arXiv subjects

Pierre Sikivie

Publications and source records attributed to Pierre Sikivie.

At least 37 records · Page 2Linked to original sources

arXiv2015

The Rethermalizing Bose-Einstein Condensate of Dark Matter Axions

The axions produced during the QCD phase transition by vacuum realignment, string decay and domain wall decay thermalize as a result of their gravitational self-interactions when the photon temperature is approximately 500 eV. They then form a Bose-Einstein condensate (BEC). Because the axion BEC rethermalizes on time scales shorter than the age of the universe, it has properties that distinguish it from other forms of cold dark matter. The observational evidence for caustic rings of dark matter in galactic halos is explained if the dark matter is axions, at least in part, but not if the dark matter is entirely WIMPs or sterile neutrinos.

astro-ph.CO↗

arXiv2015

Linear Newtonian perturbation theory from the Schrödinger-Poisson equations

We obtain solutions to the coupled Schrödinger-Poisson equations. The solutions describe the evolution of cold dark matter density perturbations in an otherwise homogeneous expanding Friedmann universe. We discuss the relationships between descriptions of cold dark matter in terms of a pressureless fluid, in terms of a wavefunction, of a classical scalar field, and a quantum scalar field. We identify the regimes where the various descriptions coincide and where they differ.

astro-ph.CO↗

arXiv2011

Axion BEC Dark Matter

Cold dark matter axions thermalize through gravitational self-interactions and form a Bose-Einstein condensate when the photon temperature reaches approximately 500 eV. Axion Bose-Einstein condensation provides an opportunity to distinguish axions from the other dark matter candidates on the basis of observation. The rethermalization of axions that are about to fall in a galactic potential well causes them to acquire net overall rotation, whereas ordinary cold dark matter falls in with an irrotational velocity field. The inner caustics of galactic halos are different in the two cases.

astro-ph.CO↗

arXiv2011

Prospects for Searching Axion-like Particle Dark Matter with Dipole, Toroidal and Wiggler Magnets

In this work we consider searches for dark matter made of axions or axion-like particles (ALPs) using resonant radio frequency cavities inserted into dipole magnets from particle accelerators, wiggler magnets developed for accelerator based advanced light sources, and toroidal magnets similar to those used in particle physics detectors. We investigate the expected sensitivity of such ALP dark matter detectors and discuss the engineering aspects of building and tuning them. Brief mention is also made of even stronger field magnets that are becoming available due to improvements in magnetic technology. It is concluded that new experiments utilizing already existing magnets could greatly enlarge the mass region in searches for axion-like dark matter particles.

physics.ins-det↗

arXiv2010

The dark matter is mostly an axion BEC

Axions differ from ordinary cold dark matter, such as WIMPs or sterile neutrinos, because they form a Bose-Einstein condensate (BEC). As a result, axions accreting onto a galactic halo fall in with net overall rotation. In contrast, ordinary CDM accretes onto galactic halos with an irrotational velocity field. The inner caustics are different in the two cases. It is shown that if the dark matter is axions, the phase space structure of the halos of isolated disk galaxies, such as the Milky Way, is precisely that of the caustic ring model for which observational support exists. The other dark matter candidates predict a far more chaotic phase space structure for galactic halos.

astro-ph.CO↗

arXiv2010

The Case for Axion Dark Matter

Dark matter axions form a rethermalizing Bose-Einstein condensate. This provides an opportunity to distinguish axions from other forms of dark matter on observational grounds. I show that if the dark matter is axions, tidal torque theory predicts a specific structure for the phase space distribution of the halos of isolated disk galaxies, such as the Milky Way. This phase space structure is precisely that of the caustic ring model, for which observational support had been found earlier. The other dark matter candidates predict a different phase space structure for galactic halos.

astro-ph.GA↗

arXiv2009

Detailed design of a resonantly-enhanced axion-photon regeneration experiment

A resonantly-enhanced photon-regeneration experiment to search for the axion or axion-like particles is described. This experiment is a shining light through walls study, where photons travelling through a strong magnetic field are (in part) converted to axions; the axions can pass through an opaque wall and convert (in part) back to photons in a second region of strong magnetic field. The photon regeneration is enhanced by employing matched Fabry-Perot optical cavities, with one cavity within the axion generation magnet and the second within the photon regeneration magnet. Compared to simple single-pass photon regeneration, this technique would result in a gain of (F/pi)^2, where F is the finesse of each cavity. This gain could feasibly be as high as 10^(10), corresponding to an improvement in the sensitivity to the axion-photon coupling, g_(agg), of order (F/pi)^(1/2) ~ 300. This improvement would enable, for the first time, a purely laboratory experiment to probe axion-photon couplings at a level competitive with, or superior to, limits from stellar evolution or solar axion searches. This report gives a detailed discussion of the scheme for actively controlling the two Fabry-Perot cavities and the laser frequencies, and describes the heterodyne signal detection system, with limits ultimately imposed by shot noise.

