As physicists continue their struggle to find and explain the origin of dark matter, the approximately 80% of the matter in the universe that we can't see and so far haven't been able to detect, researchers have now proposed a model where it is produced before the Big Bang.
Their idea is that dark matter would be produced during a infinitesimally short inflationary phase when the size of the universe quickly expanded exponentially. The new model was published in Physical Review Letters by three scientists from Texas in the US.
An intriguing idea among cosmologists is that dark matter was produced through its interaction with a thermal bath of some species, and its abundance is created by "freeze-out" or "freeze-in." In the freeze-out scenario, dark matter is in chemical equilibrium with the bath at the earliest times—the concentration of each does not change with time.
In the freeze-in picture, dark matter never comes into equilibrium with the bath. Such a suppressed interaction between dark matter and the thermal bath could be due to interactions in quantum field theories, either infrared freeze-in or ultraviolet freeze-in.
In UV freeze-in, the temperature of the thermal bath is always lower than the masses of the particles that connect dark matter to the Standard Model of particle physics. (Mass and temperature are both proportional to energy and can be related via fundamental constants.)
The theory of inflation was developed about 45 years ago now, proposing a period of exponentially fast expansion in the very early universe where the universe expanded by a factor of about 1026 in 10-36 seconds. (After that inflation ceased, the universe continued to expand, though not exponentially.)
Billions of years later dark energy started the acceleration we see today.) The inflation idea tidily explains many puzzles in cosmology, such as the flatness problem, the homogeneity problem and the monopole problem, and it explains the origin of structure in the universe as quantum fluctuations that were enormously magnified.
Although inflation is mostly accepted by cosmologists as part of the Big Bang picture based on some evidence (though there is meaningful dissent), the driver of inflation is still unknown.
Cosmologists refer to it generically as the inflaton, a hypothetical field that spans all of spacetime of some scalar (spin zero) particle, perhaps the Higgs field. (Perhaps not.) Inflation occurs so rapidly that the universe is in a supercooled expansion, where the temperature drops by a factor of roughly 100,000.
This low temperature persists through the inflationary stage. When inflation ends, the temperature returns to the pre-inflationary temperature, a process called reheating, and the inflaton field decays into the particles of the Standard Model, including photons.
Research has shown that the bath can attain much higher temperatures than the reheating temperature, and for ultraviolet freeze-in the amount of dark matter produced depends on the highest temperature of the thermal bath.
But to-date research has not considered the possibility that a significant of dark matter could be produced during the inflationary expansion and not be diluted away.
In the paper's WIFI model—Warm Inflation via ultraviolet Freeze-In—dark matter is created through small and rare interactions with particles in a hot, energetic environment. It contains a new mechanism where this production occurs just before the Big Bang, during cosmic inflation, leading to dark matter being formed much earlier than in existing theories via freeze-in.
Though it sounds unusual, many cosmologists now think that inflation happened before the Big Bang, since the existence of a Big Bang singularity with infinite density and infinite spacetime curvature seems unrealistic.
Instead, the universe would have some small size after inflation, roughly 10-26 meters in diameter, and from there the standard steps of radiation and particle production would occur, then nucleosynthesis would take place to populate the universe.
The theorists proposed a different perspective on the role of inflation in the role of dark matter via a freeze-in.
"The thing that's unique to our model is that dark matter is successfully produced during inflation," said Katherine Freese, Director of the Weinberg Institute of Theoretical Physics and the Texas Center for Cosmology and Astroparticle Physics at The University of Texas at Austin and lead author of the paper. "In most [other] models, anything that is created during inflation is then 'inflated away' by the exponential expansion of the universe, to the point where there is essentially nothing left."
In this new mechanism, all the dark matter that we observe today could have been created during that brief, pre-Big Bang period of inflation. The quantum field driving inflation, the inflaton, loses some of its energy to radiation, and this radiation, in turn, produces dark matter particles via the freeze-in mechanism. What was before inflation? Physicists have no idea.
The WIFI model cannot yet be confirmed by observations. But a key part of the scenario, warm inflation, will be tested over the next decade by the so-called cosmic microwave background experiments. Confirming warm inflation would be a significant step for the WIFI model's dark matter production scenario.
"In our study, we focused on the production of dark matter, but WIFI suggests a broader applicability," said Barmak Shams Es Haghi, a co-author on the paper along with Gabriele Montefalcone, "such as the production of other particles that could play a crucial role in the early universe's evolution. This highlights new opportunities for exploration in future research."
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