Huge Patch of Universe is Strangely Opaque Despite Its Lack of Galaxies

By Ryan F. Mandelbaum on at

The mere fact that we live in a universe boggles my mind every once in a while. But thankfully, our cosmic home is a place that follows rules; the laws of physics seem to agree everywhere, and galaxies are uniformly distributed throughout. Except for in this 300-million-light-year-long region, which seems to be missing something.

Above: Computer simulation of the cosmic web
Graphic: TNG Collaboration (Eurekalert)

Scientists observed an opaque region of space in front of a quasar, an extra bright source of light, in the far distance. They realised that, confusingly, this light-blocking region had fewer galaxies than they expected.

The region sits in front of a quasar called ULAS J0148+0600 whose light left on its journey to us when the Universe was less than a billion years old. This area seems to absorb light for a stretch of over 300 million light-years. The team of scientists used the Subaru telescope in Hawaii to hunt for distant galaxies around the opaque region, and found far fewer than average, according to the paper published in The Astrophysical Journal. This is the opposite of what you’d expect—shouldn’t an opaque area have more matter?

This seeming inconsistency could help scientists better understand the nature of the early cosmos.

“It gives us a chance to study a place in the universe lagging behind the rest of the universe,” study author George Becker, from the University of California, Riverside, told Gizmodo.

Astronomers view distant objects via their “Lyman alpha” emissions, light emitted by hydrogen atoms with a characteristic wavelength. The light traverses the expanding universe, causing its wavelength to stretch along the way. But stuff in the intervening space, specifically neutral atoms, can absorb the light and cause certain wavelengths to go missing from the signal received on Earth. By plotting the wavelengths of light coming from a distant quasar, you can make a map and timeline of the intervening stuff.

Becker and his team explained that electrically neutral gas absorbs the Lyman alpha light, but ionized gas, atoms that have missing electrons, does not. The region of the universe nearest to us is relatively uniform, and there’s lots of ultraviolet light to keep the gas ionized and prevent absorption. But in the early universe, things were perhaps not as uniform. Maybe places with more galaxies had more ultraviolet radiation, meaning more ionization, and those areas were, therefore, more transparent. But regions like the one in front of this quasar had fewer galaxies, meaning less ionization, and were therefore opaque.

It’s an interesting observation that’s counter-intuitive in several ways. “The exciting bit is that this is a very large structure over which there’s something different,” Martin Haehnelt, an astronomer at the University of Cambridge who was not involved with the study, told Gizmodo. “There should be the same number of galaxies in one region as in another,” yet in this case there are not.

Becker told Gizmodo that perhaps this region tells a story of the re-ionization epoch—the period early in the universe’s history after the first stars formed and exploded in supernovae, ionizing neutral gas molecules. Perhaps certain regions of the sky entered this period later than others.

There’s more work to be done to explain the mystery, and this region is just one skewer of the enormous sky. If this idea is correct, then regions of the sky with more galaxies at this distance should have less Lyman alpha light absorption.

But while the laws of physics remain constant, there really do seem to be parts of the universe different from the rest.

[ApJ]