Constraining Alternative Theories of Gravity Using Pulsar Timing Arrays.

Area of Science:

• Astrophysics
• Cosmology
• Gravitational Wave Astronomy

Background:

• Ground-based gravitational wave detectors have opened new avenues for testing gravity and constraining wave polarization.
• Pulsar timing arrays are poised to detect nanohertz gravitational waves, complementing existing observatories.
• Pulsar timing offers unique advantages for studying gravitational wave polarization, particularly longitudinal modes.

Purpose of the Study:

• To investigate the potential of pulsar timing arrays for constraining longitudinal gravitational waves.
• To establish limits on the energy density of longitudinal stochastic gravitational waves using existing pulsar timing data.

Main Methods:

• Analysis of existing pulsar timing array results to derive constraints on gravitational wave polarization modes.
• Utilizing pulsar-pulsar correlation patterns and response functions for longitudinal polarizations.
• Applying upper limits on pulsar timing residual power spectra to constrain wave amplitudes.

Main Results:

• Stringent upper limits are placed on the energy density of longitudinal stochastic gravitational waves.
• Amplitudes for vector longitudinal (VL) and scalar longitudinal (SL) modes at 1/year frequencies are constrained to A_{VL}<4×10^{-16} and A_{SL}<4×10^{-17}.
• Cosmological background energy density bounds are Ω_{VL}h^{2}<4×10^{-11} and Ω_{SL}h^{2}<3×10^{-13} for a scale-invariant spectrum.

Conclusions:

• Existing pulsar timing data provides significant constraints on longitudinal gravitational waves.
• Distinguishing longitudinal gravitational wave modes from noise presents a considerable challenge due to data variances.
• Future advancements in pulsar timing may lead to more definitive detections and characterizations of these waves.