First some background. Cosmologists have known for almost a century that the universe is expanding. But just over a decade ago, astronomers noticed that the most distant galaxies were accelerating away from us much faster than the ones nearby. In fact, the further they looked, the greater the acceleration seemed to be.

That’s a significant puzzle. Until then, one of the simplest and most popular models of the universe had been that gravitational attraction would eventually stop everything expanding and then cause the cosmos to contract. Instead, the new observations implied that some kind of force must be accelerating the expansion of the entire universe instead. How come?

In a state of feverish excitement, cosmologists quickly got to work dreaming up various answers. One idea is that gravity becomes much weaker over these huge cosmological distances allowing everything to fly apart faster. Another is that the universe is filled with an exotic dark energy that is pushing everything apart.

One final possibility is that the expansion of the universe isn’t accelerating at all but merely looks that way to us.

It’s this third possibility that Tardis spacetime addresses. Mikko Lavinto at the University of Helsinki in Finland and few pals say the entire accelerated expansion thing could be an optical illusion created by regions of space that are bigger on the inside than they look on the outside.

Their method is straightforward. These guys took a standard model of the expanding universe and then replaced certain regions of spacetime with other regions that had a bigger volume but the same surface area. “We have removed a portion of spacetime and ﬁtted in its place another region that ﬁts smoothly into the hole on the boundary, but has larger spatial volume than the removed part,” they say.

That’s possible because different regions of spacetime can have different curvatures. A two dimensional example would be to cut a disc out of a flat sheet and replace it with hemisphere. This hemisphere would have the same circumference as the disc but a greater surface area because it is curved. In other words, this region of space would be bigger on the inside than it looks from the outside.

Lavinto and co then calculated what our Universe would look like if it contained Tardis regions. It turns out that as the universe expands, the volume of Tardis regions grows more quickly and this makes it look as if the expansion of the entire universe is accelerating.

And there is more. Lavinto and co say that Tardis regions would look like parts of the universe that are particularly low density. If that sounds familiar, it’s because astronomers can actually see that our universe is filled with regions called voids that look just like this. These are parts of the universe that have far fewer galaxies than other parts of the cosmos.

The new theory isn’t quite a slam dunk when it comes to describing the real universe. Lavinto and co say that when light enters a Tardis region, it is deflected sharply by the greater curvature there.

That’s not what astronomers observe at all and Lavinto and co have to get around this problem with the standard solution available to all cosmologists developing new theories of the universe—they simply ignore it and continue their calculations as if the light travels in the same direction as it would ordinarily have done.

Nevertheless, the new approach is promising in that it reproduces many of the features of the universe that astronomers can observe while offering a solution to the biggest outstanding problem of all—why the expansion of the universe is accelerating. Their answer? It isn’t!

That won’t be the last word on the matter by any means. But what science fiction enthusiasts will want to know is whether these Tardis regions of spacetime can exist on much smaller scales. Specifically on the scale of the small blue Police box in Dr Who.

Ref: arxiv.org/abs/1308.6731 : Average Expansion Rate And Light Propagation In A Cosmological Tardis Spacetime