Read the first part of this research, Much Ado About Nothing, here.
The Not-So-Empty Vacuum
Nothing matters in cosmology. One of the biggest cosmic conundrums is the source of “dark energy,” the invisible unknown entity believed to be propelling empty space to expand at an ever-faster rate (see Part 1). Many physicists believe that dark energy is related to the nature of “nothing,” specifically to properties of the vacuum of empty space. Still others argue that pinning down the physical meaning of nothing has implications for understanding how the universe was born, how the cosmos will evolve—and whether empty space even exists, at all.
Currently, there is no consensus on dark energy’s identity. But many physicists believe its power to thrust space outwards is intrinsically tied to the bizarre nature of “nothing,” as laid out by the rules of quantum physics, the strange laws that govern the microscopic realm. “Why there must be some energy, rather than nothing, goes deep into quantum mechanics,” says cosmologist Michael Vogeley of Drexel University in Philadelphia.
“Essentially every quantum-mechanical system has a lowest energy state that is not zero,” explains Vogeley, and that includes the empty vacuum. Thanks to this energy, even the empty vacuum is in a constant state of writhing flux. These vacuum fluctuations may average out to zero, but every so often they can give rise to particles that appear fleetingly and then pop out of existence again, like spray from the surface of the ocean. “There are particles appearing and disappearing,” adds Miguel Aragon-Calvo, a cosmologist at the Public University of Mexico City (UNAM).
“There is a lot going on inside this apparently empty space.”
“Nothing,” in the quantum world, can push back.
It’s in this sense—as the result of quantum effects—that physicists sometimes say that particles can be created from nothing. But here the use of the word nothing is a misnomer, says philosopher of physics David Albert, of Columbia University in New York, New York. He uses the analogy of creating a fist by curling your fingers into your palm. “A fist is a certain configuration of fingers,” says Albert. “When I open my fingers again, there are no fists in the room, but that doesn’t mean that there is nothing in the room—the fingers are still there.” Similarly, there is a configuration of the quantum vacuum that corresponds to the number of particles in all of space being zero. But even when there are no particles in empty space, there is the vacuum, which is a real physical thing.
Even empty space, devoid of matter, cannot truly represent “nothing,” then.
The quest for the meaning of physical nothingness may need to push back further still, to before the Big Bang and the creation of the vacuum of empty space.
A Universe from Nothing
In 1982, Alex Vilenkin, a cosmologist at Tufts University in Medford, Massachusetts, published the provocatively titled paper, “Creation of Universes from Nothing.” His quantum calculations had shown that a bubble of vacuum could burp into existence. Speaking to me for my book A Big Bang in a Little Room, a few years ago, Vilenkin described how this represented the spontaneous creation of the vacuum of space, along with the creation of time and matter, from a state bereft of all three—a state that is perhaps as close to real nothingness as humans can comprehend.
Was this, then, a physical mechanism describing creatio ex nihilo?
No, argues Albert. “It’s not the profound sense of ‘nothing’ that people have been thinking about in the long tradition of Western philosophy and theology,” he explains. The universe may conceivably have been born from a clean slate that originally contained no vacuum of space, no time and no matter, but that clean slate still had to exist, even if it is impossible to visualize because it has no spatial or temporal extent. “There is still some other kind of physical substratum, which has two different states: one corresponding to there being space and one corresponding to there not being space,” Albert says.
Vilenkin had also conceded, when we spoke in the past, that his conception of nothing had its limitations: “Well, ‘nothing,’ of course, I had in quotation marks, because you still need the laws of physics to govern this process,” he had told me.
So even here, before space or time, the concept of “nothing” remains slippery, because the laws of physics were in existence and acted on some invisible, inconceivable physical substratum.
Something and Nothing
These musings on the nature of nothingness may seem purely academic, but being fooled that nothing is actually something could have deep consequences when it comes to understanding—or dramatically misunderstanding—the universe, says Julian Barbour, a former visiting professor in physics at the University of Oxford, UK. He feels uncomfortable that so many cosmological models are built on the assumption that dark energy arises from empty space, which, he notes, physicists cannot measure directly.
“We don’t see space,” says Barbour. “We see stars.”
