Why some physicists really think there's a 'mirror universe' hiding in space-time

Why some physicists really think there’s a ‘mirror universe’ hiding in space-time

The Cosmic Microwave Background, shown here, is the most ancient object in universe. But what lies behind it? (Image credit: ESA and the Planck Collaboration)

According to a series of viral articles, NASA detected particles from another parallel reality where time runs backward. These assertions were false. The true story, which involves a journey inside the Big Bang and out the other side, is significantly more fascinating and weird.

The sensational headlines masked the findings of an obscure 2018 paper, never published in a peer-reviewed journal, which argued that our universe may have a mirror reflection throughout time, a partner universe that extends beyond the Big Bang. If that’s the case, and a number of other extremely unusual and crazy possibilities turn out to be correct, the research argues, it could explain a mysterious signal indicating that a completely new particle is flying out of Antarctica’s ice.

The claim that NASA discovered a parallel world appeared to have originated with the British tabloid The Daily Star, which was then picked up by British and American outlets, including The New York Post.

Screenshots from numerous publications illustrate false “parallel universe” claims. (Image credit: Live Science Illustration)

Our universe’s “mirror”

To understand how The Daily Star arrived at its bizarre, viral claim, one must also understand the statements of two separate papers from 2018.

The first publication proposed a mirror universe — a reflection of our world across time — by Latham Boyle, a physicist at The Perimeter Institute in Ontario, Canada, and his colleagues. It was published in the journal Physical Review Letters in December 2018 (after an appearance on the arXiv server in March that year).

“I think nobody else understands the full sweep of what they have composed,” said John Learned, a University of Hawaii astrophysicist and the co-author of a second paper, which builds on Boyle’s theory.

Boyle’s work is a form of expansion pack meant to fill holes in the dominant creation tale of the universe: Lambda-Cold Dark Matter (CDM).

The cosmos is described by CDM using two key ideas: The universe is expanding due to an unknown dark energy. If you go far enough back in time, the entire universe occupies a single point in space. Second, unseen dark matter tugs on stuff in the universe while producing no light. According to the theory, dark matter accounts for the vast majority of the universe’s mass.

“ΛCDM is basically the only game in town,” Learned said. “It works in many cases, but there are some somewhat disturbing lapses in the modeling.”

For example, measurements of expansion do not line up across time, therefore measurements based on data from the early cosmos do not agree with measurements based on data from the present era. Furthermore, CDM cannot explain why matter exists at all, because it predicts that matter and antimatter were produced at equal rates after the Big Bang and annihilated each other, leaving nothing behind.

The new universe created by Boyle and his colleagues takes the CDM story further back in time, delving into the singularity at the beginning of time and emerging on the other side.

According to Boyle’s team, their theory is as follows: imagine today’s cosmos as a wide, flat circle sitting on top of yesterday’s slightly smaller circle, which sits on top of the day before that’s even smaller circle, according to Boyle.

(Image credit: Meghan McCarter)

Combining all the circles from today back to the Big Bang results in a cone standing on its point end.

(Image credit: Meghan McCarter)

Astronomers are effectively looking back in time when they look deep into space. GN-z11, the most distant galaxy visible to us, looks to us as it existed 13.4 billion years ago, or 400 million years after the Big Bang.

Before that, the universe went through a “dark age” that lasted millions of years, during which nothing bright enough for us to see formed. The universe created the oldest object we can see before that: the Cosmic Microwave Background (CMB), which formed 370,000 years after the Big Bang as the universe cooled from a hot, opaque plasma.

Telescopes are unable to detect anything before to the CMB.

Looking back in time is like looking down through the cosmic cone, according to Boyle.

(Image credit: Meghan McCarter)

When viewed in this way, the CDM story concludes with the universe collapsing into a single point concealed behind the CMB. Boyle’s theory examines the CMB’s opaque wall over time and comes to a different conclusion about what the CMB hides.

According to conventional wisdom, the first tiny fraction of a second following the Big Bang was a “big mess” that modern physics equations struggle to describe. We can’t observe what happened at that time since it’s hidden behind the CMB from our vantage point on the cone. This early, brief chapter in the history of the universe is chaotic and difficult to comprehend in CDM cosmology.

