Latham Boyle, Neil Turok

The standard model is a remarkably consistent and complete quantum field theory but its coupling to gravity and the Higgs field remain problematic, as reflected in the cosmological constant problem, the Weyl anomaly, and the hierarchy puzzle. We point out that 36 conformally-coupled dimension-zero scalar fields can simultaneously cancel the vacuum energy and both terms in the Weyl anomaly, if the Higgs and graviton fields are emergent. The cancellation is highly non-trivial: given the standard model gauge group $SU(3)\times SU(2)\times U(1)$, it requires precisely 48 Weyl fermions, i.e., three generations of standard model fermions, including right-handed neutrinos. The dimension-zero scalars have a four-derivative Lagrangian, usually taken to imply vacuum instability. However, using the Euclidean inner product natural in the context of our recent proposal arXiv:2109.06204, we find no negative norm or negative energy states. Hence the vacuum is stable. Moreover, the scalars possess a scale invariant power spectrum extending to long wavelengths, suggesting a new explanation for the primordial scalar perturbations in cosmology, without the need for inflation. These intriguing results, spanning a vast range of scales, suggest dimension-zero scalars may play a key role in fundamental physics. We discuss how the Higgs and graviton fields might emerge in this context.

Clifford Cheung, Andreas Helset, Julio Parra-Martinez

We derive a universal soft theorem for every scattering amplitude with at least one massless particle in an arbitrary theory of scalars. Our results follow from the geometry of field space and are valid for any choice of mass spectrum, potential terms, and higher-derivative interactions. For a vanishing potential, the soft limit of every amplitude is equal to the field-space covariant derivative of an amplitude with one fewer particle. Furthermore, the Adler zero and the dilaton soft theorem are special cases of our results. We also discuss more exotic scenarios in which the soft limit is non-trivial but still universal. Last but not least, we derive new theorems for multiple-soft limits which directly probe the field-space curvature, as well as on-shell recursion relations applicable to two-derivative scalar field theories exhibiting no symmetries whatsoever.

Yuewei Wen, Eva Nesbit, Dragan Huterer, Scott Watson

Standard cosmological data analyses typically constrain simple phenomenological dark-energy parameters, for example the present-day value of the equation of state parameter, $w_0$, and its variation with scale factor, $w_a$. However, results from such an analysis cannot easily indicate the presence of modified gravity. Even if general relativity does not hold, experimental data could still be fit sufficiently well by a phenomenological $w_0w_a$CDM, unmodified-gravity model. Hence, it would be very useful to know if there are generic signatures of modified gravity in standard analyses. Here we present, for the first time to our knowledge, a quantitative mapping showing how modified gravity models look when (mis)interpreted within the standard unmodified-gravity analysis. Scanning through a broad space of modified-gravity (Horndeski) models, and assuming a near-future survey consisting of CMB, BAO, and SNIa observations, we report values of the best-fit set of cosmological parameters including $(w_0, w_a)$ that would be inferred if modified gravity were at work. We find that modified gravity models that can masquerade as standard gravity lead to very specific biases in standard-parameter spaces. We also comment on implications for measurements of the amplitude of mass fluctuations described by the parameter $S_8$.