A Recycled Universe Could Explain Why Everything Is So Flat, Physicists Suggest

A new model suggests that the universe did not undergo a Big Bang followed by inflation, as the prevailing model suggests, but a cycle of "bounces". Though so very far from anything approaching confirmed, the authors believe it could help solve a few cosmic mysteries, including dark matter and the "flatness problem".

You may have noticed, but there are a few problems in cosmology and physics that scientists are attempting to get their heads around.  One such problem, along with dark matter and dark energy, is that the universe looks sort of flat. First, what do we mean by flat?

"The density of the universe also determines its geometry. If the density of the universe exceeds the critical density, then the geometry of space is closed and positively curved like the surface of a sphere," NASA explains . "This implies that initially parallel photon paths converge slowly, eventually cross, and return back to their starting point (if the universe lasts long enough). If the density of the universe is less than the critical density, then the geometry of space is open (infinite), and negatively curved like the surface of a saddle. If the density of the universe exactly equals the critical density, then the geometry of the universe is flat like a sheet of paper, and infinite in extent."

In a flat universe, parallel lines stay parallel. And it is this type of universe which we appear to be living in . However much simpler this makes studying the universe, it comes with its own problems. The shape of the universe depends upon the average density of matter within it.

"If the density of matter is less than the so-called critical density, the universe is open and infinite. If the density is greater than the critical density the universe is closed and finite," NASA explains . "If the density just equals the critical density, the universe is flat, but still presumably infinite."

In the current age of the universe, the density appears to be very close to the "critical density", which makes it very close to being flat. This was a bit of a problem for Big Bang models of the universe when the flatness problem was first identified, as a hot, dense Big Bang would be above that critical density, and so why did it produce the flat universe that we see today?

While inflationary models can explain this by having the inflation occur very very quickly, some scientists are still looking at other options. While Nobel Prize-winning physicist Roger Penrose suggests conformal cyclic cosmology (far from an accepted idea, but interesting nonetheless), another idea is that the universe goes through cycles of "bounces".

Like string theory or MOND, there are a number of different versions of this. In a new paper, a team looked at "non-singular bouncing cosmology" in which the universe bounces between a hot dense state and the sort of universe we see around us today.

"This cosmological scenario is free of the initial singularity problem present in inflationary cosmology, solving as well the flatness and horizon problems of the standard HBB [Hot Big Bang] theory," the team claims in their paper, "and giving rise to scale-invariant curvature power spectra on large scales, hence being compatible with [Cosmic Microwave Background] observations."

The team looked at how primordial black holes (PBHs) would fit in such a model.

"Interestingly, PBHs can serve as a novel portal in order to probe alternative cosmological and gravitational scenarios," the team explained.

"We found a novel natural model-independent mechanism for PBH formation during the HBB radiation-dominated era, within the context of non-singular matter bouncing cosmologies. In particular, the enhancement of super-horizon curvature perturbations, during a matter contracting phase in combination with a short transitory period from the matter contracting to the HBB expanding Universe, can lead to enhanced curvature perturbations on small scales during the HBB phase, collapsing to form PBHs."

The team suggests that in this model, an array of sizes of black holes would form, rather than just the stellar mass and supermassive black hole mass objects which we have confirmed with observations.

"Interestingly, the primordial black hole masses that we find can lie within the observationally unconstrained asteroid-mass window, potentially explaining the totality of dark matter."

Primordial black holes have been suggested as a potential explanation for dark matter before, but in this model, they would be the remnant of a time before the Big Bang (or bounce), caused by tiny disturbances during this phase of the universe.

While interesting, and we should be open to alternative models, it is far from "problem solved" and the inflationary model remains our best explanation of what we see so far. But the team does give potential ways to look for evidence one way or the other.

"Remarkably, the enhanced curvature perturbations, collapsing to primordial black holes, can induce as well a stochastic gravitational-wave background, being potentially detectable by future experiments, in particular by SKA, PTAs, LISA and ET, hence serving as a new portal to probe the bouncing nature of the initial conditions prevailing in the early Universe."

The study is published in the Journal of Cosmology and Astroparticle Physics .