Doctor Who’s TARDIS is distinctive in that it seems larger on the inside than it does from the outside. According to scientists of the University of Helsinki, our universe may display similar properties, a occurrence they're calling Tardis spacetime. It’s a new theory that could resolve a venerable mystery in cosmology. Scientists aren’t completely sure why the universe is accelerating. To that end, they’ve suggested a number of theories, comprising the existence of a mysterious dark energy that’s triggering space to expand at an accelerating rate.
But according to Mikko Lavinto and colleagues, the universe isn’t essentially expanding. It only looks that way from our perception. Our view of the universe, they claim, is the product of an optical illusion created by regions of space that are greater on the inside than they appear on the outside. Motivated by Doctor Who’s TARDIS, they’ve called this the Cosmological Tardis Model (also defined as a “Swiss Cheese” model of the universe spoiled with “inhomogeneous holes”).
image courtesy Rob Semenoff
The paper, has been distributed in the pre-print archive, arXiv. The Physics arXiv blog offers a good summary of the paper as:
“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 his colleagues 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 areas should seem like parts of the universe that have mainly low density. And interestingly, our universe does certainly feature such regions, which are called voids. These are gigantic bands of the cosmos that have far less celestial objects than other portions of the universe.
The explanation is far from perfect, though, in that it doesn’t precisely connect with our
observations; when light enters a Tardis region, it should bend suddenly owing to the greater curvature. But this isn’t what astrophysicists detect, so Lavinto and his associates still have their work cut out for them in explaining why we observe light that travels in the same direction.
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