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Abstract

A pseudoscalar “axionlike” field, ϕ, may explain the 3σ hint of cosmic birefringence observed in the EB power spectrum of the cosmic microwave background polarization data. Is ϕ dark energy or dark matter? A tomographic approach can answer this question. The effective mass of dark energy field responsible for the accelerated expansion of the Universe today must be smaller than m_ϕ≃10⁻³³  eV. If m_ϕ≳10⁻³²  eV, ϕ starts evolving before the epoch of reionization and we should observe different amounts of birefringence from the EB power spectrum at low (l≲10) and high multipoles. Such an observation, which requires a full-sky satellite mission, would rule out ϕ being dark energy. If m_ϕ≳10⁻²⁸  eV, ϕ starts oscillating during the epoch of recombination, leaving a distinct signature in the EB power spectrum at high multipoles, which can be measured precisely by ground-based cosmic microwave background observations. Our tomographic approach relies on the shape of the EB power spectrum and is less sensitive to miscalibration of polarization angles.