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Aliens: Yes Or No?
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<blockquote data-quote="briggart" data-source="post: 9083350" data-attributes="member: 6805135"><p>Sure, but that is not relevant to my original point. I was trying to clarify why it makes sense to consider different bubbles within an eternal inflation as forming a multiverse, even though they are part of the same continuum. The space between different bubbles is still inflating so, barring FTL, it is not possible to travel from one bubble to another. On the other hand, under some conditions, within a bubble it can be possible to travel to an arbitrarily large distance given enough times. Those conditions just happen to not be true in our Universe, but I'm not aware of any fundamental reasons while they <strong>could</strong> not be true, which to me is enough to differentiate travel within a bubble from travel between bubbles. But I feel you do not agree?</p><p></p><p>The rest in spoiler because I think it will be interesting only to the two of us.</p><p>[SPOILER]</p><p></p><p>Technically, those are 3 questions. <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite2" alt=";)" title="Wink ;)" loading="lazy" data-shortname=";)" /></p><p></p><p>Let me start from the last one. Assuming a flat or open Universe, the comoving distance a photon can travel is the integral between the emission time and infinity of</p><p></p><p><span style="font-family: 'verdana'">c dt /a(t)</span></p><p></p><p>where c is the speed of light, t is time and a is the cosmological scale factor. Assuming that asymptotically </p><p></p><p><span style="font-family: 'verdana'">a(t) ~ t ^ n</span> </p><p></p><p>with n > 0, then the requirement that asymptotically the expansion rate goes to 0 implies n < 1 and the comoving distance diverges, which is indeed the case for matter or radiation dominated universes. For a curvature dominated universe n = 1, the expansion rate does not go asymptotically to 0, but the comoving distance still diverges, albeit logarithmically. The future travel comoving distance diverging to me implies there will be no un-observable zones given enough time, but maybe I'm missing something?</p><p></p><p>Second one, I'd say no in general. If there is a cosmological constant, then the comovig distance integral does not diverge, even allowing for infinite future time.</p><p></p><p>For the first one, I'm not sure I understand what you mean by "continued indefinitely in to the past": something like cyclic cosmologies? </p><p>I know people have been working on that and figured out a way to have stable cosmological solutions that continue through the bounce, but I've not been following that line of work, but if you have an answer, I'd be happy to hear it. </p><p>[/SPOILER]</p></blockquote><p></p>
[QUOTE="briggart, post: 9083350, member: 6805135"] Sure, but that is not relevant to my original point. I was trying to clarify why it makes sense to consider different bubbles within an eternal inflation as forming a multiverse, even though they are part of the same continuum. The space between different bubbles is still inflating so, barring FTL, it is not possible to travel from one bubble to another. On the other hand, under some conditions, within a bubble it can be possible to travel to an arbitrarily large distance given enough times. Those conditions just happen to not be true in our Universe, but I'm not aware of any fundamental reasons while they [B]could[/B] not be true, which to me is enough to differentiate travel within a bubble from travel between bubbles. But I feel you do not agree? The rest in spoiler because I think it will be interesting only to the two of us. [SPOILER] Technically, those are 3 questions. ;) Let me start from the last one. Assuming a flat or open Universe, the comoving distance a photon can travel is the integral between the emission time and infinity of [FONT=verdana]c dt /a(t)[/FONT] where c is the speed of light, t is time and a is the cosmological scale factor. Assuming that asymptotically [FONT=verdana]a(t) ~ t ^ n[/FONT] with n > 0, then the requirement that asymptotically the expansion rate goes to 0 implies n < 1 and the comoving distance diverges, which is indeed the case for matter or radiation dominated universes. For a curvature dominated universe n = 1, the expansion rate does not go asymptotically to 0, but the comoving distance still diverges, albeit logarithmically. The future travel comoving distance diverging to me implies there will be no un-observable zones given enough time, but maybe I'm missing something? Second one, I'd say no in general. If there is a cosmological constant, then the comovig distance integral does not diverge, even allowing for infinite future time. For the first one, I'm not sure I understand what you mean by "continued indefinitely in to the past": something like cyclic cosmologies? I know people have been working on that and figured out a way to have stable cosmological solutions that continue through the bounce, but I've not been following that line of work, but if you have an answer, I'd be happy to hear it. [/SPOILER] [/QUOTE]
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