Imagine yourself on a boat in a great ocean, the water extends to the distant horizon, with the most subtle signs of earth beyond. It is morning, just before dawn, and a dense fog settles along the coast. As the cold squeezes you in front of you, you pick a headlamp out of the corner of your eye, flickering feebly through the haze.
And – yes – there! Another lighthouse, closer, the light a little stronger. As you examine the horizon, more lighthouses signal the dangers of the distant shore.
You know this coast, returning to the same port year after year. You know that headlights are all the same shine, made by the same manufacture and kept in good working order over the years.
And so, to pass the time you play a little game. By consulting your maps, you know the distance from each lighthouse, and how far the light of them traveled to reach your salty eyes. But its light, bright and bright on a clear night, is overshadowed and shadowed by lingering haze. You know how they shine should to be, and you can compare that glow with what you see, peering through the layers and layers of fog, to estimate how much fog is hugging the shoreline.
It's not like you have something better to do.
This is exactly the procedure that astronomers have recently used to measure the total amount of starlight in the universe – minus, of course, fog, lighthouses and salty sailors.
Our cosmic headlights are the active galaxies, the most powerful motors in the universe, where matter flowing into gigantic black holes compresses and heats, burning in a radiation flame before being swallowed up by the event horizon. At its death, these tumultuous chaotic gases emit more energy than millions of galaxies and are able to pump their light through the universe.
When they light up in the young cosmos, they appear to us as beacons, glistening but distant.
Between these headlights and our telescopes is the thing in the universe. Most of the universe is empty, but the filling of these voids is the accumulated light of all the generations of stars that have lived and died from those distant times, illuminating the cosmos in a dark, thin fog of photons.
The radiation coming from the distant active galaxies is extremely high energy – not surprising given the powerful nature of its origin. And when high-energy light explodes through the universe, you find that fine mist. Possibility of interaction by random interaction, random collision by random collision, high energy radiation loses energy and disperses.
Examining the light of more than 700 active galaxies, the team of astronomers was able to estimate all the starlight produced throughout the universe and throughout cosmic time, just after the time of the first stars, only 500 million years after the Big Bang up close to the present day. The approximate count? 4 × 10 ^ 84 photons, which is … a lot.
This estimate agrees with other calculations of this so-called extra-galactic backlight, but buried in this last observation and others is a troubling finding: our universe is dying.
By comparing the light of different active galaxies placed at different distances from each other, astronomers could not only calculate the total amount of stellar light already produced, but also trace the fluxes and ebbs of that stellar light through billions of years of cosmic history.
And the terrible news is that the lights are going out, one by one. The best we can see, through a variety of observations and estimates, is that our universe hit the peak of star formation more than 9 billion years ago when the cosmos was only a quarter of its current age.
The precise reason still deceives us. Of course, our expanding universe has something to do with it – galaxies are moving away from each other, on average, resulting in fewer fusions and less supplies of fresh material flowing into the galaxies where they can turn that gas into new stars. But why was the peak at that time, so long ago? Why has star formation declined so quickly? Or perhaps why the stars persisted for so long, despite the collapse of their once great empire?
Difficult questions without easy answers. For now, at least, we're still in the fog.
Read more: "A determination of gamma rays of the history of the formation of stars of the Universe"