The most famous image we have of the Big Bang is that of a single point that expanded and gave rise to the universe.
But what if this is just the twin of another universe that formed at the same time as this point and expanded in the opposite direction?
That’s the bold proposal that was recently published by a group of cosmologists at the Perimeter Institute for Theoretical Physics in Canada.
And they go further. In this “anti-universe” that they propose, which moves in the opposite direction to ours, time also runs in the opposite direction.
This hypothesis, as complex as it may seem, is an attempt by its authors to explain in a simpler and “economical” way, several mysteries of the cosmos – among them, the enigmatic dark matter.
On the other side of the mirror
There are two key concepts to understanding the idea of ​​an “anti-universe”.
The first has to do with the standard model of particle physics, a theory that describes the fundamental particles of which the universe is made and the forces that make them interact with each other.
According to this standard model, whenever a matter particle appears, so does its antimatter counterpart—an identical particle with a different charge. This means that during the Big Bang, the same amount of matter and antimatter was produced.
And the second concept is that of symmetry.
In cosmology, this principle indicates that any physical process remains the same, even if time goes backwards, space is reversed, or if particles are replaced by antiparticles.
Based on these two principles, the analogy that could be made is that, just as there is a universe, one would expect there to be an “anti-universe” symmetrical to what we know.
Symmetry
In a recent study by Canada’s Perimeter Institute for Theoretical Physics, the authors analyzed a type of symmetry called CPT (charge, parity, and time initials).
This symmetry indicates that if you reverse the charges, image, and time of a particle interaction, that interaction will behave in the same way.
So, this symmetry that applies to particles, according to the study authors, could also be applied to the universe as a whole, which opens up the possibility of a symmetrical universe.
“The universe in its CPT symmetric ensemble,” write the authors of the research.
Under this premise, the Big Bang is a starting point from which the universe and its mirror image (in the mirror) originate.
“We suggest that the universe before the Big Bang is the ‘antiverse’ of the universe after the Big Bang,” say the authors.
How is this “anti-universe”
Latham Boyle, one of the study’s co-authors, cautions that he is unsure about the “anti-universe” hypothesis and that his proposals will have to be verified experimentally. But he thinks his calculations give him some clues.
“Until now, we believe that the antiverse is a true mirror image reflected in time, with an exchange of particles and antiparticles,” Boyle tells BBC Mundo (the BBC’s Spanish service).
According to this view, this “anti-universe” is not an independent universe, but a mere reflection of our own.
“We have an ‘anti-me’ in the other universe, but it’s not independent,” says Boyle.
“If you choose to eat eggs for breakfast, your antiverse version cannot choose to eat bacon for breakfast. If you eat eggs for breakfast, it will have anti-eggs for breakfast,” he adds.
What happens to time in the “anti-universe”?
As proposed by Boyle and his colleagues, the Big Bang is like a mirror that reverses not only the image but also the direction of time.
On both sides of the universe, time moves away from the Big Bang—on one side, the arrow of time goes to the right, and on the other, to the left.
“Every side of the universe thinks it’s perfectly normal,” says Boyle. “Both believe that their time is advancing. From our point of view, in the antiverse time rewinds, but for them it is we who rewind.”
Boyle’s idea contains another possibility: maybe we are the ones in the “anti-universe” and we don’t know it.
And another question you may be asking yourself: is it possible to travel to this “anti-universe”?
“We can’t cross to the other side of the mirror,” says Boyle. “For that, it would have to be possible to travel to the past.”
That is, you would have to travel through space-time, go through the Big Bang singularity and come out on the other side.
minimalist solutions
But beyond these science fiction ideas, the work of Boyle and his colleagues also proposes solutions to more practical problems in physics and cosmology.
His proposal offers challenging insights into three fundamental concepts of cosmology: dark matter, post-Big Bang inflation, and gravitational waves.
Dark matter is a mysterious element that makes up 25% of the universe, but so far no one has been able to observe what it is or what it is made of.
Dark matter, however, can be noticed due to the gravitational influence it exerts on the cosmos.
For years, scientists have proposed various theories to explain what this dark matter is, but no one has a convincing answer.
Some of the possible answers point out that it is made of a particle that we don’t know yet, that is, that is outside the standard model.
Boyle’s study, however, offers a “more economical” answer to this conundrum.
His proposal is that, to explain dark matter, it is not necessary to imagine new particles.
Instead, Boyle thinks the answer may be that dark matter is made up of “right-handed neutrinos,” a variety of neutrinos that aren’t part of the standard model.
“Right-handed neutrinos” have yet to be proven, but according to Boyle, many scientists agree that they could be part of the standard model.
In this way, Boyle spares himself the effort of speculating with new particles and finds the answer in the laws of physics that we already know.
So far, known neutrinos are “left-handed”, referring to the direction in which they spin. But in a symmetrical universe, one would expect that there would also be a right-handed neutrino, that is, an antineutrino, according to astrophysicist Paul Sutter, in an article on the Live Science portal in which he reviews Boyle’s study.
These right-handed neutrinos would be virtually invisible and their presence could only be detected by gravity.
“An invisible particle that permeates the universe and only interacts through gravity looks a lot like dark matter,” explains Sutter.
Joseph Formaggio, a physicist who investigates the role of neutrinos in cosmology, says he finds Boyle’s proposal to explain dark matter interesting.
“I like the minimalist model,” Formaggio, who was not involved in the investigation, told BBC Mundo.
“Usually in particle physics you can explain a lot of phenomena by introducing new particles, interactions and fields, so it’s easy to get lost.”
“But this research takes another approach, they don’t add anything beyond what we’ve already observed,” concludes Formaggio, who directs the Division of Experimental Nuclear and Particle Physics at the Massachusetts Institute of Technology.
Formaggio points out that the idea of ​​right-handed neutrinos is very common, although it is not known if they exist.
“They’re a new particle, but they’re not really,” he says, laughing.
Neither inflation nor gravitational waves
Finally, the study raises doubts about the existence of cosmic inflation and primordial gravitational waves, which would have given rise to the universe, according to the Big Bang Theory.
Boyle’s model questions whether after the Big Bang there was a period when the universe expanded rapidly, a concept known as inflation.
That inflation, in turn, may have created primordial gravitational waves, which are ripples that travel in the fabric of spacetime, like the ripples generated by a stone thrown into a lake.
Boyle’s proposal maintains that, instead of inflation, matter in the universe expanded with less force, without the need for an “inflammatory epoch”.
So, according to this model, if there was no inflation, then there were also no primordial gravitational waves.
In 2015, gravitational waves were detected for the first time. Boyle, however, cautions that these correspond to post-Big Bang events, so they are not primordial gravitational waves.
