Magnetic fields from small-scale primordial perturbations
Bibliographic record
Abstract
Weak magnetic fields must have existed in the early Universe, as they were sourced by the cross product of electron density and temperature gradients through the Biermann-battery mechanism. In this paper we calculate the magnetic fields generated at cosmic dawn by a variety of small-scale primordial perturbations, carefully computing the evolution of electron density and temperature fluctuations, and consistently accounting for relative velocities between baryons and dark matter. We first compute the magnetic field resulting from standard, nearly scale-invariant primordial adiabatic perturbations, making significant improvements to previous calculations. This ``standard'' primordial field has a root mean square (rms) of a few times ${10}^{\ensuremath{-}15}\text{ }\text{ }\mathrm{nG}$ at $20\ensuremath{\lesssim}z\ensuremath{\lesssim}100$, with fluctuations on $\ensuremath{\sim}\mathrm{kpc}$ comoving scales, and could serve as the seed of present-day magnetic fields observed in galaxies and galaxy clusters. In addition, we consider early Universe magnetic fields as a possible probe of nonstandard initial conditions of the Universe on small scales $k\ensuremath{\sim}1\ensuremath{-}{10}^{3}\text{ }\text{ }{\mathrm{Mpc}}^{\ensuremath{-}1}$. To this end, we compute the maximally allowed magnetic fields within current upper limits on small-scale adiabatic and isocurvature perturbations. Enhanced small-scale adiabatic fluctuations below current cosmic microwave background spectral-distortion constraints could produce magnetic fields as large as $\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\text{ }\text{ }\mathrm{nG}$ at $z=20$. Uncorrelated small-scale isocurvature perturbations within current big bang nucleosynthesis bounds could potentially enhance the rms magnetic field to $\ensuremath{\sim}{10}^{\ensuremath{-}14}\ensuremath{-}{10}^{\ensuremath{-}10}\text{ }\text{ }\mathrm{nG}$ at $z=20$, depending on the specific isocurvature mode considered. While these very weak fields remain well below current observational capabilities, our work points out that magnetic fields could potentially provide an interesting window into the poorly constrained small-scale initial conditions of the Universe.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".