1,246 research outputs found
The Cosmological Constant Problems
The old cosmological constant problem is to understand why the vacuum energy is so small; the new problem is to understand why it is comparable to the present mass density. Several approaches to these problems are reviewed. Quintessence does not help with either; anthropic considerations offer a possibility of solving both. In theories with a scalar field that takes random initial values, the anthropic principle may apply to the cosmological constant, but probably to nothing else
Baryogenesis from a right-handed neutrino condensate
We show that the baryon asymmetry of the Universe can be generated by a
strongly coupled right handed neutrino condensate which also drives inflation.
The resulting model has only a small number of parameters, which completely
determine not only the baryon asymmetry of the Universe and the mass of the
right handed neutrino but also the inflationary phase. This feature allows us
to make predictions that will be tested by current and planned experiments. As
compared to the usual approach our dynamical framework is both economical and
predictive.Comment: 13 pages, 3 figures. Typos corrected and several points clarified.
Results unchanged. New references adde
Evidence for a Positive Cosmological Constant from Flows of Galaxies and Distant Supernovae
Recent observations of high-redshift supernovae seem to suggest that the
global geometry of the Universe may be affected by a `cosmological constant',
which acts to accelerate the expansion rate with time. But these data by
themselves still permit an open universe of low mass density and no
cosmological constant. Here we derive an independent constraint on the lower
bound to the mass density, based on deviations of galaxy velocities from a
smooth universal expansion. This constraint rules out a low-density open
universe with a vanishing cosmological constant, and together the two favour a
nearly flat universe in which the contributions from mass density and the
cosmological constant are comparable. This type of universe, however, seems to
require a degree of fine tuning of the initial conditions that is in apparent
conflict with `common wisdom'.Comment: 8 pages, 1 figure. Slightly revised version. Letter to Natur
Fluids in cosmology
We review the role of fluids in cosmology by first introducing them in
General Relativity and then by applying them to a FRW Universe's model. We
describe how relativistic and non-relativistic components evolve in the
background dynamics. We also introduce scalar fields to show that they are able
to yield an inflationary dynamics at very early times (inflation) and late
times (quintessence). Then, we proceed to study the thermodynamical properties
of the fluids and, lastly, its perturbed kinematics. We make emphasis in the
constrictions of parameters by recent cosmological probes.Comment: 34 pages, 4 figures, version accepted as invited review to the book
"Computational and Experimental Fluid Mechanics with Applications to Physics,
Engineering and the Environment". Version 2: typos corrected and references
expande
Hierarchies of Susy Splittings and Invisible Photinos as Dark Matter
We explore how to generate hierarchies in the splittings between
superpartners. Some of the consequences are the existence of invisible
components of dark matter, new inflaton candidates, invisible monopoles and a
number of invisible particles that might dominate during various eras, in
particular between BBN and recombination and decay subsequently.Comment: 16 pages. v3: Ref. 27 has been modified. v4: Published versio
Fermion Masses in Emergent Electroweak Symmetry Breaking
We consider the generation of fermion masses in an emergent model of
electroweak symmetry breaking with composite gauge bosons. A universal
bulk fermion profile in a warped extra dimension is used for all fermion
flavors. Electroweak symmetry is broken at the UV (or Planck) scale where
boundary mass terms are added to generate the fermion flavor structure. This
leads to flavor-dependent nonuniversality in the gauge couplings. The effects
are suppressed for the light fermion generations but are enhanced for the top
quark where the and couplings can deviate at the
level in the minimal setup. By the AdS/CFT correspondence our model
implies that electroweak symmetry is not a fundamental gauge symmetry. Instead
the Standard Model with massive fermions and gauge bosons is an effective
chiral Lagrangian for some underlying confining strong dynamics at the TeV
scale, where mass is generated without a Higgs mechanism.Comment: modified discussion in Sec 3.1, version published in JHE
Axion Protection from Flavor
The QCD axion fails to solve the strong CP problem unless all explicit PQ
violating, Planck-suppressed, dimension n<10 operators are forbidden or have
exponentially small coefficients. We show that all theories with a QCD axion
contain an irreducible source of explicit PQ violation which is proportional to
the determinant of the Yukawa interaction matrix of colored fermions.
