Probing compensated isocurvature with the 21-cm signal during cosmic dawn

Selim C. Hotinli, Thomas Binnie, Julian B. Muñoz, Bikash R. Dinda, and Marc Kamionkowski

Phys. Rev. D 104, 063536 – Published 21 September 2021

Abstract

Upcoming measurements of the 21-cm line of neutral hydrogen will open a new observational window into the early stages of structure growth, providing a unique opportunity for probing large-scale cosmological signatures using the small-scale signals from the first stars. In this paper, we evaluate the detection significance of compensated isocurvature perturbations (CIPs) from observations of the 21-cm hydrogen line during the cosmic dawn era. CIPs are modulations of the relative baryon and dark-matter density that leave the total matter density unchanged. We find that, under different assumptions for feedback and foregrounds, the ongoing HERA and upcoming SKA1-low experiments will provide constraints on uncorrelated CIPs at the level of $σ (A_{CIP}) = 10^{- 3} - 10^{- 4}$ , comparable to the sensitivity of upcoming cosmic microwave background experiments, and potentially exceeding the constraints from cosmic-variance limited baryon acoustic oscillation surveys.

Received 26 July 2021
Accepted 19 August 2021

DOI:https://doi.org/10.1103/PhysRevD.104.063536

Physics Subject Headings (PhySH)

Cosmological parameters Dark matter Evolution of the Universe Gravitation Inflation Large scale structure of the Universe Radio, microwave, & sub-mm astronomy Relativistic aspects of cosmology

Astrophysical & cosmological simulations

Gravitation, Cosmology & Astrophysics

Authors & Affiliations

Selim C. Hotinli ^1,*, Thomas Binnie^2,†, Julian B. Muñoz^3,‡, Bikash R. Dinda^4,§, and Marc Kamionkowski ^1,¶

¹Department of Physics & Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA
²Department of Physics, Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, United Kingdom
³Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, Massachusetts 02138, USA
⁴Department of Theoretical Physics, Tata Institute of Fundamental Research, Dr Homi Bhabha Road, Navy Nagar, Colaba, Mumbai-400005, India

^*selim.hotinli14@imperial.ac.uk
^†t.binnie16@imperial.ac.uk
^‡julianmunoz@cfa.harvard.edu
^§bikashd18@gmail.com
^¶kamion@jhu.edu

Article Text (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 104, Iss. 6 — 15 September 2021

Reuse & Permissions

Access Options

Article Available via CHORUS

Download Accepted Manuscript

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
The 21-cm hydrogen line brightness-temperature power spectra at $z = 14$ (in solid blue), shown with the fitted smooth contribution to the power spectra (in dashed green) in the absence of relative velocity fluctuations and the velocity contribution (dashed orange) as discussed in Sec. 3b. We have considered medium feedback and used 21cmvfast [45]. Error bars shown in the figure for the signal are Poisson errors from our simulations. Dashed green lines are fourth-order polynomial fits to the brightness temperature spectra, as discussed in Sec. 3b.
Reuse & Permissions
Figure 2
The effect of locally varying the baryon-DM ratio and the speed of sound $c_{s}$ of the baryon-photon plasma due to CIPs on the VAO signature and brightness temperature power spectrum, at redshift $z = 14$ . The effect of velocities was calculated with the medium-feedback assumption using 21cmvfast software. The blue solid line shows the 21-cm power spectrum (fitted from simulations) with zero CIP amplitude, $Δ = 0$ . The red and violet dotted/dashed lines show the power spectrum in a universe where $Δ = \pm 0.05$ . Similar to Fig. 1, the green dashed line is the fitted temperature power spectrum excluding the effect of VAOs. The gray solid line shows the contribution of the relative-velocity effect discussed in this paper, in the absence of CIPs. The orange (purple) dot-dashed (dotted) lines show the effect of CIPs on the relative velocity power spectrum, which shift the VAO peaks by locally modulating the acoustic scale $r_{drag}$ by Eq. (5). Both this shift and the overall amplitude change are included in our forecasts.
Reuse & Permissions
Figure 3
Detection significance for the CIP fluctuations from the HERA (left) and SKA1-low (right) surveys, shown as a function of the box size in comoving Mpc, using the Fisher matrix formalism defined in Eq. (15). Results are from simulations with medium baryonic-feedback levels and using 21cmsense. We find similar constraints for low and high feedback models [constraints improve (worsen) by a less than a factor of approximately 2 for low (high) feedback] as we marginalized over the smooth spectra. The Planck and CVL constraints from the CMB were calculated in ref. [26], and the BAO constraints (from galaxy surveys) were calculated in Ref. [29] (Note that the constraints in Ref. [29] are for correlated CIP fluctuations). We find SKA1-low may improve upon HERA at the optimistic foreground limit while also being more adversely affected by the baryonic feedback for pessimistic feedback scenarios. For display purposes, we omitted plotting constraints from the pessimistic-foreground scenario, which we find less than or approximately equal to $O (10)$ worse than the moderate case.
Reuse & Permissions
Figure 4
Constraints on CIPs, shown for different assumptions on parameters describing the smooth spectra and the VAO amplitude. The solid line corresponds to the prior choice made in Fig. 3 and described in Sec. 4e. The dashed line corresponds to no prior assumption on the parameters in the forecast. The dot-dashed line corresponds to the same prior choice as the solid line for the smooth part of the 21-cm spectra, but with 1% prior on the VAO amplitude $A_{vel} (z)$ (an order or magnitude better than for the solid line).
Reuse & Permissions
Figure 5
The detection significance for the CIP fluctuations from the ongoing HERA (SKA1-low) survey, shown as a function of the box size in comoving Mpc in the left (right) subfigure. Results are from simulations with medium baryonic-feedback levels using 21cmsense. The violet, blue, and red shaded regions correspond to three different considerations of priors on our phenomenological model described in Sec. 4c. The violet shaded region corresponds taking 10% priors on the parameters describing the smooth component of the spectra, $c_{i} (z)$ , which we assume can be achieved by better and theoretically motivated modeling. The blue shaded region assumes 1% prior on the same parameters. Both blue and violet regions do not assume any prior knowledge on the VAO amplitude, $A_{vel}$ . The red shaded region corresponds to 1% prior on all parameters except the CIP amplitude. The upper dot-dashed lines in each region correspond to the pessimistic scenario for foreground contamination. The middle solid and lower dashed lines correspond to moderate and optimistic scenarios of foreground contamination.
Reuse & Permissions

Physical Review D

covering particles, fields, gravitation, and cosmology