We investigate a class of slow roll inflationary models in light of the recent Cosmic Microwave Background constraints from Planck 2018, ACT DR6, DESI DR1, and BICEP/ <italic>Keck</italic> 2018. The combined dataset favors a higher value of the scalar spectral index <math altimg='si20.svg'><mrow><msub><mi>n</mi><msub><mrow /><mrow><mi mathvariant='normal'>S</mi></mrow></msub></msub><mo linebreak='goodbreak'>=</mo><mn>0.9743</mn><mo>&#177;</mo><mn>0.0034</mn></mrow></math>, which places increased pressure on several conventional inflationary scenarios. In this study, we analyze the observational viability of various well-motivated models, including the <math altimg='si2.svg'><mi>&#945;</mi></math>-attractor E- and T-models, chaotic inflation, hilltop inflation, and natural inflation. We incorporate the effects of a post-inflationary phase of reheating and examine how the dynamics of reheating influence the predictions in the <math altimg='si1.svg'><mrow><msub><mi>n</mi><msub><mrow /><mrow><mi mathvariant='normal'>S</mi></mrow></msub></msub><mo linebreak='goodbreak'>&#8722;</mo><mi>r</mi></mrow></math> plane. We also impose a lower bound on the reheating temperature based on the constraint from the effective number of relativistic species ( <math altimg='si21.svg'><mrow><mstyle mathvariant='normal'><mi>&#916;</mi></mstyle><msub><mi>N</mi><mrow><mi mathvariant='normal'>eff</mi></mrow></msub></mrow></math>) arising from primordial gravitational waves. While reheating improves agreement with observations for some models, significant regions of parameter space remain disfavored. Finally, we explore the impact of a non Bunch-Davies initial state and demonstrate that it can substantially improve the fit to the <math altimg='si1.svg'><mrow><msub><mi>n</mi><msub><mrow /><mrow><mi mathvariant='normal'>S</mi></mrow></msub></msub><mo linebreak='goodbreak'>&#8722;</mo><mi>r</mi></mrow></math> data across a broader class of inflationary models, thereby offering a potentially viable mechanism for reconciling theory with the latest observations.

India

Suvashis Maity

ACT-ing on inflation: Implications of non bunch-Davies initial condition and reheating on single-field slow roll models

<p>Understanding the loop corrections to cosmological observables is of paramount importance for having control on the quantum consistency of a theory in an expanding universe as well as for phenomenological reasons. In the present work, we begin with a systematic study of such corrections in the context scalar toy models whose perturbative Bunch-Davies wave function enjoys an intrinsic definition in terms of <i>cosmological polytopes</i>, focusing on one-loop graphs. Owing to the underlying twisted period integral representation they admit, their combinatorial structure along with their vector space structure, emerging from polynomial ideals algebra and intersection theory, are exploited to set-up and analyze the differential equations that the two- and three-site one-loop corrections have to satisfy upon variation of the external kinematic variables. We find that, while the two-site contribution can be written in terms of multiple polylogarithms, this is no longer true for the three-site case, for which elliptic structures appear. As a nontrivial check, we consider the scattering amplitude limit, recovering the known result in terms of polylogarithms only.</p>

Spain

Germany

Paolo Benincasa

Italy

France

Giacomo Brunello

Manoj K. Mandal

Pierpaolo Mastrolia

Francisco Vazão

One-loop corrections to the Bunch-Davies wave function of the universe

In this work, we discussed the Davies-type phase transitions in two distinct categories of dyonic AdS black holes (BHs). The first one is dyonic AdS BHs within four-dimensional spacetime framework. The second one is the dyonic BHs with quasitopological electromagnetism (QE) in Einstein-Gauss-Bonnet gravity (EGBG). First, we analyze the increasing and decreasing behavior of divergency depending upon different parameters which provides useful information about the region that are physical and stable. We also discuss the critical behavior of BHs in three different statistical ensembles: the canonical, mixed and grand canonical. It is noted that BHs can be classified into distinct topological classes based on their topological charge (<math><mi>Q</mi></math>) which assumes discrete values of <math><mn>0</mn><mo>,</mo><mo>+</mo><mn>1</mn><mo>,</mo><mo>−</mo><mn>1</mn></math>. This topological charge plays a pivotal role in determining the phase structure and stability of the BHs within each ensemble. A topological charge of 0 corresponds to a neutral topological state that could be indicative of a more complex underlying structure, while charges of +1 or −1 indicate the presence of additional structures such as magnetic monopoles or dyons, which significantly influence the thermodynamic properties and phase transitions of the system.

