277 research outputs found
Geochemical proxies of North American freshwater routing during the Younger Dryas cold event
Author Posting. © The Authors, 2006. This is the author's version of the work. It is posted here by permission of National Academy of Sciences for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences 104 (2007): 6556-6561, doi:10.1073/pnas.0611313104.The Younger Dryas cold interval represents a time when much of the Northern
Hemisphere cooled from ~12.9 to 11.5 kiloyears before present. The cause of this event,
which has long been viewed as the canonical example of abrupt climate change, was
initially attributed to the routing of freshwater to the St. Lawrence River with an attendant
reduction in Atlantic meridional overturning circulation. However, this mechanism has
recently been questioned because current proxies and dating techniques have been unable
to confirm that eastward routing with an increase in freshwater flux occurred during the
Younger Dryas. Here we use new geochemical proxies (ΔMg/Ca, U/Ca & 87Sr/86Sr)
measured in planktonic foraminifera at the mouth of the St. Lawrence Estuary as tracers
of freshwater sources to further evaluate this question. Our proxies, combined with
planktonic δ18Oseawater and δ13C, confirm that routing of runoff from western Canada to the
St. Lawrence River occurred at the start of the Younger Dryas, with an attendant increase
in freshwater flux of 0.06 ± 0.02 Sverdrup (1 Sverdrup (Sv) = 106 m3 s-1). This base
discharge increase is sufficient to have reduced Atlantic meridional overturning circulation
and caused the Younger Dryas cold interval. In addition, our data indicate subsequent
fluctuations in the freshwater flux to the St. Lawrence River of ~0.06 to 0.12 Sv, thus
explaining the variability in the overturning circulation and climate during the Younger
Dryas.This research was funded by the NSF Paleoclimate Program (P.U.C.) and the NSF (G.P.K.)
Ringholes and closed timelike curves
It is shown that in a classical spacetime with multiply connected space
slices having the topology of a torus, closed timelike curves are also formed.
We call these spacetime ringholes. Two regions on the torus surface can be
distinguished which are separated by angular horizons. On one of such regions
(that which surrounds the maximum circumference of the torus) everything
happens like in spherical wormholes, but the other region (the rest of the
torus surface), while still possessing a chronology horizon and non-chronal
region, behaves like a coverging, rather than diverging, lens and corresponds
to an energy density which is always positive for large speeds at or near the
throat. It is speculated that a ringhole could be converted into a time machine
to perform time travels by an observer who would never encounter any matter
that violates the classical averaged weak energy condition. Based on a
calculation of vacuum fluctuations, it is also seen that the angular horizons
can prevent the emergence of quantum instabilities near the throat.Comment: 11 pages, RevTex, 4 figures available upon reques
Quantum interest in two dimensions
The quantum interest conjecture of Ford and Roman asserts that any
negative-energy pulse must necessarily be followed by an over-compensating
positive-energy one within a certain maximum time delay. Furthermore, the
minimum amount of over-compensation increases with the separation between the
pulses. In this paper, we first study the case of a negative-energy square
pulse followed by a positive-energy one for a minimally coupled, massless
scalar field in two-dimensional Minkowski space. We obtain explicit expressions
for the maximum time delay and the amount of over-compensation needed, using a
previously developed eigenvalue approach. These results are then used to give a
proof of the quantum interest conjecture for massless scalar fields in two
dimensions, valid for general energy distributions.Comment: 17 pages, 4 figures; final version to appear in PR
From wormhole to time machine: Comments on Hawking's Chronology Protection Conjecture
The recent interest in ``time machines'' has been largely fueled by the
apparent ease with which such systems may be formed in general relativity,
given relatively benign initial conditions such as the existence of traversable
wormholes or of infinite cosmic strings. This rather disturbing state of
affairs has led Hawking to formulate his Chronology Protection Conjecture,
whereby the formation of ``time machines'' is forbidden. This paper will use
several simple examples to argue that the universe appears to exhibit a
``defense in depth'' strategy in this regard. For appropriate parameter regimes
Casimir effects, wormhole disruption effects, and gravitational back reaction
effects all contribute to the fight against time travel. Particular attention
is paid to the role of the quantum gravity cutoff. For the class of model
problems considered it is shown that the gravitational back reaction becomes
large before the Planck scale quantum gravity cutoff is reached, thus
supporting Hawking's conjecture.Comment: 43 pages,ReV_TeX,major revision
Open and Closed Universes, Initial Singularities and Inflation
The existence of initial singularities in expanding universes is proved
without assuming the timelike convergence condition. The assumptions made in
the proof are ones likely to hold both in open universes and in many closed
ones. (It is further argued that at least some of the expanding closed
universes that do not obey a key assumption of the theorem will have initial
singularities on other grounds.) The result is significant for two reasons:
(a)~previous closed-universe singularity theorems have assumed the timelike
convergence condition, and (b)~the timelike convergence condition is known to
be violated in inflationary spacetimes. An immediate consequence of this
theorem is that a recent result on initial singularities in open,
future-eternal, inflating spacetimes may now be extended to include many closed
universes. Also, as a fringe benefit, the time-reverse of the theorem may be
applied to gravitational collapse.Comment: 27 pages, Plain TeX (figures are embedded in the file itself and they
will emerge if it is processed according to the instructions at the top of
the file
Inflating Lorentzian Wormholes
It has been speculated that Lorentzian wormholes of the Morris- Thorne type
might be allowed by the laws of physics at submicroscopic, e.g. Planck, scales
and that a sufficiently advanced civilization might be able to enlarge them to
classical size. The purpose of this paper is to explore the possibility that
inflation might provide a natural mechanism for the enlargement of such
wormholes to macroscopic size. A new classical metric is presented for a
Lorentzian wormhole which is imbedded in a flat deSitter space. It is shown
that the throat and proper length of the wormhole inflate. The resulting
properties and stress-energy tensor associated with this metric are discussed.Comment: 24 pg
Chronology Protection in Generalized Godel Spacetime
The effective action of a free scalar field propagating in the generalized
Godel spacetime is evaluated by the zeta-function regularization method. From
the result we show that the renormalized stress energy tensor may be divergent
at the chronology horizon. This gives a support to the chronology protection
conjecture.Comment: Latex 6 pages, typos correcte
Massive scalar field in multiply connected flat spacetimes
The vacuum expectation value of the stress-energy tensor is calculated in several multiply
connected flat spacetimes for a massive scalar field with arbitrary curvature
coupling. We find that a nonzero field mass always decreases the magnitude of
the energy density in chronology-respecting manifolds such as ,
, , the M\"{o}bius strip, and the Klein bottle.
In Grant space, which contains nonchronal regions, whether diverges on a chronology horizon or
not depends on the field mass. For a sufficiently large mass remains finite, and the metric
backreaction caused by a massive quantized field may not be large enough to
significantly change the Grant space geometry.Comment: 19 pages, REVTeX, 5 figures in separate uuencoded compressed fil
A Self-Consistent Vacuum for Misner Space and the Chronology Protection Conjecture
In this paper we find a self-consistent vacuum for Misner space. For this
"adapted" Rindler vacuum the renormalized stress-energy tensor is zero
throughout the Misner space. A point-like particle detector traveling on a
timelike geodesic in a Misner space with this vacuum detects nothing. Misner
space with this vacuum thus creates no problems for time travel in and of
itself but a time traveler may pose a danger to himself and to the spacetime.Comment: 5 pages, received September 5, 1997, with a note added November 24,
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