Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Ultralight Scalar Decay and the Hubble Tension   We examine whether the Hubble tension, the mismatch between early and late
measurements of $H_0$, can be alleviated by ultralight scalar fields in the
early universe, and we assess its plausibility within UV physics. Since their
energy density needs to rapidly redshift away, we explore decays to massless
fields around the era of matter-radiation equality. We highlight a concrete
implementation of ultralight pseudo-scalars, axions, that decay to an abelian
dark sector. This scenario circumvents major problems of other popular
realizations of early universe scalar models in that it uses a regular scalar
potential that is quadratic around the minimum, instead of the extreme
fine-tuning of many existing models. The idea is that the scalar is initially
frozen in its potential until $H\sim m$, then efficient energy transfer from
the scalar to the massless field can occur shortly after the beginning of
oscillations due to resonance. We introduce an effective fluid model which
captures the transition from the frozen scalar phase to the radiation dark
sector phase. We perform a fit to a combined Planck 2018, BAO, SH$_0$ES and
Pantheon supernovae dataset and find that the model gives
$H_0=69.9_{-0.86}^{+0.84}$ km/s/Mpc with $\Delta\chi^2 \approx -9$ compared to
$\Lambda$CDM; while inclusions of other data sets may worsen the fit.
Importantly, we find that large values of the coupling between fields is
required for sufficiently rapid decay: For axion-gauge field models $\phi
F\tilde{F}/\Lambda$ it requires $\Lambda\lesssim f/80$, where $2\pi f$ is the
field range. We find related conclusions for scalar-scalar models
$\sim\phi\,\chi^2$ and for models that utilize perturbative decays. We conclude
that these sorts of ultralight scalar models that purport to alleviate the
Hubble tension, while being reasonable effective field theories, require
features that are difficult to embed within UV physics.

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The categories of this paper are: nucl-th
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The categories of this paper are: plasm-ph
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The categories of this paper are: solv-int
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The categories of this paper are: stat
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The categories of this paper are: mtrl-th
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The categories of this paper are: ao-sci
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The categories of this paper are: alg-geom
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The categories of this paper are: eess
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The categories of this paper are: chem-ph
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The categories of this paper are: cond-mat
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The categories of this paper are: hep-ex
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The categories of this paper are: hep-lat
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The categories of this paper are: q-bio
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The categories of this paper are: supr-con
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The categories of this paper are: dg-ga
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The categories of this paper are: nucl-ex