Constraining Baryon–Dark-Matter Scattering with the Cosmic Dawn 21-cm Signal

The recent detection of an anomalously strong 21-cm signal of neutral hydrogen from cosmic dawn by the EDGES low-band radio experiment can be explained if cold dark matter particles scattered off the baryons draining excess energy from the gas. In this Letter we explore the expanded range of the 21-cm signal that is opened up by this interaction, varying the astrophysical parameters as well as the properties of dark matter particles in the widest possible range. We identify models consistent with current data by comparing to both the detection in the low-band region and the upper limits from the EDGES high-band antenna. We find that consistent models predict a 21-cm fluctuation during cosmic dawn that is between 3 and 30 times larger than the largest previously expected without dark matter scattering. The expected power spectrum exhibits strong baryon acoustic oscillations imprinted by the velocity-dependent cross section. The latter signature is a conclusive evidence of the velocity-dependent scattering and could be used by interferometers to verify the dark matter explanation of the EDGES detection.

Figure 1

Top: The function $T_{21}(v_{\mathrm{b}\text{−}\mathrm{DM}})$ at $z=17$ in units of mK (left) and its shape (right) versus the local relative baryon-DM velocity $v_{\mathrm{b}\text{−}\mathrm{DM}}$ in units of the rms velocity. To show the shape we rescaled each $T_{21}(v_{\mathrm{b}\text{−}\mathrm{DM}})$ along the $y$ axis to range from $-1$ to 0. Bottom: Global (i.e., $v_{\mathrm{b}\text{−}\mathrm{DM}}$-averaged) 21-cm signal (left) and rms of its fluctuations (right), as a function of frequency (bottom $x$ axis) or redshift (top $x$ axis). We show the envelope of all the possible signals without baryon-DM scattering (black, 6389 models) and with the scattering included (magenta, 325 839 models). Also shown are all the models consistent [as defined in Eqs. (1, 2, 3)] with EDGES low-band and high-band data (gray lines, 2448 models in total). Out of the latter compilation we highlight three models: the model with the deepest absorption trough (blue) which is characterized by (see text) $V_c=16.5\mathrm{km}{\mathrm{s}}^{-1}$, $f_{*}=0.5\%$, $f_X=10$, $\alpha =-1$, ${\nu }_{\min }=1\mathrm{keV}$, $\tau =0.0703$, $R_{\mathrm{mfp}}=20\mathrm{Mpc}$, $m_{\chi }=0.0032\mathrm{GeV}$ and ${\sigma }_1=316\times {10}^{-20}{\mathrm{cm}}^2$; the lowest redshift of heating transition (green) with $V_c=16.5\mathrm{km}{\mathrm{s}}^{-1}$, $f_{*}=0.3\%$, $f_X=0.0721$, $\alpha =-1$, ${\nu }_{\min }=1\mathrm{keV}$, $\tau =0.0702$, $R_{\mathrm{mfp}}=30\mathrm{Mpc}$, $m_{\chi }=0.1\mathrm{GeV}$, and ${\sigma }_1=1\times {10}^{-20}{\mathrm{cm}}^2$; and the highest rms (red) with $V_c=16.5\mathrm{km}{\mathrm{s}}^{-1}$, $f_{*}=0.5\%$, $f_X=0.01$, $\alpha =-1$, ${\nu }_{\min }=0.1\mathrm{keV}$, $\tau =0.0775$, $R_{\mathrm{mfp}}=20\mathrm{Mpc}$, $m_{\chi }=0.0032\mathrm{GeV}$, and ${\sigma }_1=1\times {10}^{-20}{\mathrm{cm}}^2$. The min and max points from Eqs. (2) and (3) are shown for these three curves. Also shown are the best-fit signal to the EDGES low-band data [9] (yellow, bottom left); and the rms of the thermal noise for the SKA1 at $k=0.1{\mathrm{Mpc}}^{-1}$ assuming integration time of 1000 h and 10 MHz bandwidth (dashed black, bottom right).

Figure 2

Power spectrum of the 21-cm signal versus wave number at $z=17$. The spectra are shown for: the model with the strongest absorption (blue), the lowest $z_h$ (green) and the highest rms (red); in addition, several other random models from the ensemble compatible with EDGES are shown (gray). Also shown is the power spectrum of $v_{\mathrm{b}-\mathrm{DM}}$ (black dotted curve). In order to highlight the BAO shape, all the curves are normalized to unity at $k=0.1$ ${\mathrm{Mpc}}^{-1}$.