Indeed, even the math associated with the quantum vacuum of empty space doesn’t truly line up with observations of the dark energy it is meant to be causing. When physicists calculate how large the vacuum energy of empty space should be, using quantum physics, they come up with a value that is 10^120 (1 followed by 120 zeroes) times larger than the measured value of the dark energy, causing the universe to barrel outwards at an ever-increasing pace. So while many physicists believe that dark energy is somehow related to the vacuum energy of empty space, the numbers really don’t add up. “This tells you that we don’t understand something deep about physics,” says Vogeley.
That lack of understanding could be due to ascribing too much power to nothing. “The whole theoretical framework of cosmology is built on the idea that empty space is literally there and nobody challenges that,” says Barbour.
Barbour has spent years reformulating the laws of physics in a way that does not presuppose the existence of empty space, having been inspired by the works of seventeenth-century German polymath Gottfried Leibniz and nineteenth-century Austrian physicist Ernst Mach. In Barbour’s model, all measurements of observable objects are made in relation to other observable objects, rather than against an unseen intangible background space. In such a framework, it is simply meaningless to say that the fabric of empty space is being stretched by some mysterious dark energy. All that can be examined is how the shapes and configurations drawn out by stars and galaxies shift about each other, due to forces between them.
Albert notes that this “relationist” tradition stretches back to Aristotle and is philosophically well-motivated. “If a theory refers to things that you can’t see and that you can’t measure, then a natural question to ask in science is ‘can I reformulate the theory in such a way as to not mention them?’” says Albert. He adds that it’s not clear whether Barbour will ultimately be successful in this endeavor to rid physics of empty space, but he sympathizes with the thinking behind the effort.
Illuminating the Dark
Only time, and a careful analysis of more astronomical data, will tell if Barbour is right to do away with space entirely, whether the more mainstream approach of identifying dark energy with the quantum vacuum is correct, or if some completely different possibility holds true. This is why astronomers are making efforts to accurately map cosmic voids—the vast tracts of seeming nothingness that lie between galaxy clusters (see Part 1).
Voids are the most sensitive testing grounds for rival cosmic models, explains Vogeley. This is because within galaxy clusters, there is too much matter and gravity pulling inwards and countering dark energy’s outward push, for dark energy to make a noticeable difference. By contrast, within voids, there is very little matter or gravity to stem dark energy’s influence, so its effects are amplified. “The primary effect of dark energy is to blow up the voids that lie in between the galaxy clusters and filaments of the cosmic web,” says Vogeley. “Clusters accelerate away from each other because the voids are expanding between them.”
The aim, then, is to see how the predictions for void patterns made by computer simulations of different cosmic models compare with the real patterns seen in the sky. But analyzing the humongous amount of observational data collected from telescopes is a monumental task. For instance, DESI, the Dark Energy Spectroscopic Instrument in Tucson, Arizona, is surveying 15 million galaxies to create the largest 3-D map of the universe to date.
Most previous approaches for finding voids in large catalogues of telescope data were not very reliable, says Vogeley. The computer programs used to analyze these huge data sets worked by effectively counting the number of galaxies within a set region, then classifying the area as a ‘void’ if it didn’t contain much matter. But depending on how a “void” was defined, different astronomers using different programs could end up with vastly different results for the pattern of voids, even when using the same galaxy data.
Deep Void
That’s why Vogeley, Aragon-Calvo, and their colleagues have now developed “DeepVoid”—a deep-learning neural network, or artificial intelligence (AI), that has been trained to identify voids, not by counting the galaxy dots, but in a way that more closely mimics how humans process images and intuitively spot large gaps in a web. “We train it on a very rich dataset, with a lot of density, so that it can find the voids, and then next time the AI sees a sample that is very sparse, it knows, ‘ok I have seen this before, I know what kind of structure it corresponds to’,” says Aragon-Calvo. The team announced the tool in April 2025.
Vogeley and Aragon-Calvo describe themselves as agnostic as to what the eventual solution to the mystery of dark energy’s origin will turn out to be. Is the dark energy due to the vacuum of empty space? Do physicists need to do away with the concept of empty space entirely in order to understand cosmic evolution? Or will the answer involve one of myriad other alternatives that have been proposed over the years?
“I don’t have any prejudice with regard to what the dark energy is,” says Vogeley. “The beautiful thing is that we can learn what it is—that’s the joy of cosmology and of human exploration.”
And that’s not nothing.
Zeeya Merali is a London-based science journalist and author of the popular physics book, A Big Bang in a Little Room.