hep-ph↗

arXiv2008

A further look at particle annihilation in dark matter caustics

Dark matter caustics are small scale, high density structures believed to exist in galaxies like ours. If the dark matter consists of Weakly Interacting Massive Particles, these caustics may be detected by means of the gamma rays produced by dark matter particle annihilation. We discuss particle annihilation in outer and inner caustics and provide sky maps of the expected gamma ray distribution.

astro-ph↗

arXiv2007

Does the Second Caustic Ring of Dark Matter Cause the Monoceros Ring of Stars ?

Caustic rings of dark matter were predicted to exist in the plane of the Galaxy at radii $a_n \simeq 40 {\rm kpc}/n$ for $n = 1,2,3 ..$. The recently discovered Monoceros Ring of stars is located near the $n=2$ caustic, prompting us to consider a possible connection between these two objects. We identify two processes through which the Monoceros Ring of stars may have formed. One process is the migration of gas to an angular velocity minimum at the caustic leading to enhanced star formation there. The other is the adiabatic deformation of star orbits as the caustic slowly grows in mass and radius. The second process predicts an order 100% enhancement of the density of disk stars at the location of the caustic ring.

astro-ph↗

arXiv2006

Axion Cosmology

Outline: 1. Thermal axions 2. Axion field evolution 3. The domain wall problem 4. Cold axions 5. Axion miniclusters 6. Axion isocurvature perturbations

astro-ph↗

arXiv2006

The Search for Dark Matter Axions

Axions solve the Strong CP Problem and are a cold dark matter candidate. The combined constraints from accelerator searches, stellar evolution limits and cosmology suggest that the axion mass is in the range $3 \cdot 10^{-3} > m_a > 10^{-6}$ eV. The lower bound can, however, be relaxed in a number of ways. I discuss the constraint on axion models from the absence of isocurvature perturbations. Dark matter axions can be searched for on Earth by stimulating their conversion to microwave photons in an electromagnetic cavity permeated by a magnetic field. Using this technique, limits on the local halo density have been obtained by the Axion Dark Matter eXperiment.

hep-ph↗

arXiv2005

The Inner Caustics of Cold Dark Matter Halos

We prove that a flow of cold collisionless particles from all directions in and out of a region necessarily forms a caustic. A corollary is that, in cold dark matter cosmology, galactic halos have inner caustics in addition to the more obvious outer caustics. The outer caustics are fold catastrophes located on topological spheres surrounding the galaxy. To obtain the catastrophe structure of the inner caustics, we simulate the infall of cold collisionless particles in a fixed gravitational potential. The structure of inner caustics depends on the angular momentum distribution of the infalling particles. We confirm a previous result that the inner caustic is a "tricusp ring" when the initial velocity field is dominated by net overall rotation. A tricusp ring is a closed tube whose cross section is a section of an elliptic umbilic catastrophe. However, tidal torque theory predicts that the initial velocity field is irrotational. For irrotational initial velocity fields, we find the inner caustic to have a tent-like structure which we describe in detail in terms of the known catastrophes. We also show how the tent caustic transforms into a tricusp ring when a rotational component is added to the initial velocity field.

astro-ph↗

arXiv2005

Axions 05

The Strong CP Problem and its resolution through the existence of an axion are briefly reviewed. The combined constraints from accelerator searches, the evolution of red giants and the duration of the SN 1987a neutrino pulse require the axion mass $m_a$ to be less than $3 \cdot 10^{-3}$ eV. On the other hand, the requirement that the axion does not overclose the universe implies a lower bound on $m_a$ of order $10^{-6}$ eV. This lower bound can, however, be relaxed in a number of ways. If $m_a$ is near the lower bound, axions are an important contribution to the energy density of the universe in the form of (very) cold dark matter. Dark matter axions can be searched for on Earth by stimulating their conversion to microwave photons in an electromagnetic cavity permeated by a magnetic field. Using this technique, limits on the local halo density have been placed by the Axion Dark Matter eXperiment. I'll give a status report on ADMX and its upgrade presently under construction. I'll also report on recent results from solar axion searches and laser experiments.

