One of the great outstanding challenges of modern science is to explain the observations that point to the accelerating expansion of the universe.
These come from astronomers who say the most distant supernovas are dimmer and so further away than they should be if the universe were merely expanding. Instead, the expansion must be accelerating, they say.
The conventional explanation for this acceleration is that the universe must be filled with an unseen or dark energy that is forcing this process.
That’s something that many physicists feel uncomfortable with. Conventional calculations of the universe’s vacuum energy arrive at a number that is 120 orders of magnitude smaller than dark energy must have. Then there is the small problem of conservation of energy which dark energy seems to violate. It’s an altogether unsatisfactory state of affairs.
There’s another seemingly unconnected problem that cosmologists are wrestling with: the abundance of elements that must have been created in the Big Bang.
Our models for the Big Bang and how the universe grew in its first few minutes make very precise predictions about the abundance of elements that must have been created in this process.
For example, there must have been lots of hydrogen, deuterium and helium-4. And the measurements of this stuff more or less exactly match the predictions.
However, the theory also predicts that a certain amount of lithium must have formed too. The trouble is that, as far as we can see, the universe contains only about a third of this amount. That has caused more than a little head scratching
Now Marco Regis and Chris Clarkson from the University of Cape Town in South Africa say they can explain this shortfall in lithium. What’s extraordinary, however, is that the same thinking also explains the supernova observations without any need for an accelerated expansion or dark energy.
Their new idea is that the lithium abundance can be explained by abandoning one of the fundamental assumptions of modern cosmology: the Copernican principle. This is the notion that humans have no privileged position in the universe. For cosmologists, this means that the universe must be more or less the same everywhere and on all scales.
Various cosmologists have pointed out that if we abandoned this principle, it would be straightforward to explain the supernova data. It simply means that the universe is not homogeneous on the very largest scale. Instead, we must be sitting at the centre of some kind of giant void in a much larger universe.
Now Regis and Clarkson say the same kind of thinking-that there are various irregularities in the way that stuff is distributed in the universe–can explain the lithium shortfall.
That’s an interesting contribution to this debate. That the same idea seems to explain two seemingly unconnected observations is a powerful reason to look at it more carefully.
On the face of it, there seems little to lose from abandoning the Copernican Principle on this scale. After all, why should stuff in the universe be evenly distributed on this scale?
However, this introduces an uncomfortable problem. Regis and Clarkson’s claim is that the Universe contains a region that is short on lithium. That’s not so hard to accept. What’s difficult to swallow is that if true, the observations indicate that the Earth is at the very centre of it.
That would seem to be an extraordinary coincidence, one that Regis and Clarkson say has a chance of of only 1 in 10^8 of occurring.
But they also point out that this has to be compared with the problems with the standard model of physics which is out by 120 orders of magnitude compared with the thinking behind dark energy.
Take your pick. Either way, it looks as if cosmologists will have to do some mighty hard thinking to get us out of this bind.
Ref: arxiv.org/abs/1003.1043: Do Primordial Lithium Abundances Imply There’s No Dark Energy?