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Thread: What would the edge of the Universe look like?

  1. #1
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    What would the edge of the Universe look like?

    I've heard it said the Universe looks spatially flat in all directions, with neither positive or negative curvature. So first question, what is the observational evidence for that flatness (i.e. how do you tell it's flat rather than positively or negatively curved).

    Second question: I've heard it said the whole Universe must be hugely bigger than the observable Universe because the observable Univserse looks flat - i.e. if the whole Universe was only as big as the observable Universe you would see it curving out towards 'the edge'. Is that right (or at least a plausible hypothesis)?

    Thanks!

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    Quote Originally Posted by Cheap Astronomy View Post
    what is the observational evidence for that flatness...
    I think the answer is contained in this old thread from 2012. Basically, if the universe is flat, the angles of a triangle within it should add to 180 degrees. A really big triangle was checked and found to add up to 180 (or close to it).
    Everyone is entitled to his own opinion, but not his own facts.

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    As far as we know, it's not possible to see any "edge" to the Universe because we're viewing it from the inside .According to current models, anywhere you look, you'd see space in all directions looking pretty much the same..You'd find no center or edge.in any location.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    Quote Originally Posted by Cheap Astronomy View Post

    Second question: I've heard it said the whole Universe must be hugely bigger than the observable Universe because the observable Univserse looks flat - i.e. if the whole Universe was only as big as the observable Universe you would see it curving out towards 'the edge'. Is that right (or at least a plausible hypothesis)?

    Thanks!
    It's not that the whole universe "must be" hugely bigger than the observable universe. What is suggested is that if the universe is curved and not flat, then the observable part we see must be only a tiny fraction of the whole universe. This is because the observable appears to be flat (or as near as dam it to).
    However the universe could be in a number of possible configurations: flat and finite, flat and infinite, curved and finite, curved and infinite... all sorts of shapes, even one bubble in many. AFAIA the current consensus is that the universe is flat and is bigger than that which we can observe, but we have no idea what shape (if any) or how big it is.

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    Quote Originally Posted by Cheap Astronomy View Post
    Second question: I've heard it said the whole Universe must be hugely bigger than the observable Universe because the observable Univserse looks flat....
    I don't think that's the reason the universe is thought to be much larger than the observable universe. I'm not sure if the original reasoning still holds, but some decades ago Alan Guth came to that hypothesis based on his idea of cosmic inflation...

    Quote Originally Posted by Guth
    "...the entire universe is expected to be at least 1023 times larger than the observed universe! ...if the inflationary theory is correct, then the observed universe is only a minute speck in a universe that is many orders of magnitude larger."
    Everyone is entitled to his own opinion, but not his own facts.

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    We can only see things, whether by eyeball or instrumentation, when photons, or other particles set off events in a detector.Years ago( circa 2008) Sunyaev & Chluba summarised the image of the CMB, and how it would be distorted by the recombination of helium, known to be a small portion of the models in Big Bang cosmology( about 8 %).
    Their exhaustive treatment predicts a slight shift of the baryonic acoustic spectrum which is both time dependent, and frequency dependent. This is due to the fact that the first ionization of helium is dependent on more energetic photons than the ionization of hydrogen....thus it occurs at a higher temperature.
    As the Big Bang plasma cooled while expanding, the helium recombinedd before the hydrogen did, since the temperature dropped, lowering the number density of photons still energetic enough to ionized helium. Thus the helium recombine first and released energetic UV photons as it did.
    This process causes those photons to get reprocessed in the Big Bang plasma, and change the distribution of the various species, in that plasma. The rates for all those species are well known from decades of fusion studies, so the computer simulations of how the CMB will change are very accurate...to nanokelvins since about 2009.
    The interesting part comes in Sunyaevs concluding remarks about unexpected changes, referring to figure #20. Changes in free electron fraction due to partial frequency redistribution, including atomic recoil and doppler broadening and doppler boosting.
    This actually does now have the unexpected confirmation of a six order of magnitude increase in cross-sections for nuclear recoil due the recently confirmed CEvNS effect at Oak Ridge Spallation Neutron Source by Akimov at al.
    That should change the surface of last scattering a bit again.
    The calculation is left as an exercise for homework.
    Nice paper by Sunyaev and Chluba sets it up. Pete

    See:https://www.google.com/url?sa=t&sour...bku2rguTImDmo2

    Last edited by trinitree88; 2021-Aug-24 at 03:26 PM. Reason: smilie

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    Quote Originally Posted by trinitree88 View Post
    ...Years ago( circa 2008) Sunyaev & Chluba summarised the image of the CMB, and how it would be distorted by the recombination of helium, known to be a small portion of the models in Big Bang cosmology( about 8 %).
    Well, that's interesting. The helium-hydrogen reionization difference is illustrated in their Fig. 1:

    At high redshift the Universe was completely ionized. As it expanded and cooled down in went through several stages of recombination, starting with HE III→HE II recombination (z∼7000), HE II →HE I recombination(z∼2500), and ending with the recombination of hydrogen (z∼1000).
    Everyone is entitled to his own opinion, but not his own facts.

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