Scientists Say There’s an ‘Anti-Universe’ Running Backward in Time

[Buster's Uncle]

Scientists Say There’s an ‘Anti-Universe’ Running Backward in Time
Caroline Delbert
Popular Mechanics
Tue, August 26, 2025 at 11:30 AM EDT
4 min read



There’s an ‘Anti-Universe’ Going Backward in Time Getty Images


Here’s what you’ll learn when you read this story:

*A backward, mirror universe could 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.

“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.

https://www.yahoo.com/news/articles/scientists-anti-universe-running-backward-153000191.html

[Lorizael]

Since this is clickbait about a paper from 2018, I'll just recycle what I said in 2020 when we found some weird neutrinos people wanted to connect to the paper.

There's a balloon observatory in Antarctica (called ANITA) that detects cosmic rays. For the last couple years, they've been seeing something weird: decay products of cosmic rays (in this case, very high energy neutrinos) traveling up from the Earth and hitting the detector instead of coming down from space. Normally this wouldn't be a big deal, because neutrinos are very good at passing right through solid matter, so maybe they're just coming from space the long way around--through the Earth. But high energy neutrinos should actually be stopped, kinda like the way a strainer will stop big things but not little things.

So people have been trying to come up with possible explanations. One is a very intense emission of neutrinos, like from some giant supernova or other astrophysical event, that might have thrown so many neutrinos at the Earth that some managed to get through and be detected coming up. This seems to have been ruled out.

Another explanation is... there's some systematic problem with ANITA that makes it look as if neutrinos are coming up instead of at different angles, or that what they're detecting are not the decay products of neutrinos but something else, or some other subtle problem. The ANITA scientists believe they've accounted for all possible systematic errors, but who knows.

A third possibility is some sort of exotic new physics. There's a symmetry the universe is thought to obey called CPT symmetry. Let's say you have two particles, A and B. If B is the antimatter version of A, and it's been inverted (like a mirror image), and its momentum is in the opposite direction of A, CPT symmetry says you can't tell the difference between these two particles. If subjected to the same force, they will respond identically.

Some theorists have proposed a kind of very strong CPT symmetry that seeks to explain the fact that our universe seems to be missing a bunch of antimatter. The idea is that the Big Bang represents the creation of a universe/anti-universe pair, where the anti-universe contains all the matching B particles to our A particles. All the antimatter is just over there in the anti-chunk. These same theorists argue that such a universe would have a few new particles we haven't seen yet (to make all the symmetries work out), one of which could be a dark matter candidate. A particular dark matter candidate that seems compatible with the CPT-symmetric universe idea might possibly bunch up inside massive objects (like the Earth) and decay into, among other things, high energy neutrinos that would subsequently be detected coming up at ANITA.

So, to sum up, possible explanations for these weird upgoing neutrinos include: (a) some powerful explosion in space we just haven't detected by other means yet, (b) some tiny error in how detections are interpreted, or (c) a new kind of dark matter particle that, if you're really really intent on having the math come out perfectly and elegantly, might imply all our antimatter ended up in a separate, inaccessible chunk of the universe.

Where does the parallel time running backwards universe part of this come from? In CPT symmetry, T stands for time-reversal, and when I said above that particle B has its momentum pointing in the opposite direction from particle A, that's what I was talking about. That's time-reversal symmetry. But, as I said above, the whole point of CPT symmetry is that particle A and particle B are completely indistinguishable and behave exactly the same. So, even accepting the idea that all our antimatter is just over there, it's in a symmetric state that ends up behaving just as our universe would, with "time" "moving" "forward" and all that entails.

[Buster's Uncle]

That lasty is surely down to the writer of this article being in way over her head...