[3] The Edges of Our Universe (Ord, 2021)

Link post

Will’s quick summary/​hot takes:

For someone (e.g., me) who’s studied cosmology in college, who’s spent a bunch of free time reading futurism stuff, and who avidly follows the PBS Spacetime Youtube channel, there’s little-to-nothing novel to be found in Ord’s paper. Having said that, this paper is nice in that it collates a bunch of far future-related cosmology in one place, and the various diagrams are very pretty. Below I give my shortlist of things in the paper that might be most useful and/​or interesting to the median EA.

  • The future light cone of an event—an event being a point in space and time—is every point in spacetime that the event can causally affect.

    • i.e., it’s the region of spacetime that can be reached if one starts at the event and travels away from it at the speed of light or slower

  • The boundary of the affectable universe (from our viewpoint on Earth) is spherical in shape, is centered on Earth, has a radius of 16.5 billion light years, and is known as the universe’s event horizon.

    • not to be confused with a black hole’s event horizon

  • The affectable universe is smaller than the observable universe.

  • In 150 billion years, our local group of galaxies will be causally cut off from the entire rest of the universe.

    • because of the universe’s inflation/​expansion

    • the discussion (pp. 22-23) on how many stars/​galaxies we lose access to as we delay our space colonization reminded me of ‘Astronomical Waste’

  • Ord says that cosmologists strongly suspect the universe doesn’t have an edge, and that they instead think the universe is either 1) infinite or 2) finite but wrapped in on itself (like the surface of a 4-dimensional sphere or torus.

    • Note: “edge” here means a physical edge, rather than the time-related edges of the affectable universe and observable universe.

    • (not in the paper—this point is mine) However, some noted cosmologists would disagree with Ord’s characterization. For instance, Siegel (2017) writes:

      • “From our best observations, we know that the Universe is an awful lot bigger than the part we can observe. Beyond what we can see, we strongly suspect that there’s plenty more Universe out there just like ours, with the same laws of physics, the same types of physical, cosmic structures, and the same chances at complex life. There should also be a finite size and scale to the “bubble” in which inflation ended, and an exponentially huge number of such bubbles contained within the larger, inflating spacetime. But as inconceivably large as that entire Universe — or Multiverse, if you prefer — may be, it might not be infinite. In fact, unless inflation went on for a truly infinite amount of time, or the Universe was born infinitely large, the Universe ought to be finite in extent.” (emphasis added)

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