Just before the Big Bang, a ‘freeze-in’ event produced dark matter

Researchers from the University of Texas at Austin (UT Austin) have proposed a new model to explain the origin of dark matter. It suggests that dark matter originated during the infinitesimally short span before the Big Bang when the Universe suddenly expanded to a large extent.

In 1981, physicist Alan Guth proposed the theory of cosmic inflation. It suggests that during the earliest moments of the universe, it expanded 10²⁶ times within 10^-36 seconds. The universe continues to spread even after the inflation but not at such an unprecedented rate.

Cosmic inflation was an important event because it eliminated all the irregularities in the structure of the universe. This is why despite so much vastness and continuous expansion, the universe appears uniform and homogenous.

In their new study, the researchers propose that inflation also played a role in the formation of dark matter through the freeze-in scenario.

“The thing that’s unique to our model is that dark matter is successfully produced during inflation. 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,” Katherine Freese, lead researcher and professor of physics at UT Austin, said.

Did dark matter freeze out or freeze in?

According to the study authors, dark matter likely originated from the thermal bath during warm inflation. Thermal bath refers to the particles (like radiation and matter) that interact with each other while maintaining a certain temperature during the formation of the universe.

This thermal energy was sustained by the inflaton field (the field driving inflation) through its interactions, ensuring a warm environment during inflation (this is why referred to as warm inflation). However, there are two different possibilities for the birth of dark matter from the thermal bath.

For instance, it is possible that in the early universe, dark matter particles were in thermal equilibrium with regular matter. As the universe expanded and cooled, these particles froze out—-meaning they stopped interacting with regular matter, and their density became fixed. This is called the freeze-out scenario.

The researchers ruled out this possibility because, in a dynamic environment surrounded by intense events such as inflation and the Big Bang, thermal equilibrium would likely have been disrupted. This makes it unlikely that dark matter particles remained in equilibrium long enough for freeze-out to occur.

Another possibility called the freeze-in, implies that dark matter particles were never in thermal equilibrium. Instead, they were produced as a result of rare high-energy interactions like those involving UV radiation in the early universe.

The researchers suggest that during warm inflation, the quantum field that triggered the inflation lost a chunk of its energy to UV radiation, and this interaction caused the production of dark matter particles that eventually “froze in” as the universe cooled.

“We demonstrated that in a warm inflation setting the persistent thermal bath can source a sizable dark matter abundance via the nonrenormalizable interaction characteristic of UV freeze-in,” the study authors note.

But it was all pre-Big Bang

You may have not noticed but there is a twist in the version of freeze-in that the UT Austin team proposed.

While many scientists believe inflation and freeze-in happened after the Big Bang, according to the proposed warm inflation via ultraviolet freeze-in (WIFI) model —- dark matter production occurred before the Big Bang in the short time when the universe exponentially expanded due to cosmic inflation.

This new perspective on cosmic inflation could influence many existing theories that explain the origin of our universe.

Moreover, “WIFI suggests a broader applicability 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,” Barmak Shams, one of the study authors, said.

However, the current findings still represent a theory. The study authors are required to validate their WIFI model through observations which could take many years.

The study is published in the journal Physical Review Letters.