Top 10: Weirdest cosmology theories
NewScientist.com news service
Stephen Battersby

1. Clashing branes
2. Evolving universes
3. Superfluid space-time
4. Goldilocks universe
5. Gravity reaches out
6. Cosmic ghost
7. It's a small universe
8. Fast light
9. Sterile neutrinos
10. In the Matrix
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1. Clashing branes
Could our universe be a membrane floating in higher dimensional space,
repeatedly smashing into a neighbouring universe? According to an
offshoot of string theory called braneworld, there are large extra
dimensions of space, and while gravity can reach out into them, we are
confined to our own "brane" universe with only three dimensions. Neil
Turok of Cambridge University in the UK and Paul Steinhardt of Princeton
University in New Jersey, US, have worked out how the big bang could
have been sparked when our universe clashed violently with another.
These clashes repeat, producing a new big bang every now and then - so
if the cyclic universe model is right, the cosmos could be immortal.
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2. Evolving universes
When matter is compressed to extreme densities at the centre of a black
hole, it might bounce back and create a new baby universe. The laws of
physics in the offspring might differ slightly, and at random, from the
parent - so universes might evolve, suggests Lee Smolin of the Perimeter
Institute in Waterloo, Canada. Universes that make a lot of black holes
have a lot of children, so eventually they come to dominate the
population of the multiverse. If we live in a typical universe, then it
ought to have physical laws and constants that optimise the production
of black holes. It is not yet known whether our universe fits the bill.
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3. Superfluid space-time
One of the most outlandish new theories of cosmology is that space-time
is actually a superfluid substance, flowing with zero friction. Then if
the universe is rotating, superfluid spacetime would be scattered with
vortices, according to physicists Pawel Mazur of the University of South
Carolina and George Chapline at Lawrence Livermore lab in California –
and those vortices might have seeded structures such as galaxies. Mazur
suggests that our universe might have been born in a collapsing star,
where the combination of stellar matter and superfluid space could spawn
dark energy, the repulsive force that is accelerating the expansion of
the universe.
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4. Goldilocks universe
Why does the universe have properties that are "just right" to permit
the emergence of life? Tinker with a few physical constants and we would
end up with no stars, or no matter, or a universe that lasts only for
the blink of an eye. One answer is the anthropic principle: the universe
we see has to be hospitable, or we would not be here to observe it.
Recently the idea has gained some strength, because the theory of
inflation suggests that there may be an infinity of universes out there,
and string theory hints that they might have an almost infinite range of
different properties and physical laws. But many cosmologists dismiss
the anthropic principle as being non-science, because it makes no
testable predictions.
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5. Gravity reaches out
Dark matter might not really be "stuff" – it could just be a
misleading name for the odd behaviour of gravity. The theory called MOND
(modified Newtonian dynamics), suggests that gravity does not fade away
as quickly as current theories predict. This stronger gravity can fill
the role of dark matter, holding together galaxies and clusters that
would otherwise fly apart. A new formulation of MOND, consistent with
relativity, has rekindled interest in the idea, although it may not fit
the pattern of spots in the cosmic microwave background.
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6. Cosmic ghost
Three mysteries of modern cosmology could be wrapped up in one ghostly
presence. After making an adjustment to Einstein's general theory of
relativity, a team of physicists found a strange substance popping out
of their new theory, the "ghost condensate". It can produce repulsive
gravity to drive cosmic inflation in the big bang, while later on it
could generate the more sedate acceleration that is ascribed to dark
energy. Moreover, if this slippery substance clumps together, it could
form dark matter.
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7. It's a small universe
The pattern of spots in the cosmic microwave background has a suspicious
deficiency: there are surprisingly few big spots. One possible
explanation is that the universe is small - so small that, back when the
microwave background was being produced, it just could not hold those
big blobs. If so, space would have to wrap around on itself somehow.
Possibly the oddest suggestion is that the universe is funnel-shaped,
with one narrow end and one flared end like the bell of a trumpet. The
bent-back curvature of space in this model would also stretch out any
smaller microwave spots from round blobs into the little ellipses that
are indeed observed.
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8. Fast light
Why do opposite sides of the universe look the same? It's a puzzle
because the extremes of today's visible universe should never have been
in touch. Even back in the early moments of the big bang, when these
areas were much closer together, there wasn't enough time for light - or
anything else - to travel from one to another. There was no time for
temperature and density to get evened out; and yet they are even. One
solution: light used to move much faster. But to make that work could
mean a radical overhaul of Einstein's theory of relativity.
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9. Sterile neutrinos
Dark matter might be made of the most elusive particles ever imagined -
sterile neutrinos. They are hypothetical heavier cousins of ordinary
neutrinos and would interact with other matter only through the force of
gravity - making them essentially impossible to detect. But they might
have the right properties to be "warm" dark matter, buzzing about at
speeds of a few kilometres per second, forming the largish dark matter
clumps mapped by recent observations. Sterile neutrinos could also help
stars and black holes to form in the early universe, and give the kicks
that send neutron stars speeding around our galaxy.
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10. In the Matrix
Maybe our universe isn't real. Yale Philosopher Nick Bostrum has claimed
that we are probably living inside a computer simulation. Assuming it
ever becomes possible to simulate consciousness, then presumably future
civilisations would try it, probably many times over. Most perceived
universes would be simulated ones - so chances are we are in one of
them. In that case, perhaps all those cosmological oddities such as dark
matter and dark energy are simply patches, stuck on to cover up early
inconsistencies in our simulation.
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Stephen Battersby