Back in the old days of cosmology – say, 1990 or so – the universe was a simple place. There was this Big Bang, see, and that caused all the galaxies to fly apart, giving us the expanding universe that we actually observe. And at some future time, the universe might (or might not) slow down enough, from gravitational attraction, to cause all the galaxies to fall back together again in a hypothetical "Big Crunch".
And then something very, very odd was discovered: the universe is not only expanding, but the rate of expansion is accelerating.
And that's when cosmology began to get really, really screwy.
The problem with the accelerating expansion is this: what happened to the gravity? Since gravity pulls matter together, gravity should cause the expansion of the universe to slow down as time goes on.
But if the expansion is accelerating, that means that there must be something else out there to cause it to accelerate. And that something else must be very, very powerful. In fact, it must be huge. It must be so colossally huge that it's much, much greater than all the gravity in the universe, which means it must be much greater and stronger than all the matter and mass in the universe. The something else must be so humongous, in fact, that most of the entire universe must be made up of that something else, leaving very little for normal stuff like matter and energy.
And they called that something else Dark Energy. Dark, because nobody knows what it is, or what it's made of, or why it's there, or what its properties are, except, of course, that it causes the universe to expand, and to accelerate expanding like some giant cosmic anti-gravity force. Wow!
The problem with Dark Energy is that the whole thing seems so damned fishy. Like a just-so story for children, Dark Energy is a magical force tacked on to the universe to make the theory just exactly match the observations. Even worse, an accelerating expansion implies violating the conservation of energy — a tiny "little" problem cosmologists like to conveniently ignore. Like not thinking of an elephant.
Ugh. And ugly.
There have been a lot of challenges to this picture. So far, none has caught on. But the latest, a fascinating idea by Dr. Wun-Yi Shu of National Tsing Hua University in Taiwan, just might have legs.
Dr. Shu proposes that the expansion and acceleration of the universe are natural byproducts of the way the universe is constructed. He proposes a universe with no beginning or end, no Big Bang, and no Dark Energy. And in order to make it all work, Shu proposes that the speed of light varies with time; and further, that distance varies with time, too. Dr. Shu says:
Since there are three basic physical dimensions, any cosmological model requires two constants. Einstein took c [speed of light] and G [the gravitational constant] as the two constants, whereas we assert that the two constants are κ , the factor relating to the conversion between time and length, and τ , the conversion factor between mass and length.
In other words, c and G are not constants in Shu's cosmology, but vary with cosmological time.
Like most classic cosmologies, Shu's theory allows for three possible geometries for the universe: open, closed, and "flat" (an intermediate geometry poised on the cusp between open and closed). In his paper, Shu prefers the closed universe, which has a three-dimensional cross section that is spherical. The radius of this sphere – that is, the size of the universe – is expressed by a variable he calls a. Here's a plot showing how the size of the universe varies with time in a closed Shu cosmology:
In this diagram, t is time, a is the hyper-radius of the universe's 4-dimensional hypersphere, while M and σ are proportionality constants. On the left-hand side of the diagram, the universe is expanding, and the expansion is accelerating; that's where we are now. In this cosmology, the speed of light is increasing on the left-hand side too, until time t=0, when the speed of light instantaneously reaches infinity. Then we're on the backside of the curve, with a shrinking universe, and the pace of shrinkage decelerating.
There are similar diagrams for the flat and open (hyperboloid) geometries too.
While neither of these universes has a beginning, both of them do have an end, as each approaches an ending time asymptotically. The closed universe has no end.
Naturally we want to know: how does a Shu universe fit with actual observations? This tends to be the fly in the ointment for most non-standard cosmologies, because the observed rates of expansion and acceleration of the universe don't allow for much wiggle room. But as it turns out, when you're allowed to play with the speed of light and with intrinsic distance, you can make a lot of things fit quite nicely indeed.
The plot here shows several datasets of Type Ia supernovas, all of which should theoretically have the same intrinsic brightness. The measured brightnesses (from Earth) are on the vertical axis, and the supernovas' redshifts (i.e., their radial velocities, hence their distances) are on the horizontal axis. The black curve shows what the theoretical relationship should be in a Shu cosmology, after choosing proportionality constants to most closely match the data. As you can see, the fit is quite good.
Not only that, Shu's theory nicely avoids some other longstanding cosmological issues. From PhysOrg.com:
Shu’s models may also account for other problems faced by the standard big bang model. For instance, the flatness problem arises in the big bang model from the observation that a seemingly flat universe such as ours requires finely tuned initial conditions. But because the universe is a 3-sphere in Shu’s models, the flatness problem “disappears automatically.” Similarly, the horizon problem occurs in standard cosmology because it should not be possible for distant places in the universe to share the same physical properties (as they do), since it should require communication faster than the speed of light due to their great distances. However, Shu’s models solve this problem due to their lack of big bang origin and intrinsic acceleration.
But there are still some hills to climb for Shu's theory. First, there is no explanation for the cosmic microwave background. This has traditionally been explained as the "echo" left over from the Big Bang. Obviously, no Big Bang, no echo. Is there another possible explanation for the CMB? If Shu is right, there would have to be. But so far, we have no idea of what it might be. Also, the problem of original nucleosynthesis has not been addressed, i.e., how the universe's original hydrogen formed, and exactly what the early (or earlier, if you will, since the Shu universe has no beginning) universe looked like.
Shu's work also has not yet been peer-reviewed; it has been published on Physics ArXiv, a blog used by physicists to preview their work to others before formal publication. But it is already generating some interesting discussion there.
Given that, it's still too early to tell if Shu's cosmology will upset the apple-cart. But I for one hope so. Dark Energy is such a godawful mess.