1,928 research outputs found
On the origin of dark matter axions
We discuss the possible sources of dark matter axions in the early universe.
In the standard thermal scenario, an axion string network forms at the
Peccei-Quinn phase transition T\sim \fa and then radiatively decays into a
cosmological background of axions; to be the dark matter, these axions must
have a mass \ma \sim 100 \mu eV with specified large uncertainties. An
inflationary phase with a reheat temperature below the PQ-scale T_{reh} \lapp
\fa can also produce axion strings through quantum fluctuations, provided that
the Hubble parameter during inflation is large H_1 \gapp \fa; this case again
implies a dark matter axion mass \ma \sim 100 \mu eV. For a smaller Hubble
parameter during inflation H_1 \lapp \fa, `anthropic tuning' allows dark
matter axions to have any mass in a huge range below \ma\lapp 1 meV.Comment: to be published in the proceedings of the 5th IFT Workshop on Axion
Cosmic String Evolution in Higher Dimensions
We obtain the equations of motion for cosmic strings in extensions of the 3+1
FRW model with extra dimensions. From these we derive a generalisation of the
Velocity-dependent One-Scale (VOS) model for cosmic string network evolution
which we apply, first, to a higher-dimensional isotropic FRW model and,
second, to a 3+1 FRW model with static flat extra dimensions. In the former
case the string network does not achieve a scaling regime because of the
diminishing rate of string intersections (), but this can be avoided in
the latter case by considering compact, small extra dimensions, for which there
is a reduced but still appreciable string intercommuting probability. We note
that the velocity components lying in the three expanding dimensions are
Hubble-damped, whereas those in the static extra dimensions are only very
weakly damped. This leads to the pathological possibility, in principle, that
string motion in the three infinite dimensions can come to a halt preventing
the strings from intersecting, with the result that scaling is not achieved and
the strings irreversibly dominate the early universe. We note criteria by which
this can be avoided, notably if the spatial structure of the network becomes
essentially three-dimensional, as is expected for string networks produced in
brane inflation. Applying our model to a brane inflation setting, we find
scaling solutions in which the effective 3D string motion does not necessarily
stop, but it is slowed down because of the excitations trapped in the extra
dimensions. These effects are likely to influence cosmic string network
evolution for a long period after formation and we discuss their more general
implications.Comment: 23 pages, 8 figures. Minor updates and notational clarification
String Evolution with Friction
We study the effects of friction on the scaling evolution of string networks
in condensed matter and cosmological contexts. We derive a generalized
`one-scale' model with the string correlation length and velocity as
dynamical variables. In non-relativistic systems, we obtain a well-known
law, showing that loop production is important. For
electroweak cosmic strings, we show transient damped epoch scaling with
(or, in the matter era, ). A low initial
density implies an earlier period with . For GUT strings, the
approach to linear scaling is faster than previously estimated.Comment: 8 pages, uuencoded gziped .ps file. Paper submitted to Phys. Rev.
Let
Contribution of domain wall networks to the CMB power spectrum
We use three domain wall simulations from the radiation era to the late time
dark energy domination era based on the PRS algorithm to calculate the
energy-momentum tensor components of domain wall networks in an expanding
universe. Unequal time correlators in the radiation, matter and cosmological
constant epochs are calculated using the scaling regime of each of the
simulations. The CMB power spectrum of a network of domain walls is determined.
The first ever quantitative constraint for the domain wall surface tension is
obtained using a Markov chain Monte Carlo method; an energy scale of domain
walls of 0.93 MeV, which is close but below the Zel'dovich bound, is
determined.Comment: Submitted to Physics Letters
Axion Cosmology Revisited
The misalignment mechanism for axion production depends on the
temperature-dependent axion mass. The latter has recently been determined
within the interacting instanton liquid model (IILM), and provides for the
first time a well-motivated axion mass for all temperatures. We reexamine the
constraints placed on the axion parameter space in the light of this new mass
function. We find an accurate and updated constraint f_a \le 2.8(\pm2)\times
10^{11}\units{GeV} or m_a \ge 21(\pm2) \units{\mu eV} from the misalignment
mechanism in the classic axion window (thermal scenario). However, this is
superseded by axion string radiation which leads to f_a \lesssim
3.2^{+4}_{-2} \times 10^{10} \units{GeV} or m_a \gtrsim 0.20 ^{+0.2}_{-0.1}
\units{meV}. In this analysis, we take care to precisely compute the effective
degrees of freedom and, to fill a gap in the literature, we present accurate
fitting formulas. We solve the evolution equations exactly, and find that
analytic results used to date generally underestimate the full numerical
solution by a factor 2-3. In the inflationary scenario, axions induce
isocurvature fluctuations and constrain the allowed inflationary scale .
Taking anharmonic effects into account, we show that these bounds are actually
weaker than previously computed. Considering the fine-tuning issue of the
misalignment angle in the whole of the anthropic window, we derive new bounds
which open up the inflationary window near . In particular,
we find that inflationary dark matter axions can have masses as high as
0.01--1\units{meV}, covering the whole thermal axion range, with values of
up to GeV. Quantum fluctuations during inflation exclude dominant
dark matter axions with masses above meV.Comment: 42 pages, 12 figures, version as accepted by Phys.Rev.
Durotaxis: The Hard Path from In Vitro to In Vivo
Durotaxis, the process by which cells follow gradients of extracellular mechanical stiffness, has been proposed as a mechanism driving directed migration. Despite the lack of evidence for its existence in vivo, durotaxis has become an active field of research, focusing on the mechanism by which cells respond to mechanical stimuli from the environment. In this review, we describe the technical and conceptual advances in the study of durotaxis in vitro, discuss to what extent the evidence suggests durotaxis may occur in vivo, and emphasize the urgent need for in vivo demonstration of durotaxis
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