1,720,986 research outputs found
The quest for purely virtual quanta: fakeons versus Feynman-Wheeler particles
Full text linkThe search for purely virtual quanta has attracted interest in the past. We consider various proposals and compare them to the concept of fake particle, or “fakeon”. In particular, the Feynman-Wheeler propagator, which amounts to using the Cauchy principal value inside Feynman diagrams, violates renormalizability, unitarity and stability, due to the coexistence of the prescriptions ±iE. We contrast the Feynman, fakeon and Feynman- Wheeler prescriptions in ordinary as well as cut diagrams. The fakeon does not have the problems of the Feynman-Wheeler propagator and emerges as the correct concept of purely virtual quantum. It allows us to make sense of quantum gravity at the fundamental level, and places it on an equal footing with the standard model. The resulting theory of quantum gravity is perturbative up to an incredibly high energy
Perturbation spectra and renormalization-group techniques in double-field inflation and quantum gravity cosmology
No full textWe study primordial cosmology with two scalar fields that participate in inflation at the same time, by coupling quantum gravity (i.e., the theory R+R2+C2 with the fakeon prescription/projection for C2) to a scalar field with a quadratic potential. We show that there exists a perturbative regime that can be described by an asymptotically de Sitter, cosmic RG flow in two couplings. Since the two scalar degrees of freedom mix in nontrivial ways, the adiabatic and isocurvature perturbations are not RG invariant on superhorizon scales. It is possible to identify the correct perturbations by using RG invariance as a guiding principle. We work out the resulting power spectra of the tensor and scalar perturbations to the NNLL and NLL orders, respectively. An unexpected consequence of RG invariance is that the theory remains predictive. Indeed, the scalar mixing affects only the subleading corrections, so the predictions of quantum gravity with single-field inflation are confirmed to the leading order
Fakeons, microcausality and the classical limit of quantum gravity
Full text linkWe elaborate on the idea of fake particle and study its physical consequences. When a theory contains fakeons, the true classical limit is determined by the quantization and a subsequent process of ?classicization?. One of the major predictions due to the fake particles is the violation of microcausality, which survives the classical limit. This fact gives hope to detect the violation experimentally. A fakeon of spin two, together with a scalar feld, is able to make quantum gravity renormalizable while preserving unitarity. We claim that the theory of quantum gravity emerging from this construction is the right one. By means of the classicization, we work out the corrections to the feld equations of general relativity. We show that the fnalized equations have, in simple terms, the form F = ma, where F is an average that includes a little bit of ?future?
High-order corrections to inflationary perturbation spectra in quantum gravity
No full textWe compute the inflationary perturbation spectra and the quantity r+8nT to the next-to-next-to-leading log order in quantum gravity with purely virtual particles (which means the theory R+R2+C2 with the fakeon prescription/projection for C2). The spectra are functions of the inflationary running coupling α (1/k) and satisfy the cosmic renormalization-group flow equations, which determine the tilts and the running coefficients. The tensor fluctuations receive contributions from the spin-2 fakeon χ μν at every order of the expansion in powers of α ∼ 1/115. The dependence of the scalar spectrum on the χ μν mass mχ, on the other hand, starts from the α2 corrections, which are handled perturbatively in the ratio mφ/mχ, where mφ is the inflaton mass. The predictions have theoretical errors ranging from α4∼10-8 to α3 ∼ 10-6. Nontrivial issues concerning the fakeon projection at higher orders are addressed
Dressed propagators, fakeon self-energy and peak uncertainty
No full textWe study the resummation of self-energy diagrams into dressed propagators in
the case of purely virtual particles and compare the results with those
obtained for physical particles and ghosts. The three geometric series differ
by infinitely many contact terms, which do not admit well-defined sums. The
peak region, which is outside the convergence domain, can only be reached in
the case of physical particles, thanks to analyticity. In the other cases,
nonperturbative effects become important. To clarify the matter, we introduce
the energy resolution around the peak and argue that a "peak
uncertainty"
around energies expresses the impossibility to approach
the fakeon too closely, being the fakeon mass and being the fakeon width. The introduction of is also
crucial to explain the observation of unstable long-lived particles, like the
muon. Indeed, by the common energy-time uncertainty relation, such particles
are also affected by ill-defined sums at , whenever we separate
their observation from the observation of their decay products. We study the
regime of large , which applies to collider physics (and
situations like the one of the boson), and the regime of small , which applies to quantum gravity (and situations like the one of
the muon).Comment: 39 pages, 6 figures; v2: JHE
Improved Schwinger-DeWitt techniques for higher-derivative perturbations of operator determinants
No full textWe consider higher-derivative perturbations of quantum gravity and quantum field theories in curved space and investigate tools to calculate counterterms and short-distance expansions of Feynman diagrams. In the case of single higher-derivative insertions we derive a closed formula that relates the perturbed one-loop counterterms to the unperturbed Schwinger-DeWitt coefficients. In the more general case, we classify the contributions to the short-distance expansion and outline a number of simplification methods. Certain difficulties of the common differential technique in the presence of higher-derivative perturbations are avoided by a systematic use of the Campbell-Baker-Hausdorff formula, which in some cases reduces the computational effort considerably
Fakeons and microcausality: Light cones, gravitational waves and the Hubble constant
No full textThe concept of a fake particle, or 'fakeon', allows us to make sense of quantum gravity as an ultraviolet complete theory, by renouncing causality at very small distances. We investigate whether the violation of microcausality can be amplified or detected in the most common settings. We show that it is actually short range for all practical purposes. Due to our experimental limitations, the violation does not propagate along the light cones or by means of gravitational waves. In some cases, the Universe even conspires to make the effect disappear. For example, the positivity of the Hubble constant appears to be responsible for the direction of time in the early Universe
On the nature of the Higgs boson
Full text linkSeveral particles are not observed directly, but only through their decay products. We consider the possibility that they might be fakeons, i.e. fake particles, which mediate interactions but are not asymptotic states. A crucial role to determine the true nature of a particle is played by the imaginary parts of the one-loop radiative corrections, which are affected in nontrivial ways by the presence of fakeons in the loop. The knowledge we have today is sufficient to prove that most non-directly observed particles are true physical particles. However, in the case of the Higgs boson the possibility that it might be a fakeon remains open. The issue can be resolved by means of precision measurements in existing and future accelerators
Renormalization-group techniques for single-field inflation in primordial cosmology and quantum gravity
No full textWe study inflation as a 'cosmic' renormalization-group (RG) flow. The flow, which encodes the dependence on the background metric, is described by a running coupling a, which parametrizes the slow roll, a de Sitter free, analytic beta function and perturbation spectra that areRGinvariant in the superhorizon limit. Using RG invariance as a guiding principle, we classify the main types of flows according to the properties of their spectra, without referring to their origins from specific actions or models. Novel features include spectra with essential singularities in a and violations of the relation r + 8nt = 0 to the leading order. Various classes of potentials studied in the literature can be described by means of the RG approach, even when the action includes aWeyl-squared term, while others are left out. In known cases, the classification helps identify the models that are ruled out by data. The RG approach is also able to generate spectra that cannot be derived from standard Lagrangian formulations
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