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In the early 1990s, Ruth Gregory (now at Durham University, and recently
appointed as a Visiting Fellow at Perimeter) and Raymond Laflamme
(now an Associate Faculty member at Perimeter) found that black holes
are sometimes unstable and that singularities may not always be so
censored. Their analysis led to a long-standing question about the nature
and evolution of black hole instability.
Associate Faculty member Luis Lehner and DRC Frans Pretorius have
now beautifully resolved this question.
Their work draws on, and contributes to, string theory and multi-
dimensional models of the universe. They build on the Gregory-
Laflamme instability, which describes the state of multidimensional
holes known as “black strings.” Their simulations show how an unstable
black string in five dimensions evolves in a fractal manner. First, a thick
string evolves into a sequence of nearly spherical black holes connected
by thin strings; each thin string becomes unstable and evolves to a
sequence of smaller, self-similar spherical black holes connected by
even thinner strings. The final state violates cosmic censorship, creating
new challenges for developing a model of the universe that allows for
singularities and conventional physics to interact.
One surprising result of this new model is that a very similar fractal pattern
appears in classical physics, in the way low-viscosity fluids form droplets.
This strange parallel may help physicists reconcile our familiar world with
the counterintuitive extremes of black holes.
Postdoctoral Researcher Matthew Johnson has developed and carried
out the first systematic search for evidence of whether our observable
universe has collided with others.
The theory of cosmological inflation suggests that our observable patch
of the universe is contained within an expanding bubble – one of many
effervescing in a cosmic cauldron. If our bubble collided with another in
the past, the collision should have left a distinctive mark in the Cosmic
Microwave Background. This would make it possible to detect evidence
of a past collision through observation today – the cosmological equivalent
of checking for car damage after a fender bender.
Johnson and his colleagues developed a general algorithm for searching
the CMB for signatures of bubble collisions. An analysis of the most
recent CMB data was inconclusive, but they plan to use new data from
the European Space Agency’s Planck satellite to test the bubble collision
hypothesis more stringently in the coming year.
L. Boyle and P. J. Steinhardt, “Testing Inflation: A Bootstrap Approach,” Phys. Rev. Lett. 105, 241301
(2010), arXiv:0810.2787. Note: This paper was recognized by a “Viewpoint” article in the APS journal
Physics. Of more than 18,000 articles published in the Physical Review journals each year, only 150 are
recognized in this way.
L. Lehner and F. Pretorius, “Black Strings, Low Viscosity Fluids, and Violation of Cosmic Censorship,”
Phys. Rev. Lett. 105, 101102 (2010), arXiv:1006.5960. Note: This paper was recognized in Physical Review
Letters as an “Editor’s Suggestion” and by a “Synopsis” in the APS journal Physics.
S. M. Feeney, M. C. Johnson, D. J. Mortlock, and H. V. Peiris, “First Observational Tests of Eternal Inflation,”
Phys. Rev. Lett. 107, 071301 (2011), arXiv: 1012.1995.
My passion is what you could call “extreme gravity,”
involving interactions of massive objects at high speeds.
I grew up on the plains of Argentina and few who know
me would be surprised to learn that gravity often played
a role in my childhood. For instance, while horseback
riding when I was nine, a spooked horse, a soccer goal
post, and I all combined to produce an intense collision,
a fall, and an impromptu nose job!
My goal is to understand how gravity behaves in even
more extreme situations, involving neutron stars, or black
holes, which we believe are very important to forming
and shaping galaxies. Events such as the merging of
two black holes should produce detectable ripples in the
fabric of spacetime – these ripples would carry crucial
information about the systems that created them, and
the behaviour of spacetime itself.
We expect to detect gravitational waves using highly
sensitive detectors within the decade. When we do, an
era of gravitational wave astronomy will begin, allowing
us to peek deep into systems that are impossible to study
with current instruments. It promises to revolutionize
our understanding of astrophysics, gravity, and even
cosmology by confronting theoretical predictions with
I develop such predictions using complex simulations
to faithfully model systems of interest. Recently, for
example, my collaborators and I demonstrated that
as binary black holes come together, they induce
very powerful electromagnetic jets. By observing and
comparing gravitational and electromagnetic signals, we
will get a much richer picture, giving us clues as to the
validity of general relativity and providing guidance on
how to go beyond it.
Perimeter’s environment and research areas are ideal for
interacting with others, and stimulating thinking outside
the usual boxes. This encourages me to venture beyond
my comfort zone, which I greatly enjoy – it’s a very lively
and energizing atmosphere.
− Luis Lehner
Associate Faculty member Luis Lehner was recently elected a
Fellow of the American Physical Society and a Fellow of the
Canadian Institute for Advanced Research.