Before there was light, there was cosmic inflation.

Before life, planet Earth, the first galaxies — and even before the violent explosion of hot dense primordial stuff scientists traditionally have thought of as the Big Bang — our universe was in an exotic state, expanding exponentially at an unfathomable rate.

It expanded so fast that in about a trillionth of a trillionth of a billionth of a second, a chunk of space the size of an atom would have exploded into a size far larger than our solar system. It brought our slice of the universe — everything we can see in the night sky — from an incomprehensibly small point to a size roughly between that of a human head and a city block.

But while the modern-day universe is riddled with evidence that this strange prologue to the universe that physicists call “inflation” probably happened, scientists still don’t know exactly why it happened.

A new spacecraft from NASA’s Jet Propulsion Laboratory in La Cañada Flintridge, launching as early as Tuesday evening on a SpaceX rocket out of Vandenberg Space Force Base near Lompoc, hopes to find out.

The mission — the Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer; or SPHEREx — will examine one of the clues inflation left behind. From its data, scientists hope to gain a better understanding of the culprit (or culprits) behind the rapid expansion.

Over the course of two years, SPHEREx will create four three-dimensional maps of the spread of galaxies throughout the entire sky, allowing scientists to search for the subatomic quantum ripples created by undiscovered inflation particles 13.8 billion years ago, now etched into the large-scale structure of the universe.

“It’s zooming out to map the cosmos and see the largest structures and the biggest picture, unlike large telescopes like [the James Webb Space telescope] that will zoom in and take very detailed, exquisite pictures over specific small areas of the sky,” said James Bock, Caltech physics professor, JPL senior research scientist and SPHEREx’s principal investigator.

While the universe has been expanding ever since its first moments, physicists reserve the term “inflation” only for the rapid, exponential expansion governed by unknown physics at the start of the universe as we know it.

Inflation still has its detractors who say the inflation process would have needed incredibly unlikely circumstances to kick off in the first place and that — absent the ability to directly detect exotic inflation particles — the current indirect evidence of their existence remains insufficient. However, inflation is widely accepted in the field as the best explanation for a range of strange phenomena throughout our modern universe.

Amelia Quan, the mechanical integration lead for NASA’s SPHEREx mission, is seen with a V-groove radiator, a piece of hardware that will help keep the space telescope cold, at Jet Propulsion Laboratory.

(NASA / JPL-Caltech)

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The different inflation theories disagree on some numbers and details, but the general story goes like this: Whatever existed before inflation instantly exploded away once the great expansion began.

During inflation, no form of matter we know was present yet. Instead, the universe was filled with some unknown inflationary energy and particles. As they fluctuated, they created ripples in the energy field — pockets of higher energy and pockets of lower energy.

What’s still unclear, though, is what exactly this energy and particle field was, or if there were multiple sets of energy fields and particles at work. But whatever SPHEREx finds will almost certainly have been created by wild particles outside the realm of physics as we know it.

When inflation fizzled out, the energy field and its fluctuations transmogrified into an incredibly hot and dense soup of the stuff we know today — eventually becoming the light we see and the protons, neutrons and electrons that make up our world.

This hot, dense, nascent universe, under tremendous pressure, exploded outward, as described by the traditional hot Big Bang theory developed in the 1920s. Inflation was physicists’ revision for the first few moments of the universe, conceived of in the 1980s to account for some weird effects in the universe.

The quantum ripples in the inflation energy seemingly never went away. While they started on the subatomic scale, they’re now bigger than galaxies. The higher energy spots turned into bright and busy corners of the universe with plenty of galaxies. The lower energy spots are relative dead zones now.

The web of galaxies we see when we look to the sky is a snapshot of the drama that played out in a small subatomic section of space some 13.8 billion years ago.

The SPHEREx team thinks there’s still more to that drama hidden in the fine details of that web.

The probe will, for the first time, create three-dimensional full-sky maps with enough precision and data to tease out whether it was a single energy field responsible for inflation, or if it was multiple.

“If you throw a tiny pebble into a pond, it creates ripples,” said Spencer Everett, a Caltech research scientist working on the SPHEREx mission. “Then, inflation suddenly swells them into these massive waves in an ocean.”

While single-field inflation theories are analogous to throwing a bunch of same-sized pebbles into the pond, Everett said, multi-field theories are like throwing many different sized pebbles and rocks into the water. By looking at the resulting ripples, scientists should be able to determine whether multiple sizes of pebbles — or inflation particles, in SPHEREx’s case — created them.

Evidence from SPHEREx that the spread of galaxies in the universe does not look like ripples from a single field (or a single-sized pebble in Everett’s analogy) would not only serve as strong proof inflation did in fact happen, but it would also effectively put the single-field theories on their deathbeds.

By launching into space, SPHEREx will have unobstructed views of virtually the entire sky as it orbits Earth. SPHEREx also needs to look at infrared wavelengths of light, with slightly longer wavelengths than the color red. However, it’s also the wavelength at which most objects, including Earth’s ground, radiate heat.

“If you try to measure anything in the infrared on the ground,” said Everett, “you’re just going to see the ground. At the temperatures close to room temperature, everything is emitting in the infrared.”

For this reason, the spacecraft will operate at a brisk minus-350 degrees Fahrenheit, kept cool by concentric cone-shaped aluminum shields that look something like a dog cone for a spacecraft.

SPHEREx is a medium-class mission in NASA’s Explorers Program, designed to provide frequent flight and funding opportunities for space science missions on a less ambitious scale than NASA’s flagship missions like the James Webb Space Telescope, a $10-billion mission that launched in 2021 to explore a wide range of pressing space science research questions.

The mission will also probe how some of the first galaxies formed and how icy cosmic dust carrying important molecules for life ends up on planets.

SPHEREx will ride alongside a small-class Explorers mission — called the Polarimeter to Unify the Corona and Heliosphere, or PUNCH — that will study solar wind.