David J. E. Marsh

A search for ultralight axions using precision cosmological data

Ultra-light axions (ULAs) with masses in the range 10^{-33} eV 10^{-24} eV, ULAs are indistinguishable from standard cold dark matter on the length scales probed, and are thus allowed by these data. For m < 10^{-32} eV, ULAs are allowed to compose a significant fraction of the dark energy.

Galaxy UV-luminosity function and reionization constraints on axion dark matter

If the dark matter (DM) were composed of axions, then structure formation in the Universe would be suppressed below the axion Jeans scale. Using an analytic model for the halo mass function of a mixed DM model with axions and cold dark matter, combined with the abundance-matching technique, we construct the UV-luminosity function. Axions suppress high-z galaxy formation and the UV-luminosity function is truncated at a faintest limiting magnitude. From the UV-luminosity function, we predict the reionization history of the universe and nd that axion DM causes reionization to occur at lower redshift. We search for evidence of axions using the Hubble Ultra Deep Field UV-luminosity function in the redshift range z = 6{10, and the optical depth to reionization, , as measured from cosmic microwave background polarization. All probes we consider consistently exclude ma . 10 23 eV from contributing more than half of the DM, with our strongest constraint ruling this model out at more than 8 signicance. In conservative models of reion

An ultralight pseudoscalar boson

Using a fundamental discrete symmetry, ZN , we construct a two-axion model with the QCD axion solving the strong-CP problem, and an ultralight axion (ULA) with mULA 10 22 eV providing the dominant form of dark matter (DM). The ULA is light enough to be detectable in cosmology from its imprints on structure formation, and may resolve the small-scale problems of cold DM. The necessary relative DM abundances occur without ne tuning in constructions with decay constants fULA 10 17 GeV, and fQCD 10 11 GeV. An example model achieving this has N = 24, and we construct a range of other possibilities. We compute the ULA couplings to the Standard Model, and discuss prospects for direct detection. The QCD axion may be detectable in standard experiments through the ~ E ~ B and G ~ G couplings. In the simplest models, however, the ULA has identically zero coupling to both G ~ G of QCD and ~ E ~ B of electromagnetism due to vanishing electromagnetic and color anomalies. The ULA couples to fermions with strength g/ 1=fULA. This coupling causes spin precession of nucleons and electrons with respect to the DM wind with period t months. Current limits do not exclude the predicted coupling strength, and our model is within reach of the CASPEr-Wind experiment, using nuclear magnetic resonance.

Constraints on dark matter scenarios from measurements of the galaxy luminosity function at high redshifts

We use state-of-the-art measurements of the galaxy luminosity function (LF) at z=6, 7, and 8 to derive constraints on warm dark matter (WDM), late-forming dark matter, and ultralight axion dark matter models alternative to the cold dark matter (CDM) paradigm. To this purpose, we have run a suite of high-resolution N-body simulations to accurately characterize the low-mass end of the halo mass function and derive dark matter (DM) model predictions of the high-z luminosity function. In order to convert halo masses into UV magnitudes, we introduce an empirical approach based on halo abundance matching, which allows us to model the LF in terms of the amplitude and scatter of the ensemble average star formation rate halo mass relation, ⟨SFR(Mh,z)⟩, of each DM model. We find that, independent of the DM scenario, the average SFR at fixed halo mass increases from z=6 to 8, while the scatter remains constant. At halo mass Mh≳1012 M⊙  h−1, the average SFR as a function of halo mass follows a double power law trend that is common to all models, while differences occur at smaller masses. In particular, we find that models with a suppressed low-mass halo abundance exhibit higher SFR compared to the CDM results. Thus, different DM models predict a different faint-end slope of the LF which causes the goodness of fit to vary within each DM scenario for different model parameters. Using deviance statistics, we obtain a lower limit on the WDM thermal relic particle mass, mWDM≳1.5  keV at 2σ. In the case of LFDM models, the phase transition redshift parameter is bounded to zt≳8×105 at 2σ. We find ultralight axion dark matter best-fit models with axion mass ma≳1.6×10-22  eV to be well within 2σ of the deviance statistics. We remark that measurements at z=6 slightly favor a flattening of the LF at faint UV magnitudes. This tends to prefer some of the non-CDM models in our simulation suite, although not at a statistically significant level to distinguish them from CDM.

Future CMB tests of dark matter: Ultralight axions and massive neutrinos

Measurements of cosmic microwave background (CMB) anisotropies provide strong evidence for the existence of dark matter and dark energy. They can also test its composition, probing the energy density and particle mass of different dark-matter and dark-energy components. CMB data have already shown that ultralight axions (ULAs) with mass in the range 10 − 32     eV → 10 − 26     eV compose a fraction ≲ 0.01 of the cosmological critical density. The next Stage-IV CMB experiment (CMB-S4) (assuming a 1 arcmin beam and ∼ 1     μ K − arcmin noise levels over a sky fraction of 0.4) to the density of ULAs and other dark-sector components is assessed. CMB-S4 data should be ∼ 10 times more sensitive to the ULA energy density than Planck data alone, across a wide range of ULA masses 10 − 32 ≲ m a ≲ 10 − 23     eV , and will probe axion decay constants of f a ≈ 1 0 16     GeV , at the grand unified scale. CMB-S4 could improve the CMB lower bound on the ULA mass from ∼ 10 − 25     eV to 10 − 23     eV , nearing the mass range probed by dwarf galaxy abundances and dark-matter halo density profiles. These improvements will allow for a multi- σ detection of percent-level departures from CDM over a wide range of masses. Much of this improvement is driven by the effects of weak gravitational lensing on the CMB, which breaks degeneracies between ULAs and neutrinos. We also find that the addition of ULA parameters does not significantly degrade the sensitivity of the CMB to neutrino masses. These results were obtained using the axionCAMB code (a modification to the CAMB Boltzmann code), presented here for public use.

The effects of the small-scale DM power on the cosmological neutral hydrogen (HI) distribution at high redshifts

The particle nature of dark matter remains a mystery. In this paper, we consider two dark matter models---Late Forming Dark Matter (LFDM) and Ultra-Light Axion (ULA) models---where the matter power spectra show novel effects on small scales. The high redshift universe offers a powerful probe of their parameters. In particular, we study two cosmological observables: the neutral hydrogen (HI) redshifted 21-cm signal from the epoch of reionization, and the evolution of the collapsed fraction of HI in the redshift range

Unifying inflation and dark matter with the Peccei-Quinn field: observable axions and observable tensors

2 < z < 5

Quintessence in a quandary: prior dependence in dark energy models

. We model the theoretical predictions of the models using CDM-like N-body simulations with modified initial conditions, and generate reionization fields using an excursion-set model. The N-body approximation is valid on the length and halo mass scales studied. We show that LFDM and ULA models predict an increase in the HI power spectrum from the epoch of reionization by a factor between 2--10 for a range of scales

Searching for the QCD Axion with Gravitational Microlensing

0.1 4 \times 10^5

Search for Axionlike Dark Matter through Nuclear Spin Precession in Electric and Magnetic Fields

(for LFDM) and the axion mass