Pulsar timing signal from ultralight scalar dark matter (1309.5888v1)

Abstract: An ultralight free scalar field with mass around 10^{-23} - 10^{-22} eV is a viable dark mater candidate, which can help to resolve some of the issues of the cold dark matter on sub-galactic scales. We consider the gravitational field of the galactic halo composed out of such dark matter. The scalar field has oscillating in time pressure, which induces oscillations of gravitational potential with amplitude of the order of 10^{-15} and frequency in the nanohertz range. This frequency is in the range of pulsar timing array observations. We estimate the magnitude of the pulse arrival time residuals induced by the oscillating gravitational potential. We find that for a range of dark matter masses, the scalar field dark matter signal is comparable to the stochastic gravitational wave signal and can be detected by the planned SKA pulsar timing array experiment.

• The paper introduces pulsar timing as a method to detect oscillating gravitational potentials induced by ultralight scalar dark matter.
• Using classical scalar field models, the authors compute timing residuals of ~10⁻¹⁵ amplitude with nanohertz frequencies relevant for SKA.
• Findings suggest dark matter in the 10⁻²³–10⁻²² eV range could resolve sub-galactic discrepancies observed in conventional CDM models.

Pulsar Timing Signal from Ultralight Scalar Dark Matter

The paper by Khmelnitsky and Rubakov introduces a novel approach to the detection of scalar field dark matter through pulsar timing array observations. It focuses on ultralight free scalar fields with masses in the range of $10^{-23} - 10^{-22}$ eV, proposing them as viable dark matter candidates and discusses their potential to resolve discrepancies between cold dark matter (CDM) simulations and observations on sub-galactic scales.

Key Concepts and Findings

The primary hypothesis is that the gravitational field within galactic halos composed of ultralight scalar dark matter exhibits oscillating pressure, leading to observable oscillations of the gravitational potential. These oscillations occur with an amplitude of approximately $10^{-15}$ and a frequency in the nanohertz range. Particularly relevant to pulsar timing arrays, the paper estimates the magnitude of pulse arrival time residuals induced by this oscillating potential.

Critically, the authors find that, across a range of dark matter masses, the scalar field's signal competes with stochastic gravitational wave signals, potentially falling within detection capabilities of the planned Square Kilometer Array (SKA) pulsar timing experiment. This is a significant observation, suggesting that scalar field dark matter could be identified via pulsar timing arrays previously optimized for gravitational wave detection.

Methodology

The approach integrates classical field theory and pulsar signal modeling:

• Gravitational Potential Oscillations: The time-dependent gravitational potential is expressed using a classical scalar field model with oscillating pressure. The pressure variations induce oscillations in gravitational potentials detectable through pulsar timing.
• Detection of Pulsar Timing Signal: Using equations analogous to the Sachs-Wolfe effect, the authors relate the propagation of signals in a fluctuating metric to potential shifts in arrival frequencies observed in pulsar timing. This methodology enables the estimation of timing residuals, which serve as an indirect signal of the ultralight scalar field dark matter.

Results

The paper presents quantitative comparisons between timing residuals from gravitational waves and from scalar field-induced oscillations. It suggests that pulsar timing arrays have the sensitivity to discern between these signals for dark matter mass values of $m \lesssim 2.3\cdot10^{-23}\,\text{eV}$. At higher masses, the signal diminishes, highlighting an optimal detection range.

Implications

This research potentially impacts both cosmology and astrophysics through the proposed detection mechanism, offering:

• Revised Dark Matter Models: By demonstrating that scalar field dark matter can be detected via SKA observations, the paper opens pathways for alternative dark matter models, enriching the understanding of sub-galactic structures.
• Enhanced Pulsar Timing Analysis: The findings indicate the potential for pulsar timing arrays to discern novel cosmic phenomena beyond gravitational waves, motivating enhancements in pulsar timing techniques and technology.

Future Directions

Further exploration could focus on refining detection techniques for scalar field dark matter, optimizing pulsar timing array instrumentation, and examining the potential coexistence and interaction of ultralight scalar fields with other dark matter components. This line of research might also explore the broader implications on galaxy formation and evolution paradigms.

In conclusion, Khmelnitsky and Rubakov's paper provides a compelling argument for pursuing pulsar timing array methodologies as a means to detect ultralight scalar dark matter, promising to expand our cosmic detection capabilities and refine dark matter theories.