Physicists continue to grapple with understanding dark matter—an elusive substance that constitutes roughly 80% of the universe's matter but remains undetectable. A groundbreaking model now proposes that dark matter was produced before the Big Bang, during the universe's rapid inflationary phase.
This model, recently published in Physical Review Letters by researchers from Texas, suggests dark matter formed during a brief period when the universe expanded exponentially. Unlike traditional theories that attribute dark matter's creation to interactions with a thermal bath of particles after the Big Bang, this new approach highlights its production during cosmic inflation.
Traditional Theories: Freeze-Out and Freeze-In
Previously, scientists believed dark matter formed through two main mechanisms: freeze-out and freeze-in.
Freeze-Out: Dark matter particles were initially in chemical equilibrium with other particles in a thermal bath. As the universe cooled, interactions decreased, "freezing out" the dark matter abundance.
Freeze-In: Dark matter never reached equilibrium with the thermal bath, and its production relied on rare interactions, often governed by quantum field theories.
The new model expands on the freeze-in concept, introducing the possibility that ultraviolet (UV) freeze-in could occur during inflation, where the temperature of the thermal bath is far lower than the masses of the particles linking dark matter to the Standard Model of particle physics.
Inflation and the Pre-Big Bang Era
Inflation, a theory proposed 45 years ago, describes an exponentially rapid expansion of the universe in its earliest moments. During this phase, the universe expanded by a factor of 10^26 in just 10^{-36} seconds, solving several cosmological puzzles like the flatness and homogeneity problems.
The driver of inflation, often referred to as the "inflaton," remains hypothetical. This scalar field caused the universe's supercooled expansion, with temperatures dropping significantly. When inflation ended, reheating occurred, creating particles of the Standard Model, including photons.
The WIFI Model: A New Mechanism for Dark Matter
The new Warm Inflation via Ultraviolet Freeze-In (WIFI) model challenges the idea that inflation dilutes everything it produces. Instead, it suggests dark matter could form during inflation through rare interactions in an energetic, high-temperature environment.
Katherine Freese, the lead author and Director of the Weinberg Institute of Theoretical Physics, emphasized the novelty of the model:
> "In most models, anything created during inflation is inflated away. In our model, dark matter is successfully produced during inflation."
According to the WIFI model, the inflaton transfers some energy to radiation, which produces dark matter particles via the freeze-in mechanism. This process predates the traditional Big Bang timeline, suggesting the universe had a finite size after inflation—around 10^-26 meters—before evolving into the familiar stages of nucleosynthesis and structure formation.
Testing the Theory
While the WIFI model is not yet observable, its warm inflation mechanism will be tested in upcoming cosmic microwave background experiments over the next decade. If validated, this could provide critical evidence for the model's predictions about dark matter production.
Co-author Barmak Shams Es Haghi expressed optimism about future research, noting:
> "WIFI suggests broader applications, including the production of other particles critical to the early universe's evolution."
This innovative perspective opens new avenues for exploring dark matter and the universe's earliest moments, pushing the boundaries of current cosmological theories.
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