First Stars

When the Universe is young but not so hot anymore, after electrons decide to end their random flings and opt for the comfort of stable relationships with protons, and photons stream sad and lonely forever, everything becomes very, very boring for quite some time.

Stuff -Dark Matter and newly established neutral hydrogen atoms- is distributed in a very, very, very, very uniform way, its density ripples only one part in ten thousand. And everything is so cold that nothing happens for a while. As time goes by, the tiny ripples in density (we call them anisotropies) grow and attract more material from the neighboring regions. As the Universe keeps expanding some of these bubbles gather enough material and detach from the general drifting flow, forming a unit of their own: a stable, self-gravitating halo.

As they fall into the newly formed home -whose gravity is dominated by Dark Matter- the gas atoms acquire energy and heat up, a process that in a seemingly paradoxical way allows them to eventually lose their energy, and collapse toward the center of the halo. This process is star formation, taking place for the first time in the Universe, thus giving birth to the First Stars ever, sometimes dubbed Population III for reasons not so exciting to be worth mentioning here.

The conditions in which the First Stars form are very peculiar and rather different from those of the stars you and I are used to (the Sun, first among them, and all the stars in our Galaxy the Milky Way, and all the stars in the Milky Way’s sister galaxies like the Large and Small Magellanic Cloud, and all the stars in all the galaxies…). We have reasons to believe that these conditions imprint them with very peculiar properties, such as being typically more massive and possibly hotter than their modern analogues.
We still don’t know for sure, as this happened a very long time ago and these stars are either dead or missing, and one other interesting game we are playing is to find them and check if our theories are correct, or disprove them and move on to refine our understanding.

But how do you nail them? Once you find a strange, isolated, very old star, how can you be sure that’s one of those firstborns? On the basis of what we know about stellar evolution, and the conditions of the Universe when the First Stars were born, we are trying to figure out peculiar signatures, birthmarks that these ancient ancestors may be carrying on them. There is a host of these possible observables, including some that may not seem too obvious at first.

I have tried to understand what is the neutrino trace they leave behind, as well as worked out a scenario in which some peculiar candidates of Dark Matter interact with the environment and heavily affect the stars’ formation, as well as their life and death. I wrote a critical overview on the subject which you can find here and in my opinion not much has changed since then. There is much more on the plate, but for that you have to read the papers, and ask me questions. Don’t hesitate to contact me!