Constraining isocurvature perturbations with the 21cm emission from minihaloes

Abstract

We investigate the effects of isocurvature perturbations on the 21cm radiation from minihaloes (MHs) at high redshifts and examine constraints on the isocurvature amplitude and power spectrum using the next generation of radio telescopes such as the Square Kilometre Array. We find that there is a realistic prospect of observing the isocurvature imprints in the 21cm emission from MHs, but only if the isocurvature spectral index is close to 3 (i.e. the spectrum is blue). When the isocurvature fraction increases beyond ∼ 10% of the adiabatic component, we observe an unexpected decline in the 21cm fluctuations from small-mass MHs, which can be explained by the incorporation small MHs into larger haloes. We perform a detailed Fisher-matrix analysis, and conclude that the combination of future CMB and 21cm experiments (such as CMBPol and the Fast-Fourier-Transform Telescope) is ideal in constraining the isocurvature parameters, but will stop short of distinguishing between CDM and baryon types of isocurvature perturbations, unless the isocurvature fraction is large and the spectrum is blue.

Bump in the blue axion isocurvature spectrum

“…For example, setting Φ+ (η i ) M p at initial times, the constraint on k is where N e is the number of e-folds between t i and the end of inflation, T rh is the reheating temperature, g * S (t 0 ) is the effective number of entropy degrees of freedom today. 7…”

“…If these fields are sufficiently secluded from both the inflaton sector and the SM sector (i.e. they are only very weakly interacting), the blue spectrum will survive long enough for them to be observable today [6] in the form of isocurvature perturbations [7][8][9][10]. 1 However, if the masses of order H do not undergo a transition to a different value at some point during inflation, the energy density dilution during inflation can make these noninflaton fields' isocurvature perturbations nearly impossible to observe directly even if they had a large amplitude blue isocurvature spectrum [15].…”

Large blue isocurvature spectral index signals time-dependent mass

“…In [73], it was shown that spectral indices as large as n I = 3.8 (but not n I = 4) can be achieved in the context of a dynamical VEV breaking the Peccei-Quinn (PQ) symmetry. This is of interest because n I = 3 is considered to be observable for example by the Square Kilometer Array [72].…”

“…1 Scale-invariant isocurvature perturbations with negligible correlations with curvature perturbations are well constrained to be less than 3% of the adiabatic power [12,15,19,[65][66][67][68][69][70]. However, isocurvature spectra with very blue spectral indices can be unobservably small on long wavelengths, for which the measurements are strongly constraining, but have large amplitudes on short wavelengths, where the measurements are less constraining [71][72][73]. The case of a blue spectrum is qualitatively different from a "bump" in the spectrum because bumps usually involve a red part as well as a blue part, and because the blue spectrum here is envisioned to have a qualitatively extended k-space range over which an approximately constant blue spectral index persists.…”

“…The number 2.4 is interesting because there are claims in the literature [72,[75][76][77] that future experiments may be able to measure spectral indices of n I 3. The results in this work demonstrate that if any of these experiments detect a blue isocurvature spectrum, then they may have uncovered evidence for a dynamical degree of freedom with a time-dependent mass.…”