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Before we can determine the size of the Universe, do we not have to determine first if the Universe is infinite or finite because an infinite Universe would have no size ?

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    $\begingroup$ That is very clearly true. An infinite universe can't be measured in size because it just keeps going forever. Why would you think otherwise? $\endgroup$
    – James K
    Jun 1 at 19:59
  • $\begingroup$ If the big bang theory is correct then the universe was a finite size. How can something of a finite size become infinite in size? $\endgroup$
    – Aaron F
    Jun 3 at 16:03
  • $\begingroup$ also, relevant recent xkcd cartoon: xkcd.com/2622 $\endgroup$
    – Aaron F
    Jun 3 at 16:04
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    $\begingroup$ @Aaron The big bang theory doesn't rule of an infinite universe, but if the universe is infinite, then it was already infinite at the moment of the big bang. A good explanation is given here: physics.stackexchange.com/questions/136860/… $\endgroup$
    – D. Halsey
    Jun 5 at 22:48

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We do not know whether the universe is infinite in size.

However, we have a strong suspicion, based on evidence that supports the big bang theory, that it is finite in age, at roughly 13.7 billion years. In that time, light - or any other effect that we know about, such as gravity - can only have travelled a certain distance, so we can only see so much of the universe. This is usually called the observable universe, and it is the size of that which is usually quoted.

The cosmic microwave background (CMB) is the effective edge of what we can see using electromagnetic radiation (light, radio waves etc) - the visible universe. It is an event that emitted light in all directions in all places at once (within some differences much smaller than the total age). That's not quite the beginning of the universe, but it is pretty close at around 400,000 years since the beginning. The CMB is estimated as around 45 billion light years away. The discrepancy here with the age of the universe is that the universe is also expanding, and when the effective distance to the objects is taken into account due to this expansion, it can give measurements larger than the distance light can travel during the age of the universe.

As the universe ages, the distance we can observe expands further. Interestingly, objects outside this horizon are also moving away from us at a significant fracton of the speed of light, so they get revealed at a slower and slower pace (there is some evidence that this rate could be expanding, so our effective observable horizon could even shrink in future).

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    $\begingroup$ " As the Universe ages, the distance we can observe expands further " The distance we can observe does not expand further. Quite the opposite is true because as the Universe is expanding these outermost Galaxies become lost to us forever . $\endgroup$
    – Peter U
    Jun 2 at 18:48
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    $\begingroup$ @PeterU Yes, we're losing the furthest galaxy clusters, but the distance still increases. OTOH, there are various definitions of distance that are applicable at the cosmological scale. physics.stackexchange.com/a/63780/123208 $\endgroup$
    – PM 2Ring
    Jun 6 at 18:29

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