How dark is the universe? – New measurement of the light from the sky speaks for fewer galaxies in the cosmos than expected


Diffuse glow:  The  cosmos could be darker and fewer galaxies than previously thought. This is indicated by measurements of the optical background lighting with the New Horizons space probe. Because it moves its orbit beyond Pluto, it can observe the sky without light pollution. According to your measurements, space is ten times darker than assumed. Instead of around two trillion galaxies, the universe could contain only a few hundred billion galaxies.

Even if the night sky seems dark to us, the cosmos is filled with a weak, diffuse light. This optical backlight is what is left over when the point light sources of stars, galaxies and other cosmic objects are blocked out. ©

 The  extremely weak, even carpet of photons consists of scattered light that emanates, among other things, from distant galaxies that are beyond the range of even the best telescopes.

Light pollution in space too


 The  optical background light thus provides valuable information about how many galaxies there could be in the cosmos as a whole, and also which light sources there were in the distant, early early days. “While the cosmic microwave background tells us what the cosmos looked like 380,000 years after the Big Bang, the optical background radiation tells us something about the number of stars that have formed since then and when this happened,” explains lead author Marc Postman from Space Telescope Science Institute in Baltimore.


 The  problem, however, is that in order to measure this weak background light, one must avoid any light pollution from the sun, the reflective dust particles of the inner solar system or other disruptive effects. Even the Hubble space telescope is still exposed to these disruptive effects, so that it is difficult for astronomers to calculate the amount of background lighting from its deep-field images.

Ten times darker than measured with Hubble

But there is an outpost in space that can carry out this measurement more undisturbed than any other: NASA’s New Horizons space probe. It flies beyond Pluto through dark areas of our solar system that are barely disturbed by scattered solar light. Postman and his team have therefore evaluated sky recordings from New Horizons as the basis for a new measurement of the optical background lighting.


 The  surprising result: space is apparently much darker than previously assumed. “Expressed in magnitudes, the sky is around ten times darker than the darkest sky images of the Hubble telescope suggested,” report Postman and his team. Overall, they determined a value of 33.2 nanowatts per square meter per steradian for the cosmic background light.

Fewer galaxies in the cosmos

But what does that tell us? As the astronomers explain, their measurements indicate that there could be significantly fewer galaxies in the distant, invisible space than expected. Based on the Hubble measurements, researchers have so far assumed a proportion of 90 percent of such invisible galaxies and thus at least two trillion galaxies in the cosmos. A large part of it would be fainter than 30 magnitudes and therefore not directly visible.


 The  new measurement data from New Horizons paint a different picture: “You can take all the galaxies that Hubble sees and then double that number,” explains co-author Tod Lauer from the NOIRLab in Tucson. “That is what we measure – but not more.” According to this, the total number of galaxies in the cosmos is more like a few hundred billion instead of two trillion.

Light residue of unexplained origin

What is interesting, however, is that if you take out all the light that comes from known sources and the invisible galaxies, a small residue remains: “©

 The re remains a diffuse component of unknown origin in the range of 8.8 to 11.9 nanowatts per square meter per steradian “, Report the researchers. This could be light that was generated by processes that were previously unknown and considered.

“Such a diffuse component of the optical background light could reflect more exotic production processes of photons, for example through the annihilation or decay of dark matter particles,” speculate Postman and his team. (Astrophysical Journal, accepted; 237th Meeting of the American Astronomical Society, abstract)

Quelle: Space Telescope Science Institute, NOIRLab

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