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.
This timeline shows the history of our universe, beginning with the era of the big bang and ending with the modern day. The chart is split to show two different cosmic inflation models being tested by NASA’s SPHEREx mission. One goal of the project is to ask whether cosmic inflation occurred from a single field or multiple interacting fields. If the latter is true, the models predict that galaxies, as seen on very large scales, should clump together into more dense clusters, with bigger voids, than would be the case in the single-field model. This is reflected in the artist’s concept showing galaxy structures in the lower half of the drawing. SPHEREx is uniquely capable of probing this question because it is creating a huge 3D map of hundreds of millions of galaxies in our universe.Credit: Caltech/Robert Hurt(IPAC)
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/.