However, the CMB is not that chaotic. According to CDM, its simple structure was formed following an intense flattening process that cleared away the chaos by the end of the universe’s first second. The tidy universe thought to have emerged from that brief mess is captured in the CMB.

“We were interested in exploring a simpler picture where you take the evidence more at face value,” he said. “You say ‘Okay, we can’t see all the way down to the Bang, but we can look darned close, and as close as we look, things look super simple. What if we take those observations at face value?'”

This vision of space-time still has a Big Bang hiding behind the CMB, he said.

But “it’s much simpler than most of the singularities that arise in Einstein’s theory of gravity,” he said. “It’s a very special type of ultra-simple singularity, where you can follow the solution [to the equations governing space-time] through the singularity.”

Whereas observations only go back to the CMB, conventional cosmological theories go back a little further but still tend to come to a halt at the Big Bang. Not in Boyle’s plan.

“You find that it extrapolates, it extends — it analytically continues, physicists would say, to this double cone,” he explained, referring to the second universe, which extends in time away from the Big Bang.

(Image credit: Meghan McCarter)

“It just seems to be the natural, simplest extension of the equations that seem to describe the universe as we see it,” he said.

We can’t see the universe inside the “second cone” because it’s too far down space-time. From our perspective, time may appear to move backwards there, according to Learned. However, beings in that universe, like us, would sense cause coming before effect. Time in that universe, like time in ours, runs away from the Big Bang. The direction of time in that universe is the opposite of the direction of time in our universe. However, it does not run “backward” as we might expect.

Our universe exists on the opposite side of the ancient history of that universe, and that world lives on the other side of ours.

The “zero particle state”

According to Boyle, we have no evidence that this reflected universe exists. However, he said, “once you have it, it turns out this universe has an extra symmetry, which you didn’t see when you were just looking at the top half of the cone.”

Symmetries “ring a loud bell” for physicists, Boyle said. They imply a deeper truth. And this double-cone universe could help physicists repair a symmetry defect that has bothered them for years.

The Charge, Parity, Time (CPT) symmetry states that if you flip a particle to its antimatter twin — say, an electron into a positron — or make it right-handed instead of left-handed, or move it backward through time instead of forward, that particle should still behave and obey the same laws as it did before being flipped. (Right-handed or left-handed refers to a particle’s spin and travel direction.)

“Everybody thought these were fundamental symmetries that could not be escaped,” Learned said.

In 1956, Columbia University physicist Chien-Shiun Wu led an experiment that proved CPT symmetry was not absolute. (The two male colleagues who offered the fundamental idea to Wu were awarded a Nobel Prize for her discovery in 1957, but she was not.)

Wu’s experiment showed that the “C” in CPT symmetry is incorrect. Further research showed that some particles break both “C” and “P.” However, despite being cracked, most physicists believe that CPT symmetry still holds in general, and no particle has been discovered that violates all three aspects of this symmetry. The universe appears CPT symmetric at the particle level.

However, because the CDM model of the cosmos lacks CPT symmetry, its “zero particle state,” or the structure of space-time when empty of particles, is unclear. This suggests that CPT symmetry is violated at the global scale.

According to Boyle, his model retains the universe’s CPT symmetry in a way that the CDM cosmology does not. With the addition of a second cone to space-time, the zero particle state is no longer unclear. The CPT asymmetry of the cosmos has been repaired.

“We thought, ‘Wait a minute. It seemed like the universe violated CPT symmetry, but actually we just weren’t looking at the whole picture,” he said. What does it entail for the rest of physics if the universe is truly CPT-symmetric, if it consists of two space-time cones rather than one?

The truth behind what those “NASA scientists” really detected

The most practical implication of the CPT-symmetric universe is a straightforward explanation for dark matter.

One popular theory about the unseen stuff is based on the existence of an undetected fourth form of neutrino, known as a sterile neutrino. Boyle’s CPT symmetry appears to suggest this. The three known neutrino flavors, electron, muon, and tau neutrinos, are all left-handed. That means that they fly around without a matching right-handed partner. The Standard Model assumes that neutrinos, unlike other particles, do not have such partners. The CPT-symmetric universe, on the other hand, believes they should have those partners.

