publications

2022

1. MNRAS

   Using the cosmological recombination radiation to probe early dark energy and fundamental constant variations

   Hart, Luke, and Chluba, Jens

   Monthly Notices of the Royal Astronomical Society Dec 2022

   Abs arXiv PDF

   The cosmological recombination radiation (CRR) is one of the guaranteed spectral distortion signals from the early Universe. The CRR photons from hydrogen and helium pre-date the last scattering process and as such allow probing physical phenomena in the pre-recombination era. Here we compute the modifications to the CRR caused by early dark energy models and varying fundamental constants. These new physics examples have seen increased recent activity in connection with the Hubble tension, motivating the exploratory study presented here. The associated CRR responses are spectrally-rich but the level of the signals is small. We forecast the possible sensitivity of future spectrometers to these effects. Our estimates demonstrate that the CRR directly depends to changes in the expansion history and recombination physics during the pre-recombination era. However, futuristic sensitivities are required for spectrometer-only constraints that are competitive with other cosmological probes. Nevertheless, measurements of the CRR can directly reach into phases that otherwise remain inaccessible, highlighting the potential these types of observations could have as a probe of the early Universe. A combination with Planck data further shows that a synergistic approach is very promising.

2. JHEAp

   Cosmology intertwined: A review of the particle physics, astrophysics, and cosmology associated with the cosmological tensions and anomalies

   Abdalla, Elcio, Abellán, Guillermo Franco, and Aboubrahim, Amin et. al.

   Journal of High Energy Astrophysics Jun 2022

   Abs arXiv PDF

   The standard Λ Cold Dark Matter (ΛCDM) cosmological model provides a good description of a wide range of astrophysical and cosmological data. However, there are a few big open questions that make the standard model look like an approximation to a more realistic scenario yet to be found. In this paper, we list a few important goals that need to be addressed in the next decade, taking into account the current discordances between the different cosmological probes, such as the disagreement in the value of the Hubble constant H0, the σ8-S8 tension, and other less statistically significant anomalies. While these discordances can still be in part the result of systematic errors, their persistence after several years of accurate analysis strongly hints at cracks in the standard cosmological scenario and the necessity for new physics or generalisations beyond the standard model. In this paper, we focus on the 5.0 σ tension between the Planck CMB estimate of the Hubble constant H0 and the SH0ES collaboration measurements. After showing the H0 evaluations made from different teams using different methods and geometric calibrations, we list a few interesting new physics models that could alleviate this tension and discuss how the next decade’s experiments will be crucial. Moreover, we focus on the tension of the Planck CMB data with weak lensing measurements and redshift surveys, about the value of the matter energy density Ωm, and the amplitude or rate of the growth of structure (σ8 , fσ8). We list a few interesting models proposed for alleviating this tension, and we discuss the importance of trying to fit a full array of data with a single model and not just one parameter at a time. Additionally, we present a wide range of other less discussed anomalies at a statistical significance level lower than the H0-S8 tensions which may also constitute hints towards new physics, and we discuss possible generic theoretical approaches that can collectively explain the non-standard nature of these signals. Finally, we give an overview of upgraded experiments and next-generation space missions and facilities on Earth that will be of crucial importance to address all these open questions.

2021

1. MNRAS

   Varying fundamental constants principal component analysis: additional hints about the Hubble tension

   Hart, Luke, and Chluba, Jens

   Monthly Notices of the Royal Astronomical Society Oct 2021

   Abs arXiv PDF

   Varying fundamental constants (VFC) [e.g., the fine-structure constant, αEM] can arise in numerous extended cosmologies. Through their effect on the decoupling of baryons and photons during last scattering and reionisation, these models can be directly constrained using measurements of the cosmic microwave background (CMB) temperature and polarization anisotropies. Previous investigations focused mainly on time-independent changes to the values of fundamental constants. Here we generalize to time-dependent variations. Instead of directly studying various VFC parameterizations, we perform a model-independent principal component analysis (PCA), directly using an eigenmode decomposition of the varying constant during recombination. After developing the formalism, we use Planck 2018 data to obtain new VFC limits, showing that three independent VFC modes can be constrained at present. No indications for significant departures from the standard model are found with Planck data. Cosmic variance limited modes are also compared and simple forecasts for The Simons Observatory are carried out, showing that in the future improvements of the current constraints by a factor of ≃3 can be anticipated. Our modes focus solely on VFC at redshifts z≥300. This implies that they do not capture some of the degrees of freedom relating to the reionisation era. This aspect provides important new insights into the possible origin of the Hubble tension, hinting that indeed a combined modification of recombination and reionisation physics could be at work. An extended PCA, covering both recombination and reionisation simultaneously, could shed more light on this question, as we emphasize here.

