The universe may seem shapeless because it’s so vast, but it has a shape that astronomers can observe. So what is its shape?
Physicists believe that universe is flat. Several clues point to this flat universe: the light left by the big Bangthe rate of expansion of the universe in different places and how the universe “appears” from different angles, experts have told Live Science.
David Spegel (opens in a new tab), a theoretical astrophysicist and professor emeritus of astrophysical sciences at Princeton University, has probed the shape of the universe for decades. In a 2003 study published in The Astrophysical Journal (opens in a new tab)Spergel measured irregularities in the cosmic microwave background (CMB), the afterglow of the Big Bang, which was observed by NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) and later by the Space Agency’s Planck spacecraft European.
The amounts of positive and negative energy in a flat universe are exactly the same and therefore cancel each other out. If the universe had a curvature, one would be higher than the other. “A flat universe corresponds to a universe without energy,” Spergel told Live Science.
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In this case, WMAP measurements of CMB fluctuations suggested that the universe was both infinite and flat. Spergel also compared these measurements with those made by the European Space Agency’s Planck spacecraft, which further limited the possible shapes the universe could take.
“We can measure curvature with some uncertainty, so we can say curvature is zero with some uncertainty,” Spergel said. “Although we can reduce the uncertainty, we restrict the geometry as best we can.”
Another reason why Spergel is convinced that the universe is flat is its rapid expansion, which is captured by the Hubble constant. Because the universe has gone from birth to a compact ball of question to expand outward at remarkable speeds, all that stretching made it flat, or at least as flat as possible.
The proof of the flatness of the universe also appears in what is called the critical density. At critical density, a hypothetical universe would be flat and eventually stop expanding, but only after an infinite time, according to the Swinburne University of Technology (opens in a new tab) in Australia. If a hypothetical universe were denser than this, it would be curved like a sphere and eventually collapse in on itself due to its gravity – a proposed phenomenon known as “Big Crunch”.
But all measurements of our real universe suggest it is just below critical density, which means the universe is both flat and will expand indefinitely.
Yet another source of evidence suggests that the universe is flat: it’s isotropic, meaning it looks alike from every angle. Anton Chudaykin (opens in a new tab)a physicist at the Institute of Nuclear Research in Russia, and his colleagues examined data on oscillations in regular or “baryonic” matter, as well as models of how atomic nuclei heavier than hydrogen were created shortly after the Big Bang, to estimate the curvature of the universe.
“In different geometries, matter and light evolve differently, which allows us to extract the three-dimensional shape of the universe from observational data,” Chudaykin told Live Science.
The research, published in the journal Physical examination D (opens in a new tab), found that with an accuracy margin of 0.2%, the universe was flat. “The data we collected indicate that spatial curvature is consistent with zero,” the researchers wrote in the study. “This implies that our universe in statistical uncertainty is infinite.”