Destiny of this Universe
Will this universe exist forever or will its present expansion be replaced by a contraction ending in a big crunch? To this question contemporary cosmology can only answer: It all depends! - Technically, it depends on the change with time of the radius of 3-dimensional space. This again depends on the change with time of the mean matter-density and the universal radiation pressure together with the curvature of space. Thus, if one can determine these three quantities, the future destiny of the universe can be decided.
The images above serve to illustrate the curvature of 3-dimensional space. However, the depicted objects are not 3-dimensional, they are just 2-dimensional - like a thin sheet of paper. The reason, that you can see the differing curvatures, is that you are sitting in a 3-dimensional world. The concept of curvature also applies to higher dimensions. If the universal curvature is negative or zero the expansion will continue forever - the universe is said to be open. If the curvature is positive the universe is closed and the expansion will, sooner or later, be replaced by a contraction. - Unfortunately, one needs a 4-dimensional outlook in order to be able to see the curvature of a 3-dimensional object. Science has not yet learned how to utilize the circumstance that we live in a 4-dimensional space-time continuum, so there is no presently known method to determine the curvature of the universe directly.
Now, here the mathematics of General Relativity come to our aid. Combining equations, it turns out that the curvature of the universe can be determined if one knows its present matter-density and the overall radiation pressure as well as the change with time of these quantities. Essentially, the issue is to determine the amount of matter in the universe as compared to the number of photons. This, however, is not an easy task. Although physics is close at having a pretty good picture of the whole set of physical realms throughout cosmos, there still are several riddles and unresolved questions left. How far or how close physics is to having it all worked out is hard to tell. However, remedies are continuously improving.
Is the universe a vacuum fluctuation?
Imagine that we live in a negatively curved universe which thus continues to expand forever, forever and forever. Isn't it boring? Besides, if one thinks deeply, it really doesn't make much sense, does it? - No, let's think positively and assume that the universe is closed. A natural consequence is that the overall energy of the universe must equal zero. As explained in detail in the quantum physics section of this site, all physical phenomena in the universe are essentially just various manifestations of energy. In this context matter and gravitation count as negative energy while other kinds of energy count as positive. Summing it all up the result must be zero in a closed universe. As also explained in the quantum physics section, the various kinds of energy are just different manifestation of a universal energy field - the vacuum. Consequently, if the universe is closed, it is merely a vacuum fluctuation. This means that not only the various appearances in the universe originate from the vacuum field, as revealed by particle physics and Quantum Field Theory, but also the very universe itself originated from vacuum and ends up as vacuum.
Assuming that the universe is closed - what would happen at the ultimate end? This question - or rather the equivalent question of what happened at the ultimate beginning - is the topic of the next couple of pages. - But first, let us recall what happened in the very early eras which most probably is what will happen in reverse order at the assumed end.
Towards the Planck Era
Imagine that we either rewind the clock or travel forwards in time. One hundredth of a second before we reach the ultimate extremity, the universe is a fireball in which the energy is so high and dense that no compound bodies exist any longer. Elementary particles and antiparticles are swirling around under influence of the four fundamental forces which still can be distinguished from each other. However, as far as particle interactions go, gravity is insignificant. We move on and 10^-10 (10 billionth) of a second before we reach the extremity, we see the electromagnetic and the weak force unite and become indistinguishable. By now only three forces exist - electro-weak, strong and gravitation. At -10^-32 second the fireball, which is now the size of an orange, undergoes an exponential implosion and collapses to a size 10^+25 times smaller than an atom. Now the energy-density is so high that at -10^-35 second the strong force unites with the unified electro-weak force. This is the GUT Era, which is characterized by particle annihilation. Elementary particles and antiparticles find each other and vanish in bursts of energy. By the end of the GUT Era no matter exists - not even in the shape of elementary particles. At -10^-43 second gravitation unites with the unified GUT-force - and we have reached the Planck Era. (The time intervals are as measured in our usual frame of reference but, according to General Relativity, time slows down when space-time is curled up. Thus, in the local frame of reference, the tiny time intervals are actually very large. An identical comment applies to the 3-dimensional space).
Up to this point, we know what is going on by virtue of Quantum Field Theoretical extrapolations of results from high-energy particle physics laboratories. However, by reaching the Planck Era we enter a no mans land, where every known law of physics has broken down. - Before we continue our time-travel we shall, on the next page, have a theoretical interlude dealing with the crucial concept of space-time singularities.
