Backreaction Issues Research Articles

Could PBHs and secondary GWs have originated from squeezed initial states?

Recently, the production of primordial black holes (PBHs) andsecondary gravitational waves (GWs) due to enhanced scalar power on smallscales have garnered considerable attention in the literature.Often, the mechanism considered to arrive at such increased power involvesa modification of the standard slow roll inflationary dynamics, achievedwith the aid of fine-tuned potentials.In this work, we investigate another well known method to generate featuresin the power spectrum wherein the initial state of the perturbations isassumed to be squeezed states.The approach allows one to generate features even in slowroll inflation with a specific choice for the Bogoliubov coefficientscharacterizing the squeezed initial states.Also, the method is technically straightforward to implement since theBogoliubov coefficients can be immediately determined from the formof the desired spectrum with increased scalar power at small scales.It is known that, for squeezed initial states, the scalar bispectrum isstrongly scale dependent and the consistency condition governing the scalarbispectrum in the squeezed limit is violated.In fact, the non-Gaussianity parameter characterizing the scalarbispectrum proves to be inversely proportional to the squeezed mode andthis dependence enhances its amplitude at large wave numbers making ithighly sensitive to even a small deviation from the standard Bunch-Daviesvacuum.These aspects can possibly aid in leading to enhanced formation of PBHsand generation of secondary GWs.However, we find that: (i) the desired form of thesqueezed initial states may be challenging to achieve from a dynamicalmechanism, and (ii) the backreaction due to the excited states severelylimits the extent of deviation from the Bunch-Davies vacuum at large wavenumbers.We argue that, unless the issue of backreaction is circumvented,squeezed initial states cannot lead to a substantial increase in poweron small scales that is required for enhanced formation of PBHs and generationof secondary GWs.

Backreaction issues in axion monodromy and Minkowski 4-forms

We clarify the differences between the usual Kaloper-Sorbo description of axion monodromy and the effective axionic potential in terms of Minkowski 4-forms derived in string compactifications. The fact that the metric of the 3-form fields coming from string theory is field dependent (unlike in Kaloper-Sorbo) leads to the backreaction issues recently studied in axion monodromy models within string theory. We reanalyse these problems in terms of the 4-forms focusing on the case in which the non-periodic scalars backreact on the Kahler metric of the inflaton reducing the physical field range. In the closed string sector of Type II Calabi-Yau compactifications with fluxes the metric becomes field dependent precisely when Δϕ ∼ Mp, independently of the choice of fluxes. We propose, however, some counter-examples to this universal behaviour by including open string fields.

Magnetogenesis in bouncing cosmology

We consider the process of magnetogenesis in the context of nonsingular bounce cosmology. We show that large primordial magnetic fields can be generated during contraction without encountering strong coupling and backreaction issues. The fields may seed large-scale magnetic fields with observationally interesting strengths. This result leads to a theoretical constraint on the relation of the energy scale of the bounce cosmology to the number of effective e-folding of the contracting phase in the case of scale invariance for the power spectrum of primordial magnetic fields. We show that this constraint can be satisfied in a sizable region of the parameter space for the nonsingular bounce cosmology.

A gradient expansion for cosmological backreaction

We address the issue of cosmological backreaction from non-linearstructure formation by constructing an approximation for the time evolved metric of a dust dominated universe based on a gradient expansion. Our metric begins as a perturbation of a flat Friedmann-Robertson-Walker state described by a nearly scale invariant, Gaussian, power-law distribution, and evolves in time until non-linear structures have formed. After describing and attempting to control for certain complications in the implementation of this approach, this metric then forms a working model of the universe. We numerically calculate the evolution of the average scale factor in this model and hence the backreaction. We argue that, despite its limitations, this model is more realistic than previous models that have confronted the issue of backreaction. We find that the instantaneous effects of backreaction in this model could be as large as ∼ 10% of the background. This suggests that a proper understanding of the cumulative effects of backreaction could be crucial for precision cosmology and any future exploration of the dark sector.

On inflating magnetic fields, and the backreactions thereof

We investigate in more depth the issue of backreaction in models that attempt at generating cosmological magnetic fields at inflation. By choosing different, physically motivated, parametrisations, we are able to isolate the heart of the problem, namely the existence, alongside the wanted magnetic field, of its electric counterpart, which turns out quite generally to be stronger and redder. We were also able to identify a few more interwoven weak spots (the typically very high scale of inflation, the width of the spectrum of modes processed by inflation, the blindness of the amplification mechanism to the energy scale processed), in a way independent on the specifications of the coupling between inflation and electromagnetism. Despite having stripped down the problem to the core, the obstacles encountered appear insurmountable, thereby posing a challenge to inflation as the incubator of cosmological magnetism.

Tailoring three-point functions and integrability II. Weak/strong coupling match

We compute three-point functions of single trace operators in planar N=4 SYM. We consider the limit where one of the operators is much smaller than the other two. We find a precise match between weak and strong coupling in the Frolov-Tseytlin classical limit for a very general class of classical solutions. To achieve this match we clarify the issue of back-reaction and identify precisely which three-point functions are captured by a classical computation.

Optical geometry for gravitational collapse and Hawking radiation

The notion of optical geometry, introduced more than 20 years ago as a formal tool in quantum field theory on a static background, has recently found several applications to the study of physical processes around compact objects. In this paper we define optical geometry for spherically symmetric gravitational collapse, with the purpose of extending the current formalism to physically interesting spacetimes which are not conformally static. The treatment is fully general but, as an example, we also discuss the special case of the Oppenheimer-Snyder model. The analysis of the late-time behavior shows a close correspondence between the structure of optical spacetime for gravitational collapse and that of flat spacetime with an accelerating boundary. Thus, optical geometry provides a natural physical interpretation for derivations of the Hawking effect based on the ``moving mirror analogy.'' Finally, we briefly discuss the issue of back reaction in black hole evaporation and the information paradox from the perspective of optical geometry.

Semiclassical approximations for gravity and the issue of backreaction

Semiclassical approximations, which are useful in the study of a quantum system interacting with a classical system, are studied and compared. A toy quantum mechanical model with two degrees of freedom (which mimics the features of gravity interacting with quantum fields) is used for illustration. In particular, the author considers the Born-Oppenheimer approximation (BOA) (corresponding to G to 0 at fixed h(cross)), the effective action approach (h(cross) to 0 at fixed G) and their combinations. He shows that in the strict BOA limit there is no backreaction on gravity. Gravity is described by classical equations and the fields are quantised in that background. In the effective action approach one can obtain a semiclassical description for gravity, if certain stringent requirements are satisfied. In most situations of interest these conditions will not be met and the O(h(cross)) contribution from gravitons will be comparable to that from quantum fields. He studies the system using both the Schrodinger equation and path integrals and indicates the correspondence.