What created more light, the Big Bang or the stars?

The Universe has been creating stars for almost all of its 13.8 billion year history. But those photons can’t match the light from the Big Bang.

It’s been 13.8 billion years since the Big Bang, and our entire cosmos has evolved quite significantly in that time. At the moment, our cosmic vision extends for approximately 46.1 billion light years in all directions. Scientists have estimated that there are between 6 and 20 trillion galaxies. Among typical large galaxies, there are an average of hundreds of billions of stars contained within them. Although most galaxies are small and low-mass, this still translates to a total of 2 × 10 ²¹ place.

Inside them, each star is made up of about 10 ⁵⁷ atoms on average. We can reconstruct the entire star formation history of the Universe through a variety of methods, including examining the stars and galaxies found in all different eras of cosmic history. Important evidence confirming these estimates comes from the Fermi gamma-ray telescope, which in 2018 measured for the first time the star formation history of the entire Universe throughout cosmic time. Not only that, this type of measurement also allows us to answer another question: what produced more light, the Big Bang or the cumulative amount of stars formed over the course of cosmic history? The answer is the Big Banghere’s how we know.

What happens when stars form

When , several important processes occur sequentially.

• A molecular cloud of raw material, mainly hydrogen, will collapse under its own gravity.
• During the collapse, the cloud fragments, quickly giving rise to star systems and clusters of stars; the birth of stars officially coincides with the ignition of nuclear fusion in the cores of those stars.
• So, with so much high-energy (i.e. ultraviolet) radiation escaping from these newborn stars, molecules left in the surrounding cloud are ionized from that energetic radiation, stripping electrons from their atoms.
• Once the surrounding circumstellar medium becomes ionized, a special type of radiation begins to appear: emission lines, as radiation is emitted when electrons fall onto ionized atomic nuclei and cascade down the various energy levels.
• This starlight then travels through the Universe, where it interacts with all the atoms it encounters, leaving further absorption imprints imprinted on that light.
• Finally, starlight has a finite, non-zero probability of interacting with gamma rays, which are the highest-energy photons, to produce a specific species of new particles: electron-positron pairs.

The importance of blazars

Whenever you look at anything in the Universe, you have to recognize that there are clouds of gas that absorb a fraction of the light; we can take this into account by examining the absorption lines. There are also galaxies and galaxy clusters that often come between us and what we are observing. We can measure their brightness, density and other properties to calibrate each individual blazar (among the most violent phenomena in the Universe, associated with black holes) that we examine. Blazars are found throughout the sky and each individual blazar, at source, will have energy and flow properties that are unique.

By doing proper accounting of what exists in the Universe—at the source, along the line of sight, and at the very point where we ultimately received the light—we can determine the source properties of the blazar we are examining. There’s a lot of work behind it, but it’s worth it in the end: we’ll have a starting point to start from.

Measuring all the light in the Universe

There is a method to measure all the starlight in the Universe.

• First, let’s start by measuring and identifying all the blazars, across the Universe, wherever they are located.
• We then measure of each blazar, which can be determined by measuring at least one absorption or emission line inside it, in order to have a good indicator of the distance at which it is located.
• Once you have this information, this is where it comes into play gamma rays: we measure the number of gamma rays received by the gamma ray telescope we are using (like the Fermi, in fact) of two specific properties of each blazar, the red shift of the blazar and its brightness.
• Remember that gamma rays, when colliding with the cumulative amount of extragalactic background starlight, have a certain probability (depending on the starlight) of producing electron-positron pairs.
• And then, finally, instead of looking for that characteristic energy signal of electron-positron annihilation, that 511 keV signal (redshifted relative to the expansion of the Universe at the moment Fermi sees it), we instead use the deficit of gamma rays from the source (because gamma rays are converted into electron-positron pairs) to calculate how much background starlight must be present, as a function of redshift/distance, to account for the loss of gamma rays.

How we understood that the Big Bang created more light

This was something that, until recently, we would not have been able to do. But with the advent of NASA’s Fermi satellite, and in particular the Fermi-LAT collaboration (the Large Area Telescope instrument on board Fermi), we were able to make these measurements for the first time, for all the known blazars that gamma rays appear in the sky: all 739 of them. Well, we’ll spare you further calculations to tell you that these measurements confirmed that the Universe had its star formation peak when it was about 3 billion years old and since then the formation rate stellar is declining. Today, it stands at just 3% of that maximum initial rate, and the rate at which new stars are forming in the Universe continues to decline over time.

The total amount of starlight created since the start of the Big Bang corresponds to a total of about 4 × 10⁸⁴ photons, which is an incredibly large number. But how does this number compare to all the photons present in the Universe as part of the leftover radiation from the Big Bang? The answer comes directly from the measurement of the cosmic background radiation, the afterglow of the Big Bang. Putting all this information together, it turns out that the total number of photons left over from the Big Bang, in our observable Universe today, is about 10 ⁸⁹ -10 ⁹⁰: Hundreds of thousands of times more photons than have ever been created by stars!

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