The universe has been expanding we think since the beginning Big Bang about 13.6 billion years ago. However, it now appears that the universe is expanding more rapidly than expected. Two competing ideas address this issue. Neither is easily understood.
First, to understand the idea that motion of galaxies are not understood relative to their measured mass, astronomers have invented the idea that there may be missing or unaccounted for mass associated with them. This mass is referred to as “dark matter.” This is a form of matter that seems to be there, but for some reason we can’t see it or account for how it affects the motion of the major objects in the universe that we can see–namely galaxies. Over 80% of the matter in the universe is dark matter. On the face of it, if true, this is an alarming number, but this missing mass is needed to account for the observed velocities of stars in galaxies. The consensus view is that dark matter is composed of new, but not yet identified subatomic particles. However, the existence of dark matter is totally inferred from unaccounted for gravitational effects on the motion of galaxies. The view is that there just has to be more mass out there than there seems to be.
Accompanying dark matter there must be a form of dark energy to which dark matter apparently responds. Together dark matter and dark energy may (if they really exist) represent most of the mass-energy of the universe. In truth, we don’t understand the more motions of the galaxies and the stars that are both more rapid that we can account for from what we can see. However, inventing additional forms of matter and energy to adjust something we don’t understand seems to be a bit like the invention of epicycles for planetary motion to save the Ptolemy’s geocentric theory.
An alternative explanation for a universe which is now beginning to expand beyond a rate that we expect comes from new theories about the structure of space that may ultimately replace relativity. These ideas are referred to as “string theory” and “loop quantum gravity.” I’m not sure that I understand either theory–but I am equally uncertain about who, if anyone, beyond those who have presented them, understand them any better.
One idea coming from these theories is that at some point in the universe (perhaps now) that the discrete structure of space will begin to break down and space will begin to expand because space itself will appear to expand. One outcome will be that the universe itself will, after this new inflationary period begins, cease to appear to age. We wouldn’t really know how to measure that anyway, at least not in any short term sense. We might need something on the order of 100 million years to pass before we could be certain that the age of the universe was not really changing (or in fact slowing down).
While these are certainly interesting ideas and may tell us that we have come to a crossroads in our real understanding of the universe, it is clear that we are at a point where a testable hypothesis relevant to our continuing existence may no longer be possible. What do we look for? What do we measure, indeed what can we measure that is relevant to the current state of the universe.
Since we have only been around for 2-3 million years and only been an interesting species for far less than a million of that period, where will we take these ideas. They are to be sure interesting ideas and we will want to understand them as best we can, but where will we take them?