Key Takeaways:

  • Our universe might have an antimatter counterpart that existed before the Big Bang.
  • This theory proposes a universe-antiuniverse pair that obeys CPT symmetry.
  • The antiuniverse would be dominated by antimatter and its time would run backward.
  • The new model explains dark matter with sterile neutrinos.
  • Quantum uncertainty prevents the universe and antiuniverse from being perfect mirror images.

It’s possible that our universe is the antimatter counterpart of an antimatter universe that existed earlier in time than the Big Bang. So claim physicists in Canada, who have devised a new cosmological model positing the existence of a “antiuniverse” which, paired to our own, preserves a fundamental rule of physics called CPT symmetry. Though many details in their theory still need to be worked out, the researchers claim that it naturally explains the existence of dark matter.

According to standard cosmological models, the universe—which consists of space, time, and mass/energy—exploded into being about 14 billion years ago. Since then, it has expanded and cooled, causing subatomic particles, atoms, stars, and planets to gradually form.

But according to Neil Turok of the Perimeter Institute for Theoretical Physics in Ontario, these models are beginning to resemble Ptolemy’s solar system description more and more because they rely on ad hoc parameters. The brief period of rapid expansion known as inflation, he claims, is one such parameter that can explain the large-scale uniformity of the universe. “There is this frame of mind that you explain a new phenomenon by inventing a new particle or field,” he says. “I think that may turn out to be misguided.”

Rather, Turok and his colleague Latham Boyle at the Perimeter Institute set out to create a universe model that relies solely on known particles and fields to explain all observable phenomena. They questioned whether the cosmos could naturally extend beyond the Big Bang, the singularity at which general relativity breaks down, and continue on the other side. “We found that there was,” he says.

The answer was to assume that the universe as a whole obeys CPT symmetry. According to this fundamental principle, if space is inverted, time is reversed, and particles are switched out for antiparticles, then the physical process stays the same. Turok says that this is not the case for the universe that we see around us, where time runs forward as space expands, and there’s more matter than antimatter.

Rather, a universe-antiuniverse pair is the entity that honors the symmetry, according to Turok. According to Turok, the antiuniverse would expand larger and further back in time from the Big Bang, dominated by antimatter, with its spatial properties inverted in comparison to our universe. This would be similar to the formation of electron-positron pairs in a vacuum.

In a CPT-symmetric universe, time would run backwards from the Big Bang and antimatter would dominate (Courtesy: L Boyle/Perimeter Institute of Theoretical Physics)

Turok, who also collaborated with Kieran Finn of Manchester University in the UK, acknowledges that the model still needs plenty of work and is likely to have many detractors. Indeed, he says that he and his colleagues “had a protracted discussion” with the referees reviewing the paper for Physical Review Letters – where it was eventually published – over the temperature fluctuations in the cosmic microwave background. “They said you have to explain the fluctuations and we said that is a work in progress. Eventually they gave in,” he says.

Turok says that the fluctuations can be broadly attributed to the quantum-mechanical properties of space-time in the vicinity of the Big Bang singularity. Though fixed (classical) points would exist in the far future of our universe and the far past of the antiuniverse, every possible quantum-based permutation would exist in the middle. He and his colleagues determined which CPT pair configuration is most likely to exist by calculating the instances of each possible configuration. “It turns out that the most likely universe is one that looks similar to ours,” he says.

In order to avoid difficult issues like free will, Turok continues, quantum uncertainty implies that the universe and antiuniverse are not perfect mirror images of one another.

But problems aside, Turok says that the new model provides a natural candidate for dark matter. The finite (very small) mass of more prevalent left-handed neutrinos is thought to be explained by this candidate, an extremely elusive, massive particle known as a “sterile” neutrino. Turok claims that the abundance of right-handed neutrinos in our universe can be calculated from first principles using CPT symmetry. He claims that when the observed density of dark matter is taken into account, the right-handed neutrino’s mass is estimated to be around 5×108 GeV, or 500 million times the mass of a proton.

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