How big is Eridanus Supervoid? | Scale of the Universe

The Eridanus Supervoid is a vast region of space with fewer galaxies than usual, seen in the direction of the constellation Eridanus. It is roughly 2 billion light-years across, where a light-year is the distance light travels in one year. It lies about 3 billion light-years from Earth, in the same direction as an unusually cold patch in the faint glow left over from the Big Bang.

Size

A supervoid has no sharp edge. Astronomers measure its size by where the number of galaxies drops below the average. In 2015, a team led by István Szapudi of the University of Hawaii mapped galaxies in this part of the sky with the Pan-STARRS1 telescope and NASA's WISE satellite. They found a region with about 14% less matter than average.

The team's announcement described the supervoid as 1.8 billion light-years across. Their paper gives its radius as 220 megaparsecs, give or take 50, in units that depend on how fast the universe is expanding. A megaparsec is about 3.26 million light-years. With the expansion rate measured by the Planck satellite, that radius gives a width of about 2.1 billion light-years, somewhere between 1.6 and 2.6 billion.

Later studies disagree about its shape. A 2016 study found it stretched out along the line of sight: about 1.9 billion light-years wide across the sky, but about 4.8 billion light-years deep. A 2017 survey, which measured galaxy distances more precisely, found several smaller voids along the same line instead. Small regions with more galaxies than average lie between them.

Size comparisons

The supervoid is about 20,000 times as wide as the disk of the Milky Way, which is about 100,000 light-years across.
If the Milky Way were shrunk to the size of a basketball, about 24 centimeters (9.4 inches) across, the supervoid would be about 4.8 kilometers (3 miles) wide.
It is about twice the length of the Pisces-Cetus Supercluster Complex, the band of galaxy clusters that includes the Milky Way, which is about 1 billion light-years long.
The observable universe, the part of space from which light has had time to reach Earth, is about 93 billion light-years across. That is about 46 times the width of the supervoid.
Seen from Earth, the supervoid covers a patch of sky more than 30 degrees across, almost 60 times the width of the full Moon.

The Cold Spot

The cosmic microwave background is the oldest light in the universe, released when the universe was about 380,000 years old. Today it has a temperature of about 2.7 degrees above absolute zero, almost the same in every direction. In 2004, astronomers studying maps from NASA's WMAP spacecraft found a large patch in Eridanus that was colder than expected.

The center of this Cold Spot is about 70 to 150 millionths of a degree colder than average, depending on how it is measured. The European Space Agency's Planck satellite confirmed in 2013 that the spot is real and not a flaw in the measurements.

A supervoid could make light look colder. As light enters a void, it loses a little energy. Because the universe's expansion is speeding up, the void flattens out while the light crosses it, so the light does not regain all it lost on the way out. This is called the integrated Sachs-Wolfe effect.

An open question

The 2015 team suggested that the supervoid could explain the Cold Spot. The 2016 study estimated that its stretched-out void would cool the light by only about 40 millionths of a degree, not enough to account for the spot.

The 2017 survey concluded that the voids it found were far too small to explain the Cold Spot. Its authors suggested that the spot may instead date from the very early universe, rather than from voids between it and Earth.