The fate of the cosmos could lie in your bathtub. Or perhaps in your kitchen sink. At least that’s the view of Joseph Samuel and Supurna Sinha, physicists at the Raman Research Institute in Bangalore, India. They believe one of the most puzzling aspects of the universe could be explained by something as down-to-earth as soap bubbles.
It’s all to do with the cosmological constant, a measure of the energy inherent in empty space. This “vacuum energy” causes space-time to push outwards on itself, a phenomenon that astronomers believe explains why the universe is expanding at ever faster rates. Its value determines whether the universe will accelerate gently forever or eventually rip itself to pieces.
The trouble is, none of our best theories can explain the value of the cosmological constant: it is small, ridiculously small, but not zero. This has led Samuel and Sinha to draw an analogy between space-time and the surface tension of bubbles. It’s not as strange as it might seem. After all, “the cosmological constant is a kind of tension pervading space-time”, says Rafael Sorkin, a physicist at Syracuse University in New York state.
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Soap suds have been used to visualise the universe before. In 1986 astronomers Margaret Geller and John Huchra at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, analysed thousands of images of distant galaxies, revealing a structure that resembled a kind of cosmic foam. Geller compared it to a collection of soapy bubbles in a kitchen sink. The “bubbles” that make up this celestial foamy filament are hundreds of millions of light years across. Their “skins” – or surface membranes – are made of galaxies that enclose a volume of space, linked together into gigantic clusters by their mutual gravitational attraction.
It might be bubbles all the way down. Physicists from John Wheeler to Stephen Hawking have famously likened space-time at the smallest scales to a kind of quantum foam, in which tiny bubbles of virtual particles continually pop in and out of existence.
Now Samuel, a gravitational physicist, and Sinha, who specialises in “soft” condensed matter, are adopting a more literal take on this frothy concept. The inspiration for their analogy arose, in part, from an invitation to submit scientific contributions in honour of Sorkin’s 60th birthday two years ago.
Back in 1987, Sorkin tackled the problem of how to unite general relativity with quantum mechanics in order to explain gravity at the smallest possible scales. This led him to develop an approach called causal set theory. Samuel and Sinha noticed striking parallels between certain aspects of Sorkin’s theory and the bubbles in foam. “In putting our discussion in print, we realised how good the analogy was,” says Samuel. “We thought we could exploit it to gain insight into both fields.”
Causal set theory and general relativity differ in their basic descriptions of space-time. General relativity treats it as a smooth four-dimensional fabric, with gravity being a result of its geometry or curved shape. The fabric is frozen in the sense that it does not evolve: the past, present and future are all laid out in space-time. Causal set theory argues that space-time can be dynamic and evolves according to the order in which things happen in time. This is called the causal order of space-time. Sorkin’s theory says we don’t need to know the geometry of space-time. Simply by knowing the causal order of all points within it, we can almost reconstruct it from scratch.
Because causal set theory is a quantum approach to gravity, it assumes that space-time will come in discrete chunks or quanta and orders these elements into sets called causets. They are similar to the branching networks of a family tree, except instead of being related by blood, the elements in the causet tree are related by causation. So as space-time grows, element by element, the quanta link together like bubbles in an ever-evolving and expanding foam.
Although causal set theory is not yet a full quantum gravity theory, it is its predictions for the cosmological constant that set it apart from more popular attempts to unite gravity and quantum mechanics, namely string theory and loop quantum gravity. Other theories predict values for the cosmological constant that are either so high the universe would long have torn itself apart or zero. Sorkin’s theory happens to give a value that ties in well with observations made in 1998 of the expansion rate of the universe and its acceleration. This is due to the quantum nature of the causets, which allows the vacuum energy to fluctuate by tiny amounts.
More than froth
Despite this success, causal set theory attracts the same criticism frequently lobbed at string theory. “It is beautiful mathematics without a shred of experimental evidence,” says Sidney Perkowitz of Emory University in Atlanta, Georgia, an expert on the physics of foam.
Samuel and Sinha are hoping to change all that. To start, they have proposed a theoretical model of soap suds that could yield useful insights into causal set theory. Every bubble in a foam is made of a thin membrane enclosing a volume of gas. Surface tension along the membrane makes the bubbles clump together, forcing them to adopt a shape with the smallest surface area possible in such a clump, somewhere between a sphere and a cube.
In this model, the causet elements are like the molecules in the surface membrane of a soap bubble. As well as drawing parallels between surface tension and the cosmological constant, this likens the length of soap molecules to the smallest possible subdivision of space-time allowed by quantum mechanics, the Planck length (10-35 metres).
Samuel and Sinha’s calculations reveal a critical insight: the surface tension in foam fluctuates before stabilising to zero, just as causal set theory predicts will happen billions of years ahead for the cosmological constant.
It’s a promising start, but not everyone is convinced. Cosmologist Joe Polchinski of the Kavli Institute for Theoretical Physics in Santa Barbara, California, is sceptical about the usefulness of soap bubbles. “I am leery of analogies because they often miss key aspects of a problem,” he says. For instance, while some researchers have compared the cosmos to an expanding bubble of space-time – or to a balloon filling with air – this can be a bit misleading. Unlike real bubbles or balloons that expand or pull in particular directions, the universe doesn’t expand “into” anything, and the push or pull is uniform in every direction. Also, bubbles and balloons eventually pop – a fate no one wants for our universe.
Perkowitz is intrigued by the fact that random bubbles of space-time could lead to accelerated expansion in the cosmos. But he points out that no known physical mechanism for the interaction of such space-time elements could produce a cosmological constant that is on the point of vanishing.
To address this, Samuel and Sinha have designed experiments they believe could be used to test Sorkin’s predictions in the laboratory. These would use lipid membranes of the sort found in ordinary shaving foam or beaten eggs because they have a nearly vanishing surface tension, just like Sorkin’s predicted value for the cosmological constant. They believe that isolating the mechanism behind the membrane’s vanishing surface tension could yield insights into the mysterious mechanism propelling cosmic expansion.
As for Sorkin himself, he considers the analogy “a beautiful idea that opens up the possibility to test the causet explanation of acceleration”. In fact, Samuel and Sinha suspect that their concept might not be unique to causets and that other approaches to quantum gravity could also predict a tiny cosmological constant.
If they’re right, shampooing your hair could take on a whole new meaning. The key to the nature of ever-expanding universe could be at your fingertips.
“Shampooing your hair could take on a whole new meaning”
