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Hubble volume: the observable part of the ►universe. Due to the finite ►speed of light, only a spherical region of outer space is visible to us. This is because light rays emitted by celestial bodies beyond that sphere would take longer than the entire age of the universe to reach us. If the universe did not expand, the radius of this sphere (13.7 giga-lightyears) would correspond precisely to its age. Since, however, the universe does expand with increasing speed and as a consequence all distances continually grow, distant objects are as a rule no longer at the place at which we see them. They may in the meantime be located far beyond the limit of 13.7 giga-lightyears. Hence, the Hubble volume has by now acquired a radius of about 46 giga-lightyears* or 4.4 ∙ 1026 meters.* This volume contains about 100 billion galaxies, 1021 solar systems, 1078 atoms as well as 1088 photons (light particles). Its mass thus amounts to about 1053 kg and its density to 2.3 ∙ 10-26 kg per cubic meters. Light from the most distant visible objects, the ►quasars, requires up to 13 billion years to reach us. Thus, when we see a quasar, we see it in the state it was in when it emitted the light. What it looks like today, or even whether it still exists at all, cannot be known, since the quasar has in the meantime moved about 40 billion light-years further away from us due to the expansion of space.** Sometimes this fact results in ambiguities when we try to state the ►distances of far-away celestial bodies. Frequently, only our distance from its earlier location (where it was at the time it emitted the light we now see) is stated, not our distance from its present position. Additional confusion arises from the fact that in the literature, the Hubble volume is often confused with the universe itself. The speric shell of the Hubble volume is an ►event horizon not unlike that of a ►black hole, since anything located beyond this shell remains forever inaccessible to observation. To be sure, what part of the universe is included in the Hubble volume depends on the location of the observer. Since the latter is always at the center of this region, residents of the ►Andromeda galaxy, for example, will see a somewhat different Hubble volume from ours. The Universe on Our Desk The following comparison may serve to illustrate the proportions within the Hubble volume. Let's assume that the observable part of the universe is right in front of us on our desk. In this case it would have a diameter of about 2 meters. Galaxy clusters create faint spots and ribbons in the millimeter region. With a strong magnifying glass or a simple microscope, we can see the individual galaxies as tiny light spots; a galaxy is about the size of a dust particle (0.002 millimeters) and located about a tenth of a millimeter from its neighboring galaxy. From this point of view, cosmic distances are not that great after all. Not even a scanning electron microscope, however, would suffice to observe our solar system on this scale; having a diameter of about 10-11 millimeters, it would be far smaller than an atom. * This value is based on the wavelength of cosmic ►background radiation. You'll find a formula for its calculation in the footnote of the article on ►distance. ** Thus, your departure from the celestial object is faster than that of the light. This does not contradict ►special relativity theory, according to which the speed of light is the greatest possible speed. That restriction applies only to motion within space, not to the expansion of space itself.
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