BBN constraints on dark radiation isocurvature

Adshead, Peter; Holder, G. P.; Ralegankar, Pranjal

doi:10.1088/1475-7516/2020/09/016

Cited by 9 publications

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References 33 publications

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“…Furthermore, several authors have pointed out that small regions with very large values of η could lead to the primordial production of elements much heavier than in standard BBN, including even r-process elements [17][18][19][20][21][22].However, spatial variations in η are by no means the only inhomogeneous BBN models that have been considered. Various authors have examined BBN with inhomogeneous variations in the cosmological shear [23], inhomogeneous neutrino chemical potentials [24][25][26], inhomogeneous values of the baryon/dark matter ratio [27], inhomogeneous primordial magnetic fields [28], and inhomogeneous fluctuations in the dark radiation density [29]. Models with small-scale antimatter domains have also been discussed [30][31][32][33][34].Most of these studies of inhomogeneous BBN tacitly assume a uniform distribution of elements at present.…”

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“…Papers that assume that inhomogeneities in BBN are preserved today include Refs. [19,22,27,29].This dichotomy of approaches leads to an obvious question: are the spatial inhomogeneities produced in inhomogeneous BBN preserved at present, or are they erased by subsequent element diffusion? Clearly, inhomogeneities will be erased on sufficiently small scales and preserved on sufficiently large scales, so we can define a critical comoving length, d com , that separates these two regimes.…”

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“…However, spatial variations in η are by no means the only inhomogeneous BBN models that have been considered. Various authors have examined BBN with inhomogeneous variations in the cosmological shear [23], inhomogeneous neutrino chemical potentials [24][25][26], inhomogeneous values of the baryon/dark matter ratio [27], inhomogeneous primordial magnetic fields [28], and inhomogeneous fluctuations in the dark radiation density [29]. Models with small-scale antimatter domains have also been discussed [30][31][32][33][34].…”

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“…[46] for a recent discussion). Indeed, Adshead et al [29] have suggested that the appropriate length scale for primordial element inhomogeneities to be erased is on the scale of individual galaxies, with the inhomogeneities preserved between different galaxies. While galaxy formation (along with subsequent chemical evolution) will certainly smooth out primordial inhomogeneities on scales larger than those given by Eqs.…”

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“…Hence, our results add support to the approach taken in Refs. [19,22,27,29]. Unless inhomogeneities in BBN are taken to be sufficiently small (completely sub-horizon at BBN), one cannot assume that the present-day element abundances are simply a homogeneous average of the originally inhomogenous distribution of elements.…”

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Does inhomogeneous big bang nucleosynthesis produce an inhomogeneous element distribution today?

Scherrer

2021

Preprint

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Inhomogeneous big bang nucleosynthesis (BBN) produces a spatially inhomogeneous distribution of element abundances at T ∼ 10 9 K, but subsequent element diffusion will tend to erase these inhomogeneities. We calculate the cosmological comoving diffusion length for the BBN elements. This diffusion length is limited by atomic scattering and is therefore dominated by diffusion when the atoms are neutral, between the redshifts of recombination and reionization. We find that the comoving diffusion length today is dcom ≈ 70 pc for all of the elements of interest except 7 Li, for which dcom is an order of magnitude smaller because 7 Li remains ionized throughout the relevant epoch. This comoving diffusion length corresponds to a substellar baryonic mass scale and is roughly equal to the horizon scale at BBN. These results lend support to the possibility that inhomogeneities on scales larger than the horizon at BBN could lead to a spatially inhomogeneous distribution of elements today, while purely subhorizon fluctuations at BBN can result only in a homogeneous element distribution at present.Since its development in the 1960s [1-4] big bang nucleosynthesis (BBN) has become one of the main pillars of modern cosmology. While the baryon-photon ratio, η, was once treated as the main quantity to be determined by comparing the predictions of BBN to the observed element abundances, the independent measurement of η from CMB observations has left the theory of BBN with no free parameters. Given the allowed range for η derived from the CMB, the predicted BBN abundances of deuterium and 4 He are in excellent agreement with the observations. However, the predicted 7 Li abundance remains a factor of three larger than the abundance inferred from observations; this discrepancy remains unexplained at present [5]. For a recent review of BBN, see Ref. [6].Despite the successes of BBN, many modifications to the theory have been proposed. Perhaps the most widely investigated of these modified theories are models involving some sort of inhomogeneity at the epoch of BBN. The most basic of such models postulate isocurvature fluctuations, for which η varies from one region of the Universe to another [7][8][9][10][11][12][13]. Most of these investigations simply calculate the element abundances as a function of η in separate domains and average the final results. However, an important effect arises when the length scale of the fluctuations is longer than the proton diffusion length but shorter than the neutron diffusion length; in this case neutron diffusion from high-density to low-density regions will strongly modify the neutron-proton ratio. These models were initially inspired by the possibility of such density fluctuations arising naturally in the QCD phase transition [14,15], and this line of investigation yielded an extensive literature (see, e.g., Ref.[16] and references therein). Furthermore, several authors have pointed out that small regions with very large values of η could lead to the primordial production of elements much heavier ...

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Does inhomogeneous big bang nucleosynthesis produce an inhomogeneous element distribution today?

Scherrer

2021

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Diraxiogenesis

Berbig

2024

J. High Energ. Phys.

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The family of Dirac Seesaw models offers an intriguing alternative explanation for the smallness of neutrino masses without necessarily requiring microscopic lepton number violation, when compared to the more familiar class of Majorana Seesaws. A global U(1)D symmetry, that is explicitly broken by a higher dimensional scalar operator, ensures that the right handed neutrino does not couple directly to the Standard Model like Higgs and an exact gauged or residual lepton number symmetry prohibits all Majorana masses. We demonstrate that all three Dirac Seesaws possess a Pseudo-Nambu-Goldstone boson associated with the U(1)D symmetry, that we call the Diraxion, whose cosmological dynamics have so far been left unexplored. Furthermore we illustrate that a Dirac-Leptogenesis version of the recently proposed Lepto-Axiogenesis scenario can be realized in this class of models, leading to a unified origin of the observed baryon asymmetry and dark matter relic abundance. Explaining only the baryon asymmetry can lead to potentially observable amounts of right handed neutrino dark radiation with ∆Neff. ≲ 0.028. On the other hand, if we only fix the dark matter abundance via the kinetic misalignment mechanism, this set-up could lead to detectable signatures in proposed cosmic neutrino background experiments via decays of eV-scale Diraxions to neutrinos. Here there is no domain wall problem, since topological defects decay to a subleading fraction of relic Diraxions. A key ingredient of all Axiogenesis scenarios is the dynamics of relatively light scalar called the Saxion, that in our case has a mass at the GeV-scale and which might reveal itself in heavy meson decays or collider searches. Our setup predicts isocurvature perturbations in baryons, dark matter and dark radiation sourced by fluctuations of the Saxion.

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