[HN Gopher] A small Universe
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A small Universe
Author : bookofjoe
Score : 31 points
Date : 2023-09-12 19:12 UTC (3 hours ago)
(HTM) web link (arxiv.org)
(TXT) w3m dump (arxiv.org)
| lainga wrote:
| One may also read https://phys.org/news/2023-09-case-small-
| universe.html to get an overview of what the mentioned
| "swamplands programme" is
| zuminator wrote:
| Nice summary, although it has a rather glaring "thinko" in an
| early paragraph, where it points out that, because of the age
| of the visible universe being around 13 billion years, a naive
| calculation of the universe's diameter would be 26 billion
| light years across, but that because of cosmic inflation, it's
| "46 billion light years across [sic]." That should be ~46 bly
| in _radius_ , in other words, ~93 bly across.
| hprotagonist wrote:
| where "small" here means the many trillions of stars in the
| observed universe.
| pdonis wrote:
| Yes, still very big. You might think it's a long way round the
| corner to the chemist, but that's just peanuts to space.
| sfink wrote:
| [dead]
| zuminator wrote:
| More or less, although the paper is talking about somewhere
| between "a few times larger than the currently observable size"
| (p.3) to "not more than a few orders of magnitude larger" (p.6)
| Either of which counts as "small" in comparison with the
| generally _infinite_ size implied by current solutions of our
| flat curvature models.
|
| (Totally unrelated question: How are you getting proper open-
| closed quotes?)
| virtualritz wrote:
| > Totally unrelated question: How are you getting proper
| open-closed quotes?
|
| On a Mac you have them on some keys by holding down a
| modifier.
|
| On Windows and Linux you need to use a utility of some sorts
| or modify key bindings for your desktop environment.
|
| On Android you hold the resp. quote key on the virtual
| keyboard to get a selection of resp. quotes to choose from.
|
| On that note: a good typographer will use single quotes for
| quoting 'single' words.
| checkyoursudo wrote:
| "On my Mac, Swedish keyboard, it is option-shift-n and -m to
| get these quotation marks."
|
| Otherwise, the shift-2 quote marks are " straight. Mac
| English surely has them somewhere, but not necessarily those
| keys.
|
| If you have linux, then you can use the compose key, like
| this (according to the internet, my linux box is 1000 km away
| right now so can't test)
|
| [Compose] < " for "
|
| [Compose] > " for "
|
| No idea about Windows.
| causality0 wrote:
| _generally infinite_
|
| How infinite is "generally infinite"? Are we talking infinite
| enough it's not productive to talk about its size, or so
| infinite there are physical copies of the solar system,
| earth, and everyone it it by sheer chance?
| zuminator wrote:
| My apologies. By "generally infinite" I merely meant
| "generally thought of as infinite," not like I was invoking
| some special class of nigh-infinitude. By the way, I
| wouldn't say an infinite universe doesn't necessarily imply
| that everything exactly repeats. Take any irrational number
| like pi. We know for a fact that it doesn't repeat itself
| exactly because if it did, it would not be irrational.
| Although it seems likely that pi is "normal" and that any
| finite segment of it will repeat infinitely many times,
| that is not a requirement of all infinite numbers. One
| could imagine a number called _pu_ where every 9 but the
| first is replaced with a 0. So, 3.1415926535807032, etc.
| What can we say about pu? It a) is an actual number with a
| precise definition, b) is infinitely long, c) is aperiodic
| thus irrational, but d) only has one 9 in its entire
| infinitely long decimal expansion. Similarly, the cosmos
| might conceivably have some "one off" features that never
| repeat, even if it is infinite.
|
| But I still haven't answered your question. My answer is,
| yes, if spacetime curvature is exactly 0, then it's my
| understanding (as a non-cosmologist) that the equations
| imply that the universe is _that_ infinite such that
| conceivably everything could in principle repeat an
| infinite number of times. Although I myself am curious what
| the estimated size of just the non-repeating part would be.
| mika69 wrote:
| I never heard of the term 'swampland conjectures' before.. Any
| resources to dive in to this topic?
| jug wrote:
| https://en.wikipedia.org/wiki/Swampland_(physics)
| anon____ wrote:
| The galaxy is on Orion's belt.
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