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The Universe, Its Way Past, and Its Way Future


by Sarah Scoles


Today, Scientific American came out with a graphical cosmic timeline that places 16 universal events in chronological order with convenient (if uncertainty-laden) timestamps. The timeline, as all good timelines do, starts at the beginning and ends at the end, whatever meaning those words have in this context.

While the graphics were pretty, and the information was as sound as it could be, there were no references, so the reader was just left having to take on some amount of faith SciAm's from-on-high knowledge about the distant past and more distant future. So I decided to dig up some papers about these astrophysical milestones, because, like life and liberty, the pursuit of primary source material is a basic human right.

Below are the three timeline slides, and below each one are links to the papers and a brief summary of their content.


Cosmic inflation ends: The inflationary epoch was when the universe was expanding exponentially. As you can see, it didn't last long. And not to start out with something that is not a journal article, but this is an interview with Alan Guth, the theory's baby-daddy, about his baby, the theory.

Protons form: Protons are not fundamental particles, but are hadrons--they're made out of quarks. This paper describes how quarks might have gone from hanging out as individuals to hanging out so close together that they become something else entirely (and positively).

Deuterium, helium, and lithium are synthesized: The formation of the first atoms that were not hydrogen-one atoms is referred to as Big Bang Nucleosynthesis (BBN). It didn't last very long, as the universe continued to expand, and its temperature fell below that necessary for nuclear fusion. This paper discusses the current state of BBN and what it means for the abundance of baryons (regular atoms) in the universe today.

First stars form: The first stars were once thought to be "monstrous"--hundreds of times the mass of the Sun. However, these stars should have died in a particular kind of supernova (a pair-instability supernova), which would have left particular chemical signatures--signatures we don't see. This paper describes a scenario in which the first stars don't have to be so massive, and in which they do not undergo pair-instability supernovae, which fits better with what we observe today.

Earliest known galaxy: This one is pretty straightforward. It's a galaxy, and it's the oldest known. It was found in the Hubble Ultra Deep Field. Here's the discovery paper.

Star formation peaks: We can figure out at what rates stars were forming at different points in the universe's life by looking at up-and-coming stellar populations in different galaxies, since looking at galaxies at different distances from us is like looking at different times. This paper discusses the star formation rate as a function of time using a different method--looking at the entire stellar populations--and stirs up controversy about when the peak actually was.



Cosmic expansion begins to accelerate: Everybody's favorite website, PNAS (say it out loud), hosts a review article about the expansion of the universe and how we know what we know.

Solar system forms: At what point in the solar system's formation do we say that it "formed"? Turns out, it's not when the Sun became a proto-star, or when the planets were planets, but when the first solid "grains" formed out of the disk surrounding the proto-Sun. This article discusses when, exactly, that was, based on a Methuselah-esque meteorite's composition.

Today: Hi.


Inner planet orbits destabilize: This is a really old paper, but it contains proof of something that most people find disturbing: that the solar system is a fundamentally chaotic place (in the mathematical sense, meaning that tiny differences in initial conditions lead to radically different outcomes, making precise/accurate long-term prediction [such as, perhaps, this SciAm timestamp] impossible).

Sun goes white dwarf: The Sun will eventually, like human beings as they age, expand and then contract. This paper describes a model of the Sun that starts at its conception, shows that at this (now) point in its life it would have as much mass as the Sun does and shine at the brightness that it does, and ends at its final stage as a white dwarf.

Milky Way and Andromeda collide: Instead of a paper, here's a simulation of the collision.

Last massive stars explode: BOOM. Massive stars can't form forever, and they can't live forever. Here's a paper, but I'm going to talk more about this same paper later, as it's relevant to stuff below.


Galaxies beyond ours become invisible: Isn't that crazy? Even though it won't happen for 100 billion years, it's very strange to think that at some point all the other galaxies will have been carried so far away by cosmic expansion that we won't be able to see them. As far as scientific observation goes, it is conceivable that we will not be able to prove that there is anything in the universe but our own galaxy. Will, then, the idea of a universe outside ourselves be in the realm only of belief, and of religion? Perhaps not, according to this paper, which suggests we can do cosmology with hypervelocity stars ejected from our own galaxy.