Pakistan

Aqsa Mehmood

Saudi Arabia

Nazek Alessa

Azerbaijan

M. Umair Shahzad

Russia

Greece

Euaggelos E. Zotos

Davies-type phase transitions in 4D Dyonic AdS black holes from topological perspective

Topological arguments are currently being used as a novel scheme to discern the properties of black holes while ignoring their detailed structure and specific field equations. Among various avenues of black hole physics, where this novel approach is being utilized, the phase transition in black hole thermodynamics lies at the forefront. There are several types of phase transition in black holes; such as the van der Waals type phase transition, Davies-type phase transition, extremal phase transition, and Hawking-Page (HP) transition. So far, the topological interpretation, where the critical point has been identified with the non-zero topological charge, has been obtained only for the van der Waals type phase transition and HP transition in different spacetimes. To complete the picture, here we provide the same interpretation for two other phase transitions: Davies-type phase transition and extremal phase transition. The entire analysis is general and is valid for any spacetime where these types of phase transitions are observed. More importantly, our analysis suggests that amid the apparent differences in these phase transitions, they share the same topological characteristics, i.e. non-zero topological charge arising from different thermodynamic potentials in different types of phase transition.

Japan

Krishnakanta Bhattacharya

Kazuharu Bamba

United States

Douglas Singleton

Topological interpretation of extremal and Davies-type phase transitions of black holes

We describe a method for defining dynamical black hole entropy in gravitational effective field theories. The entropy is constructed order by order in derivatives. For any fixed number of derivatives, the entropy satisfies a nonperturbative second law of black hole mechanics if the black hole remains within the regime of validity of effective field theories. In equilibrium the entropy reduces to the Wald entropy. It reduces to the entropy defined by Hollands et al. in theories of vacuum gravity with up to ten derivatives.

United Kingdom

Iain Davies

Harvey S. Reall

Nonperturbative Second Law of Black Hole Mechanics in Effective Field Theory

In this work, we present a quintessential interpretation of having a blue-tilted tensor power spectrum for canonical single-field slow-roll inflation to explain the recently observed Pulsar Timing Array (NANOGrav 15-year and EPTA) signal of Gravitational Waves (GW). We formulate the complete semi-classical description of cosmological perturbation theory in terms of scalar and tensor modes using the Non-Bunch Davies initial condition. We found that the existence of the blue tilt  $$(n_t)$$  <math> <mrow> <mo>(</mo> <msub> <mi>n</mi> <mi>t</mi> </msub> <mo>)</mo> </mrow> </math>   within the favoured range  $$1.2&lt;n_t&lt;2.5$$  <math> <mrow> <mn>1.2</mn> <mo>&lt;</mo> <msub> <mi>n</mi> <mi>t</mi> </msub> <mo>&lt;</mo> <mn>2.5</mn> </mrow> </math>   can be explained in terms of a newly derived consistency relation. Further, we compute a new field excursion formula using the Non-Bunch Davies initial condition, that validates the requirement of Effective Field Theory in the sub-Planckian regime,  $$|\Delta \phi |\ll M_{\textrm{pl}}$$  <math> <mrow> <mrow> <mo>|</mo> <mi>Δ</mi> <mi>ϕ</mi> <mo>|</mo> </mrow> <mo>≪</mo> <msub> <mi>M</mi> <mtext>pl</mtext> </msub> </mrow> </math>   for the allowed value of the tensor-to-scalar ratio,  $$r&lt;0.06$$  <math> <mrow> <mi>r</mi> <mo>&lt;</mo> <mn>0.06</mn> </mrow> </math>   from CMB observations. In our study, we refer to this result as Anti Lyth bound as it violates the well-known Lyth bound originally derived for Bunch Davies initial condition. Further, we study the behaviour of the spectral density of GW and the associated abundance with the frequency, which shows that within the frequency domain  $$10^{-9}~{\textrm{Hz}}&lt;f&lt;10^{-7}~{\textrm{Hz}}$$  <math> <mrow> <msup> <mn>10</mn> <mrow> <mo>-</mo> <mn>9</mn> </mrow> </msup> <mspace width="3.33333pt"></mspace> <mtext>Hz</mtext> <mo>&lt;</mo> <mi>f</mi> <mo>&lt;</mo> <msup> <mn>10</mn> <mrow> <mo>-</mo> <mn>7</mn> </mrow> </msup> <mspace width="3.33333pt"></mspace> <mtext>Hz</mtext> </mrow> </math>   the outcome obtained from our analysis is completely consistent with the Pulsar Timing Array (NANOGrav 15-year and EPTA) signal. Also, we found that the behaviour of GW spectra satisfies the CMB constraints at the low frequency,  $$f_*\sim 7.7\times 10^{-17}~{\textrm{Hz}}$$  <math> <mrow> <msub> <mi>f</mi> <mrow> <mrow></mrow> <mo>∗</mo> </mrow> </msub> <mo>∼</mo> <mn>7.7</mn> <mo>×</mo> <msup> <mn>10</mn> <mrow> <mo>-</mo> <mn>17</mn> </mrow> </msup> <mspace width="3.33333pt"></mspace> <mtext>Hz</mtext> </mrow> </math>   corresponding to the pivots scale wave number,  $$k_*\sim 0.05~{\textrm{Mpc}}^{-1}.$$  <math> <mrow> <msub> <mi>k</mi> <mrow> <mrow></mrow> <mo>∗</mo> </mrow> </msub> <mo>∼</mo> <mn>0.05</mn> <mspace width="3.33333pt"></mspace> <msup> <mrow> <mtext>Mpc</mtext> </mrow> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> <mo>.</mo> </mrow> </math>   Finally, the sharp falling behaviour of the GW spectra within the frequency domain  $$10^{-7}~{\textrm{Hz}}&lt;f&lt;1~{\textrm{Hz}}$$  <math> <mrow> <msup> <mn>10</mn> <mrow> <mo>-</mo> <mn>7</mn> </mrow> </msup> <mspace width="3.33333pt"></mspace> <mtext>Hz</mtext> <mo>&lt;</mo> <mi>f</mi> <mo>&lt;</mo> <mn>1</mn> <mspace width="3.33333pt"></mspace> <mtext>Hz</mtext> </mrow> </math>   validates our theory in the comparatively high-frequency regime as well.

