Linking Cosmic Inflation and the Big Bang with ‘Reheating’ period

The origins of the universe can be traced to two distinct events, cosmic inflation and the Big Bang — these periods are so different they are difficult to link. But, an added phase of ‘reheating’ can resolve this issue.

Scientists believe that the origin of the universe is comprised of two distinct stages, before the inflation period that is generally referred to as the Big Bang, there was another shorter but much more violent period of rapid expansion — known as cosmic inflation.

During this initial period of cosmic inflation — which lasted less than a trillionth of a second — the matter in the universe inflated exponentially quickly before the slower process of the Big Bang took over.

Even though recent observations of the cosmos have supported the concept of these two separated periods of inflation — a problem remains. The two stages are so distinct and radically different that uniting them in a sequence presents quite the challenge for cosmology. Enter physicists from MIT, Kenyon College.

They build upon the suggestion that there may have been an intermediary phase in the early universe linking the period of cosmic inflation to the period of the Big Bang. This bridging phase — known as ‘reheating’ — occured at the end of cosmic inflation — occurring at about 10 –³⁵ to 10 -¹⁰ seconds. The researchers suggest that reheating consisted of processes that changed to the cold, uniform homogenous matter of this initial period into the ultrahot, complex matter in place by the onset of the Big Bang.

David Kaiser, the Germeshausen Professor of the History of Science and professor of physics at MIT, explains: “The post inflation reheating period sets up the conditions for the Big Bang, and in some sense puts the ‘bang’ in the Big Bang.

“It’s this bridge period where all hell breaks loose and matter behaves in anything but a simple way.”

In order to obtain their results and build a picture of this reheating period, the researchers simulated in painstaking detail the interactions between multiple forms of matter during this chaotic stage in the origin of the cosmos. The team’s simulations revealed that the extreme energy which drove inflation could have been redistributed in a way that would produce the conditions required for the onset of the Big Bang within a tiny fraction of a second.

This period of extreme transformation— already shorter than the trillionth of a second that the period of cosmic inflation lasted — could nave been shortened even further if quantum effects modified the way matter and gravity interacted at high energies.

“This enables us to tell an unbroken story, from inflation to the post inflation period, to the Big Bang and beyond,” Kaiser continues. “We can trace a continuous set of processes, all with known physics, to say this is one plausible way in which the universe came to look the way we see it today.”

Syncing up cosmic inflation and the Big Bang

The theory of cosmic inflation — first put forward in the 1908s by Alan Guth — suggests that the universe began its existence as a tiny speck of matter approximately hundred-billionth of the size of a proton. The speck was filled with extremely energetic matter, so energetic that the pressures within it drove a repulsive effect. The driving force of rapid inflation.

This repulsive force inflated this proto-matter outwards at an incredible rate. So fast, that it reached 10²⁶ its initial size in less than a period of a trillionth of a second. After this came the first phases of reheating which Kaiser and his team attempted to reconstruct.

The team believes that earliest phases of reheating should be marked by resonances, caused by one form of high-energy matter dominating and shaking back and forth in sync with itself across large expanses of space. This lead to the explosive production of new particles.

“That behaviour won’t last forever, and once it starts transferring energy to the second form of matter, its own swings will get more choppy and uneven across space,” Kaiser explains. “We wanted to measure how long it would take for that resonant effect to break up, and for the produced particles to scatter off each other and come to some sort of thermal equilibrium, reminiscent of Big Bang conditions.”

Working from initial conditions based on predictions made from measurements of the Cosmic Microwave Background (CMB) — the radiation leftover from an event known as the ‘last scattering’ 3.8 x 10⁵ after the Big Bang which permiates the entire universe— the team’s computer simulation presented a large lattice upon which multiple forms of matter could be mapped. The team then tracked how the energy and distribution of these forms of matter changed throughout space over time as they varied certain conditions.

Tweaking the Cosmos

The team used their simulation to track the behaviour associated with two dominant forms of matter during the period of inflation. But, before running said simulates they added a slight ‘tweak’ to how the model describes the gravity.

This modification diverged the response of matter to gravity away from Einstein’s theory of general relativity. This was to represent the high energy conditions that existed during the period of cosmic inflation. Matter at these energies should have their response to gravity altered to reflect quantum effects and interactions on an atomic scale.

In general relativity, the strength of gravity is represented by a constant that with minimal coupling — meaning that a particle will interact with gravity in a set way, no matter what energy it possesses. But at higher energy levels predicted by cosmic inflation, the way matter behaves in response to the spatial distortion that is gravity becomes more complex. This is known as ‘non-minimal coupling.’

Kaiser’s team incorporated this non-minimal coupling effect into their simulations observing how it changed the distribution of matter and energy as they increased and decreased this quantum effect.

The team found that the stronger they made the quantum-modified gravitational effect, the faster the universe transitioned cold, homogeneous matter of cosmic inflation to the much hotter, diverse forms of matter characteristic of the Big Bang.

By tuning this quantum effect, they could make this crucial transition take place over 2 to 3 “e-folds” — the amount of time it takes for the universe to triple in size. In this case, they managed to simulate the reheating phase within the time it takes for the universe to triple in size two to three times. By comparison, inflation itself took place over approximately 60 e-folds.

“Reheating was an insane time, when everything went haywire,” Kaiser concludes. “We show that matter was interacting so strongly at that time that it could relax correspondingly quickly as well, beautifully setting the stage for the Big Bang.