Let us think about the rigidity of space and Universal expansion again. As we discussed in a prior post, spacetime is incredibly rigid, but it expands nonetheless. And, despite this unrelenting and ever-accelerating expansion, the matter within it condenses into Galaxies rather than evaporating into space. Strange. Perhaps another analogy would be useful here.
Imagine taking a typical balloon filled with air, and then drawing several quarter-sized circles on it (the circles will be our galaxies). If we then let the air out of the balloon, the circles will get smaller and move closer together until they become a cluster of small dots like Cheerios.
Now, what if we refill the balloon? As the balloon grows in size, the circles will expand away from one another in much the same way that galaxies in the observable Universe do. Interestingly, as this happens there will be no apparent central point away from which the circles move; instead, they will all simply drift away from one another.
Actually, the central point could be the center of the balloon, but this analogy is meant to be reflected by the balloon’s surface.This analogy is quite good for conceptualizing Universal Expansion, except for one problem: as the balloon swells in size, the circles also enlarge. Conversely, galaxies do not grow as the Universe expands. Imagine blowing up this same balloon, but rather than the circles growing as the balloon gets larger, they move away from one another but remain the same size or even shrink.
Counter-intuitive, to say the least.
Since we have seen that familiar terms such as warping, stretching and bending can be applied to Spacetime (think Gravity Wells), we may wonder whether other physical attributes can also be applied, if only in metaphor. We know, for instance, that there are a couple ways to make water boil. One is to heat it up, but another is to subject it to near-zero pressure. If water is subjected to a near-perfect vacuum, it will begin to boil as the oxygen and hydrogen within it evaporate into gas. So, what appears to be a very stable substance under normal environmental conditions (at least in the typical kitchen), can be placed under more extreme conditions that cause it to break down in some way.
If you place the palms of your hands together so that they form a somewhat airtight seal and then cup them, you will experience the suction of air as the pressure between your palms drops below the pressure of the surrounding area. Of course, no matter how hard you try, you will be unable to produce a complete vacuum - or even a marginally strong one.
Yet, there are more extreme conditions under which cavitation occurs in more noticeable, and in some cases, damaging ways; such as on propellers and in pumps. Part of the trick to making submarines stealthy, for instance, lies in minimizing cavitation that can result when propeller blades slice through water at high speeds. Low pressure builds on the backside of the propellers, which produces noisy bubbles – a bad thing if you hope to remain undetected.
Could there be a counterpart to this in the realms of spacetime? Everything has boundaries at which it will succumb to stress and begin behaving outside of what we may think of as its typical character. Even a simple piece of steel will break if subjected to a greater force than it can withstand. Burning wood produces heat energy, but also destroys the wood. Compressing matter beyond a certain point results in fusion. In truth, the natural behavior of any substance is as much a function of environment as anything intrinsic.
So, in any case and for any substance, the characteristics of an object are changed by pressing it beyond certain limits - by changing its environment. Now, how would this apply to Universal expansion? What happens when something as incomprehensibly stiff as spacetime is eternally and relentlessly stretched? Can the fabric of space - spacetime itself - be stressed? And if so, what would happen?