J. Shterenberg, D. Garfinkle, A. I. Rosenzweig, D. Shlivko and P.J. Steinhardt
Anti-ultralocality and plateau models of inflation | arXiv:2608.24997
Anti-ultralocality refers to the growth of spatial gradient terms relative to velocity terms in the coupled Einstein–scalar field equations. It is a characteristic feature of decelerated expansion before the onset of inflation. We find that, starting from generic initial conditions, the growth of gradient terms in the Einstein equations either prevents inflation from lasting for enough e-folds or triggers a phase of quantum runaway and an unpredictive multiverse. We show that the fine-tuning of initial conditions necessary to avoid these issues becomes more severe as the energy scale of inflation is made smaller, disfavoring the current leading approaches for reducing the tensor-to-scalar ratio and/or the energy scale of inflation.
M. Khaldieh, A.I. Rosenzweig and P.J. Steinhardt
Causal horizons, geodesic completeness and stability in slow contraction cosmology
Phys. Lett. B 880 (2026) 140851, arXiv:2607.07815
We show that cosmological models with a semi-infinite phase of slow contraction (ekpyrosis) possess a combination of properties that can address several fundamental problems in cosmology, otherwise faced in contracting de Sitter phases or standard big bang expansion. In particular, slow contraction admits a stable past attractor asymptoting to Minkowski space, as well as a stable, flat, homogeneous, and isotropic future attractor with negligible Weyl curvature (and, therefore, negligible gravitational entropy).
N. Patino and P.J. Steinhardt.
Particle productiion by time-varying dark energy and the end of cosmic expansion | arXiv:2512.15860
We consider various possible consequences of time-varying dark energy due to a quintessence scalar field whose energy density is partially converted to particles as the field evolves down its potential. This particle production acts as a source of thermal friction on the field that can make it difficult to distinguish whether dark energy is due to a radiating field rolling down a steep potential, a purely self-interacting field moving down a flatter potential, or a cosmological constant. We take special interest in the case where a steep potential rapidly changes from positive to negative as the field evolves, resulting in the end of cosmic expansion and the beginning of contraction. We show that particle production can provide an alternative avenue for detecting the transition by generating a background of thermal dark radiation, partly comprised of neutrinos or other particles.
A. Bedroya and P.J. Steinhardt.
Holographic constraints on the string landscape | arXiv:2511.15784
We show that holography imposes strong and general constraints on scalar field potentials in the string landscape, ruling out potentials with extended plateaus, excessively steep or shallow asymptotics, certain zero crossings, and specific alignments of stable AdS minima in moduli space. Taken together, these constraints exclude the leading proposals to realize scale-separated AdS vacua and metastable de Sitter vacua in the string theory landscape such as DGKT and KKLT.
A.Bedroya and P.J. Steinhardt.
Holography vs. scale separation | arXiv:2509.25313
In this work, we point out a contradiction between holography and scale-separated AdS (i.e. parametrically large mass gap) in string theory. Namely, there must exist a scalar field trajectory that follows the gradient of the scalar potential to the asymptotic region of the scalar field space in which limit V∼m^α, where α≤2 is a constant and m is the mass scale of the lightest tower of states. Barring fine-tuning, our conclusions apply to other Ricci-flat flux compactifications including the KKLT scenario which relies on scale separation to obtain a metastable de Sitter uplift.
A. Bedroya, H. Lee and P.J. Steinhardt.
A species scale-driven breakdown of effective field theory in time-dependent string backgrounds | arXiv:2504.13260
We present a novel way in which effective field theory (EFT) can break down in cosmological string backgrounds depending on the behavior of the quantum gravity cutoff in infinite distance limits, known as the species scale. Namely, EFT can break down if the species scale falls off so rapidly as the Friedmann-Robertson-Walker (FRW) scale factor grows from some initial value to some final value such that the physical momentum of an initial Hubble-sized perturbation grows to exceed the species scale. Our bound is proportional to the Trans-Planckian Censorship Conjecture (TCC) bound, with the advantage that it applies to any FRW solution in the string landscape. Finally, we distinguish EFT breakdown from TCC violation, the latter being a quantum gravity constraint rather than an EFT limitation. Perhaps our most surprising finding is that in any flat FRW solution that develops a weakly coupled string at future infinity the EFT inevitably breaks down.