The bright and invisible universe


Optical imaging reveals glowing hydrogen as far as the eye can see. Andrew Masterson reports.


Imaging using the the MUSE spectrograph on the European Southern Observatory's Very Large Telescope reveals a universe full of gently glowing hydrogen clouds.

Imaging using the the MUSE spectrograph on the European Southern Observatory's Very Large Telescope reveals a universe full of gently glowing hydrogen clouds. 

ESA/Hubble & NASA, ESO/ Lutz Wisotzki et al

Seen through the lens of the Multi Unit Spectroscopic Explorer (MUSE) spectrograph on the European Southern Observatory’s Very Large Telescope, in Chile, the vast expanse of the night sky is revealed to be glowing – the result of excited electrons in hydrogen atoms dating from the very early universe.

Invisible to the naked eye, the glow is the result of a process known as Lyman-alpha emission – in which electrons in the vast clouds of hydrogen that suffuse the universe are kicked from low to higher energy states.

The existence of Lyman-alpha emissions has been previously well established, but it wasn’t until an international team of researchers used the MUSE to map its prevalence in distant galaxies that its virtually ubiquitous distribution was revealed.

“Realising that the whole sky glows in optical when observing the Lyman-alpha emission from distant clouds of hydrogen was a literally eye-opening surprise,” explains one of the researchers, Kasper Borello Schmidt from the Leibniz-Institut für Astrophysik Potsdam in Germany.

“This is a great discovery!” added team member Themiya Nanayakkara from Leiden University in The Netherlands.

“Next time you look at the moonless night sky and see the stars, imagine the unseen glow of hydrogen: the first building block of the universe, illuminating the whole night sky.”

The research is published in the journal Nature.

  1. https://www.eso.org/sci/facilities/develop/instruments/muse.html
  2. http://w.astro.berkeley.edu/~jcohn/lya.html
  3. https://www.nature.com/articles/s41586-018-0564-6
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