Cosmic voids as a probe of the nature of dark matter: simulations and galaxy survey forecasts

Abstract

Voids are parts of the cosmic web least affected by non-linearities and baryonic feedback. We thus calculate the sensitivity of voids to the nature of dark matter (DM), using ultra-light axions as a concrete model and the ongoing Dark Energy Spectroscopic Instrument (DESI) and \textit{Euclid} galaxy surveys as observational settings. We simulate axion effects on voids using mass-peak patch simulations and find that: (i) axions suppress the formation of lower-mass halos leading to the merging of smaller (radius $< 25\,\mathrm{Mpc}/h$) voids into fewer larger (radius $> 25\,\mathrm{Mpc}/h$) voids; and (ii) voids in the presence of axions are emptier of halos, thereby suppressing the void-halo correlation function. These effects strengthen as axion particle mass $m_\mathrm{a}$ decreases. We forecast improvements in axion constraints from the void size function (VSF; the void number density as function of their radius). A \textit{Euclid}-like survey (effective volume of $73\,\mathrm{Gpc}^3$ with a prior on the other $Λ$CDM cosmological parameters from the Simons Observatory cosmic microwave background experiment) can limit the axion energy density (for $m_\mathrm{a} = 10^{-25}\,\mathrm{eV}$) to $< 4.6\%$ of the DM (at $95\%$ credibility), about two times stronger than current limits. Conversely, we show that a Universe with a dark sector consisting of axions at the $10\%$ level, as motivated by the string axiverse, can be recovered with $\sim 2 σ$ preference. A DESI-like survey achieves comparable results. Axion and $Λ$CDM parameters have different degeneracies given VSF and galaxy power spectrum data, indicating future combined analyses will be most powerful in disentangling the DM nature. We anticipate our results will extend to other (e.g., warm or interacting) DM models.

Figures (26)

• Figure 1: Voids (their positions and radii indicated by open green circles) identified (see § \ref{['sec:voidfinder']}) in slices of our halo simulations (#3 and #6-9, see Table \ref{['parameters-table']}) with the same random seed for the $\Lambda$CDM setting (top) and axion masses decreasing from $10^{-23}\,\mathrm{eV}$ to $10^{-26}\,\mathrm{eV}$ (top to bottom). Halos and their masses are indicated by the colored points. The slices are $50\,\mathrm{Mpc}/h$ thick in the Z direction (into the page). The brightness of each circle increases as the center of the void is closer to the center of the simulation slice in the Z direction. As axion mass decreases, there are on average fewer low-mass halos and voids increase in size. We highlight one such void in white.
• Figure 2: The halo mass function (HMF) of simulation #3 and #6-9 (see Table \ref{['parameters-table']}), showing the effect of decreasing axion mass. The upper panel shows the cumulative HMF (number density of halos $>$ halo mass $M$). The bottom panel shows the ratio of the HMF in the presence of axions to the $\Lambda$CDM HMF. We indicate the sample variance by the 68% confidence error bars. As axion mass decreases, there are fewer low-mass halos.
• Figure 3: In the top panel, the halo-halo correlation function $\xi (r)$ as a function of separation $r$ for simulation # 3 and 6-9 (see Table \ref{['parameters-table']}), showing the effect of decreasing axion mass. In the bottom panel, we show the ratio to the $\Lambda$CDM case. We indicate the sample variance by the 68% confidence limit error bars. As axion mass decreases, the halo bias increases and so the correlation function increases. The dashed line indicates the halo bias ratio $\left(\frac{b_{\mathrm{h},10^{-25}\,\mathrm{eV}}}{b_{\mathrm{h},\Lambda\mathrm{CDM}}}\right)^2$, which we will use in § \ref{['sec:voidforecast']}.
• Figure 4: A single void (the same as highlighted in white in Fig. \ref{['fig:Visualization']}) identified from our halo simulations (#3 and #6-9, see Table \ref{['parameters-table']} for details) with the same random seed for the $\Lambda$CDM setting (top left) and axion masses decreasing from $10^{-23}\,\mathrm{eV}$ to $10^{-26}\,\mathrm{eV}$ (top to bottom, left to right). Halos within the void and their local number density (specifically, the mean distance to the fifteen nearest halos) are indicated by the colored points. The center and radius of the void (defined in the main text) are respectively indicated by the red points and circles. As axion mass decreases, voids increase in size and get emptier of halos.
• Figure 5: In the left panels, the void mass function (number density of voids per unit void mass) of simulation #3 and #6-9 (see Table \ref{['parameters-table']} for simulation parameters), showing the effect of decreasing axion mass. In the right panels, we show the void size function (number density of voids per unit void radius) for the same simulations. The upper panels show the absolute mass and size functions, while the bottom panels show the ratio to the $\Lambda$CDM case. We indicate the sample variance by the 68% confidence limit error bars and the average void mass and radius by dotted lines. While the void size function shifts to larger voids with decreasing axion mass, the void mass function reduces largely in amplitude. Thus, voids grow in size with decreasing axion mass, but they get emptier of halos and fewer in number.