FOSSIL: A future mission for CMB spectral distortion measurements

The universe is filled with a faint, ancient afterglow known as the cosmic microwave background. This radiation is the oldest light we can see, a snapshot of the cosmos when it was only a few hundred thousand years old. For decades, astronomers have studied the tiny temperature variations in this glow, which act like a map of the early universe's structure. These maps have told us a great deal about how galaxies formed and what the universe is made of. However, this light is not perfectly uniform in its energy. Just as a perfectly smooth surface might have microscopic ripples, this ancient light carries subtle distortions in its spectrum—the specific mix of colors or frequencies that make up the light. These distortions are not random; they are the fingerprints of violent and energetic events that happened long before the first stars ignited, such as the dissipation of sound waves in the primordial plasma or the heating of gas by the first black holes.

While the temperature map has been explored in great detail, the spectrum itself has remained largely unmeasured with high precision since the 1990s. The current understanding is that the universe should have left behind specific, tiny deviations from a perfect thermal glow, but these signals are incredibly faint, buried beneath the noise of our instruments and the bright foreground of our own galaxy. To find them, scientists need a tool far more sensitive than anything currently in space. This is the challenge that a new proposal, called FOSSIL, aims to meet. The paper outlines a plan for a space mission designed to listen to the universe's thermal history with unprecedented clarity, looking for the faint echoes of physics that occurred in the first moments after the Big Bang.

The proposal describes a spacecraft that would carry a specialized instrument capable of measuring the intensity of the sky's light across a vast range of frequencies, from 50 gigahertz to 2 terahertz. This range covers everything from radio waves to far-infrared light. The core of the mission is a device known as a Fourier Transform Spectrometer. Instead of taking a picture of the sky, this instrument acts like a super-sensitive scale, weighing the energy of light at different frequencies. To do this with the necessary precision, the entire instrument must be cooled to a temperature just above absolute zero, roughly 4.5 Kelvin, to prevent the device itself from glowing and drowning out the faint cosmic signal. Even colder, at 50 thousandths of a Kelvin, sit the detectors that catch the light. These detectors are so sensitive they can register the energy of individual photons.

A critical part of the design is how the instrument compares the sky to a known reference. The spacecraft will constantly switch its view between the cold depths of space and an internal blackbody reference—a small, controlled source of heat that mimics the temperature of the cosmic background. By comparing the light from the sky to this internal reference, the instrument can cancel out its own imperfections and measure the absolute brightness of the universe with extreme accuracy. The paper details a strategy where the spacecraft spins slowly, scanning the entire sky over the course of a year. This spinning motion, combined with the switching between the sky and the internal reference, allows the team to build a complete, high-fidelity map of the cosmic spectrum.

The authors present detailed forecasts showing that this mission would be roughly one thousand times more sensitive than the previous best measurements taken by the COBE satellite in the 1990s. This leap in sensitivity is not just a technical achievement; it opens a window into physics that is currently invisible. The paper suggests that by measuring these spectral distortions, scientists could test theories about the very first moments of the universe, specifically the period of rapid expansion known as inflation. They could determine if the universe was smooth on very small scales or if there were bursts of energy that created ripples in the fabric of space-time. Furthermore, the mission could reveal the nature of dark matter. If dark matter particles decay or annihilate, they would release energy that heats the early universe, leaving a specific signature in the spectrum that FOSSIL could detect.

The paper also explores how this data could help solve the mystery of how the first massive black holes formed. Some theories suggest these giants began as seeds created by huge fluctuations in the early universe, which would leave a distinct mark on the cosmic spectrum. By measuring the amount of energy injected into the gas of the early universe, the mission could also tell us how stars and galaxies heated their surroundings, a process known as feedback. This would help astronomers understand how the universe evolved from a hot, uniform soup into the complex web of galaxies we see today.

Beyond the deepest cosmological questions, the mission would also provide a treasure trove of data for other areas of astronomy. The instrument's wide frequency coverage would allow it to map the dust in our own galaxy with great precision, helping to understand the lifecycle of stars and the formation of planets. It could also measure the total glow of the universe from all the dust-obscured star formation that has ever happened, a quantity known as the cosmic infrared background. The paper notes that the mission would be able to detect the integrated light from specific chemical lines, such as those from carbon monoxide and ionized carbon, providing a census of gas in distant galaxies during the "cosmic noon," a period when star formation was at its peak.

The proposal is a collaborative effort involving hundreds of researchers from nineteen countries, aiming to launch around 2041 as part of the European Space Agency's medium-class mission program. The authors emphasize that while the technology relies on proven principles, the scale and sensitivity required are new. They have modeled the instrument's performance extensively, showing that it can distinguish between different theoretical models of the early universe. The paper concludes that spectral distortions are a powerful, complementary tool to the temperature maps we already have. By measuring the energy content of the universe with this new level of precision, FOSSIL has the potential to confirm the standard model of cosmology or, more excitingly, to reveal new physics that challenges our current understanding of how the universe began and evolved.