Have you ever wondered exactly how much light had been produced by all the stars of the universe during the time elapsed? Well, now you can not ask anymore. An international team of astronomers has actually calculated the amount of stellar light in the cosmos.
And that teaches us new things about the early years of our universe.
Since the Big Bang – about 13.7 billion years ago – our Universe has produced many galaxies and many other stars. Maybe about two billion galaxies, containing about a trillion billion stars.
For decades, scientists have known that knowing the amount of light produced by these stars during the life of the Universe would be a powerful tool for understanding the first universe and the history of the formation of the Universe. stars.
But hey, it's not really easy to measure. Although there are many stars producing many photons, the space is incredibly vast and the light of the stars incredibly low. There is also interference from the zodiacal light and the faint glow of the Milky Way. The light of the stars of the universe can not really be observed directly.
But astrophysicist Marco Ajello of Clemson University and his team discovered an indirect method of quantification of starlight. They used gamma-ray photons.
"These are high-energy photons, typically a billion times the energy of visible light," Ajello told ScienceAlert.
"By traveling in space, gamma rays can be absorbed by interactions with star photons, and if there are a lot of star photons, there will be more absorptions, so we can count the number of absorptions we see to understand the density of the photon field between us and the gamma ray source ".
Drawing on nine years of data from NASA's Fermi gamma-ray space telescope, Ajello and his team analyzed the light of 739 blazars (powerful gamma ray sources) throughout the universe to determine the rate Absorption in extragalactic background light (EBL). , accumulated background radiation from the universe.
This gave them the photon density of starlight in the EBL layer – and, as the blazars are at different distances, they were able to do so over several periods.
Once they have taken into account and subtracted light from other sources, such as incandescent accretion disks around supermassive black holes, they could multiply that density by the volume of the light. Universe to obtain the number of photons produced by the stars since the beginning of time.
"We basically have a tool, like a book, to tell the starlight stories through the history of the Universe, and finally we found it, and we can just read it," he said. said Ajello.
"We did it.We measured the entire history of star formation of the universe."
It's pretty simple to explain, but it was painful and complex to do. It took the team three years – and it was worth it.
We now know that from the moment the Fermi data was collected, the stars of the Universe produced 4×1084 photons.
Do you need this specified? Here: 4,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000
Yes, it's a four followed by 84 zeros. Or four octillions of octodecillion. It's an interesting fact, so put it in your sleeve for later.
But you bet that science becomes much colder. Because this cool figure unveils a particularly mysterious period in the history of our universe: the era of reionization, which began about 500 million years after the Big Bang. We often hear about "Holy Grail", but the EoR is really one.
It is essentially when the lights of the Universe are on.
Before the EoR, the space was opaque. Then something happened and ionized all the neutral hydrogen, so that radiation – including light – can flow freely in the Universe.
"Our measurements allow us to reach the very first billion years of the Universe, a very interesting time in the Universe, so remote from us that all very powerful telescopes can not really see. we can not really see them, instead we always see the light of these objects, "Ajello explained.
The team discovered two interesting things at that time: a very large number of UV photons, which was planned for the reionization process; and that the sources of these UV photons were populations of irregular galaxies – small asymmetrical "blobby" galaxies that produce a lot of UV radiation.
These could be the drivers of reionization. The James Webb Space Telescope, scheduled for launch in 2021, is expected to tell us more about the EoR network.
Meanwhile, Ajello and his team will apply their star book to further study of the cosmos, such as the pace of expansion of the Universe, the Hubble constant, which has been really hard to pin down .
"It turns out that our measurement is very sensitive to the rate of expansion of the Universe," Ajello said. "This can be used to make a measure of Hubble's constant now, so that's something we're going to do."
The team's research was published in the journal Science.