Did LZ see modified gravity?

Deep within the Earth, shielded from the cosmic noise of the surface, sits a massive tank of liquid xenon. This is the LUX-ZEPLIN experiment, a device designed to catch the faintest whisper of dark matter, the invisible substance that makes up most of the universe's mass. For years, scientists have waited for a particle of dark matter to bump into an atom of xenon, creating a tiny flash of light and a ripple of energy. Recently, the experiment reported a single, unusual event: a heavy nucleus in the tank recoiled with an energy of 248 keV. This energy is far higher than what most standard theories of dark matter predict for a single collision, creating a puzzle. If this event is real and not a mistake or a background glitch, it suggests that the invisible matter around us behaves in ways we have not yet imagined, perhaps interacting with ordinary matter more strongly or differently than expected.

A researcher named Basabendu Barman has proposed a new way to solve this puzzle, one that connects the behavior of these invisible particles to the very history of how the universe expanded. The standard story of the universe suggests that after the Big Bang, it cooled down in a predictable rhythm, allowing dark matter to form and settle into the amounts we see today. However, Barman suggests that in the very early moments of the universe, before the formation of the first atomic nuclei, the expansion of space might have been much faster than this standard story allows. This idea, rooted in modified theories of gravity, changes the rules of the game. If the universe expanded more rapidly, it would have been harder for dark matter particles to interact with the hot soup of other particles, meaning they would have needed a stronger connection to be created in the first place.

The paper explores a specific type of dark matter called a "feeble interacting massive particle," or FIMP. In the usual view, these particles are so shy that they barely touch anything, making them nearly impossible to detect. This shyness is a problem for explaining the recent event in the xenon tank, because if the particles are too shy, they shouldn't be able to hit the xenon hard enough to create that 248 keV signal. Barman's work shows that if the early universe expanded faster than we thought, it would allow these particles to have a stronger connection to ordinary matter while still remaining rare enough to match the total amount of dark matter we observe today. This stronger connection is exactly what is needed to make the collision in the xenon tank possible.

To test this idea, the study builds a model where dark matter consists of two types of particles that are almost identical in weight but slightly different, separated by a tiny gap of about 280 keV. These particles interact through a new, invisible force carrier called a dark photon. The researcher calculated how often these particles would hit the xenon nuclei under different conditions. The results show that if the universe's expansion rate was boosted by a factor related to modified gravity, the parameters required to explain the single event in the LUX-ZEPLIN data fit perfectly with the amount of dark matter we see in the sky. The study identifies specific ranges for the mass of the dark matter and the strength of its interaction that satisfy both the single collision event and the cosmic abundance of dark matter.

The analysis also checks these ideas against other known facts. The proposed particles must not have been seen in other experiments, such as those looking for new particles at the Large Hadron Collider or in beam-dump experiments. The study finds that while some parts of the proposed explanation are already ruled out by existing data, a significant portion of the possible settings remains open and untested. This means the theory is still alive and waiting for more data. The researcher notes that if future experiments can detect the dark photon or if the LUX-ZEPLIN experiment collects more data, they could confirm or rule out this specific explanation. Furthermore, if the early universe did expand this quickly, it might have left behind a faint background of gravitational waves that future detectors could find, offering a second way to test this story.

Ultimately, this work suggests that a single strange event in a deep underground tank could be a clue pointing toward a fundamental change in our understanding of gravity and the early universe. It does not prove that modified gravity is real, nor does it confirm that the LUX-ZEPLIN event is definitely dark matter. Instead, it demonstrates that if we assume the universe expanded differently in its infancy, the strange event becomes a plausible signal of a specific type of dark matter that would otherwise be too weak to detect. The paper offers a coherent path forward, showing how a single anomaly can drive us to rethink the history of the cosmos and the nature of the invisible matter that holds it together.