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Title: Alternative dark matter candidates: Axions
Author: Andreas Ringwald

he axion is arguably one of the best motivated candidates for dark matter. For a decay constant greater than about 10^9 GeV, axions are dominantly produced non-thermally in the early universe and hence are "cold", their velocity dispersion being small enough to fit to large scale structure. Moreover, such a large decay constant ensures the stability at cosmological time scales and its behaviour as a collisionless fluid at cosmological length scales. Here, we review the state of the art of axion dark matter predictions and of experimental efforts to search for axion dark matter in laboratory experiments.

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Title: The Lifetime of Axion Stars
Author: Joshua Eby, Peter Suranyi, L.C.R. Wijewardhana

We investigate the decay of condensates of scalars in a field theory defined by V (A) = m f [1 - cos(A / f)], where m and f are the mass and decay constant of the scalar field. An example of such a theory is that of the axion, in which case the condensates are called axion stars.

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Axion Stars
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Tiny dark matter stars would harbour particles that act as one

One theory is that dark matter could be made of particles called axions. Unlike protons, neutrons and electrons that make up ordinary matter, axions can share the same quantum energy state. They also attract each other gravitationally, so they clump together. Together, those two properties mean that the clumps would exist as a Bose-Einstein condensate (BEC) - a state of matter in which all the particles occupy the same quantum state, according to calculations by Chanda Prescod-Weinstein at the Massachusetts Institute of Technology and her colleagues.

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RE: axions
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Title: Axion as a cold dark matter candidate: Analysis to third order perturbation for classical axion
Author: Hyerim Noh, Jai-chan Hwang, Chan-Gyung Park

We investigate aspects of axion as a coherently oscillating massive classical scalar field by analyzing third order perturbations in Einstein's gravity in the axion-comoving gauge. The axion fluid has its characteristic pressure term leading to an axion Jeans scale which is cosmologically negligible for a canonical axion mass. Our classically derived axion pressure term in Einstein's gravity is identical to the one derived in the non-relativistic quantum mechanical context in the literature. We show that except for the axion pressure term, the axion fluid equations are exactly the same as the general relativistic continuity and Euler equations of a zero-pressure fluid up to third order perturbation. The general relativistic density and velocity perturbations of the CDM in the CDM-comoving gauge are exactly the same as the Newtonian perturbations to the second order (in all scales), and the pure general relativistic corrections appearing from the third order are numerically negligible (in all scales as well) in the current paradigm of concordance cosmology. Therefore, here we prove that, in the super-Jeans scale, the classical axion can be handled as the Newtonian CDM fluid up to third order perturbation. We also show that the axion fluid supports the vector-type (rotational) perturbation from the third order. Our analysis includes the cosmological constant.

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Title: Axions and Dark Matter
Author: Qiaoli Yang

Dark matter particles constitute 23% of the total energy density of our universe and their exact properties are still unclear besides that they must be very cold and weakly interacting with the standard model particles. Many beyond standard model theories provide proper candidates to serve as the dark matter. The axions were introduced to solve the strong CP problem and later turned out to be a very attractive dark matter candidate. In this paper, we briefly review the physics of axions and the axion dark matter.

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Science shines light on dark matter

Scientists from the University of Leicester say they may have solved one of the most enduring mysteries in modern physics - the nature of dark matter.
Researchers say they have identified a signal which - if confirmed - would lay the blame on 'axions', emitted from stars.

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Previously undiscovered particles could be detected as they accumulate around black holes say Scientists at the Vienna University of Technology.

Finding new particles usually requires high energies - that is why huge accelerators have been built, which can accelerate particles to almost the speed of light. But there are other creative ways of finding new particles: At the Vienna University of Technology, scientists presented a method to prove the existence of hypothetical "axions". These axions could accumulate around a black hole and extract energy from it. This process could emit gravity waves, which could then be measured.
Axions are hypothetical particles with a very low mass. According to Einstein, mass is directly related to energy, and therefore very little energy is required to produce axions.

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Title: The Axion Dark Matter eXperiment
Authors: Dmitry Lyapustin

The Axion is a particle arising from the Peccei-Quinn solution to the strong CP problem. Peccei-Quinn symmetry breaking in the early universe could produce a large number of axions which would still be present today, making the axion a compelling dark matter candidate. The goal of the Axion Dark Matter eXperiment (ADMX) is to detect these relic axions through their conversion to photons in a strong magnetic field. Results are presented from a recent ADMX data-taking, along with plans for the next phase of ADMX, which will allow the experiment to explore a significant fraction of the favoured dark matter axion mass and coupling phase space.

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Spinning black holes could expose exotic particles

Black holes do not have a reputation for giving up their secrets, but they could prove instrumental in uncovering exotic particles that are difficult to detect on Earth.
If conditions are right, a particle scattering from a spinning black hole will trigger the formation of a new particle. This also causes the black hole to lose a little angular momentum, an effect known as "superradiance".
Asimina Arvanitaki of the University of California, Berkeley, and colleagues say this loss of angular momentum could be exploited in the hunt for hypothetical particles called axions, which could constitute the invisible cold dark matter that appears to hold galaxies together.

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Title: Dark Matter Axions Revisited
Authors: Luca Visinelli, Paolo Gondolo
(Version v2)

We study for what specific values of the theoretical parameters the axion can form the totality of cold dark matter. We examine the allowed axion parameter region in the light of recent data collected by the WMAP5 mission plus baryon acoustic oscillations and supernovae, and assume an inflationary scenario and standard cosmology. If the Peccei-Quinn symmetry is restored after inflation, we recover the usual relation between axion mass and density, so that an axion mass m_a =67 2{\mu eV} makes the axion 100% of the cold dark matter. If the Peccei-Quinn symmetry is broken during inflation, the axion can instead be 100% of the cold dark matter for m_a < 15{meV} provided a specific value of the initial misalignment angle \theta_i is chosen in correspondence to a given value of its mass m_a. Large values of the Peccei-Quinn symmetry breaking scale correspond to small, perhaps uncomfortably small, values of the initial misalignment angle \theta_i.

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