2. Exp. Astron.

   New horizons in cosmology with spectral distortions of the cosmic microwave background

   Chluba, J., Abitbol, M. H., and Aghanim, N. et. al.

   Experimental Astronomy May 2021

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   This Voyage 2050 paper highlights the unique science opportunities using spectral distortions of the cosmic microwave background (CMB). CMB spectral distortions probe many processes throughout the history of the Universe, delivering novel information that complements past, present and future efforts with CMB anisotropy and large-scale structure studies. Precision spectroscopy, possible with existing technology, would not only provide key tests for processes expected within the cosmological standard model but also open an enormous discovery space to new physics. This offers unique scientific opportunities for furthering our understanding of inflation, recombination, reionization and structure formation as well as dark matter and particle physics. A dedicated experimental approach could open this new window to the early Universe in the decades to come, allowing us to turn the long-standing upper distortion limits obtained with COBE/FIRAS some 25 years ago into clear detections of the expected standard distortion signals and also challenge our current understanding of the laws of nature.

2020

1. MNRAS

   Updated fundamental constant constraints from Planck 2018 data and possible relations to the Hubble tension

   Hart, Luke, and Chluba, Jens

   Monthly Notices of the Royal Astronomical Society Feb 2020

   Abs arXiv PDF

   We present updated constraints on the variation of the fine structure constant, α_EM, and effective electron rest mass, m_e, during the cosmological recombination era. These two fundamental constants directly affect the ionization history at redshift z ≃ 1100 and, thus, modify the temperature and polarization anisotropies of the cosmic microwave background (CMB) measured precisely with Planck . The constraints on α_EM tighten slightly due to improved Planck 2018 polarization data but otherwise remain similar to previous CMB analysis. However, a comparison with the 2015 constraints reveals a mildly discordant behaviour for m_e, which from CMB data alone is found below its local value. Adding baryon acoustic oscillation data brings m_e back to the fiducial value, m_e=(1.0078\pm 0.0067) m_e,0, and also drives the Hubble parameter to H_0 = 69.1 \pm 1.2(in units of {km s\^{-1} Mpc\^{-1} }). Further adding supernova data yields m_e=(1.0190\pm 0.0055) m_e,0 with H_0 = 71.24 \pm 0.96. We perform several comparative analyses using the latest cosmological recombination calculations to further understand the various effects. Our results indicate that a single-parameter extension allowing a slightly increased value of m_e (≃3.5σ above m_e, 0) could play a role in the Hubble tension.

2. MNRAS

   Improved model-independent constraints on the recombination era and development of a direct projection method

   Hart, Luke, and Chluba, Jens

   Monthly Notices of the Royal Astronomical Society May 2020

   Abs arXiv PDF Code

   The unparalleled precision of recent experiments such as Planck have allowed us to constrain standard and non-standard physics (e.g. due to dark matter annihilation or varying fundamental constants) during the recombination epoch. However, we can also probe this era of cosmic history using model-independent variations of the free electron fraction, X_e, which, in turn, affects the temperature and polarization anisotropies of the cosmic microwave background. In this paper, we improve on the previous efforts to construct and constrain these generalized perturbations in the ionization history, deriving new optimized eigenmodes based on the full Planck 2015 likelihood data, introducing the new module FEAREC+. We develop a direct likelihood sampling method for attaining the numerical derivatives of the standard and non-standard parameters, and discuss complications arising from the stability of the likelihood code. We improve the amplitude constraints of the Planck 2015 principal components constructed here, μ_1 = -0.09 \pm 0.12, μ_2 = -0.17 \pm 0.20, and μ_3 = -0.30 \pm 0.35, finding no indication for departures from the standard recombination scenario. The error constraint on the third mode has been improved by a factor of 2.5. We utilize an efficient eigenanalyser that keeps the cross-correlations of the first three eigenmodes to \rm Corr\left(μ μ^\prime \right) 0.1 per cent after marginalization for all the considered data combinations. We also propose a new projection method for estimating constraints on the parameters of non- standard recombination scenarios. As an example, using our eigenmode measurements, this allows us to recreate the Planck constraint on the two-photon decay rate, A_2s1s = 7.60 \pm 0.64, giving an error estimate to within ≃ 0.05σ of the full MCMC result. The improvements on the eigenmode analysis using the Planck data will allow us to implement this new method for analysis with fundamental constant variations in the future.

