Can the unsolved string theory turn the tide and save the universe from collapse?

Can string theory save the universe from a devastating phase transition?

Can ideas inspired by string theory save the universe from collapse?

(Photo credit: Pasheka, Getty Images)

Our universe may be fundamentally unstable. In a split second, the vacuum of spacetime could find a new ground state, triggering a catastrophic shift in the physics of the universe.

Or perhaps new ideas inspired by string theory suggest that our universe may be more stable than we previously thought.

Broken Universe

In the first few microseconds after the Big Bang, the universe went through a series of dramatic phase transitions. The four forces of nature—electromagnetism, gravity, and the strong and weak nuclear forces—were unified into a single force. Physicists don't know the identity or nature of this force, but they know it won't last long.

As the Universe expanded and cooled, gravity was the first to separate from the other three. Then, the strong nuclear force became independent. The last two forces to split were electromagnetism and the weak nuclear force. This last split is actually within experimental reach: in our largest particle accelerators, we can recreate the conditions of the early Universe and (briefly) obtain the energy needed to reunite the two forces.

From then on, everything settled down. The four forces of nature remain exactly as they are today. Elementary particles combined to form nuclei, atoms, and molecules. Eventually, stars were born and planets rose from the ashes. Compared to the first few microseconds of the Big Bang, the past 13.8 billion years have been boring.

Variety of changes

But the apparent stability of the universe may be an illusion caused by its long existence. In fact, every phase transition that occurred in the early universe completely changed the nature of reality, obliterating the old order and replacing it with new forces and new particles.

Scientists can assess the stability of the current vacuum in spacetime by measuring the mass of the Higgs boson. The Higgs boson permeates all of space and time, and it plays a very important role. In addition to providing mass to many fundamental particles, it also acts as a wedge driving between the weak nuclear force and the electromagnetic force. In other words, in the early, hot, dense universe, the faint Higgs particle lurked behind the scenes, allowing the two forces to combine. But as the universe cooled, the Higgs particle strengthened and pushed the two apart. (The mechanisms that pull the other forces of nature apart are an ongoing area of ​​research in modern physics.)

Clearly, the universe is not unstable; otherwise, it would have transitioned to a new epoch long ago. But the mass of the Higgs particle can help us tell whether the universe is completely stable or merely metastable — meaning, it is temporarily stable until something causes a random phase transition.

Current measurements of the Higgs mass suggest that we were right: the Universe appears to be metastable and could enter a new phase at any time.

To describe the process of the spacetime vacuum phase transition to a new ground state as catastrophic is an understatement. At some random point, a random quantum fluctuation might trigger the phase transition. From there, it would spread out like an expanding soap bubble. Outside the bubble, life and the universe would continue to function as normal. But inside the bubble, a whole new set of physical laws would emerge.

Given that our entire existence depends on the stability of natural laws—the arrangement of forces and the number of known particles—if a phase transition swept us away, we would be… gone.

Combination with string theory

Not necessarily. After all, this is all highly speculative physics. A new paper recently posted to the preprint database arXiv paints a more optimistic view.

Our knowledge of physics is incomplete. The combination with the Higgs particle taught us how the electromagnetic force and the weak nuclear force merge. But we have yet to find a consistent, coherent theory of how the strong nuclear force merges with the other forces. A fully unified force that includes gravity in a complete quantum description of nature is beyond our comprehension.

However, string theory is an attempt to unify all the forces under one framework. In string theory, the fundamental particles of our existence are viewed as collections of tiny vibrating strings.

While string theory is not yet complete (and some believe it never will be), it does allow researchers to develop tools to study thorny problems, such as the physics of phase transitions that could end the universe.

The authors of the new paper studied a version of string theory that includes nonlocal effects — meaning that strings in one region of space can appear to instantaneously influence strings in another part of space, even though they are far away. (If that sounds like a radical idea, it’s not that far-fetched: The phenomenon of quantum entanglement is also nonlocal.)

The researchers found that as the bubble expanded, nonlocal effects in this version of string theory tended to flatten the bubble walls. In some cases, the bubble walls were stretched so long that they dissolved entirely. This means that if our reality is indeed metastable and undergoes random phase transitions, the fundamental string physics laws of the universe would prevent the phase transition from engulfing the entire universe; the bubble would burst before it had a chance to expand beyond microscopic sizes.

This is still very speculative thinking, but at least it can provide a degree of comfort while we continue to understand the basic workings of the universe.

BY:Paul Sutter

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