Our universe may be fundamentally unstable. In a flash, the vacuum of spacetime can find a new ground state, triggering a cataclysmic transformation of the physics of the universe.
Or not. A new understanding inspired by string theory shows that our universe may be more stable than previously thought.
A broken universe
Within her first microseconds Big explosion, the universe underwent a series of radical phase transitions. The four forces of nature — electromagnetism, gravity, the strong nuclear force and the weak nuclear force — were once unified into a single force. Physicists do not know the character or nature of this force, but they do know that it did not last long.
Related: The Big Bang: What Really Happened at the Birth of Our Universe?
As the universe expands and cools, at first gravity separates from the other three. Then the powerful nuclear power became independent. Finally, the last two forces to decay were electromagnetism and the weak nuclear force. This last separation is actually within experimental range: Within us larger particle acceleratorswe can recreate the conditions of the early universe and achieve the energies required to (briefly) recombine these two forces.
Since then, everything has been pretty stable. The four forces of nature have remained the four forces of nature. The fundamental particles joined together to form nuclei, atoms and molecules. After all, stars were born and planets arose from their ashes. The last 13.8 billion years have been downright boring compared to the first microseconds of the Big Bang.
But the apparent stability of the universe due to its long lifetime may be an illusion. Each of the phase transitions that occurred in the infant world completely reworked the nature of reality, with the old order disappearing and new forces and new particles appearing to replace them.
Scientists can estimate the current stability of the spacetime vacuum by measuring its mass Higgs boson. The Higgs boson permeates all space and year, and plays a very important role. In addition to providing mass for many fundamental particles, it also does the work of creating a wedge between the weak nuclear force and the electromagnetic force. In other words, in the early, hot, dense universe, the Higgs remained in the background, allowing the two forces to merge. But as the universe cooled, the Higgs gained strength and separated the two. (The mechanism that separated the other forces of nature is an ongoing direction of modern physics research.)
It is obvious that the universe is not unstable. Otherwise, he probably would have switched to a new reality a long time ago. But the Higgs mass can tell us whether the universe is fully stable or just metastable, meaning it’s stable for now, until something causes a random phase transition.
Current measurements of the Higgs mass show that we are on the line: The universe appears to be metastable and could flip into a new phase transition at any time.
To say that a phase transition to a new ground state of the vacuum of spacetime would be catastrophic would be an understatement. At some random point, a random quantum fluctuation could cause the phase transition. From there, it will spread like an expanding soap bubble. Outside the bubble, life and the universe would proceed as normal. But inside the bubble, a whole new set of physical laws would emerge.
Considering that our entire existence depends on the stability of the laws of nature – the arrangement of forces and the zoo of known particles – if the phase transition were to overtake us, we would simply … disappear.
Tied together with strings
Or not. This is all highly speculative physics, after all. And a new paper recently published in the arXiv preprint database presents a somewhat more optimistic view.
Our knowledge of physics is incomplete. We know how the electromagnetic and weak nuclear forces merge thanks to work with the Higgs. But we have yet to find a consistent, coherent theory of how the strong nuclear force merges with the others. And a fully unified force, with gravity included in a complete quantum description of nature, is well beyond our grasp.
String theory, however, is an attempt to unify all forces under a single framework. In string theory, the fundamental particles of our existence are instead seen as a collection of tiny, vibrating strings.
While string theory is not yet complete (and some argue it never will be), it allows researchers to develop tools to study difficult problems, such as the physics of the phase transition that might end the universe.
The authors of the new paper studied a version of string theory that included non-local effects – meaning that strings in one region of space can seemingly momentarily affect strings in another part of space despite their distance. (If you’re wondering how radical this idea is, it’s not that far-fetched: the phenomenon of quantum entanglement is also non-local.)
The researchers found that when the bubble expanded, the nonlocal effects in this version of string theory tended to smooth the bubble wall. In some cases, the wall of the bubble stretched so much that it completely dissolved. This means that if our reality were indeed metastable and undergoing a random phase transition, the basic string physics 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 tiny sizes.
This is still highly speculative physics, but at least it can provide a measure of comfort while we continue to understand the fundamental workings of the universe.
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