Theoretical study predicts "awakening" of the vacuum with macroscopic effects

A model proposed by Brazilians has undergone successive refinements and could contribute to explaining “dark energy”.

By José Tadeu Arantes

Agência FAPESP – For physicists, the notion of a vacuum is markedly different from the commonsense understanding of empty space. Physicists find that empty space has structure and teems with activity in the form of quantum fluctuations that become more marked through the production and obliteration of virtual particles. This notion, which arises directly from quantum theory, is highly familiar to scientists in the field. In fact, one of the consequences of the fluctuations of the vacuum, the Casimir effect, has already been observed and measured in the laboratory.

The new theoretical possibility is that these fluctuations, once considered to be too subtle to exert any macroscopic effect, may be amplified to the point where their energy exceeds the energy of the concentrations of mass and produces results such as the destruction of a star.

The discovery, made from a purely mathematical theoretical approach and yet to be confirmed through observation, is one of the principal findings of the recently concluded FAPESP-funded thematic project “Physics in curved spacetimes” coordinated by George Emanuel Avraam Matsas, full professor at the Institute of Theoretical Physics of the Universidade Estadual Paulista (IFT-Unesp).

“Right in the middle of the project, in approximately 2010, Daniel Augusto Turolla Vanzella, professor and PhD in the Physics Institute of São Carlos at the University of São Paulo (IFSC-USP), and William Couto Corrêa de Lima, at that time the doctoral student of Prof. Vanzella, discovered that if there is a certain class of quantum fields, the density of the vacuum energy of these fields will grow exponentially in the vicinity of a very dense clump of matter,” Matsas told Agência FAPESP.

The work of Vanzella and Lima, entitled “Gravity-Induced Vacuum Dominance”, was published in the journal Physical Review Letters. It established the bases of this process in such generality that it was possible to apply the knowledge to several areas of scientific research, including cosmology and astrophysics.

“Later, I got together with both of them, and we applied the effect to a particular gravitational field created by a neutron star. This enabled us to understand the phenomenon more clearly,” added Matsas. A new article, entitled “Awaking the Vacuum in Relativistic Stars”, by Lima, Matsas and Vanzella, was then published in Physical Review Letters.

Neutron stars

There are currently nearly 2,000 neutron stars in the Milky Way (the galaxy to which the Solar System belongs) and in the Magellanic Clouds (galaxies near ours). They consist of extremely dense objects with masses of more than one and one-half times the solar mass squeezed into a spherical body with a radius of only a few dozen kilometers. Their gravitational fields are so intense that they strongly curve spacetime in the regions in which they are found, according to general relativity. It is this curving that distorts the quantum vacuum, causing the vacuum energy density to grow exponentially.

“If there were a hypothetical field in which to base our model, when the neutron star becomes sufficiently dense, the fluctuations of the vacuum would grow so that in just a short time there would be, point to point, more vacuum energy than energy from the star itself. And this vacuum energy would curve spacetime even more, leading to the star’s possible destruction at the edge,” Matsas said.

Why does the distortion of spacetime amplify the vacuum fluctuations to the point that they produce macroscopic and even catastrophic effects? “We don’t have a good answer for that, and I doubt we ever will,” Vanzella responded. “If there was an intuitive way to understand this effect, other people would have made the discovery years ago. When people do the math, the effect appears. It was a strictly mathematical approach to equations that led us to discover this theoretical possibility,” he went on to say.

The researcher reported that the first mathematical finding that signaled the effect appeared in a side calculation performed in a study he conducted during his doctoral work. This intriguing finding was literally hibernating in a drawer until a decade later, when he managed to apply it and realize that one of its consequences was the growth of vacuum fluctuations.

The first two articles in Physical Review Letters were followed by a third article, this one published in the journal Physical Review D, written in collaboration with Andre Gustavo Scagliusi Landulfo, currently an assistant professor at the Federal University of the ABC (UFABC), entitled “Particle creation due to tachyonic instability in relativistic stars”.

“The third study began from the belief that the growth in density of the vacuum energy unleashed by a neutron star cannot continue indefinitely because, since this energy retroacts in the spacetime, curving it even more at the edges, it could lead to the collapse of the universe. At some point, something has to occur to stabilize the system. We’ve theoretically determined that when the process is interrupted and a new state of equilibrium occurs, part of the existing vacuum energy is released in the form of actual particles that escape the system. In this case, then, there would be a profusion of particles,” Vanzella explained.

According to Matsas, in the actual event of a collision between two neutron stars, for example, the energy balance of this production of particles would provide an observational condition that would confirm the proposed model.

Given the major difficulties involved in the calculations, the model was built based on various simplifications: spacetime was conceived as a static reality, the retroaction of the spacetime vacuum energy was not calculated and the star was designed as a perfectly spherical non-rotating object. Of course, this is not how things really are. However, the researchers determined that, even in this ultrasimplified scenario, the effect was already occurring. Later studies conducted with the collaboration of Raissa Mendes, a doctoral student of Matsas, attempted to approximate an actual situation even more closely by theoretically investigating what would occur if the star was not perfectly symmetrical or if it rotated.

Two articles from this stage of the study have already been published in Physical Review D: “Awakening the vacuum with spheroid shells” and “Quantum versus classical instability of scalar fields in curved backgrounds”. A third article is undergoing mathematical review for publication.

Cosmological context

The task of analyzing this effect in the context of cosmology as a possible explanation for the accelerated expansion of the Universe, one of cosmology’s most important enigmas, is something that is on Vanzella’s list of priorities. “I started thinking about this when I did my post-doc under the supervision of Leonard Parker at the University of Wisconsin in Milwaukee. It was Parker who developed quantum field theory in curved spacetimes, in which we were working. And he thought that the so-called ‘dark energy’ responsible for the accelerated expansion of the Universe could be a vacuum energy.”

The possible application of the effect discovered by the Brazilians in the cosmological scenario would be a spectacular achievement. However, the effect itself is already a remarkable discovery. “The possibility that spacetime curvature exacerbates the vacuum fluctuations is neither trivial nor intuitive. We were surprised to discover it,” said Matsas.

“Even though the total vacuum energy was zero and remained zero, the fluctuations cause the energy to present locally extreme variations, increasing or decreasing exponentially. Quantum vacuum effects were expected but with much more subtle expression. What Vanzella and Lima have shown was that these effects can assume catastrophic proportions,” the researcher said.