Imagine that you are in a boat on a big ocean, the water reaching to the far horizon, with the slightest hints of land immediately behind it. It is morning, just before sunrise, and thick fog has formed along the coast. While the cold holds you to your early watch, you catch a lighthouse that flickers faintly through the fog out of the corner of your eye.
And – yes – there! Another lighthouse closer, its light a little bit stronger. As you scan the horizon, more lighthouses signal the dangers of the distant shore.
You know this coast and return year after year to the same port. You know, the lighthouses all have the same brightness, were made by the same manufactory and kept in perfect condition over the years.
To pass the time playing a little game. As you look at your charts, you know how far each lighthouse has gone and how far its light has gotten to your salty eyes. However, their bright and bright light on a clear evening is muted and shaded by the persistent fog. You know how bright they should be and you can compare that brightness to what you see by looking through the layers and layers of fog to gauge how much fog the coast is surrounding.
It's not like you have something better to do.
This is precisely the process by which astronomers have recently measured the total amount of starlight in the Universe – without the fog and the lighthouses and salty sailors, of course.
Our cosmic lighthouses are the active galaxies, the most powerful engines in the Universe, where matter that flows into giant black holes is compressed and heated, radiating in a ray of flame before being swallowed up by the event horizon. In their death throes, these swirling, chaotic gas blocks release more energy than millions of galaxies and can pump their light through the universe.
When lit in the young cosmos, they appear to us as luminous lighthouses 
Between these lighthouses and our telescopes is all in the universe. Most of the universe is empty, but filling those cavities is the accumulated light of all generations of stars that have lived and died since those distant ages, letting the cosmos shine in a faint and thin mist of photons.
The radiation comes from the distant active galaxies is an extremely high energy – no surprise given the vast nature of their origin. And when that high-energy light shoots through the universe, it hits that thin mist. Random interaction through random interaction, accidental collision by accidental collision, the high-energy radiation loses energy and scatters.
Examining the light of over 700 active galaxies, the astronomical team was able to estimate all of the stellar light generated throughout the universe and throughout the cosmic period, just after the time of the first stars, only 500 million years after the Big Bang close to the present. The rough count 4 × 10 ^ 84 photons, which is … very much.
This estimate is consistent with other calculations of this so-called extragalactic background light, but in this recent observation and in other terms there is a disturbing finding: Our universe dies
By comparing the light of different active galaxies at different distances from us Not only could astronomers calculate the total amount of starlight ever produced, but they could also track the ebb and flow of that starlight through billions of years of cosmic history
And the terrible news is that the lights go out one by one. As we can say from various observations and assessments, more than nine billion years ago, our universe reached a star when the cosmos was only a quarter of its present age.
The exact reason is still missing. Surely, our expanding universe has something to do with it – galaxies on average continue to move away, causing fewer fusions and less material to flow into galaxies where they can throw that gas into new stars. But why was the summit so long ago? Why did star formation go down so fast? Or maybe why, despite the collapse of their once-great empire, why do the stars last?
Difficult questions without simple answers. At least we are still in the fog.
Read more: "A gamma-ray determination of the history of the star formation of the universe"