اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدTopological Defects in Contracting Universes
61
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
We study the behaviour and consequences of cosmic string networks in contracting universes. They approximately behave during the collapse phase as a radiation fluids. Scaling solutions describing this are derived and tested against high-resolution numerical simulations. A string network in a contracting universe, together with the gravitational radiation it generates, can affect the dynamics of the universe both locally and globally, and be an important source of radiation, entropy and inhomogeneity. We discuss possible implications for bouncing and cyclic models.
قيم البحث
اقرأ أيضاً
We study the behaviour of cosmic string networks in contracting universes, and discuss some of their possible consequences. We note that there is a fundamental time asymmetry between defect network evolution for an expanding universe and a contractin
g universe. A string network with negligible loop production and small-scale structure will asymptotically behave during the collapse phase as a radiation fluid. In realistic networks these two effects are important, making this solution only approximate. We derive new scaling solutions describing this effect, and test them against high-resolution numerical simulations. A string network in a contracting universe, together with the gravitational radiation background it has generated, can significantly affect the dynamics of the universe both locally and globally. The network can be an important source of radiation, entropy and inhomogeneity. We discuss the possible implications of these findings for bouncing and cyclic cosmological models.
We discuss the possibility of quantum transitions from the string perturbative vacuum to cosmological configurations characterized by isotropic contraction and decreasing dilaton. When the dilaton potential preserves the sign of the Hubble factor thr
oughout the evolution, such transitions can be represented as an anti-tunnelling of the Wheeler--De Witt wave function in minisuperspace or, in a third-quantization language, as the production of pairs of universes out of the vacuum.
We study codimension-even conical defects that contain a deficit solid angle around each point along the defect. We show that they lead to a delta function contribution to the Lovelock scalar and we compute the contribution by two methods. We then sh
ow that these codimension-even defects appear as Euclidean brane solutions in higher dimensional topological AdS gravity which is Lovelock-Chern-Simons gravity without torsion. The theory possesses a holographic Weyl anomaly that is purely of type-A and proportional to the Lovelock scalar. Using the formula for the defect contribution, we prove a holographic duality between codimension-even defect partition functions and codimension-even brane on-shell actions in Euclidean signature. More specifically, we find that the logarithmic divergences match, because the Lovelock-Chern-Simons action localizes on the brane exactly. We demonstrate the duality explicitly for a spherical defect on the boundary which extends as a codimension-even hyperbolic brane into the bulk. For vanishing brane tension, the geometry is a foliation of Euclidean AdS space that provides a one-parameter generalization of AdS-Rindler space.
An important, and potentially detectable, signature of a non-trivial topology for the universe is the presence of so called circles-in-the-sky in the cosmic microwave background (CMB). Recent searches, confined to antipodal and nearly antipodal circl
es, have however failed to detect any. This outcome, coupled with recent theoretical results concerning the detectability of very nearly flat universes, is sufficient to exclude a detectable non-trivial cosmic topology for most observers in the inflationary limit ($0< |Omega_{tot}-1| lesssim 10^{-5}$). In a recent paper we have studied the consequences of these searches for circles if the Universe turns out to be exactly flat ($Omega_{tot} = 1 $) as is often assumed. More specifically, we have derived the maximum angles of deviation possible from antipodicity of pairs of matching circles associated with the shortest closed geodesic for all multiply-connected flat orientable $3$-manifolds. These upper bounds on the deviation from antipodicity demonstrate that in a flat universe for some classes of topology there remains a substantial fraction of observers for whom the deviation from antipodicity of the matching circles is considerably larger than zero, which implies that the searches for circles-in-the-sky undertaken so far are not enough to exclude the possibility of a detectable non-trivial flat topology. Here we briefly review these results and discuss their consequences in the search for circles-in-the-sky in a flat universes.
We investigate the spectrum of stochastic gravitational wave background generated by hybrid topological defects: domain walls bounded by strings and monopoles connected by strings. Such defects typically decay early in the history of the universe, an
d their mass scale is not subject to the constraints imposed by microwave background and millisecond pulsar observations. Nonetheless, the intensity of the gravitational wave background from hybrid defects can be quite high at frequencies above $10^{-8} Hz$, and in particular in the frequency range of LIGO, VIRGO and LISA detectors.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
