Cosmological Recombination Spectrum Research Articles

Cosmology with recombination spectrum

Precision measurement of the cosmological recombination spectrum can provide an entire new window to look at the early universe. We aim to quantify the information hidden in the cosmological recombination spectrum and for this purpose we have developed a new code following the algorithm proposed in [1,2]. Our code is closely based on the COSMOSPEC [3] code. We find, using Fisher information matrix and assuming that the foregrounds can be subtracted by using higher or lower frequency channels and spatial information, that going beyond the detection will need an experiment with sensitivity 25× better compared to the proposed experiment PIXIE. Such an experiment will be able to measure the cosmological parameters with a precision that is competitive with the CMB anisotropy experiments. The best constrainted parameter is baryon energy density, Ωb, which can be nailed down with incredible precision in principle. We also show that the shape of the hydrogen lines is connected to the speed of the hydrogen recombination, with the peaks of the recombination lines coinciding with the peak of the recombination rate. In general, the shape of the lines encodes information about the rate of recombination as a function of redshift.

Recombinations to the Rydberg states of hydrogen and their effect during the cosmological recombination epoch

In this paper we discuss the effect of recombinations to highly excited states (n > 100) in hydrogen during the cosmological recombination epoch. For this purpose, we developed a new ordinary differential equation solver for the recombination problem, based on an implicit Gear's method. This solver allows us to include up to 350 l-resolved shells or ∼61 000 separate levels in the hydrogen model and to solve the recombination problem for one cosmology in ∼27 h. This is a huge improvement in performance over our previous recombination code, for which a 100-shell computation (5050 separate states) already required ∼150 h on a single processor. We show that for 350 shells down to redshift z∼ 200, the results for the free electron fraction have practically converged. The final modification in the free electron fraction at z∼ 200 decreases from about ΔNe/Ne∼ 2.8 per cent for 100 shells to ΔNe/Ne∼ 1.6 per cent for 350 shells. However, the associated changes in the cosmic microwave background power spectra at large multipoles l are rather small, so that for accurate computations in connection with the analysis of Planck data already ∼100 shells are expected to be sufficient. Nevertheless, the total value of τ could still be affected at a significant level. We also briefly investigate the effect of collisions on the recombination dynamics. With our current estimates for the collisional rates we find a correction of ΔNe/Ne∼−8.8 × 10−4 at z∼ 700, which is mainly caused by l-changing collisions with protons. Furthermore, we present results on the cosmological recombination spectrum, showing that at low frequencies collisional processes are important. However, the current accuracy of collisional rates is insufficient for precise computations of templates for the recombination spectrum at ν≲ 1 GHz, and also the effect of collisions on the recombination dynamics suffers from the uncertainty in these rates. Improvements in collisional rates will therefore become necessary in order to obtain a final answer regarding their effects during recombination.

Could the cosmological recombination spectrum help us understand annihilating dark matter?

In this paper we explore the potential effects of DM annihilations on the cosmological recombination spectrum. With this example we want to demonstrate that the cosmological recombination spectrum in principle is sensitive to details related to possible extra energy release during recombination. We restrict ourselves to DM models which produce a negligible primordial distortion of the CMB energy spectrum. However, since during the epoch of cosmological recombination a large fraction of the deposited energy can directly go into ionizations and excitations of neutral atoms, both the cosmological recombination spectrum and ionization history can still be affected significantly. We compute the modifications to the cosmological recombination spectrum using our multi-level HI and HeI recombination code, showing that additional photons are created due to uncompensated loops of transitions which are induced by DM annihilations. As we illustrate here, the results depend on the detailed branching of the deposited energy into heating, ionizations and excitations. This dependence in principle should allow us to shed light on the nature of the underlying annihilating DM model (or more generally speaking, the mechanism leading to energy injection) when measuring the cosmological recombination spectrum. However, for current upper limits on the potential DM annihilation rate during recombination the cosmological recombination spectrum is only affected at the level of a few percent. Nevertheless, we argue here that the cosmological recombination spectrum would provide another independent and very direct way of checking for the presence of sources of extra ionizing or exciting photons at high redshifts. This would open an new window to possible (non-standard) processes occurring (abridged)

Cosmological recombination: feedback of helium photons and its effect on the recombination spectrum

