A new study in quantum cosmology proposes an innovative approach to address one of the oldest puzzles in physics: the transition from quantum to classical. This transition is crucial for understanding how our universe, full of well-defined stars and galaxies, emerged from a primitive quantum state where things could be in multiple states at once.

The study’s researchers point out that the classical decoherence model, which explains how a quantum system can lose its quantum nature due to interactions with its environment, is not sufficient for the universe. Since the universe doesn’t have an “external observer” to trigger this transition, we must seek other mechanisms that allow quantum probabilities to collapse into a defined reality.

To understand the paradox of quantum superposition, the famous thought experiment of Schrödinger’s cat is often mentioned. Imagine a cat in a box with a device that can release poison if a radioactive atom decays. Until the box is opened, the cat is both alive and dead, depending on whether the atom has decayed. This phenomenon of quantum superposition illustrates the peculiarity of quantum mechanics: things can exist in multiple states at once until they are observed or measured.

In the universe, however, no one “opens the box”. So, how do we go from a state of multiple possibilities to a universe where things are solid and predictable?

Expansion of the Universe
Expansion of the Universe. Credit: NASA / Public domain / Wikimedia Commons

Scientists propose a new model called “spontaneous wave function collapse”. This model suggests that quantum probabilities collapse on their own without the need for an external observer. It’s as if Schrödinger’s cat decides for itself whether it is alive or dead, without anyone opening the box to check.

The study shows that spontaneous collapse can be applied to general relativity models, where the universe is seen as evolving spacetime. From a quantum superposition of different spacetime geometries, spontaneous collapse can select a single well-defined geometry. This might explain how the universe transformed from a chaotic and quantum state into something classical and tangible.

Additionally, this idea might offer a solution to the cosmological constant problem, which is the dark energy that seems to drive the universe’s accelerated expansion. Instead of relying on complex multiverse theories or anthropic principles, spontaneous collapse suggests that the cosmological constant emerges as a result of the collapse of a superposition of different possible values.

The authors of the study point out that this approach isn’t limited to a specific model but can be applied to other contexts in quantum cosmology. While it doesn’t completely solve the problem of time in quantum gravity, it provides a promising framework for exploring the universe and its origins.


Sources

Gaona-Reyes, J.L., Menéndez-Pidal, L., Faizal, M. et al. Spontaneous collapse models lead to the emergence of classicality of the Universe. J. High Energ. Phys. 2024, 193 (2024). doi.org/10.1007/JHEP02(2024)193


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