Our universe has an anti-universe twin moving backwards in time, study finds

Seconds after the Big Bang, the Universe exhibited an astonishing simplicity. Observations reveal a spatially flat, radiation-dominated cosmos described by a Friedmann–Robertson–Walker (FRW) metric. This early state included small, Gaussian, and nearly scale-invariant scalar perturbations.

However, there is no evidence for primordial vector or tensor perturbations, nor for cosmic defects. These observations align with the prevailing inflationary model, which suggests that an era of rapid expansion preceded the Universe's observable state.

Despite its utility, inflation theory introduces complexities and arbitrary parameters, which many physicists view as unnecessary. A team led by Neil Turok and Latham Boyle challenges this conventional framework, offering an alternative grounded in the symmetry of the Universe itself.

Their hypothesis explores the idea that the Universe obeys an unbroken CPT symmetry—a principle stating that the laws of physics remain unchanged under simultaneous transformations of charge, parity, and time.

Turok critiques inflation theory as a modern equivalent of Ptolemaic epicycles, dependent on hypothetical constructs to align with observations. Instead, his approach extends beyond the Big Bang singularity, which marks a breakdown in general relativity.

By applying CPT symmetry, Turok and Boyle propose that the Universe, along with an anti-universe, forms a symmetric pair. In this model, the anti-universe stretches backward in time from the Big Bang, mirroring the properties of our Universe with reversed spatial configurations and dominated by antimatter.

This concept offers a profound reinterpretation of cosmological phenomena. Turok explains, “The most natural assumption is that the Universe as a whole respects CPT symmetry, encompassing not just our universe but an anti-universe as well.”

In an FRW metric, the Universe’s scale factor is proportional to conformal time, introducing time-reversal symmetry. When extended through the Big Bang, the metric exhibits an additional isometry, creating a CPT-invariant vacuum.

Unlike the Minkowski vacuum, which is empty, the CPT-symmetric vacuum contains a finite density of particles. This extension provides a new mechanism for dark matter production.

Dark matter, a mysterious substance that constitutes about 27% of the Universe, remains one of cosmology’s greatest puzzles. Turok and Boyle’s model eliminates the need for exotic particles by proposing that dark matter could arise from the same right-handed neutrinos already suggested by the Standard Model of particle physics. These sterile neutrinos, extraordinarily massive and elusive, naturally emerge in a CPT-symmetric framework.

Turok describes this as “the most economical explanation for dark matter,” adding that it aligns with all known particles and fields. Their calculations suggest that the mass of these neutrinos is approximately 5×108 GeV5 \times 10^8 \, \text{GeV}5×108GeV—500 million times the mass of a proton.

Supporting evidence for this hypothesis comes from unexpected sources, such as the Antarctic Impulsive Transient Antenna (ANITA) experiment. ANITA detected anomalous radio signals suggesting particles traveling upward through Earth.

These particles, with masses between 2×108 GeV2 \times 10^8 \, \text{GeV}2×108GeV and 10×108 GeV10 \times 10^8 \, \text{GeV}10×108GeV, may be consistent with decaying right-handed neutrinos. While some scientists argue these neutrinos should be stable in a CPT-symmetric Universe, Turok notes that slight adjustments could account for their decay over cosmic timescales.

Cosmic microwave background (CMB) fluctuations, a cornerstone of inflation theory, also present challenges for this new model. These temperature variations are attributed to quantum mechanics near the Big Bang singularity. According to Turok, quantum uncertainty ensures that the Universe and anti-universe are not perfectly symmetrical, resolving philosophical concerns about determinism and free will.

This CPT-symmetric model not only redefines the origin of dark matter but also extends the scope of quantum field theory to curved spacetime. It introduces a preferred vacuum state for quantum fields that respects the full symmetry of the background metric.

Despite its elegance, the model faces scrutiny. Critics argue that it lacks a definitive explanation for certain phenomena well-accounted for by inflation, such as the observed large-scale structure of the Universe. However, Turok’s team views this as a work in progress. Theoretical refinements and future observations may validate their predictions or reveal new directions for exploration.

Turok and his collaborator Kieran Finn acknowledge the model’s unfinished nature but remain optimistic. “Our findings suggest that the most likely universe is one very similar to our own,” Turok states. This perspective aligns with statistical analyses showing the likelihood of a symmetric universe.

Beyond theoretical physics, the implications of this model could revolutionize our understanding of the cosmos. If CPT symmetry holds, it reshapes the narrative of the Universe’s birth, evolution, and composition, eliminating the need for speculative constructs and expanding the reach of established physics.

The path forward involves rigorous testing of the model’s predictions. Experiments targeting the mass and properties of sterile neutrinos, as well as high-energy cosmic-ray studies, may provide critical evidence. Moreover, advances in quantum cosmology could refine our understanding of symmetry in curved spacetime.

While the debate continues, Turok and Boyle’s framework represents a bold step in cosmology. By adhering to CPT symmetry, their model integrates simplicity with scientific rigor, offering a unified explanation for dark matter and the Universe’s fundamental structure.

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