Borders of the Universe

When we look out on this strange, apparitional universe, we see that all the distant galaxies appear to be moving away from us, and the farther away they appear to be from us, the faster they appear to be moving away. The evidence for this expansion, and it is usually thought of as an expansion, is the red shift of the spectral lines in the radiation from those distant galaxies. (When a fire engine is approaching us at high speed, we hear the sounds of the bell and the siren at a higher pitch than the pitch which is heard by the firemen on the engine. But, when the engine has passed us and is receding from us, we hear the sounds at a lower pitch. This is called the Doppler shift, and we see the same thing in radiation. The spectral lines of an approaching star are shifted toward the blue end of the spectrum while the spectral lines of a receding star are shifted toward the red end, the low energy end, and are said to be red shifted.) Now this cosmological expansion, as it is called, imposes a boundary on the observable universe, because beyond a certain distance, even if there were galaxies, we couldn't see them. They would be moving away at speeds in excess of the speed of light and it would be impossible for us to see them. It would be impossible for us, by any measurement, to determine the existence of such matter. Radiation from there could never reach us, the gravitational fields could never reach us, nor could any message however contrived. Actually, it is the red shift itself, rather than our interpretation of the red shift, which imposes this boundary, and it is an observational boundary, not an actual boundary. It is not a boundary which could be visited. The observer is always at the same distance from it, in all directions, and, at the present observed rate of expansion, this boundary should be about fifteen thousand million light years away (fifteen American billion). Are there any messages from the borders of the universe which reach us here, and which could be interpreted as evidence that such a boundary does, indeed, exist? Yes, there are. There are several such messages, and one of them is apparent even to the unaided eye. The night sky is dark. We must not let the familiarity of the observation keep us from understanding its significance. If the observable universe were infinite in extent, and if it were speckled with stars as we see it nearby, and if the stars were "forever n then as Kepler and others long ago pointed out, the entire night sky should be as bright as the face of the sun. Under such conditions, looking in any direction which we choose to look, we would see the face of a star with a surface brightness at least approximately equal to the surface brightness of the sun. Partly the night sky is dark because the red shift of the radiation from the distant galaxies which we can see robs that radiation of some of its energy. But mostly the night sky is dark because the red shift of the radiation from beyond about fifteen billion light years away would rob that radiation of all of its energy so that we could see nothing at all. A second such message is related to the rest mass of matter nearby and is apparent to the unaided hand. If the observable universe were infinite in extent, and of a mean density comparable to the mean density nearby, then the rest mass of matter would be infinite and it would be impossible to shake a stick. Each proton sees itself separated from all other protons in the observable universe, and the gravitational energy involved in this separation is the gravitational rest mass of the proton. It is matched, of course, by its electrical rest mass due to the smallness of the electrical charge. They are two sides of the same coin. Now, with the present known strength of the gravitational field, if the number of protons from which each proton saw itself separated were infinite, then its gravitational energy (its mass) would, likewise, be infinite. Once again, we must not allow ourselves to be thrown by our familiarity with the observation. A finite rest mass can arise only in a finite universe. There is a third message, not so obvious, arising from the extreme red shift of the radiation from very near the border. If, as seen by us, most of the energy of that radiation is red shifted away, then, as seen by us, most of the energy of the particles giving rise to that radiation will also be red shifted away. Then, since E = m, the rest mass of those particles will be seen to be very low, and the radiation moving through the vicinity of those low rest mass electrical particles will be so often absorbed and re-radiated as to reach us thermalized to a black body radiation at about 3 degrees Kelvin. If an observational boundary such as we have suggested does really exist, then this thermalized black body radiation should reach us from all directions in space. Such a microwave background radiation was discovered in the 1960s and is interpreted by the proponents of the "big bang" hypothesis as the radiation of the fireball cooled by some fifteen billion years of expansion. But it is unavoidable even in the steady state model. A fourth message, if it may be considered to be a message at all, depends, as so many cosmological messages do depend, on the model of the universe that is assumed in the interpretation of the evidence. It is related to the density of the universe and, once again, it is not immediately apparent. For a "big bang" model, a model which explains the apparent expansion of the universe as due to a cosmic explosion, a gradual decrease in the overall density of matter in the universe is acceptable. It is not acceptable, however, for a steady state model which assumes that the expansion is beginningless and is driven by the energy which the radiation loses in its long traverse of the vast, expanding spaces of the universe. A universe of finite density cannot result from a beginningless expansion without some mechanism to prevent its decrease in density. Either there must be a mechanism for the creation of new matter within it or there must be a mechanism for the recycling of material from the boundary back into the observable universe. Is there such a mechanism? Curiously enough, there is. And it arises through Heisenberg's uncertainty principle. As the rest mass of the particles near the boundary is seen to approach zero, the momentum of those particles is also seen to approach zero, and if the momentum approaches zero, then our uncertainty in that momentum must also approach zero. But, by the uncertainty principle, if our uncertainty in the momentum of a particle approaches zero, then our uncertainty in its position must approach infinity, and there is then no measurement whatsoever by which we could determine that the particle is near the boundary. If the uncertainty in the position goes to infinity, the particle may be found anywhere. This should give rise to a rain of "brand new" hydrogen throughout the observable universe. It should be noted that this steady state model does not suggest that the expansion of the universe should be constant in time or homogeneous throughout space. Nor does it suggest that the size of the observable universe should remain constant. It only suggests that there should be some sort of mechanism to bring it back to some sore of norm. If, for instance, the expansion rate were somehow doubled, the receding galaxies would reach the speed of light at about seven and a half billion light years from us instead of the currently estimated fifteen billion. Then the protons would see themselves separated from a smaller number of other protons, and their rest mass would thus decrease. But if their rest mass decreases, the rate at which they would fall together by gravity would likewise decrease. Then the radiation rate would go down and the boundaries of the observable universe would again recede, raising the rest mass of the protons. Similarly, if the expansion rate were slowed, the boundaries of the observable universe would recede from us. The proton mass would consequently rise, increasing the radiation rate, which, in turn, would increase the expansion rate and bring the boundary back in.