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article image'Tardis regions of spacetime' could exist, say cosmologists

By Mathew Wace Peck     Oct 4, 2013 in Science
Cosmologists have theorised the existence of “Tardis regions of spacetime,” in an attempt to explain the observed expansion of our universe.
People familiar with Doctor Who, the BBC’s ongoing science-fiction drama series, are well aware of the concept of something being “bigger on the inside than the outside,” through the show’s oldest constant: the Doctor’s space-time vehicle, the TARDIS.*
On the outside, the TARDIS looks like an old-fashioned British police public-call box, the type of which were common in parts of the UK during the early to mid-20th century. However, on the inside, it is far more than that.
In fictional terms, TARDIS stands for “time and relative dimension in space,” the Ship being dimensionally transcendental — that is to say, the aforementioned “bigger on the inside.” On entering the TARDIS, you are transported into a place of almost infinite size.
Now cosmologists say that parts of the real universe could have exactly the same property as the Doctor’s TARDIS.
In their paper, “Average Expansion Rate And Light Propagation In A Cosmological Tardis Spacetime” — which has been submitted to the Journal of Cosmology and Astroparticle Physics and is detailed online on the preprint website Arxiv — the cosmologists have identified “Tardis regions,” which they say could explain our universe’s ever-accelerating expansion rate.
Accelerated rate of expansion
For the best part of a century, cosmologists have known that the universe is expanding. However, in 1998, it was observed for the first time that the more distant galaxies were expanding at an accelerated rate.
Three ideas have since emerged as to how that might be. The first theorises that gravity becomes much weaker over huge cosmological distances. The second, that everything in the universe is being pushed apart by “dark energy.” The third says that the expansion of the universe is not accelerating after all, but that it merely looks that way when observed.
It’s this third theory that “Tardis spacetime” has attempted to address, as explained to SPACE.com by Syksy Rasanen, a lecturer in theoretical physics at the University of Helsinki, Finland:
The idea was to get this proof of principle, that this is possible that you can do it. We’re not claiming the inside of the hole is realistic, but it’s the first model where we have an exact solution where structures that are distributed randomly in space have a significant effect on the expansion rate.
According to SPACE.com, the model looked at by Rasanen’s team in Helsinki assumes that “small perturbations in the structure of the universe at the age of 10 million years [an arbitrary starting age for the model] could alter the universe’s growth in a few billion years.”
Tardis regions of spacetime graph
Tardis regions of spacetime graph
Syksy Rasanen / Helsinki University
Rasanen explians, “In this model, at early times, the holes expand [at the same rate] as the background, but, as the universe becomes older and older, the hole expansion becomes more and more. The expansion rate is bigger than what you expect.”
Swiss cheese
Rasanen and his team built on a cosmological model – first proposed by Albert Einstein and Willem de Sitter — that “portrays the universe as mostly homogeneous, but peppered with regions that are different than the background.” A bit like Swiss cheese.
Other variations of the Einstein–de Sitter model assume that the regions and the background grow at the same rate. However, Rasanen put in variables to make those inhomogeneities grow independently. Rasanen again:
With the holes in the “Swiss cheese,” we have built them so they are specially curved such that they get the expansion rate that we want. Though the “Tardis regions” we used are not realistic, the property that regions can have larger volume than expected based on their surface area is a general feature of gravity.
This is an expression of the fact that according to general relativity, the geometry of space is not Euclidean. Different regions of space are curved differently: some have smaller volume than in the Euclidean case, others are larger. In the case of our model, we only have regions that are larger. When you take a realistic model, it is not clear whether the regions that are smaller balance out the regions that are larger.
Fact of fiction
This isn’t the first time that scientists have talked about the real universe in similar terms to the fictional universe of Doctor Who. In 2012, NASA confirmed the existence of a real-universe analogue of the Doctor’s homeworld, Gallifrey.
* This year, Doctor Who is celebrating its 50th anniversary. The series was first broadcast on Saturday, 23 November 1963, making that day the show’s official birthday. However, its unofficial birthday is Thursday, 19 September, which marks 50 years to the very day that Doctor Who went before the cameras for the very first time.
The first sequence ever to be filmed for the sci-fi series was that of the police-box exterior of the TARDIS. The scene shows the TARDIS landing on Stone Age Earth, then a mysterious shadow falling across it. It was filmed at London’s famous Ealing Studios and broadcast at the end of the very first episode, "An Unearthly Child."
More about Tardis regions, Tardis regions of spacetime, Tardis, Doctor Who, Cosmology
 
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