Stellar lifetimes decrease: The more heavy elements stars have, the shorter their lives are. The older the universe gets, the more heavy elements there are. The more heavy elements there are, the more they are present in stars, and the sooner the stars die. This is a great review paper discussing "the long-term fate and evolution of astrophysical objects," including "the evolution of planets, stars, stellar populations, galaxies, and the universe itself over time scales that greatly exceed the current age of the universe." It addresses topics both above and below this one and is definitely worth a read, if you'd like to learn about the death of everything as you know it.

Last star burns out: So sad. So lonely. It's discussed in the paper above.

Protons decay: Remember how a few minutes ago, we were talking about protons forming? Turns out, they unform, too. Don't be afraid. Embrace it.

Galactic-scale black holes evaporate: It will be a cold, dark universe at this point, not made darker by the evaporation of supermassive black holes through their slow secretion of secreted Hawking radiation. This evaporation is discussed in the "long-term fate" paper above.



It's a good thing we're alive now, instead of then or then or then, but, then again, we're only alive now to think about how lucky we are to be alive now in particular because we are, in fact, alive now.

REFERENCES (in order of appearance)


Fries, R., Greco, V., & Sorensen, P. (2008). Coalescence Models for Hadron Formation from Quark-Gluon Plasma Annual Review of Nuclear and Particle Science, 58 (1), 177-205 DOI: 10.1146/annurev.nucl.58.110707.171134Gary Steigman (2003). Big Bang Nucleosynthesis: Probing the First 20 Minutes Carnegie Observatory Astrophyisics Series arXiv: astro-ph/0307244v1Hosokawa, T., Omukai, K., Yoshida, N., & Yorke, H. (2011). Protostellar Feedback Halts the Growth of the First Stars in the Universe Science, 334 (6060), 1250-1253 DOI: 10.1126/science.1207433Bouwens, R., Illingworth, G., Labbe, I., Oesch, P., Trenti, M., Carollo, C., van Dokkum, P., Franx, M., Stiavelli, M., González, V., Magee, D., & Bradley, L. (2011). A candidate redshift z ≈ 10 galaxy and rapid changes in that population at an age of 500 Myr Nature, 469 (7331), 504-507 DOI: 10.1038/nature09717Heavens, A., Panter, B., Jimenez, R., & Dunlop, J. (2004). The star-formation history of the Universe from the stellar populations of nearby galaxies Nature, 428 (6983), 625-627 DOI: 10.1038/nature02474Kirshner, R. (1999). Supernovae, an accelerating universe and the cosmological constant Proceedings of the National Academy of Sciences, 96 (8), 4224-4227 DOI: 10.1073/pnas.96.8.4224Bouvier, A., & Wadhwa, M. (2010). The age of the Solar System redefined by the oldest Pb–Pb age of a meteoritic inclusion Nature Geoscience, 3 (9), 637-641 DOI: 10.1038/ngeo941Laskar, J. (1989). A numerical experiment on the chaotic behaviour of the Solar System Nature, 338 (6212), 237-238 DOI: 10.1038/338237a0Sackmann, I., Boothroyd, A., & Kraemer, K. (1993). Our Sun. III. Present and Future The Astrophysical Journal, 418 DOI: 10.1086/173407Adams, F., & Laughlin, G. (1997). A dying universe: the long-term fate and evolutionof astrophysical objects Reviews of Modern Physics, 69 (2), 337-372 DOI: 10.1103/RevModPhys.69.337Abraham Loeb (2011). Cosmology with Hypervelocity Stars Journal of Cosmology and Astroparticle Physics arXiv: 1102.0007v2 Nishino, H., et al. (2009). Search for Proton Decay via p→e+π0 and p→μ+π0 in a Large Water Cherenkov Detector Physical Review Letters, 102 (14) DOI: 10.1103/PhysRevLett.102.141801

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