The Early Universe

In the beginning, there was dark matter, plasma, and radiation. After the beginning, the universe cooled enough that it become neutral and formed hydrogen and helium atoms. Their formation was largely spatially homogeneous, but there were some density variations in the plasma. While these began as tiny babies, gravity eventually grew them into raging adolescents--adolescents that gravity then began to collapse.

A visualization of the structural

evolution of the early universe.

Credit: Norman, et al. (SDSC)

Collapsing basically means clumping, and the clumps are what turned into stars (which produced light!), and the prevailing idea is/was that light (radiation) from these stars is what ionized (charged) the once neutral plasma between the early galaxies in which these stars cohabitated.

NOTE TO YOU: IF YOU WANT TO KNOW WHY YOU SHOULD CARE ABOUT WHY THE PLASMA GOT ALL IONIZIFIED, SKIP TO THE PLACE BELOW WITH THE RED 'WHY SHOULD I CARE' SIGN. CAPS LOCK.

And cohabitate they did, apparently. New three-dimensional simulations (Abel, et al., 2002; Bromm, et al., 2009) suggest that gravitational collapsing often led to close-together clumps at the centers of dense regions, suggesting that the first stars formed in little cliques.

Sources of (Re)ionization

However, is ultraviolet radiation from these binary, tertiary, quaternary, quintuplary (I could go on and on with my numerical prefixes) star systems really what heated and ionized all the space in between the galaxies (intergalactic medium, or IGM)? Recent Hubble Observations of galaxies from this time period suggest, simply, "No, man. No." The truth of it is, there was not enough UV radiation that could travel far enough to make the universe the way we observe it.

So people are coming up with ideas to answer this question: If not UV radiation from early massive stars, then what? What made the intergalactic medium charged instead of neutral? WHAT?

Well, in case you haven't heard, UV rays aren't the most energetic kind of rays that exist. I'm talking about X-rays, you guys.

X-rays, as they are more energetic than UV rays, can travel farther before an interaction changes their characteristics, such as direction or energy. The average distance they can travel between successive interactions is called a "mean free path." Because of X-rays' long mean free path, Madau et al. (2004) and Ricotti & Ostriker (2004) hypothesize that some of the photons doing the ionizing were from X-rays that shot out of black-hole-high-mass-X-ray-binary systems (BH-HMXBs).

The Paper



A recent Astronomy and Astrophysics paper by Mirabel, et al. (2011), backs up this claim by synthesizing the idea that many early star systems were binary systems and the idea that X-rays could have played a role in ionizing the IGM. This paper demosntrates that the rate at which binary systems containing one black hole (the rotting corpse of what used to be a high-mass star) and one high-mass star (that is still a high-mass star) was high in the early universe, and that the high-energy radiation from these knids of systems could explain the ionization of material far, far away from the edges of galaxies.





SCIENCE has a few ratio-related things to say about black hole abundance and placement:

1. the ratio of black holes to neutron stars should become larger as you go farther back in the universe's timeline (because there were more stars that were more massive in the early universe)

2. the ratio of black hole binaries to solitary black holes should also become larger as you go farther back in the universe's timeline (because there were more multiple-star systems in the early universe).





1 + 2 --> the number of BH-HMXBs used to be much larger than it is now.





While Mirabel, et al., calculated that the number of photons produced by a BH-HMXB and the number of photons that would have been produced when the black hole was a massive star, they found that the number of photons was the same. However, the photons coming from the BH-HMXB would have more energy and thus a longer mean free path and would ionize more atoms. What does this mean? We now know that it wasn't just reg'lar old stars that heated and charged up the ancient plasma: it was also black holes in binary systems, a kind of binary system that used to be more abundant than it is now. More black holes used to be in partnerships. Married, even. Or at least civilly unionized (with, you know, full tax and legal and health care benefits, so what's the difference?...psych. Black holes don't stand for separate-but-equal treatment). Imagine a universe full of BH-HMXBs. That's a lot of consonants. That's a lot of radiation. That's a lot of universe.





WHY IS THIS IMPORTANT PLEASE STOP TALKING SARAH

through it, rather than jumping on board with atoms. That means that we (well, our telescopes, and the aliens' telescopes) can see the photons. When the photons were busy ionizing, we couldn't see them--they never got to us. But once there was nothing to ionize, they could zip on through space to our lenses. Reionization is what allows the field of astronomy to exist. When the plasma was neutral, there were lots of atoms with which the photons could interact. These interactions caused them either to lose energy or to be absorbed completely. As the plasma becomes ionized, though, there are fewer and fewer neutral atoms, which means there are fewer and fewer atoms with which the photons can interact. This is why the reionization era of the universe is important: once the plasma was ionized, photons could travelit, rather than jumping on board with atoms. That means that we (well, our telescopes, and the aliens' telescopes) can see the photons. When the photons were busy ionizing, we couldn't see them--they never got to us. But once there was nothing to ionize, they could zip on through space to our lenses.









REFERENCES

Astronomy & Astrophysics, 528 DOI: Mirabel, I., Dijkstra, M., Laurent, P., Loeb, A., & Pritchard, J. (2011). Stellar black holes at the dawn of the universeDOI: 10.1051/0004-6361/201016357

Abel, T., Bryan, G. L. & Norman, M. L. Science 295, 93–98 (2002).





Bromm, V., Yoshida, N., Hernquist, L. & McKee, C. F. Nature 459, 49–54 (2009).





Madau, P., Rees, M. J., Volonteri, M., Haardt, F., Oh, S. P., et al. 2004, ApJ, 604, 484.





Ricotti, M., & Ostriker, J. 2004, MNRAS, 352, 547.