# From Big Bang to Big Rip: how will the universe end?

The universe is a very sticky place, but how sticky is it? Could it favour the “Big Rip” demise of the universe?

All good things come to an end. And the universe is no exception. In about 10^{100 }years (that’s 10x10, 100 times over – so a long time), the universe will be dead, and all that will be left is a relic of the life and energy it once birthed.

Aeons from now, the universe will be nothing but bits of dead photons, electrons and neutrinos – that’s about it, sadly.

But how will the universe end? Will it freeze to death (the Big Freeze), get crunched to bits (the Big Crunch), or even more radically, get ripped apart (the Big Rip)?

It is not known how to describe viscous fluids (yep, sticky fluids) in the context of Einstein’s general relativity. Over the years, different approaches have been made, from Eckart’s theory to Mueller-Israel-Stewart theory.

Recently, researchers at Vanderbilt University have discovered a new mathematical formulation that may help draw a line between the two notions. This new formula ties in well with one of the more radical scenarios of how the universe with end – the "Big Rip".

The Big Rip is a cosmological hypothesis first published in 2003, which proposes the possibility of dark energy accelerating, and consequently the universe expanding, without limit. The dark energy will eventually become unbearably strong that the universe (gravitational, electromagnetic and weak nuclear forces) rips itself into pieces, hence the name.

The maths also sheds new light on the fundamental properties of dark energy, a force that is still largely unknown. The new approach is described in the *Physical Review D**. *

Cosmological viscosity (ie. the stickiness of the universe) isn’t like the viscosity of ketchup, for example, which is a measure of a fluid’s resistance to flowing through small openings like the neck of a ketchup bottle. It’s the measure of a fluid’s resistance to expansion or contraction.

Marcelo Disconzi, an assistant professor of mathematics, started off by tackling the problem of fluid dynamics - the natural science of fluids in motion - which happens quite frequently in the universe from supernovae (exploding stars) to neutron stars (stars double the size of the Sun that have crushed down to the size of cities).

Although scientists have successfully modelled what happens when ideal fluids with no stickiness move to near-light speeds, no one has managed to come up with a generally accepted way of dealing with sticky fluids travelling at near-light speed.

This is because the models haven’t made much sense: the most confusing models predict conditions where these fluids travel faster than the speed of light, defying the laws of physics.

These problems inspired Disconzi to reformulate an old proposal by André Lichnerowic from 1955 (a proposal that remained mostly unnoticed until Disconzi’s 2014 paper) in a way that does not exhibit the flaw of allowing faster-than-light speeds.

After showing that Lichnerowicz's approach is potentially a viable candidate, Disconzi teamed up with Robert Scherrer and Thomas Kephart from the Vanderbilt Physics Department, finding that a Big Rip scenario is a natural consequence of the equations. The results included some potential new insights into the mysterious nature of dark energy.

Most dark energy theories so far haven’t taken cosmic stickiness into consideration, despite the fact that the repulsive effect is strikingly similar to that of dark energy.

“It is possible, but not very likely, that viscosity could account for all the acceleration that has been attributed to dark energy," Disconzi said to Vanderbilt University News. "It is more likely that a significant fraction of the acceleration could be due to this more prosaic cause. As a result, viscosity may act as an important constraint on the properties of dark energy,” he adds.

"In previous models with viscosity the Big Rip was not possible," Scherrer said to Vanderbilt University News."In this new model, viscosity actually drives the universe toward this extreme end state."

I ask Disconzi about the accuracy of his results. He replies:

My result by no means settles the question of what the correct formulation of relativistic viscous fluids is. What it shows is that, under some assumptions, the equations put forward by Lichnerowicz have solutions and the solutions do not predict faster-than-light signals. But we still don’t know if these results remain valid under the most general situations relevant to physics."

He also reflects on how his new formula may change the way we see cosmology:

It’s too early to tell. What is known from current observational data is that a Big Rip scenario is possible, although the available data is far from conclusive.What our paper brings to the discussion is a mechanism that yields a Big Rip in a fairly natural way, in contrast of most models of the Big Rip where unnatural assumptions have to be introduced."

So the Big Rip idea isn’t so far-fetched after all.