Our common understanding of the universe tells us that all matter and energy were created at the beginning of time during a period of rapid inflation called the Big Bang.
However, in 2023, Katherine Freese, director of the Texas Center for Cosmology and Astroparticle Physics, and Martin Wolfgang Winkler of the University of Texas suggested a radical new idea: a "second Big Bang." This "Dark Big Bang" would have given rise to the universe's most mysterious "stuff," known as dark matter.
Now, two scientists from Colgate University have expanded on the concept of the Dark Big Bang, which may have flooded the universe with dark matter at the same time as the standard Big Bang up to one year after the primary cosmic creation event. They set out all the possible scenarios for a Dark Big Bang that would keep it consistent with our observations of the universe. The duo also determined how evidence of a Dark Big Bang could be gathered.
"We have shown that a Dark Big Bang has many more possible realizations than those identified by Freese and Winkler in their seminal work," researcher author and Colgate Assistant Professor of Physics and Astronomy Cosmin Ilie told Space.com. "As such, one of the implications of our work is to make such a scenario more plausible.”
Is dark matter so strange it needs a Big Bang of its own?
One of the reasons that ordinary matter and dark matter have long been proposed to share the same origin is because this is the simplest or "most parsimonious" idea — one that conforms to the principle of Occam's Razor, which suggests the theory that posits the least number of additional mechanisms is likely the right one.
But the universe doesn't have to conform to this adage, and it frequently doesn't.
“The assumption that dark matter and regular matter originated in the same event, the Big Bang, is natural, given its simplicity. That is why it remained unchallenged for so long," Ilie said. "However, nature does not need to be parsimonious, abide by Occam’s razor, or by any of our aesthetic preferences."
Dark matter is troubling for scientists because it doesn't seem to interact with particles of light (photons) or particles of "ordinary" matter that compose the atoms that make up everything we see around us. This makes dark matter effectively invisible.
That tells us that whatever particles comprise dark matter can't be electrons, protons, or neutrons, particles of the baryon family found in atoms, which do interact with light and with each other. Dark matter particles greatly outweigh ordinary particles by around 5 to 1, meaning that every star, planet, moon, life form, and physical object accounts for just 15% of matter in the cosmos, with dark matter composing the other 85%.
The only way of detecting dark matter is via its interaction with gravity. Ilie added that the beauty of a Dark Big Bang theory is that this event would create dark matter particles that would not interact with regular matter except via gravity.
"Thus, this model could explain why all attempts at detecting dark matter, directly, indirectly, or via particle production, have failed," the researcher said. "Therefore, a Dark Big Bang scenario for the origin of dark matter is not only possible but perhaps more likely than the alternative!”
Ilie explained how the original Big Bang and the Dark Big Bang may have differed.
"The first phase in the evolution of the universe, or the Big Bang, is believed to be a very brief inflationary period that ended in what is called reheating," he explained. "During reheating, the energy stored in the field that drives cosmic inflation is converted into a plasma of highly energetic particles."
In standard cosmology, dark matter and regular matter were both produced during the Big Bang. In the Dark Big Bang scenario with two Big Bangs, however, dark matter particles can be produced later via the decay of a separate field that only interacts with the so-called "Dark Sector."
The "Dark Sector" would be the set of dark matter particles and their interactions.
“The standard model of particle physics characterizes the particles we know of and the forces that mediate their interactions," research author and Colgate University scientist Richard Casey told Space.com. "Who is to say that dark matter — all the yet undiscovered particles that make up most of the universe — does not have its own complicated set of particles and interactions?"
Casey explained that as the Dark Big Bang theory stands so far, it can give rise to many different kinds of dark sectors.
The major result of the work conducted on the Dark Big Bang by Ilie and Casey is to define its parameters and where it could fit within the history of the universe. This included a never-before-explored mathematical space. This large new region is especially exciting to the duo because the equations that convert inputs to physical quantities can be drastically simplified.
It also gave the Colgate researchers a hint at how we could hunt for evidence of the Dark Big Bang. The key to this detection could be faint ripples in space and time called "gravitational waves," first predicted by Einstein in 1915.
"An event of the scale of the Dark Big Bang will produce gravitational waves," Ilie said. "We implicitly determined the gravitational wave signals of any possible Dark Big Bang. For certain benchmark scenarios, we show that those gravitational waves could be detected by ongoing or upcoming experiments such as the International Pulsar Timing Array (IPTA) or the Square Kilometer Array (SKA).
"In the future, more precise data could be used to constrain the possible realizations of a Dark Big Bang, or perhaps even confirm it as the theory behind the origin of the mysterious dark matter."
Ilie and Casey think that if the Dark Big Bang theory is correct, evidence for it could be detected as a result of the Dark Sector and ordinary matter communicating with each other.
"What has yet to be considered is what the Dark Big Bang would look like when considering how the dark and visible sectors might talk to each other through forces that are not already part of the standard model," Casey said. "If the Dark Big Bang remains possible when introducing new interactions between the two sectors, gravitational waves from the phase transition could be used in addition to particle accelerators to explore Dark Sector models."
The duo's research was published in the journal Physical Review D.
