3,625 research outputs found
What can the observation of nonzero curvature tell us?
The eternally inflating multiverse provides a consistent framework to
understand coincidences and fine-tuning in the universe. As such, it provides
the possibility of finding another coincidence: if the amount of slow-roll
inflation was only slightly more than the anthropic threshold, then spatial
curvature might be measurable. We study this issue in detail, particularly
focusing on the question: "If future observations reveal nonzero curvature,
what can we conclude?" We find that whether an observable signal arises or not
depends crucially on three issues: the cosmic history just before the
observable inflation, the measure adopted to define probabilities, and the
nature of the correlation between the tunneling and slow-roll parts of the
potential. We find that if future measurements find positive curvature at
\Omega_k < -10^-4, then the framework of the eternally inflating multiverse is
excluded with high significance. If the measurements instead reveal negative
curvature at \Omega_k > 10^-4, then we can conclude (1) diffusive (new or
chaotic) eternal inflation did not occur in our immediate past; (2) our
universe was born by a bubble nucleation; (3) the probability measure does not
reward volume increase; and (4) the origin of the observed slow-roll inflation
is an accidental feature of the potential, not due to a theoretical mechanism.
Discovery of \Omega_k > 10^-4 would also give us nontrivial information about
the correlation between tunneling and slow-roll; e.g. a strong correlation
favoring large N would be excluded in certain measures. We also ask whether the
current constraint on \Omega_k is consistent with multiverse expectations,
finding that the answer is yes, except for certain cases. In the course of this
work we were led to consider vacuum decay branching ratios, and found that it
is more likely than one might guess that the decays are dominated by a single
channel.Comment: 46 pages, 5 figures; reference updates and typo corrections arising
from final Phys. Rev. D copy editin
What Makes Complex Systems Complex?
This paper explores some of the factors that make complex systems complex. We first examine the history of complex systems. It was Aristotle’s insight that how elements are joined together helps determine the properties of the resulting whole. We find (a) that scientific reductionism does not provide a sufficient explanation; (b) that to understand complex systems, one must identify and trace energy flows; and (c) that disproportionate causality, including global tipping points, are all around us. Disproportionate causality results from the wide availability of energy stores. We discuss three categories of emergent phenomena—static, dynamic, and
adaptive—and recommend retiring the term emergent, except perhaps as a synonym for creative. Finally, we find that virtually all communication is stigmergic
The Eastward Enlargement of the Eurozone: The Shaping of Capital Markets
Capital Markets, Transition Economies, EMU
Slow dynamics, aging, and glassy rheology in soft and living matter
We explore the origins of slow dynamics, aging and glassy rheology in soft
and living matter. Non-diffusive slow dynamics and aging in materials
characterised by crowding of the constituents can be explained in terms of
structural rearrangement or remodelling events that occur within the jammed
state. In this context, we introduce the jamming phase diagram proposed by Liu
and Nagel to understand the ergodic-nonergodic transition in these systems, and
discuss recent theoretical attempts to explain the unusual,
faster-than-exponential dynamical structure factors observed in jammed soft
materials. We next focus on the anomalous rheology (flow and deformation
behaviour) ubiquitous in soft matter characterised by metastability and
structural disorder, and refer to the Soft Glassy Rheology (SGR) model that
quantifies the mechanical response of these systems and predicts aging under
suitable conditions. As part of a survey of experimental work related to these
issues, we present x-ray photon correlation spectroscopy (XPCS) results of the
aging of laponite clay suspensions following rejuvenation. We conclude by
exploring the scientific literature for recent theoretical advances in the
understanding of these models and for experimental investigations aimed at
testing their predictions.Comment: 22 pages, 5 postscript figures; invited review aricle, to appear in
special issue on soft matter in Solid State Communication
Can the Universe Create Itself?
The question of first-cause has troubled philosophers and cosmologists alike.
Now that it is apparent that our universe began in a Big Bang explosion, the
question of what happened before the Big Bang arises. Inflation seems like a
very promising answer, but as Borde and Vilenkin have shown, the inflationary
state preceding the Big Bang must have had a beginning also. Ultimately, the
difficult question seems to be how to make something out of nothing. This paper
explores the idea that this is the wrong question --- that that is not how the
Universe got here. Instead, we explore the idea of whether there is anything in
the laws of physics that would prevent the Universe from creating itself.
Because spacetimes can be curved and multiply connected, general relativity
allows for the possibility of closed timelike curves (CTCs). Thus, tracing
backwards in time through the original inflationary state we may eventually
encounter a region of CTCs giving no first-cause. This region of CTCs, may well
be over by now (being bounded toward the future by a Cauchy horizon). We
illustrate that such models --- with CTCs --- are not necessarily inconsistent
by demonstrating self-consistent vacuums for Misner space and a multiply
connected de Sitter space in which the renormalized energy-momentum tensor does
not diverge as one approaches the Cauchy horizon and solves Einstein's
equations. We show such a Universe can be classically stable and
self-consistent if and only if the potentials are retarded, giving a natural
explanation of the arrow of time. Some specific scenarios (out of many possible
ones) for this type of model are described. For example: an inflationary
universe gives rise to baby universes, one of which turns out to be itself.
Interestingly, the laws of physics may allow the Universe to be its own mother.Comment: 48 pages, 8 figure
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