Generically, this contribution is of low operator dimension and will
drastically destabilize the axion potential, so its suppression is a necessary
condition for solving the strong CP problem. We propose a mechanism whereby the
PQ symmetry is kept exact up to n=12 with the help of the very same flavor
symmetries which generate the hierarchical quark masses and mixings of the SM.
This "axion flavor protection" is straightforwardly realized in theories which
employ radiative fermion mass generation and grand unification. A universal
feature of this construction is that the heavy quark Yukawa couplings are
generated at the PQ breaking scale.Comment: 16 pages, 2 figure
Very special relativity as relativity of dark matter: the Elko connection
In the very special relativity (VSR) proposal by Cohen and Glashow, it was
pointed out that invariance under HOM(2) is both necessary and sufficient to
explain the null result of the Michelson-Morely experiment. It is the quantum
field theoretic demand of locality, or the requirement of P, T, CP, or CT
invariance, that makes invariance under the Lorentz group a necessity.
Originally it was conjectured that VSR operates at the Planck scale; we propose
that the natural arena for VSR is at energies similar to the standard model,
but in the dark sector. To this end we provide an ab initio spinor
representation invariant under the SIM(2) avatar of VSR and construct a mass
dimension one fermionic quantum field of spin one half. This field turns out to
be a very close sibling of Elko and it exhibits the same striking property of
intrinsic darkness with respect to the standard model fields. In the new
construct, the tension between Elko and Lorentz symmetries is fully resolved.
We thus entertain the possibility that the symmetries underlying the standard
model matter and gauge fields are those of Lorentz, while the event space
underlying the dark matter and the dark gauge fields supports the algebraic
structure underlying VSR.Comment: 19 pages. Section 5 is new. Published version (modulo a footnote, and
a corrected typo
Spontaneous Parity Violation in SUSY Strong Gauge Theory
We suggest simple models of spontaneous parity violation in supersymmetric
strong gauge theory. We focus on left-right symmetric model and investigate
vacuum with spontaneous parity violation. Non-perturbative effects are
calculable in supersymmetric gauge theory, and we suggest two new models. The
first model shows confinement, and the second model has a dual description of
the theory. The left-right symmetry breaking and electroweak symmetry breaking
are simultaneously occurred with the suitable energy scale hierarchy. The
second model also induces spontaneous supersymmetry breaking.Comment: 14 page
d=3 Bosonic Vector Models Coupled to Chern-Simons Gauge Theories
We study three dimensional O(N)_k and U(N)_k Chern-Simons theories coupled to
a scalar field in the fundamental representation, in the large N limit. For
infinite k this is just the singlet sector of the O(N) (U(N)) vector model,
which is conjectured to be dual to Vasiliev's higher spin gravity theory on
AdS_4. For large k and N we obtain a parity-breaking deformation of this
theory, controlled by the 't Hooft coupling lambda = 4 \pi N / k. For infinite
N we argue (and show explicitly at two-loop order) that the theories with
finite lambda are conformally invariant, and also have an exactly marginal
(\phi^2)^3 deformation.
For large but finite N and small 't Hooft coupling lambda, we show that there
is still a line of fixed points parameterized by the 't Hooft coupling lambda.
We show that, at infinite N, the interacting non-parity-invariant theory with
finite lambda has the same spectrum of primary operators as the free theory,
consisting of an infinite tower of conserved higher-spin currents and a scalar
operator with scaling dimension \Delta=1; however, the correlation functions of
these operators do depend on lambda. Our results suggest that there should
exist a family of higher spin gravity theories, parameterized by lambda, and
continuously connected to Vasiliev's theory. For finite N the higher spin
currents are not conserved.Comment: 34 pages, 29 figures. v2: added reference
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