Cosmic Scales

Cosmic Scales[1]

Cosmological simulations try to replicate the universe and its evolution, and early ones produced remarkable matches to large-scale structures (top boxes in in the top illustration.) But for many years astronomers were forced to fudge what was happening at the smallest scales (second row of illustrations)—processes that, it turns out, heavily influence the larger structures. A parsec (pc) is 3.26 light-years.

What is pretty astounding is that cosmologists can start with some fundamental assumptions, and then, applying the laws of physics, simulate the growth of the universe and find, matching it with observation, that pretty good agreement can be achieved. Although there is huge diversity in the cosmological simulations, they all share the same roots. In the 1980s a concept of cold dark matter (CDM) was advanced. This concept described how the universe went from a smooth initial state at early times to the large-scale, intricate structure we see today, assuming that most matter is cold (i.e., moves slowly) and dark (i.e., does not emit electromagnetic radiation or scatter light). With the addition of a force that seems to be there, a kind of anti-gravity force called Λ (lambda, or nowadays, “dark energy”) the current  idea also accounts for the accelerating expansion of the universe.

There is probably plenty of room for a slightly more complicated paradigm, in which dark energy evolves, or where dark matter is not perfectly cold and collisionless. There is still an even more extreme possibility. Dark matter implies that most of the universe is composed of a particle we have never seen, and a type of energy which we understand only from its effects: dark energy. Maybe neither of these proposed ark entities exist, and that alternatives the laws of gravity are more likely.

[1] Benjamin Skuse, “Baking a Universe,” Sky and Telescope (133, 5, May, 2017, pp. 34-40)

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