Cosmic Voids

“our galactic neighborhood is enveloped by a cosmic void — an enormous, almost unfathomably empty pocket of space”

“sky surveys have spotted thousands more of these vacant bubbles”

“researchers have found a way to pull information out of these cosmic voids: By counting how many of them exist in a volume of space, scientists have devised a new way to explore two of the thorniest questions in cosmology”

“The first, and most perplexing, is the rate at which the universe expands, a value known as the Hubble constant”

“In addition, researchers have conflicting measurements of the clumpiness of cosmic matter —the average density of large-scale structures, dark matter, galaxies, gas and voids distributed throughout the universe as a function of time”

“Typically, astronomers measure those values in two complementary ways. Curiously, these two methods produce different values for both the Hubble constant and the so-called matter clustering strength”

“Pisani and her colleagues use cosmic voids to estimate both values. And their early results, which seem to agree much more closely with one of the traditional methods than the other, are now contributing their own complexities to an already fraught disagreement”

“Voids are regions of space that are less dense than the universe, on average. Their boundaries are defined by the immense sheets and filaments of galaxies that are woven throughout the cosmos”

“The bubbles have a tendency to expand because inside them, there isn’t much matter to exert an inward gravitational pull. The stuff outside them tends to stay away. And any galaxies that start inside a void get tugged outward by the gravitational pull of the structures defining a void’s edge”

“Because of this, in a void “very little happens,” Pisani said. “There are no mergers, no complicated astrophysics. This makes them very easy to deal with.””

“How empty does it have to be, and how do I measure it?”

“It turns out that the definition of “nothing” depends on the type of information astronomers want to extract”

“Pisani and colleagues started with a mathematical tool called a Voronoi diagram, which identifies the shapes that make up a 3D mosaic. These diagrams are typically used to study things like bubbles in foams and cells in biological tissues”

“In the current work, Pisani and her colleagues tailored their Voronoi tessellations to identify about 6,000 voids in the data from an enormous galactic mapping project called the Baryon Oscillation Spectroscopic Survey (BOSS)”

““Voids are complementary to the catalog of galaxies,” said Benjamin Wandelt, an astrophysicist at Sorbonne University in Paris who was not involved in the study. “They are a new way to probe cosmic structure.””

“Every cosmic void is a window on a great cosmic conflict. On one side, there’s dark energy, the mysterious force that causes our universe to expand ever more quickly. Dark energy is present even in empty space, so it dominates the physics of the void. On the other side of the conflict there’s gravity, which attempts to pull the void together. And then matter’s clumpiness adds wrinkles to the voids”

“a slower expansion rate produced a higher density of smaller, more crumpled voids”

“if expansion was faster and matter didn’t clump as readily, they expected to find more large, smooth voids”

“The group then compared their model predictions with observations from the BOSS survey. From this, they were able to estimate both clumpiness and the Hubble constant”

“They then juxtaposed their measurements with the two traditional ways to measure these values. The first method uses a type of cosmic explosion called a Type Ia supernova. The second relies on the cosmic microwave background (CMB), the radiation left over from the Big Bang”

“The void data revealed a Hubble constant that varied by less than 1% from the CMB’s estimate. The result for clumpiness was more muddled, but it also aligned more closely with the CMB than with Type Ia supernovas”

“Inside voids, structures never formed and evolved, so voids “are time capsules of the early universe”

“if the physics of the early universe was different from the physics of the present day, the voids may have preserved it”

“Even with thousands of voids, the study’s error bars are still too large to say anything conclusive”