On Tuesday, February 18, 2025 at 3:42:56 PM UTC-7 Brent Meeker wrote:
On 2/17/2025 5:08 PM, Alan Grayson wrote: On Monday, February 17, 2025 at 12:49:27 PM UTC-7 Liz R wrote: Apparently the simplest model of the universe is one that is infinite at all times (according to Max Tegmark) - the "concordance" model? There is no reason an infinite universe couldn't have undergone scale expansion, an (in this case, presumably uncountably) infinite thing remains infinite no matter how much you expand it. Whether the universe is a continuum is key to this, so it depends on an as-yet unknown TOE. If spacetime is quantised - in some sense - then whether it could be infinite, and whether it could expand, might still be up for grabs. Note that a quantised spacetime would presumably only contain a finite number of possible states inside any volume (e.g. our cosmological horizon) and hemce an infinite universe would eventually repeat itself across sufficiently large distances. Again quoting Max Tegmark, this would occur for our Hubble sphere at something like a distance of 10^10^37 metres (from memory - the actual figures don't really matter much for any practical purpose, just note that they make a googol look ultramicroscopic). Assuming that repeated identical quantum states are indistinguishable, an infinite quantised universe would in fact be "piecewise finite" on Vast scales - repeating arbitrarily large identical volumes would be not just indistinguishable in principle but actually identical. So on this basis, the universe would be a sort of Library of Babel in which all possible quantum states exist (including "Harry Potter" and "White Rabbit" universes) - but since the number of possible states is finite for a given volume, it would eventually run out of combinations and repeat itself. Quite what this would look like on "hyperastronomical scales" I leave to the mathematicians. *FWIW, our best current measurements fail to show any quantization of spacetime. This was discussed here by Lawrence Crowell a long time ago, and I don't recall the level of fineness of these results. AG * *It was a paper I cited years ago that noted there was no dispersion of gamma rays from very distant galaxies which ruled out on discrete structure to space down to less than the Planck scale. Here's a more recent paper: https://physics.aps.org/articles/v17/s99 <https://physics.aps.org/articles/v17/s99> Brent* *Do you have any idea how has this result has effected the quest for a quantum theory of gravity? AG* By the way the Cosmological Principle is an observation / assumption, not an actual principle based on any physical laws. *Not exactly. Physical observation do play a significant role in generating principles. Faraday's observations of the behavior of magnetic fields comes to mind, and the MM experiment, which Einstein was aware of, which showed the velocity of light is independent of an observer's motion. In the case of the CC, we have ambiguous results. The CMBR suggests the universe was very close to homogeneous and isoptropic when its age was about 380,000 years old, but measurements much later in time show it's actually lumpy, with ultra long filaments containing galaxies, separated by huge voids. I'd go with the later, showing that the CC is false. AG * -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion visit https://groups.google.com/d/msgid/everything-list/6e755514-fabe-4be7-84af-5833d858a251n%40googlegroups.com.
