactic maps will be looking at a later stage of evolution that took place after the hot spots grew gravitationally into galaxies.
"It's not as clean of a signal to study these galaxies over the hot spots, but there is a lot more data," says Mark Wise, Caltech's John A. McCone Professor of High Energy Physics, who has developed theories of inflation (and who is not on the SPHEREx team). "SPHEREx will give us another window into inflation, and there aren't a lot of windows. Its data will be very precious."
Using SPHEREx's galaxy maps, scientists will be able to look for a tantalizing feature of many theories of inflation that has been nearly impossible to address until now—namely, whether or not the distribution of tiny ripples of matter formed at the time of inflation follows a so-called Gaussian distribution. A Gaussian distribution, more commonly known as the bell curve, is a concept used in statistics. As an example, if you plotted out the heights for hundreds of adult women in the United States, the results would follow a bell shape, with most women measuring close to an average height of about 5'4", and fewer women being shorter or taller. This is a Gaussian distribution. But if you plotted out the sizes of all women, including children, you would not see a bell shape because the shorter sizes of the many children would skew things. The results would be non-Gaussian.
Whether the distribution of the primordial ripples of matter is Gaussian or not has profound implications for the first moments of our universe. Physicists think that inflation was caused by a repulsive blast that came from a high-energy field referred to as the inflaton—in other words, from a single field. A single field, according to theorists, would generally lead to a simple, Gaussian distribution. But more complex models of inflation invoke multiple fields that would interact with each other to produce a non-Gaussian distribution.
"There may be small-scale variations from one field, let's say, and then large ones from another field. Those fluctuations can interact, so that the amount of small-scale variation is bigger or smaller on the large sizes. This effect can give you non-Gaussianity," Bock says.
These primordial ripples from the big bang are still visible in how galaxies are distributed across our universe. By measuring the degree to which galaxies clump together across the sky, researchers can test complex non-Gaussian models of inflation against the simpler Gaussian ones.
The task is similar to analyzing where people live across a country. How tightly are people clustered into cities versus the countryside? A non-Gaussian signature would reveal itself as denser clumps of galaxies than what is predicted from simple inflation models—or, in the language of our metaphor, as more jam-packed cities.
However, it is not only the strength of galaxy clustering in a particular region of the sky that is important. Because the imprints of inflation will be the strongest on the largest scales, the best information on inflation comes from mapping a large volume of the cosmos. Going back to the city metaphor, finding a non-Gaussian signature would be like mapping larger and larger areas of Earth and uncovering even bigger megacities with sparser voids between them.
"The largest sizes also give us a window into inflation because they haven't been complicated by other physics," Bock says. "At smaller scales, for example, the gravitational interaction between galaxies is more intense and can conceal the imprints of the primordial universe."
SPHEREx is ideally suited to mapping these large scales because it will be in space, where the instrument is unaffected by the Earth's atmosphere and extremely stable, and because it will observe in infrared light.
"Dust in our galaxy absorbs light and can mess up large scales, but the effect is a lot weaker in the infrared compared to the optical," Bock says.
Doré adds: "This is why we need SPHEREx. We are after the unique imprint on the cosmic web that can only be seen by mapping galaxies in a gigantic sphere around us. Seeing imprints from the birth of the universe in this structure is mind boggling, beautiful, and magical. This is the unique power of physics."
The team will also study triangles among galaxies to measure the clumping of galaxies.
"Squeezed triangles, which are those that connect three galaxies where one end is very short, are ideal to find the coupling between large and small scales that comes from multiple fields," Bock says.
Chen Heinrich, a Caltech research scientist on the SPHEREx team, notes that the kinds of quantum-scale particle and field interactions they are studying cannot be reproduced in a lab on Earth. "The universe has done the experiment for us," she says. "We can learn about the earliest moments of our universe by analyzing the cosmic web of galaxies. It's crazy cool."
The Biggest Map of All
To capture such a gigantic 3D sky map, SPHEREx needed to make a trade-off between the numbers of galaxies it can observe and the accuracy of their measured distances. The galaxies' distances are determined through a phenomenon known as redshift, which occurs when light from the galaxies is shifted to longer wavelengths due to the expansion of the universe.
"One of the innovations for SPHEREx is low-resolution spectroscopy, which we use to get large numbers of redshifts," Bock says. "On the one hand, you can't see many spectral lines, but you can see more of the sky faster with lower-resolution spectroscopy. We will see hundreds of millions of galaxies with low accuracy, and tens of millions with high accuracy."
Korngut explains that SPHEREx is essentially doing the opposite of what NASA's James Webb Space Telescope (JWST) does so well. "JWST can go really deep on little chunks of sky and explore galaxies in detail," he says. "For us, galaxies are just points in space."
For comparison, JWST's field of view, from the perspective of its NIRCam (Near Infrared Camera) instrument, is roughly 1 percent of the area of the full moon, while SPHEREx's field of view is equivalent to a sky area of about 200 moons. "The ratio between the solid angle in SPHEREx's field of view and NIRCam on JWST is 14,000," Korngut says.
Part of the challenge in building SPHEREx was to create a thermally stable spacecraft. The spacecraft, which will orbit Earth, must contend with the heat of our planet as well as that of the Sun. "The temperature of the detectors should be the same no matter where you are pointing," Korngut says. The instrument itself, which was primarily tested at Caltech, will be maintained at a chilly 45 Kelvin, or minus 228 degrees Celsius. That temperature is maintained by a set of three nested, martini-shaped cones that surround the entire spacecraft and passively radiate excess heat back out into space.
After SPHEREx launches, it will continuously collect data that will become public within two months of collection.The mission will run for a total of two years. One year after that, the full package of data analyzed by the science team will be released. Like other all-sky missions, such as NASA's WISE, the maps promise to lead to a bonanza of discoveries, both near and far. Astronomers will use the mission's bounty of data to study comets, asteroids, stars, our Milky Way, other galaxies, and more. What the mission will reveal about cosmic inflation remains to be seen. "From this small telescope, we can study the largest-scale structure of galaxies and learn about the primordial universe," Bock says. "It's pretty amazing."
NASA and SpaceX are targeting the end of February for SPHEREx's launch. For updates, visit https://www.jpl.nasa.gov/missions/spherex/.
This picture shows the SPHEREx observatory and its nested cones, which passively radiate excess heat back out into space.
Credit: NASA/JPL-Caltech
Jamie Bock, the mission's principal investigator at Caltech, with a stack of proposals that he and his team wrote as part of NASA's mission selection process.
Credit: Lance Hayashida/Caltech
Phil Korngut works on SPHEREx.
Credit: Rocco Ceselin for Nature
Olivier Doré, the project scientist for SPHEREx at JPL.
Chen Heinrich, a Caltech research scientist on the SPHEREx team.
The SPHEREx instrument team during an operation readiness test. From top to bottom: Phil Korngut, Chi Nguyen, and Howard Hui of Caltech.
Credit: NASA/JPL-Caltech
Amelia Quon and Konstantin Penanen of JPL work on the SPHEREx instrument at Caltech.
Credit: Caltech