3. MNRAS

   Sensitivity forecasts for the cosmological recombination radiation in the presence of foregrounds

   Hart, Luke, Rotti, Aditya, and Chluba, Jens

   Monthly Notices of the Royal Astronomical Society Aug 2020

   Abs arXiv PDF

   The cosmological recombination radiation (CRR) is one of the inevitable Lambda cold dark matter spectral distortions of the cosmic microwave background (CMB). While it shows a rich spectral structure across dm-mm wavelengths, it is also one of the smallest signals to target. Here, we carry out a detailed forecast for the expected sensitivity levels required to not only detect but also extract cosmological information from the CRR in the presence of foregrounds. We use CosmoSpec to compute the CRR including all important radiative transfer effects and modifications to the recombination dynamics. We confirm that detections of the overall CRR signal are possible with spectrometer concepts like SuperPIXIE. However, for a real exploitation of the cosmological information, an ≃ 50 times more sensitive spectrometer is required. While extremely futuristic, this could provide independent constraints on the primordial helium abundance, Y_p, and probe the presence of extra relativistic degrees of freedom during BBN and recombination. Significantly improving the constraints on other cosmological parameters requires even higher sensitivity (another factor of ≃5) when considering a combination of a CMB spectrometer with existing CMB data. To a large part, this is due to astrophysical foregrounds which interestingly do not degrade the constraints on Y_p and N_eff as much. A future CMB spectrometer could thus open a novel way of probing non-standard BBN scenarios, dark radiation and sterile neutrinos. In addition, inflation physics could be indirectly probed using the CRR in combination with existing and forthcoming CMB anisotropy data.

2018

1. MNRAS

   New constraints on time-dependent variations of fundamental constants using Planck data

   Hart, Luke, and Chluba, Jens

   Monthly Notices of the Royal Astronomical Society Oct 2018

   Abs arXiv PDF

   Observations of the cosmic microwave background (CMB) today allow us to answer detailed questions about the properties of our Universe, targeting both standard and non-standard physics. In this paper, we study the effects of varying fundamental constants (i.e. the fine-structure constant, α_EM, and electron rest mass, m_e) around last scattering using the recombination codes COSMOREC and RECFAST++. We approach the problem in a pedagogical manner, illustrating the importance of various effects on the free electron fraction, Thomson visibility function and CMB power spectra, highlighting various degeneracies. We demonstrate that the simpler RECFAST++ treatment (based on a three-level atom approach) can be used to accurately represent the full computation of COSMOREC. We also include explicit time-dependent variations using a phenomenological power-law description. We reproduce previous Planck 2013 results in our analysis. Assuming constant variations relative to the standard values, we find the improved constraints α_EM/α_EM, 0 = 0.9993 \pm 0.0025 (CMB only) and m_e/m_e, 0 = 1.0039 \pm 0.0074 (including BAO) using Planck 2015 data. For a redshift-dependent variation, α_EM(z) = α_EM(z_0) [(1 + z)/1100]^p with α_EM(z_0) ≡ α_EM, 0 at z_0 = 1100, we obtain p = 0.0008 \pm 0.0025. Allowing simultaneous variations of α_EM(z_0) and p yields α_EM(z_0)/α_EM , 0 = 0.9998 \pm 0.0036 and p = 0.0006 \pm 0.0036. We also discuss combined limits on α_EM and m_e. Our analysis shows that existing data are not only sensitive to the value of the fundamental constants around recombination but also its first time derivative. This suggests that a wider class of varying fundamental constant models can be probed using the CMB.