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Diameter (radius) refers here to the distance the laser beam can travel to reach matter before Dark Energy (and the expansion of the universe) will make this impossible.

Hubble's constant implies an expanding universe in which clusters of galaxies are moving away from each other. After certain distance travelled the laser beam will not be able to reach the next galaxy cluster anymore because dark energy will have taken over, and even gravity will not be enough to hold galaxies together.

Is this distance bigger/equal/smaller than the observable universe? Can we already determine which galaxies are the candidates when the laser beam reaches this distance?

The observable Universe is according to Wikipedia: Diameter: $8.8 \times 10^{26}~\mathrm{m}$ (28.5 Gpc or 93 Gly)

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  • $\begingroup$ I don't understand what you mean by "... because Dark Energy will have taken over." Can you clarify? $\endgroup$ – garyp Jul 29 '16 at 12:29
  • $\begingroup$ Try googling event horizon of the universe $\endgroup$ – Ihle Jul 29 '16 at 14:49
  • $\begingroup$ @garyp I suppose they means "until the accelerating expansion of the Universe will not let it reach further" (in comoving coordinates). $\endgroup$ – Thriveth Jul 29 '16 at 17:12
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The distance is much smaller than the currently observable Universe, and it is possible to estimate which galaxies lie within it, except it's a lot of galaxies.

As @Ihle says in their comment above, the distance you are looking for is equal to our cosmic event horizon. This is normally defined as the distance beyond which light emitted now will never reach us, but by symmetry, it should be straightforward to see that it is the same.

Color version of spacetime diagram from Davis & Lineweaver 2003

The figure above by Tamara Davis (color version of figure from this paper ) shows that this distance is currently $\sim 15$ billion light years,a good deal smaller than the observable Universe. Furthermore, the size of the observable Universe will grow indefinitely in both absolute and comoving coordinates, while our event horizon will approach a finite size of $\sim 18$ gigalightyears in absolute size and zero in comoving size.

As the figure caption says, the distance currently corresponds to a redshirt of around 1.8, which is easily observable with current technology.

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  • $\begingroup$ Great figure found, it explains everything on cosmological horizons $\endgroup$ – Bob Bee Jul 30 '16 at 1:51

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