13 research outputs found
Inflationary spacetimes are not past-complete
Many inflating spacetimes are likely to violate the weak energy condition, a
key assumption of singularity theorems. Here we offer a simple kinematical
argument, requiring no energy condition, that a cosmological model which is
inflating -- or just expanding sufficiently fast -- must be incomplete in null
and timelike past directions. Specifically, we obtain a bound on the integral
of the Hubble parameter over a past-directed timelike or null geodesic. Thus
inflationary models require physics other than inflation to describe the past
boundary of the inflating region of spacetime.Comment: We improve the basic argument to apply to a wider class of
spacetimes, use a better title and add a discussion of cyclic models. 4
pages, 1 figure, RevTe
Online Cycle Detection for Models with Mode-Dependent Input and Output Dependencies
In the fields of co-simulation and component-based modelling, designers
import models as building blocks to create a composite model that provides more
complex functionalities. Modelling tools perform instantaneous cycle detection
(ICD) on the composite models having feedback loops to reject the models if the
loops are mathematically unsound and to improve simulation performance. In this
case, the analysis relies heavily on the availability of dependency information
from the imported models. However, the cycle detection problem becomes harder
when the model's input to output dependencies are mode-dependent, i.e. changes
for certain events generated internally or externally as inputs. The number of
possible modes created by composing such models increases significantly and
unknown factors such as environmental inputs make the offline (statical) ICD a
difficult task. In this paper, an online ICD method is introduced to address
this issue for the models used in cyber-physical systems. The method utilises
an oracle as a central source of information that can answer whether the
individual models can make mode transition without creating instantaneous
cycles. The oracle utilises three types of data-structures created offline that
are adaptively chosen during online (runtime) depending on the frequency as
well as the number of models that make mode transitions. During the analysis,
the models used online are stalled from running, resulting in the discrepancy
with the physical system. The objective is to detect an absence of the
instantaneous cycle while minimising the stall time of the model simulation
that is induced from the analysis. The benchmark results show that our method
is an adequate alternative to the offline analysis methods and significantly
reduces the analysis time.Comment: \c{opyright} 2021. This manuscript version is made available under
the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0
Eternal inflation and the initial singularity
It is shown that a physically reasonable spacetime that is eternally
inflating to the future must possess an initial singularity.Comment: 11 pages, Tufts University cosmology preprin
Violations of the Weak Energy Condition in Inflating Spacetimes
We argue that many future-eternal inflating spacetimes are likely to violate
the weak energy condition. It is possible that such spacetimes may not enforce
any of the known averaged conditions either. If this is indeed the case, it may
open the door to constructing non-singular, past-eternal inflating cosmologies.
Simple non-singular models are, however, unsatisfactory, and it is not clear if
satisfactory models can be built that solve the problem of the initial
singularity.Comment: 18 pages, 1 figure (which emerges automatically if you use dvips
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