In his 1980 book Cosmos, Carl Sagan famously wrote, “If you wish to make an apple pie from scratch, you must first invent the universe.”
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| Credit: Moritz Kindler on Unsplash |
If you follow their threads back in time, the ingredients for a deliciously satisfying apple pie – apples, flour, cinnamon, heat, etc. – wind their way back to before the observable universe. Existence was contained in a vacuum then, a void empty except for quantum fluctuations.
According to leading cosmological models, the vacuum was fairly stable and may have existed in this state for a very long time, but it had a weakness. When quantum fluctuations caused a region of space to spontaneously become more stable (to have lower energy) than its surroundings, a “bubble” of greater stability formed. According to cosmological models, this “bubble” of stability rapidly expanded at nearly the speed of light. Multiple bubbles may have occurred at the same time, coalescing and bringing the entire vacuum into a more stable existence. An existence in which a complex state surrounded a more pure vacuum than what it replaced.
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| 3D visualization of quantum fluctuations. Credit: Ahmed Neutron (CC BY-SA 4.0). |
Bubble formation could be responsible for our existence, according to Jonathan Braden, a cosmologist at University College London. “Our entire observable cosmos may have originated through a phase transition in which a bubble universe nucleated,” he says. Like the phase change that transforms water into steam, the transition may have turned a practically empty vacuum into a galaxy-filled universe.
Braden recently led a collaborative research effort exploring this possibility. The results, he says, “may give us a way to simulate the moment of creation for our Universe as a whole.” The team—which includes Braden, Matthew Johnson (York University and the Perimeter Institute for Theoretical Physics), Hiranya Peiris (UCL and the Oskar Klein Center), Andrew Pontzen (UCL), and Silke Weinfurtner (University of Nottingham)—found a new avenue to bubble formation that bypasses some of the complications of existing models. Their work was published in a recent issue of the American Physical Society’s journal Physical Review Letters.
The formation and evolution of bubbles are traditionally studied using a technique that yields snapshots of the activity at different instances in time. In contrast, Braden and his team used a technique that examines the process as time evolves smoothly, like a video. They started by setting up a framework for the vacuum and generating initial conditions for quantum fluctuations. Then, they let the fluctuations evolve in time according to the classical equations of motion.
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| Close-up of a bubble’s edge. Credit: Eliška Motisová on Unsplash. |
The prevailing notion among cosmologists has been that the paths to bubble formation are quantum mechanical in nature. However, this new work reveals that classical, non-quantum paths exist too. It tells the story of a quantum vacuum evolving according to the laws of classical physics and ending up at the same place as the quantum version of the story, but without requiring any “quantum weirdness” along the way.
Furthermore, the rate of bubble formation that emerges from this approach agrees well with the rate predicted by the traditional snap-shot technique. This suggests that the classical path isn’t just one possible path to bubble formation, it may be the primary path.
If this seems a little too theoretical for you, stay tuned. According to the researchers, there’s an experimental way to investigate the classical path to bubble formation. Several physical systems undergo a bubble-like process of transitioning from one state to another. Bose-Einstein condensates are a particularly interesting example, with parallels that suggest they could be a good testbed for this theory. The team is actively exploring this possibility now—perhaps we’ll soon be closer to the most comprehensive apple pie recipe yet.
-Kendra Redmond