Doctor Who’s TARDIS is unique in that it appears larger on the inside than it does from the outside. According to scientists from the University of Helsinki, our universe may exhibit similar properties, a phenomenon they're dubbing Tardis spacetime. It’s a new theory that could solve a longstanding mystery in cosmology.
Top image courtesy Rob Semenoff.
Scientists aren’t entirely sure why the universe is accelerating. To that end, they’ve posited a number of theories, including the presence of a mysterious dark energy that’s causing space to expand at an accelerating rate.
But according to Mikko Lavinto and colleagues, the universe isn’t actually expanding. It only looks that way from our perspective. Our view of the cosmos, they argue, is the product of an optical illusion created by regions of space that are bigger on the inside than they appear on the outside.
Inspired by Doctor Who’s TARDIS, they’ve dubbed this the Cosmological Tardis Model (also described as a “Swiss Cheese” model of the universe littered with “inhomogeneous holes”).
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.
In addition, Tardis regions should appear like parts of the universe that are particularly low density. And fascinatingly, our universe does indeed feature such regions, which are called voids. These are huge swaths of the cosmos that have far fewer celestial objects than other parts of the universe.
The solution is far from perfect, however, in that it doesn’t exactly mesh with our observations; when light enters a Tardis region, it should bend sharply owing to the greater curvature. But this isn’t what astronomers observe, so Lavinto and colleagues still have their work cut out for them in explaining why we (appear to) observe light that travels in the same direction.