On Tuesday, May 19, 2020 at 11:18:15 PM UTC-6, Alan Grayson wrote:
>
>
>
> On Tuesday, May 19, 2020 at 6:26:08 PM UTC-6, Lawrence Crowell wrote:
>>
>> You cannot of course circumnavigate the spatial manifold of the universe.
>> Anything beyond the cosmological horizon moves away faster than you can
>> ever catch up. It is a bit like the part in the movie The Shining with Jack
>> Nicholson where the hotel hallway expanded faster than he could run. If we
>> could though observe this, say analogous to Jack Nicholson in the film,
>> there would be optical effects. The spatial manifold could be a k = 1
>> closed or k = -1 hyperbolic or the dodecahedral tessellated universe of
>> Poincaré. Yet so far data is not forthcoming.
>>
>> A Planck energy of quanta, say a UV graviton, could have causal influence
>> on us is it expands to the cosmological horizon or near so. The B-modes of
>> inflation, which are still being pursued, represent Planck units redshifted
>> to some appreciable scale comparable to the cosmological horizon. This is a
>> z factor z = 10^{10}ly/ℓ_p = 6.3×10^{60}, where taking the nat-log of this
>> and multiplying by the horizon scale 1.3×10^{10}ly we get 1.8×10^{12}ly.
>> The furthest out anything can have traversed at the speed of light to reach
>> is from that distance and from the earliest near Planck time in the
>> universe. What this means is the source or emitter of this graviton was
>> early on close to our region and the source is not that incredible distance
>> away.
>>
>> LC
>>
>
> Is this estimate reasonable, also from
>
> https://www.forbes.com/sites/startswithabang/2020/05/19/would-a-long-journey-through-the-universe-bring-us-back-to-our-starting-point/#fe376fef6c50
>
>
> The appearance of different angular sized of fluctuations in the CMB
> results in different spatial curvature scenarios. Presently, the Universe
> appears to be flat, but we have only measured down to about the 0.4% level.
> At a more precise level, we may discover some level of intrinsic curvature,
> after all, but what we've observed is enough to tell us that if the
> Universe is curved, it's only curved on scales that are ~(250)^3 times (or
> more than 15 million times) larger than our presently-observable Universe
> is.
>
> AG
>
What I'm asking is whether, based on current measurements, if the universe
is curved, can we conclude that the universe is *15 million times larger*
than our presently observable universe? TIA, AG
>
>
>
>
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>
>
>>
>>
>> On Tuesday, May 19, 2020 at 1:41:45 AM UTC-5, Philip Thrift wrote:
>>
>>>
>>>
>>> *Would traveling out in a "straight" line bring you back to where you
>>> started?*
>>>
>>>
>>> https://www.forbes.com/sites/startswithabang/2020/05/19/would-a-long-journey-through-the-universe-bring-us-back-to-our-starting-point/#1781c2ccf6c5
>>>
>>> In the writer's (Ethan Siegel's) *opinion*:
>>>
>>>
>>> On a cosmic scale, there is no indication that the Universe is anything
>>> other than infinite and flat. There is no evidence that features in one
>>> region of space also appear in any other well-separated region, nor is
>>> there evidence of a repeating pattern in the Universe's large-scale
>>> structure or the Big Bang's leftover glow. The only way we know of to turn
>>> a freely moving object around is via gravitation slingshot, not from cosmic
>>> curvature.
>>>
>>> And yet, it's a legitimate possibility that the Universe may, in fact,
>>> be finite in extent, but larger than our observations can currently take
>>> us. As the Universe unfolds over the coming billions of years, more and
>>> more of it (about 135% more, by volume) will become visible to us. If
>>> there's any hint that a long-distance journey would bring us back to our
>>> starting point, that's the only place we'll ever find it. Our only hope for
>>> discovering a finite but traversible Universe lies, quite ironically, in
>>> our far distant future.
>>>
>>> @philipthrift
>>>
>>
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