If you asked an astrophysicist what is yet to be discovered in the universe, there’s a good chance they’d paraphrase Donald Rumsfeld: It’s full of known unknowns.
Besides the regular matter and energy we know and love, the universe is also composed of two big mysteries: dark energy and dark matter. Theoretical astrophysicists aim to tackle these unknowns by mathematically dreaming up objects that could address the gaps in our knowledge. Thus, in research published in Classical and Quantum Gravity, a team of astrophysicists propose an exotic type of object called a “nestar” as a solution to a blind spot of Einstein’s general theory of relativity.
A nestar is similar to a gravastar, which is short for gravitational vacuum condensate star. The gravastar is a theoretical object proposed in the early 2000s that has an exterior of super-thin matter and a core made up of dark energy.
Like the gravastar, the nestar is an extreme theoretical object, but with a twist. In their work, researchers Daniel Jampolski and Luciano Rezzolla propose that gravastars could be nested—hence, “nestars—and “extended to an arbitrarily large number of shells,” they write, yielding an object that offers “a new solution of the Einstein equations.”
“The nestar is like a matryoshka doll,” said Jampolski, a researcher at the Institute for Theoretical Physics in Frankfurt, in a Goethe University Frankfurt am Main release. “Our solution to the field equations allows for a whole series of nested gravastars.”
Einstein devised his equations over a century ago; over the intervening decades, discoveries by other researchers proved them out. In 1916, he predicted the existence of black holes, and in 1971, a black hole was identified for the first time. But at the center of a black hole, general relativity breaks down. At the cores of these objects, with gravity so strong that not even light can escape them, lies a point where time slows to a halt and space is compressed into a point of infinite density.
Enter gravastars: Nearly as compact as black holes, with similar gravitational force on their surfaces. However, gravastars lack an event horizon—meaning outside observers can receive information from them—and their core does not have a singularity. Rather, their (again, theoretical) cores are made of dark energy, which counteracts the immense gravitational force pulling matter inward toward the core.
Exact solutions to Einstein’s field equations in the general theory of relativity have been found before; in fact, the first one was found by Karl Schwarzschild the same year Einstein introduced the theory. Nestars present a new solution to the old problem… if they exist, of course.
“It’s great that even 100 years after Schwarzschild presented his first solution to Einstein’s field equations from the general theory of relativity, it’s still possible to find new solutions,” said Rezzolla, a theoretical astrophysicist at Goethe University, in the same release. “It’s a bit like finding a gold coin along a path that has been explored by many others before.”
“Unfortunately, we still have no idea how such a gravastar could be created,” Rezzolla added. “But even if nestars don’t exist, exploring the mathematical properties of these solutions ultimately helps us to better understand black holes.”