hep-ph↗

arXiv2005

Robustness of Discrete Flows and Caustics in Cold Dark Matter Cosmology

Although a simple argument implies that the distribution of dark matter in galactic halos is characterized by discrete flows and caustics, their presence is often ignored in discussions of galactic dynamics and of dark matter detection strategies. Discrete flows and caustics can in fact be irrelevant if the number of flows is very large. We estimate the number of dark matter flows as a function of galactocentric distance and consider the various ways in which that number can be increased, in particular by the presence of structure on small scales (dark matter clumps) and the scattering of the flows by inhomogeneities in the matter distribution. We find that, when all complicating factors are taken into account, discrete flows and caustics in galactic halos remain a robust prediction of cold dark matter cosmology with extensive implications for observation and experiment.

astro-ph↗

arXiv2004

Diurnal and Annual Modulation of Cold Dark Matter Signals

We calculate the diurnal and annual modulation of the signals in axion and WIMP dark matter detectors on Earth caused by a cold flow of dark matter in the Solar neighborhood. The effects of the Sun's and the Earth's gravity, and of the orbital and rotational motions of the Earth are included. A cold flow on Earth produces a peak in the spectrum of microwave photons in cavity detectors of dark matter axions, and a plateau in the nuclear recoil energy spectrum in WIMP detectors. Formulas are given for the positions and heights of these peaks and plateaux as a function of time in the course of day and year, including all corrections down to the 0.1% level of precision. The results can be applied to an arbitrary dark matter velocity distribution $f(\vec{v})$ by integrating the one-flow results over velocities. We apply them to the set of flows predicted by the caustic ring model of the Galactic halo. The caustic ring model predicts the dark matter flux on Earth to be largest in December $\pm$ one month. Nonetheless, because of the role of energy thresholds, the model is consistent with the annual modulation results published by the DAMA collaboration provided the WIMP mass is larger than approximately 100 GeV.

astro-ph↗

arXiv2003

Why a ring of stars at r = 20 kpc?

The recently discovered ring of stars near Galactocentric distance $r = 20$ kpc is interpreted as baryonic matter accreted onto the second caustic ring of dark matter in our galaxy. Caustic rings of dark matter were predicted in the Galactic plane at radii $a_n \simeq 40 kpc/n$ where $n=1,2,3 ..$. They can reveal themselves by their gravitational influence on the distribution of baryonic matter. There is additional evidence for caustic rings of dark matter in the Milky Way from a series of sharp rises in the Galactic rotation curve. The positions of the rises are consistent at the 3% level with the above law for the caustic ring radii. Also, a triangular feature in the IRAS map of the galactic plane is consistent with the imprint of the caustic ring of dark matter nearest to us ($n=5$) upon dust and gas in the Galactic plane. These observations imply that the dark matter in our neighborhood is dominated by a single flow whose density and velocity vector are estimated.

astro-ph↗

arXiv2003

Evidence for Ring Caustics in the Milky Way

The late infall of cold dark matter onto our galaxy produces discrete flows and caustics in its halo. The recently discovered ring of stars near galactocentric distance 20 kpc and a series of sharp rises in the Milky Way rotation curve are interpreted as due to the presence of caustic rings of dark matter in the galactic plane. Their locations are consistent at the 3% level with the predictions of the self-similar infall model for the caustic ring radii. Also, a triangular feature in the IRAS map of the galactic plane is consistent with the imprint of a caustic ring of dark matter upon the baryonic matter. These observations imply that the dark matter in our neighborhood is dominated by a single flow whose density and velocity vector are estimated.

astro-ph↗

arXiv2002

Solar Wakes of Dark Matter Flows

We analyze the effect of the Sun's gravitational field on a flow of cold dark matter (CDM) through the solar system in the limit where the velocity dispersion of the flow vanishes. The exact density and velocity distributions are derived in the case where the Sun is a point mass. The results are extended to the more realistic case where the Sun has a finite size spherically symmetric mass distribution. We find that regions of infinite density, called caustics, appear. One such region is a line caustic on the axis of symmetry, downstream from the Sun, where the flow trajectories cross. Another is a cone-shaped caustic surface near the trajectories of maximum scattering angle. The trajectories forming the conical caustic pass through the Sun's interior and probe the solar mass distribution, raising the possibility that the solar mass distribution may some day be measured by a dark matter detector on Earth. We generalize our results to the case of flows with continuous velocity distributions, such as that predicted by the isothermal model of the Milky Way halo.

astro-ph↗