What is the world like out of our sight? There seems to be no answer to this question. Some scientists suspect that our universe is an expanding bubble and there are more foam universes beyond the universe. They all exist in an ever expanding ocean full of energy, which is the multiverse.
Some physicists accept that multiverse can explain why our universe is different, while others reject this theory because it only predicts the imaginable universe and has no way to test it.
Now, teams are looking for new ways to infer how the universe expands like bubbles and what happens when these bubbles collide.
Random quantum events may lead to the birth of the universe
In the early 1980s, when physicists studied how the universe started or stopped inflation, they found that although our universe or other universes may have stopped inflation, quantum effect should make most of the space continue to expand, which is called eternal inflation theory.
In the final analysis, the difference between the bubble universe and its surroundings is reflected in the energy of space itself. When the space is as empty as possible and it is impossible to lose more energy, it reaches what physicists call "true vacuum". Imagine a ball on the floor. It's impossible to keep falling. But the system can also have a "pseudo vacuum" state. Imagine that a ball is placed in a bowl on the table, and the ball can roll back and forth, while more or less staying in the same place. But with enough vibration, the ball can fall to the ground - a true vacuum.
But physicists have made great efforts to predict the behavior of vacuum bubbles. What a vacuum bubble becomes depends on the overall influence of countless tiny details. At the same time, the bubbles also have the randomness and fluctuation of quantum mechanics. Different assumptions about these processes will lead to contradictory predictions, and it is impossible to judge which one is closest to the real situation.
Quantum entanglement is lost in the simulated vacuum bubble experiment
At first, this simple setting was not expressed according to the actual situation. When the bubble walls collide with each other, they rebound perfectly, and there is no expected complex reverberation or particle outflow (represented by a ripple like arrow flip). However, with the deepening of the research, the team observed that the outer wall of the collision ejected high-energy particles. The more intense the collision, the more particles ejected.
However, the researchers found that when the resulting particles are mixed together, they entangle and share the quantum state. Each additional particle increases the complexity of their state exponentially.
Researchers say that because quantum computers rely on quantum entanglement to complete their calculations, it is only when mature quantum computers appear that the research on bubble behavior can achieve a breakthrough.
Quantum computing or blowing a quantum bubble
This kind of device is called quantum annealing machine, which is a limited quantum computer. It is used to solve the problem of algorithm optimization by finding the lowest energy state of qubits. This process is similar to the pseudo vacuum decay.
Abel and spanovsky used the commercial quantum annealing machine d-wave to program a series of 200 qubits to simulate a high-energy or low-energy quantum field, similar to the pseudo vacuum and the true vacuum. Then they watched how the former decayed into the latter, leading to the birth of the vacuum bubble.
This experiment only verified the known quantum effect, and made no new discovery about vacuum decay. But researchers hope to eventually use d-wave to surpass current theoretical predictions.
Another way is to blow bubbles directly.
Quantum bubbles that expand at nearly the speed of light are not so easy to get, but in 2014, physicists in Australia and New Zealand proposed a way to make such bubbles in the laboratory, using a strange state of matter, the Bose Einstein condensate (BEC). A thin mass of gas can condense into BEC when it is cooled to near absolute zero. It has unusual quantum mechanical properties, including the ability to interfere with another bec (similar to the interference of two lasers). If the two condensates interfere in the right way, the team predicts, the experimenters should be able to capture direct images of bubbles that are generated in the condensate, similar to those assumed in the multiuniverse.
Perris led a team of physicists to study how to stabilize the condensed matter mixture to prevent collapse caused by unrelated effects. After years of hard work, she and her colleagues are finally ready to build a prototype experiment. They hope to blow out condensing bubbles in the next few years.
If the experiment goes well, they can answer two questions: the rate of bubble formation, and how the expansion of one bubble changes the probability of the expansion of another nearby.
This information will help cosmologists figure out how the impact from the nearby bubble universe long ago caused our universe to shake. The scar left by this collision may be a circular cold spot in the universe. Other details, such as whether the collision will produce gravitational waves, also depend on the specific situation of the unknown bubble.
(translated by Dong Wanyu)