To a sound wave, the cosmos has the consistency of chocolate syrup.
That’s one discovery that scientists investigating the Big Bang have made using a new approach that treats the matter in the universe as a peculiar kind of fluid. They have calculated properties that characterize the universe’s behavior and evolution, including its viscosity, or resistance to deformation by sound waves and other disturbances.
“Twenty pascal-seconds is the viscosity of the universe,” said Leonardo Senatore, an assistant professor of physics at Stanford University — just as it is for the ice cream topping.
The viscosity calculation could help cosmologists sleuth out the details of the Big Bang, and possibly someday identify its trigger, by enabling them to track the fluidlike flow of the cosmos back 13.8 billion years to its initial state.
As other techniques for probing the Big Bang reach their limits of sensitivity, cosmologists are co-opting the fluid approach, called “effective field theory,” from particle physics and condensed matter physics, fields in which it has been used for decades. By modeling the matter swirling throughout space as a viscous fluid, the cosmologists say they can precisely calculate how the fluid has evolved under the force of gravity — and then rewind this cosmic evolution back to the beginning. “With this approach, you can really zoom in on the initial conditions of the universe and start asking more and more precise questions,” said Enrico Pajer, a postdoctoral research fellow at Princeton University with a recent paper on the technique that has been accepted by the Journal of Cosmology and Astroparticle Physics.
The more information that astronomers gather about the distribution of galaxies throughout space — known as the “large-scale structure” of the universe — the more accurate the fluid model becomes. And the data are pouring in. The sketchy scatter plot of several thousand nearby galaxies that existed in the 1980s has given way to a far richer map of millions of galaxies, and planned telescopes will soon push the count into the billions. Proponents believe that tuned with these data points, the fluid model may grow precise enough within 10 or 15 years to prove or refute a promising Big Bang theory called “slow-roll inflation” that says the universe ballooned into existence when an entity called an inflation field slowly slid from one state to another. “There has been a big community trying to do this type of calculation for a long time,” said Matias Zaldarriaga, a professor of cosmology at the Institute for Advanced Study in Princeton, N.J. Further in the future, the researchers say, applying effective field theory to even bigger datasets could reveal properties of the inflation field, which would help physicists build a theory to explain it.
“It’s obviously the right tool to be using,” said John Joseph Carrasco, a theoretical physicist at Stanford. “And it’s the right time.”
Senatore, Carrasco and their Stanford collaborator Mark Hertzberg first proposed the fluid approach to modeling the universe’s large-scale structure in a 2012 paper in the Journal of High Energy Physics, motivated by the Big Bang details it could help them glean from the increasingly enormous data sets. Other researchers have since jumped on board, helping to hone the method in a slew of papers, talks and an upcoming workshop. “We’re a small, plucky band of people who are convinced this is the way forward,” said Sean Carroll, a theoretical cosmologist at the California Institute of Technology.
A Fluid Cosmos
In water, chocolate syrup and other fluids, matter is smoothly distributed on large scales and partitioned into chunks, such as atoms or molecules, on small scales. To calculate the behavior of water on the human scale, where it is a fluid, it isn’t necessary to take into account every collision between H₂O molecules on the atomic scale. In fact, having to do so would render the calculation impossible. Instead, the collective effects of all the molecular interactions at the atomic scale can be averaged and represented in the fluid equations as “bulk” properties. (Viscosity, for example, is a measure of the friction between particles and depends on their size and shape as well as the forces between them.)
A similar trick works for modeling the evolution of the universe’s large-scale structure.
Just like water, the universe is smooth on large scales: The same amount of matter exists in one billion-light-year-wide region as the next. Slight variations in the matter distribution, such as more- and less-dense patches of galaxies, appear when you zoom in. At short distances, the variation becomes extreme: Individual galaxies are surrounded by voids, and within the galaxies, stars pinprick empty space. The matter distribution is constantly changing at every scale as gravity causes stars, galaxies and galaxy clusters alike to clump together and dark energy stretches the space between them. By modeling these changes, cosmologists can use the output — galaxy distribution data — to deduce the input — the initial conditions of the universe.
