Why are there so many galaxies
Ten times more galaxies in the cosmos
The question of the number of galaxies and stars in the universe is one of the most fundamental in astronomy. With ever more powerful telescopes, researchers try to look far into space in order to carry out a “census” not only in our cosmic neighborhood, but above all in the distant cosmos. A first milestone was achieved in the mid-1990s with the first "deep field" images from the Hubble space telescope: Overlaying 342 images created a portrait of more than 3,000 roughly twelve billion year old galaxies - and thus a first glimpse into the young universe. From these and later, even higher-resolution subsequent images, astronomers concluded that there must be around 100 billion galaxies in the universe.
Now Christopher Conselice from the University of Nottingham and his team have carried out another cosmic census. To do this, they combined deep space images from Hubble and other telescopes and created a three-dimensional model of a tiny section of the observable universe. This gave them a 3D image that not only showed the spatial distribution, but indirectly also the chronological sequence of the various generations of galaxies. With the help of mathematical models, the researchers calculated the average density of the galaxies at different times and were thus able to infer the number of galaxies in space that are not visible to us.
Two trillion galaxies - at least
The result: the universe is much fuller than previously thought. In total there must be at least two trillion galaxies in it - ten times more than previously assumed. Most of these star clusters are so small, old and faint that they cannot be directly observed with today's telescopes, as the researchers explain. "It's just incredible that more than 90 percent of the galaxies in the universe have yet to be explored," says Conselice. “Who knows what exciting properties we will find when we explore these galaxies with future generations of telescopes?” Most of these galaxies formed in the early universe and are more similar to the dwarf galaxies that surround our Milky Way as satellites.
Astronomers have long suspected that there were more, but smaller galaxies in the cosmos than today. In the course of time, many of them then merged to form larger clusters of stars, so that the galaxy density in space decreased. "Our results are strong evidence that there has been an evolution of galaxies in the history of the universe," explains Conselice. "Through mutual mergers, it led to a reduction in the number of galaxies."
The result of the new “population census” in the cosmos also sheds new light on an old paradox: As early as the early 19th century, the German astronomer Heinrich Olbers asked why the night sky appears dark even though the universe must contain an infinite number of stars. Assuming the number and density of galaxies that have now been determined, every part of the sky should actually be occupied by at least one part of a galaxy. So why doesn't the sky appear bright and completely filled with stars? Several factors are responsible for this, as the researchers explain. On the one hand, the light of the very distant galaxies is stretched by the expansion of space and therefore shifted into areas of light that are not visible to us. On the other hand, intergalactic dust and gas clouds absorb parts of the light. Hence, although the entire sky is filled with galaxies and stars, we only see a tiny fraction of them.
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