Earth's billion parsec bubble

Does the Earth exist in an enormous cosmic void?

That's the radical idea being put forward by Oxford physicists Tim Clifton, Pedro Ferreira and Kate Land in a paper in Physical Review Letters that led Space.com to write 'Do we live in a giant cosmic bubble?' OxSciBlog caught up with Tim Clifton and quizzed him about the work:

OxSciBlog: What are 'abnormal bubbles of space-time'?
Tim Clifton: The 'abnormal bubble of space-time' referred to is a region of the Universe that is about 1,000,000,000 parsecs wide, about 2/3 underdense in the centre, and with us living somewhere near the middle. Such an enormous structure is very unlikely to have formed in the standard cosmological model of the early Universe, and having us living in a special place (near the centre) is in disagreement with the Copernican Principle that "the Earth is not in a special place". Nevertheless, if such a bubble existed then we could explain the observations of distant supernovae without the need for any mysterious Dark Energy.

OSB: What evidence is there that we are trapped in one?
TC: The observations of distant supernovae could be thought of as pointing towards this. When interpreted in the usual cosmological models these observations show that the Universe is accelerating in its expansion, and so there must exist an exotic substance (Dark Energy) making it accelerate. An observer in a giant underdense region of the Universe, however, would expect to see something similar to the observed data, and would not have to invoke any new types of matter in order to explain it. In this sense, the accelerating expansion of the Universe would be a type of mirage, resulting from interpreting the data in an incorrect model. Of course, there are lots of other reasons for thinking that the standard cosmological model is a good one - the microwave background, large-scale structure in the Universe, primordial nucleosynthesis and many other phenomena that astronomers observe.

OSB: What would be the implications if we were?
TC: If it should turn out that we really were in a giant underdense region of the Universe then there would be many stunning implications. Firstly, there would be no need for Dark Energy. Most would consider this a positive step. Secondly, the Copernican Principle would have to revisited. If we lived near the centre of a Universe that was approximately symmetric around us, then the Copernican Principle may have to be re-thought. This would be a radical departure from our normal picture of the Universe, and one that most would find very unattractive. Another implication would be that we would have to rethink our picture of the early Universe, and the formation of structure in the Universe. A structure the size of this bubble would be very unlikely to occur in the usual model, although there are some reasons to think that it is not entirely implausible. In short, the 'bubble' Universe can remove the need for Dark Energy, but if found to be true would require a radical re-thinking of a lot of the rest of our current model of the Universe.

OSB: How could such an idea be tested?
TC: Although the 'bubble' model reproduces approximately the same supernova data as the Dark Energy model, the details of what one should expect to observe will be different. At present the supernovae observations are not quite good enough to tell the difference between the two models, but the upcoming supernovae missions that are being proposed should be able to gather enough data to decisively tell the two models apart. There are other data sets that could be used to test this model as well. The microwave background, for example. As yet the calculations of what one should expect the microwave radiation to look like in the 'bubble' model are not well understood. We are working on this now, and hope to have results soon. Others have suggested using distant galaxies as mirrors, so that by looking at radiation that is deflected off them towards us, we should be able to tell something about the existence, or not, of a giant 'bubble'. This is a very promising idea, but again the current observations do not yet appear quite good enough to decisively distinguish between the two models.

Dr Tim Clifton is a postdoctoral researcher in Oxford's Department of Physics

Image: Soap bubble by Interiot courtesy of Wikimedia Commons.