In this paper we consider the re-processing of high frequency photons emitted by HeII and HeI during the epoch of cosmological recombination by HeI and HI. We demonstrate that, in comparison to computations which neglect all feedback processes, the number of cosmological recombination photons that are related to the presence of helium in the early Universe could be increased by ~40%-70%. Our computations imply that per helium nucleus ~3-6 additional photons could be produced. Therefore, a total of ~12-14 helium-related photons are emitted during cosmological recombination. This is an important addition to cosmological recombination spectrum which in the future may render it slightly easier to determine the primordial abundance of helium using differential measurements of the CMB energy spectrum. Also, since these photons are the only witnesses of the feedback process at high redshift, observing them in principle offers a way to check our understanding of the recombination physics. Here most interestingly, the feedback of HeII photons on HeI leads to the appearance of several additional, rather narrow spectral features in the HeI recombination spectrum at low frequencies. Consequently, the signatures of helium-related features in the CMB spectral distortion due to cosmological recombination at some given frequency can exceed the average level of ~17% several times. We find that in particular the bands around nu ~10GHz, ~35GHz, ~80GHz, and ~200GHz seem to be affected strongly. In addition, we computed the changes in the cosmological ionization history, finding that only the feedback of primary HeI photons on the dynamics of HeII-->HeI recombination has an effect, producing a change of DN_e/N_e ~+ 0.17% at z~2300. This result seems to be ~2-3 times smaller than the one obtained in earlier computations for this process (abridged).

Signals from the epoch of cosmological recombination – Karl Schwarzschild Award Lecture 2008

AbstractThe physical ingredients to describe the epoch of cosmological recombination are amazingly simple and well‐understood. This fact allows us to take into account a very large variety of physical processes, still finding potentially measurable consequences for theenergy spectrumandtemperature anisotropiesof the Cosmic Microwave Background (CMB). In this contribution we provide a short historical overview in connection with the cosmological recombination epoch and its connection to the CMB. Also we highlight some of the detailed physics that were studied over the past few years in the context of the cosmological recombination ofhydrogenandhelium. The impact of these considerations is two‐fold:(i) The associated release of photons during this epoch leads to interesting andunique deviationsof the CosmicMicrowave Background (CMB) energy spectrumfrom a perfect blackbody, which, in particular at decimeter wavelength and the Wien part of the CMB spectrum, may become observable in the near future. Despite the fact that the abundance of helium is rather small, it still contributes a sizeable amount of photons to the full recombination spectrum, leading to additional distinct spectral features. Observing the spectral distortions from the epochs of hydrogen and helium recombination, in principle would provide an additional way to determine some of the key parameters of the Universe (e.g. the specific entropy, the CMB monopole temperature and the pre‐stellar abundance of helium). Also it permits us to confront our detailed understanding of the recombination process withdirect observational evidence. In this contribution we illustrate how the theoreticalspectral templateof the cosmological recombination spectrum may be utilized for this purpose. We also show that because hydrogen and helium recombine at very different epochs it is possible to address questions related to thethermal historyof our Universe. In particular the cosmological recombination radiation may allow us to distinguish between Comptony‐distortions that were created by energy releasebeforeorafterthe recombination of the Universe finished.(ii) With the advent of high precision CMB data, e.g. as will be available using the PLANCK Surveyor or CMBPOL, a very accurate theoretical understanding of theionization historyof the Universe becomes necessary for the interpretation of the CMB temperature and polarization anisotropies. Here we show that the uncertainty in the ionization history due to several processes, which until now were not taken in to account in the standard recombination code RECFAST, reaches the percent level. In particular He II → He I recombination occurs significantly faster because of the presence of a tiny fraction of neutral hydrogen atz∼ 2400. Also recently it was demonstrated that in the case of H I Lymanαphotons thetimedependenceof the emission process and theasymmetrybetween the emission and absorption profile cannot be ignored. However, it is indeed surprising howinertthe cosmological recombination history is even at percent‐level accuracy. Observing the cosmological recombination spectrum should in principle allow us to directly check this conclusion, which until now is purely theoretical. Also it may allow toreconstruct the ionization historyusing observational data (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Lines in the cosmic microwave background spectrum from the epoch of cosmological helium recombination

The main goal of this work is to calculate the contributions of bound-bound transitions of helium to the cosmological recombination spectrum. We show that helium in the early Universe causes unique features to appear in the total cosmological recombination spectrum. These may provide a unique observational possibility to determine the relative abundance of primordial helium, well before the formation of first stars. We include the effect of the tiny fraction of neutral hydrogen atoms on the dynamics of He II He I recombination at redshifts z ~ 2500. As discussed recently, this process significantly accelerates He II He I recombination, resulting in rather narrow and distinct features in the associated recombination spectrum. In addition this process induces some emission within the hydrogen Lyman-α line, before the actual epoch of hydrogen recombination around z ~ 1100-1500. We also show that some of the fine-structure transitions of neutral helium appear in absorption, again leaving unique traces in the cosmic microwave background blackbody spectrum, which may allow confirmation of our understanding of the early Universe and of detailed atomic physics.