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Register a new userApproaching Lambda without fine-tuning
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We address the fine-tuning problem of dark energy cosmologies which arises when the dark energy density needs to initially lie in a narrow range in order for its present value to be consistent with observations. As recently noticed, this problem becomes particularly severe in canonical Quintessence scenarios, when trying to reproduce the behavior of a cosmological constant, i.e. when the dark energy equation of state w_Q approaches -1: these models may be reconciled with a large basin of attraction only by requiring a rapid evolution of w_Q at low reshifts, which is in conflict with the most recent estimates from type Ia Supernovae discovered by Hubble Space Telescope. Next, we focus on scalar-tensor theories of gravity, discussing the implications of a coupling between the Quintessence scalar field and the Ricci scalar (``Extended Quintessence). We show that, even if the equation of state today is very close to -1, by virtue of the scalar-tensor coupling the quintessence trajectories still possess the attractive feature which allows to reach the present level of cosmic acceleration starting by a set of initial conditions which covers tens of orders of magnitude; this effect, entirely of gravitational origin, represents a new important consequence of the possible coupling between dark energy and gravity.
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We investigate the $H_0$ tension in a range of extended model frameworks beyond the standard $Lambda$CDM without the data from cosmic microwave background (CMB). Specifically, we adopt the data from baryon acoustic oscillation, big bang nucleosynthesis and type Ia supernovae as indirect measurements of $H_0$ to study the tension. We show that the estimated value of $H_0$ from indirect measurements is overall lower than that from direct local ones regardless of the data sets and a range of extended models to be analyzed, which indicates that, although the significance of the tension varies depending on models, the $H_0$ tension persists in a broad framework beyond the standard $Lambda$CDM model even without CMB data.
Invariant Set (IS) theory is a locally causal ontic theory of physics based on the Cosmological Invariant Set postulate that the universe $U$ can be considered a deterministic dynamical system evolving precisely on a (suitably constructed) fractal dynamically invariant set in $U$s state space. IS theory violates the Bell inequalities by violating Measurement Independence. Despite this, IS theory is not fine tuned, is not conspiratorial, does not constrain experimenter free will and does not invoke retrocausality. The reasons behind these claims are discussed in this paper. These arise from properties not found in conventional ontic models: the invariant set has zero measure in its Euclidean embedding space, has Cantor Set structure homeomorphic to the p-adic integers ($p ggg 0$) and is non-computable. In particular, it is shown that the p-adic metric encapulates the physics of the Cosmological Invariant Set postulate, and provides the technical means to demonstrate no fine tuning or conspiracy. Quantum theory can be viewed as the singular limit of IS theory when when $p$ is set equal to infinity. Since it is based around a top-down constraint from cosmology, IS theory suggests that gravitational and quantum physics will be unified by a gravitational theory of the quantum, rather than a quantum theory of gravity. Some implications arising from such a perspective are discussed.
The form of the inflationary potential is severely restricted if one requires that it be natural in the technical sense, i.e. terms of unrelated origin are not required to be correlated. We determine the constraints on observables that are implied in such natural inflationary models, in particular on $r$, the ratio of tensor to scalar perturbations. We find that the naturalness constraint does not require $r$ to be lare enough to be detectable by the forthcoming searches for B-mode polarisation in CMB maps. We show also that the value of $r$ is a sensitive discriminator between inflationary models.
Discrepant measurements of the Universes expansion rate ($H_0$) may signal physics beyond the standard cosmological model. Here I describe two early modified gravity mechanisms that reconcile the value of $H_0$ by increasing the expansion rate in the era of matter-radiation equality. These mechanisms, based on viable Horndeski theories, require significantly less fine-tuned initial conditions than early dark energy with oscillating scalar fields. In Imperfect Dark Energy at Equality (IDEE), the initial energy density dilutes slower than radiation but faster than matter, naturally peaking around the era of equality. The minimal IDEE model, a cubic Galileon, is too constrained by the cosmic microwave background (Planck) and baryon acoustic oscillations (BAO) to relieve the $H_0$ tension. In Enhanced Early Gravity (EEG), the scalar field value modulates the cosmological strength of gravity. The minimal EEG model, an exponentially coupled cubic Galileon, gives a Planck+BAO value $H_0=68.7 pm 1.5$ (68% c.l.), reducing the tension with SH0ES from $4.4sigma$ to $2.6sigma$. Additionally, Galileon contributions to cosmic acceleration may reconcile $H_0$ via Late-Universe Phantom Expansion (LUPE). Combining LUPE, EEG and $Lambda$ reduces the tension between Planck, BAO and SH0ES to $2.5sigma$. I will also describe additional tests of coupled Galileons based on local gravity tests, primordial element abundances and gravitational waves. While further model building is required to fully resolve the $H_0$ problem and satisfy all available observations, these examples show the wealth of possibilities to solve cosmological tensions beyond Einsteins General Relativity.
Recently, the formation of primordial black holes (PBHs) from the collapse of primordial fluctuations has received much attention. The abundance of PBHs formed during radiation domination is sensitive to the tail of the probability distribution of primordial fluctuations. We quantify the level of fine-tuning due to this sensitivity. For example, if the main source of dark matter is PBHs with mass $10^{-12}M_odot$, then anthropic reasoning suggests that the dark matter to baryon ratio should range between 1 and 300. For this to happen, the root-mean-square amplitude of the curvature perturbation has to be fine-tuned within a $7.1%$ range. As another example, if the recently detected gravitational-wave events are to be explained by PBHs, the corresponding degree of fine-tuning is $3.8%$. We also find, however, that these fine-tunings can be relaxed if the primordial fluctuations are highly non-Gaussian, or if the PBHs are formed during an early-matter-dominated phase. We also note that no fine-tuning is needed for the scenario of a reheating of the universe by evaporated PBHs with Planck-mass relics left to serve as dark matter.
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