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## Main Question or Discussion Point

I read that the radius of the universe is around 1.3x10^26 meters but I did not find how this size has been calculated.

Thanks if you can explain to me

Thanks if you can explain to me

- Thread starter denism
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I read that the radius of the universe is around 1.3x10^26 meters but I did not find how this size has been calculated.

Thanks if you can explain to me

Thanks if you can explain to me

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No one knows the size of the universe. The Observable universe is some 90 billion light years in diameter and various estimates put the minimum size of the universe as being that much to many orders of magnitude larger. Some believe it is infinite.

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It's like religions. There are many of them and they contradict each other so I ignore them all. My own belief is that the universe is likely infinite but I have no facts to back that up so it is just an unfounded personal opinion. It may be finite but unbounded.

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bapowell

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The issue is that there is absolutely no reason to believe that there

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Chronos

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There is plenty beyond the hubble sphere - all the way to the particle horizon.

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That's not an assumption or a "postulate", it is fact. The galaxies at the edge of our observable universe are receding from us at about 3c.In fact,this belief is only based on the assumption that recessional velocities can exceed light speedbut I never saw any observational support to this postulate

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thanks

how this fact has been determined?...or indirectly infered?

how this fact has been determined?...or indirectly infered?

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http://en.wikipedia.org/wiki/Redshifts

it is stated that for a flat Minkowski space and for light motion in the transverse direction

1+z=1/sqrt(1-(v/c)^2)

from which z is infinite when v=c

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bapowell

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Of course, if postulating Vrec=HD and H constant, one easily obtains Vrec > c beyond a certain distance, but this appears purely theoretical to me. I am afraid that there is not a single supporting observation in this story

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bapowell

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And you say this why? Because wikipedia hasn't presented you with any? It would be wise to delve deeper before making such sweeping, contrarian statements.I am afraid that there is not a single supporting observation in this story

What would a single supporting observation look like to you?

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I did not want to be contrarian. Considering my poor knowledge in this field, I am sure that data (more convincing than the theoretical extension of Vrec = HD) should exist. This was precisely the purpose of my question

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George Jones

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Good models of the universe are (perturbed) Friedmann-Robertson-Walker (FRW) spacetimes, and, properly interpreted, the Hubble relationship is predicted by FRW models. Most cosmologist think that there are gigabytes of observational data that support FRW models.

Also. you might be concerned about recessional velocities greater than c, because a law of physics is that "No physical velocity can exceed c."

1) This a property of special relativity, and, as you have already been told, the universe follows general relativity, not special relativity. Special relativity is a local approximation to general relativity. In general relativity, no observer (including those in FRW spacetimes) ever measures the speed of a local particle to be greater than c. This is consistent with both the "no speed greater than c" aspect of special relativity, and the "special relativity is a local approximation in general relativity" aspect of general relativity.

2) The term "velocity" is used in different ways in special and general relativity. The way "velocity" is used in general relativity is something like the way the term "velocity parameter" is used in special relativity, and, even in special relativity, velocity parameters can take on values that are greater than c.

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I have an other question with respect to your answer: I wonder why SR is not sufficient for studying the geometry of expanding universe models, supposed to be isotropic and homogeneous as empty. General relativity deals with gravity?

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Chronos

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SR does not take gravity into account. That was the whole point of GR.

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George Jones

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I am not sure what you mean. How is this necessarily a redefinition of the line element of SR?^{2}=(cdt)^{2}+a(t)^{2}dl^{2})

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Furthermore, even SR seems to be not observed: calculations using speed substractions such as c-Vrec, rather resemble to classical mechanics ... even if I understood that Vrec is not a genuine speed.

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There's no evidence - yet - that there's anything far beyond the present horizon. An infinite flat space-time is just the simplest topology to assume. Alternative theories exist for a range of different space-time structures which are consistent with the data, some of which are finite and some are actually smaller than the current apparent horizon. The problem with assuming the limits of what we currently see are the actual physical limits is the implication we exist in a privileged moment of history - a possibility that needs a theoretical explanation that'd convince other cosmologists or they'll stick to their current assumptions.

I did not want to be contrarian. Considering my poor knowledge in this field, I am sure that data (more convincing than the theoretical extension of Vrec = HD) should exist. This was precisely the purpose of my question

The limits of what we can see are about x3 times the current Hubble light-travel-time. A good place to learn about all the different cosmological distances is Professor Ned Wright's Cosmology Tutorial...

Ned Wright's Cosmology Tutorial

...which features a handy Java calculator of all sorts of cosmic parameters. So handy that people have referenced it in their papers.

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does "infinite flat" means euclidean 3D?An infinite flat space-time is just the simplest topology to assume. .

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bapowell

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"Flat" means Euclidean 3D. It need not be infinite.does "infinite flat" means euclidean 3D?

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