Boyle and his colleagues discovered that their cosmology needs a right-handed companion in our cosmos for every left-handed neutrino in the Standard Model. However, unlike left- and right-handed quarks, these mirror particles would not stick together. Instead, two of the right-handed partner neutrinos would have been lost to space-time long ago, decaying out of sight in the early universe. However, a third right-handed partner would have remained as a result of the equations governing the beginning of time.

Boyle said it’s unclear which of the three known neutrinos it would have partnered with. It would, however, have had a distinct energy signature: 480 picoelectronvolts (PeV), a measure of particle mass. And that 480PeV neutrino could account for all of the universe’s missing dark matter.

The details of how the CPT-symmetric world leads to a 480 PeV neutrino are complex — so complicated, according to Learned, that few physicists other than Boyle and his team understand them at all.

“But these guys are not nutcases,” he said. “They’re respected members of the field and they know what they’re doing. Whether all of that complicated field theory is correct or not, I can’t say.”

Nonetheless, Learned was struck by the prediction of a 480 PeV particle.

Four years ago, a particle detector hanging from a balloon over Antarctica detected something physics could not explain: Twice, the Antarctic Impulsive Transient Antenna (ANITA) instrument picked up signals of high-energy particles that seemed to shoot straight up out of the Antarctic ice. (Most researchers involved in ANITA aren’t “NASA scientists,” but the project does receive NASA funding.)

The Antarctic Impulsive Transient Antenna (ANITA) experiment, which picked up signals of seemingly impossible particles as it dangled from its balloon over Antarctica, is about to be launched. (Image credit: NASA)

Such particles should not exist. None of the known Standard Model particles should have been able to fly through the Earth and explode out the other side at such high energy, but that’s what ANITA appeared to be sensing.

The most popular explanation as of June 2020 is that ANITA discovered sterile neutrinos. Learned, who was involved in the early days of the ANITA research, noticed that the 480 PeV number matched the ANITA findings nicely.

If the particles arrived from space and plunged through the Earth to cause the anomaly, they must have decayed just below the Antarctic surface, resulting in a shower of lighter particles that ANITA detected rising up through the ice. Boyle’s 480 PeV dark matter neutrino fell just into the mass range that may explain ANITA’s decaying mystery particle.

Learned and a team of four other researchers cooked up a scheme where this 480 PeV dark matter neutrino might have pulled off this trick, which they wrote up in a 2018 paper titled “Upgoing ANITA events as evidence of the CPT symmetric universe” and published to the arXiv database. This is the newspaper that The Daily Star transformed into a confusing headline.

If the ANITA particle indeed suited Boyle’s design, Learned believes it would be a strong weight on the balance in favor of the two-cone cosmos. However, it’s a long shot. The most pressing issue was getting the particles close enough to Antarctica. According to models, dark matter candidate particles like this 480 PeV neutrino would fall to the Earth’s core soon after colliding with it, leaving none close enough to cause the ANITA anomaly.

These researchers theorize that a recent encounter with a massive, unseen disk of dark matter had agitated the Earth’s 480 PeV neutrinos, causing some to wander around near the planet’s surface. It was an intriguing idea to experiment with, according to Learned, but even he is not convinced by his own paper.

“That was our feeble excuse, not thinking of any other good way to do the job [of getting Boyle’s neutrinos close enough to Antarctica to trip ANITA’s sensors],” Learned said. Though Learned and his colleagues worked hard on the article, he believes its conclusions are undoubtedly incorrect.

“Amongst cosmology folks there’s … an idea that you get to use a ‘tooth fairy’ once in your cosmology model but twice is simply not credible,” he said. “And I think we needed the tooth fairy two or three times to make this one work, so, oh well.”

Boyle agreed. While the concept of using his team’s ideas to explain ANITA appeal to him, he noted the numbers don’t stack up. But he remains convinced that the core concept of a CPT-symmetric world is sound.

“My personal hunch is that whether or not it’s exactly correct, it’s on the right track,” he said. “I’m very excited about that.”

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