517 research outputs found
Imprints of a Primordial Preferred Direction on the Microwave Background
Rotational invariance is a well-established feature of low-energy physics.
Violations of this symmetry must be extremely small today, but could have been
larger in earlier epochs. In this paper we examine the consequences of a small
breaking of rotational invariance during the inflationary era when the
primordial density fluctuations were generated. Assuming that a fixed-norm
vector picked out a preferred direction during the inflationary era, we explore
the imprint it would leave on the cosmic microwave background anisotropy, and
provide explicit formulas for the expected amplitudes of
the spherical-harmonic coefficients. We suggest that it is natural to expect
that the imprint on the primordial power spectrum of a preferred spatial
direction is approximately scale-invariant, and examine a simple model in which
this is true.Comment: 7 pages, no figures; v5: Corrections, as well as use of more standard
convention, in section I
Axions and Cosmic Magnetic Fields
We argue that if axions are the dark matter, their coupling to
electromagnetism results in exponential growth of a helical magnetic field when
the axion field first rolls down its potential. After an inverse cascade, the
relevant length scales to day are of order 10-100 kpc, of astrophysical
interest. Our mechanism for allowing the field to grow relies on a nuance of
MHD. Faraday's Law says that an electric field is needed to create a magnetic
field. Previous authors relied on conventional Ohm's law to calculate E, but
the resistivity is negligible and therefore they assume E is as well. We use a
modified Ohm's Law that includes the effects of self-induction in limiting the
current driven by a given E, which allows a magnetic field to grow
Dark Matter and Dark Radiation
We explore the feasibility and astrophysical consequences of a new long-range
U(1) gauge field ("dark electromagnetism") that couples only to dark matter,
not to the Standard Model. The dark matter consists of an equal number of
positive and negative charges under the new force, but annihilations are
suppressed if the dark matter mass is sufficiently high and the dark
fine-structure constant is sufficiently small. The correct relic
abundance can be obtained if the dark matter also couples to the conventional
weak interactions, and we verify that this is consistent with particle-physics
constraints. The primary limit on comes from the demand that the
dark matter be effectively collisionless in galactic dynamics, which implies
for TeV-scale dark matter. These values are
easily compatible with constraints from structure formation and primordial
nucleosynthesis. We raise the prospect of interesting new plasma effects in
dark matter dynamics, which remain to be explored.Comment: 14 pages, 6 figures Updated equations and figure
Translational Invariance and the Anisotropy of the Cosmic Microwave Background
Primordial quantum fluctuations produced by inflation are conventionally
assumed to be statistically homogeneous, a consequence of translational
invariance. In this paper we quantify the potentially observable effects of a
small violation of translational invariance during inflation, as characterized
by the presence of a preferred point, line, or plane. We explore the imprint
such a violation would leave on the cosmic microwave background anisotropy, and
provide explicit formulas for the expected amplitudes of
the spherical-harmonic coefficients.Comment: Notation improve
Rapid Evolution of Sex-Pheromone-producing Enzyme in Drosophila
A wide range of organisms use sex pheromones to communicate with each other and to identify appropriate mating partners. While the evolution of chemical communication has been suggested to cause sexual isolation and speciation, the mechanisms that govern evolutionary transitions in sex pheromone production are poorly understood. Here, we decipher the molecular mechanisms underlying the rapid evolution in the expression of a gene involved in sex pheromone production in Drosophilid flies. Long-chain cuticular hydrocarbons (e.g., dienes) are produced female-specifically, notably via the activity of the desaturase DESAT-F, and are potent pheromones for male courtship behavior in Drosophila melanogaster. We show that across the genus Drosophila, the expression of this enzyme is correlated with long-chain diene production and has undergone an extraordinary number of evolutionary transitions, including six independent gene inactivations, three losses of expression without gene loss, and two transitions in sex-specificity. Furthermore, we show that evolutionary transitions from monomorphism to dimorphism (and its reversion) in desatF expression involved the gain (and the inactivation) of a binding-site for the sex-determination transcription factor, DOUBLESEX. In addition, we documented a surprising example of the gain of particular cis-regulatory motifs of the desatF locus via a set of small deletions. Together, our results suggest that frequent changes in the expression of pheromone-producing enzymes underlie evolutionary transitions in chemical communication, and reflect changing regimes of sexual selection, which may have contributed to speciation among Drosophila
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