“WE NEED A PARTNER ANTI-UNIVERSE WHOSE TIME FLOW IS OPPOSITELY RELATED TO OUR UNIVERSE.”

The Upside Down

The current model scientists use to explain the universe around us is made up of three components: regular matter, cold dark matter (CDM), and the cosmological constant, or lambda, a coefficient first introduced by Albert Einstein in 1917 that’s related to dark energy.

This standard model of cosmology has served as a way to explain how dark energy causes our universe to expand at an accelerating rate. But while the model has been around for decades, mysteries remain. For one, we have yet to observe dark matter directly. And the exact nature of dark energy, which is estimated to make up 68 percent of the universe, has also proven controversial, with many scientists coming forward with alternate explanations.

Now, a PhD student at the Indian Institute of Technology, Gandhinagar named Naman Kumar has proposed a “new model” that does away with this uncertainty by throwing out dark energy from the equation altogether — which has a highly unusual implication.

“However, there is a price to pay,” he wrote in a statement accompanying a paper he recently published in the journal Gravitation and Cosmology. “We need a partner anti-universe whose time flow is oppositely related to our universe.”

Dark Mirror

Kumar isn’t the first to suggest our universe has a mirrored twin that flows backwards in time. In February, an international team of researchers similarly suggested that dark matter, the mysterious stuff that makes up around 27 percent of the universe, resides in a mirror universe in which atoms never formed.

Of course, Kumar’s finding isn’t anything more than a working hypothesis regarding the observed accelerated expansion of the universe, which is in his words “one of the greatest puzzles in our understanding of the cosmos.”

But as our methods to observe and study the universe improve, astronomers are ever-so-slowly inching towards a possible explanation for this discrepancy — and sometimes, a solution as elegant as an upside-down world where time flows backwards, is our best bet.

“The beauty of this idea lies in its simplicity and naturalness, setting it apart from existing explanations,” Kumar argued.

If true, it could explain where dark matter comes from, and a possible twist in our understanding of neutrinos.

• Could a backward, mirror universe explain the existence of dark matter?
• If an anti-universe exists, it would run backward in time, before the Big Bang.
• Dark matter, then, could be right-handed neutrinos implied by the mirror universe.

Scientists believe there could be an “anti-universe” somewhere out there that looks like the mirror image of our own universe, reciprocating almost everything we do. If this theory holds true, it could explain the presence of dark matter.

First, some background: the “Big Bang” is a collective term that includes a variety of theories studied by cosmologists, the scientists who try to rewind the clock as close to the very beginning of the universe as possible. Most agree that matter exploded forth, but there are different opinions on, for example, whether the temperature was extremely hot or absolute-zero cold at that initial moment.

There are also disagreements about what may have happened prior to the bang itself. Could it be that what we call the Big Bang was the inflection point of an even bigger bounce in progress? Think of the point when you bounce on the trampoline and your feet almost touch the ground beneath—then imagine only seeing the subsequent bounce upward; it’s meaningless without the first, downward half of the bounce!

Dark matter is, if such a thing exists, maybe even more perplexing to scientists than the Big Bang. That’s because dark matter is a key piece that helps to complete an unclear puzzle— the question of what forms the universe around us today, not billions of years ago. Dark matter forms the bulk of the matter in the universe, but we’ve never been able to see it anywhere.

How is dark matter hidden in plain sight, and what are its qualities? These are huge mysteries upon which a ton of other ideas must rest. For the time being, one way to describe dark matter is very literal: by “dark,” we mean that it is not luminous, which is the technical term for matter that doesn’t reflect or emit any photons in a way we can identify. But we can measure the physical (not visual) effects of dark matter in things like gravitational waves.

Now we arrive back at the theory. Could it be that an “anti-universe” might run parallel to our own universe, but backward in time? If so, it would essentially spread out “backward” in time, prior to the Big Bang, in the same way our universe progressed “forward” in time. In a paper published in 2018 in the journal Annals of Physics, researchers from the Perimeter Institute for Theoretical Physics in Ontario, Canada, suggest that the Big Bang might have been smaller and more symmetrical than we think.

Is There an “Anti-Universe” Running Backward in Time?

“Among other things, we shall describe in detail a remarkable consequence of this hypothesis, namely a highly economical new explanation for the cosmological dark matter,” the researchers write.

One cool thing about this model of the Big Bang is that it removes the need for what scientists call “inflation,” a period of time in which the universe massively expanded in order to account for its size soon after birth. Instead, the matter could have naturally expanded over time in a less forceful way, which could simplify our explanation for what happened

And in order for these two before-and-after universes to be truly symmetrical, we would need to add a particle to our existing understanding of the universe around us. Today, we know about neutrinos, extra-tiny mysterious particles involved in gravity and weak interaction only. If our universe is mirrored by a similar universe running backward in time from the Big Bang, then what we call dark matter could actually be a version of a neutrino that is “right-handed,” a term that refers to the direction of motion in the neutrino. It would be the natural opposite of the left-handed neutrinos in the other universe.

If this sounds like wild and heady stuff, you’re absolutely right. But iteration using this kind of new theory is a critical part of cosmology, because scientists must have existing, published theories in order to study them and decide what their next theoretical step is. It’s so much easier to do that by responding publicly using your own observations and measurements, and that leaves a beautiful trail of ideas over time as we refine our understanding and develop more sophisticated ways to observe the universe.