To a first approximation, the matter distribution at each distance scale (from large to small) can be treated as if it evolves independently. However, just as small ripples in the surface of water can affect the evolution of bigger waves, smaller clumps of matter in the universe (such as galaxy clusters) gravitationally influence the larger clumps that encompass them (such as superclusters). Accounting for this interplay in models of cosmic evolution is problematic because the gravitational effects at the shortest distance scales — at which the universe is not smooth like a fluid but rather condensed into isolated, particlelike objects — sabotage the calculation.
Effective field theory fixes the problem by accounting for the interplay between scales only down to a few times the distance between galaxies. “Everything smaller than that length scale, we treat as complicated and hard to understand, and whatever goes on at those small scales can be bundled up into one big effect,” Carroll explained. The average gravitational effect of matter on small scales is represented as a fluid’s viscosity; hence, the connection between the cosmos and chocolate syrup.
Although the former is sparse and cold while the latter is thick and usually served warm, their viscosities are calculated from data and simulations to be almost exactly equal. The number means both fluids immediately damp out an incident sound wave. “It just goes ‘dum,’ and then it disappears,” Pajer said.
The Ultimate Probe
“It’s still early days for the effective field theory of large-scale structure,” said Marc Kamionkowski, a professor of physics and astronomy at Johns Hopkins University who is not involved in developing the approach. While “it certainly does present some advantages,” he said, much work is needed before the tool can be used to extract new discoveries from astronomical data.
For example, so far, cosmologists have only developed an effective field theory model of the evolution of dark matter, an invisible substance that makes up roughly six-sevenths of the matter in the universe. Visible matter is slightly more complicated, and researchers say its behavior on short distance scales might be more difficult to represent as bulk properties of a fluid. “That is the next challenge,” said Zaldarriaga, who co-authored a November 2013 paper on the effective field theory approach. “We are doing one thing at a time.”
The researchers’ ultimate goal is to measure so-called “non-Gaussianities” in the initial conditions of the universe. If inflation theory is correct and an inflation field briefly transitioned to an unstable state, causing space to balloon 1078 times in volume, random ripples of energy called quantum fluctuations would have surfaced in the field and later grown into the large-scale structure that exists today. These ripples would be expected to follow a “Gaussian” distribution, in which energy is evenly distributed on both sides of a bell curve. Cosmologists look for non-Gaussianities, or subtle biases in the energy distribution, as signs of other, more meaningful events during inflation, such as interactions between multiple inflation fields. The recently released Planck satellite image of the cosmic microwave background indicated that energy fluctuations in the primordial universe followed a Gaussian curve to at least one part in 100,000, compatible with the slow-roll model in which the universe arose from a single inflation field. But alternative models that would have produced even smaller amounts of non-Gaussianity have not yet been ruled out.
By tuning the effective field theory model with galaxy distribution data from imminent sky surveys such as the Large Synoptic Survey Telescope project and Euclid mission, cosmologists estimate that it may be possible to improve detection of non-Gaussianities by a factor of 10 or 20. If none is detected at that sensitivity level, “we can be sure it is standard slow-roll inflation,” Senatore said. “This is extremely exciting.”
If it can be proved that the Big Bang began with slow-roll inflation, the next task would be to probe the properties of the “inflaton” — the particle associated with the inflation field, and a component of an all-encompassing theory of nature. During inflation, the inflaton must at least have interacted with itself and gravity, and both interactions would nudge the inflation field’s energy distribution ever so slightly to one side or another. Planned sky surveys will not be sensitive enough to detect such subtle non-Gaussianities, but researchers expect them to be imprinted on a signal emitted by hydrogen gas in the early universe. “This is the ultimate probe,” Pajer said.
Telescopes should detect this hydrogen signal, called the 21-centimeter line, in approximately 30 or 40 years, and effective field theory will be used to try to tease out the non-Gaussianities. “While we’re old,” said Senatore, who is 35, “we will for sure detect something.”
Reprinted with permission from Quanta Magazine, an editorially independent division of SimonsFoundation.org whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.