The universe presents us with a tapestry of mysteries. Our telescopes can map the radiant glow of stars and swirling galaxies, yet most of the universe’s mass-energy density is tied up in an invisible substance known as dark matter. This dark matter betrays its presence only through its gravitational pull, remaining stubbornly impervious to detection by any other means. A peculiar observation further deepens the enigma: there’s a striking, perhaps overly coincidental, similarity between the abundance of this dark matter and the familiar matter that makes up our world. Could this seemingly strange alignment contain a vital clue to deciphering the true nature of dark matter?

In a recent paper, Arushi Bodas, Manuel A. Buen-Abad, Anson Hook, and Raman Sundrum propose a captivating explanation for this observed cosmic coincidence. They advocate for the existence of a shadowy “dark sector,” a hidden world that remarkably mirrors the structure of our own Standard Model of particle physics. In this paradigm, dark matter isn’t some incomprehensible exotic material but rather arises from a parallel set of particles – imagine “dark protons,” “dark neutrons,” and even “dark electrons.”

At the core of their proposal lies the concept of a fundamental symmetry known as a Z2 exchange symmetry. This symmetry would allow for a seamless swapping of particles between the Standard Model and their shadowy counterparts in the dark sector. The observed matter/dark matter similarity could then be an elegant consequence of analogous processes known as “WIMP baryogenesis” operating within both sectors. WIMPs (Weakly Interacting Massive Particles) are, as the name implies, extremely weakly interacting, and as a result, are theorized to have ‘frozen out’ (ceased interacting with other forms of matter) very early in the history of the universe, essentially fixing their abundance at that time. These parallel mechanisms acting in both the visible and dark sectors could very well have generated the rough balance we observe between regular and dark matter in the current cosmos.

A fascinating and far-reaching consequence of this proposed Z2 symmetry is that it necessitates addressing the perplexing problem known as the “electroweak hierarchy problem” not once, but twice – first in our sector and then replicated in the mirrored dark sector. The hierarchy problem stems from the immense and unexplained difference between the strengths of the weak nuclear force and gravity. While physicists continue to grapple with this puzzle in our own sector, it seems a solution must exist independently in the dark sector as well.

Some theorists speculate that the origins of this symmetry may lie hidden in the realm of extra dimensions. If our familiar universe exists within a higher-dimensional framework, the relatively weak coupling between ordinary and dark matter, along with the subtle breaking of the Z2 symmetry, could arise naturally from the positioning of particles within these extra dimensions.

To fully validate this dark sector idea, it’s necessary to meticulously track the evolutionary pathways of both ordinary and dark matter throughout the universe’s history, taking great care to address any potential inconsistencies or oversights in previous assessments. One crucial aspect to consider is that an “asymmetric reheating” of the two sectors, perhaps tied to the necessary Z2-breaking, may help explain how the abundance of relativistic dark particles can be kept within acceptable bounds based on cosmological observations.

Remarkably, while interactions between the sectors must be rigorously suppressed after this primordial asymmetric reheating phase, tantalizing possibilities for experimentally probing the hidden sector might still remain. The authors speculate that particles roughly in the TeV (Tera electron Volt) mass-energy range could act as mediators, enabling high-energy collider experiments to potentially catch glimpses of the otherwise elusive dark sector. Direct detection of dark matter signals might be challenging due to the unavoidable background ‘noise’ produced by neutrinos, but such mediating particles could still provide vital indirect evidence.

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The work of Bodas and collaborators sheds new light on the enduring mystery of dark matter, painting a potentially transformative picture of our universe. If a hidden dark sector does mirror our own, we may be embarking on a profound advancement in our understanding of the cosmos and its most fundamental building blocks.

Read the full paper here

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