Sayantan Choudhury

Single field inflation in the light of Pulsar Timing Array Data: quintessential interpretation of blue tilted tensor spectrum through Non-Bunch Davies initial condition

We investigate the cosmological collider (CC) signal arising from the tree-level exchange of a scalar spectator particle with a non-Bunch-Davies (BD) initial state. We decompose the inflaton correlators into seed integrals, which we compute analytically by solving the bootstrap equations. We show that the non-BD initial state eliminates the Hubble-scale Boltzmann suppression <math><msup><mi>e</mi><mrow><mo>−</mo><mi>π</mi><mi>m</mi><mo>/</mo><mi>H</mi></mrow></msup></math> that usually affects the CC signal. Consequently, in this scenario, the CC can probe an energy scale much higher than the inflationary Hubble scale <math><mi>H</mi></math>.

China

Yuan Yin

Cosmological collider signal from non-Bunch-Davies initial states

In recent work, Hollands, Kovács and Reall have built on previous work of Wall to provide a definition of dynamical black hole entropy for gravitational effective field theories (EFTs). This entropy satisfies a second law of black hole mechanics to quadratic order in perturbations around a stationary black hole. We determine the explicit form of this entropy for the EFT of 4d vacuum gravity including terms in the action with up to 6 derivatives. An open question concerns the gauge invariance of this definition of black hole entropy. We show that gauge invariance holds for the EFT of vacuum gravity with up to 6 derivatives but demonstrate that it can fail when 8 derivative terms are included. We determine an entropy for Einstein-Gauss-Bonnet theory by treating it as an EFT with vanishing 6 derivative terms.

Harvey Reall

Dynamical Black Hole Entropy in Effective Field Theory

In this Letter, we study the quantum nature of non-Bunch–Davies states in de Sitter space by evaluating CHSH inequality on a localized two-atom system. We show that quantum nonlocality can be generated through the Markovian evolution of two-atom, witnessed by a violation of CHSH inequality on its final equilibrium state. We find that the upper bound of inequality violation is determined by different choices of de Sitter-invariant vacua sectors. In particular, with growing Gibbons–Hawking temperature, the CHSH bound degrades monotonously for Bunch–Davies vacuum sector. Due to the intrinsic correlation of non-Bunch–Davies vacua, we find that the related violation of inequality can however drastically increase after certain turning point, and may persist for arbitrarily large environment decoherence. This implies that the CHSH inequality is useful to classify the initial quantum state of the Universe. Finally, we clarify that the witnessed intrinsic correlation of non-Bunch–Davies vacua can be utilized for quantum information applications, e.g., surpassing the Heisenberg uncertainty bound of quantum measurement in de Sitter space.

Australia

Jun Feng

Xiaoyang Huang

Yao-Zhong Zhang

Heng Fan

Bell inequalities violation within non-Bunch–Davies states

In this work, we study the quantum entanglement and compute entanglement entropy in de Sitter space for a bipartite quantum field theory driven by an axion originating from Type IIB string compactification on a Calabi–Yau three fold ( CY3 ) and in the presence of an NS5 brane. For this computation, we consider a spherical surface S2 , which divides the spatial slice of de Sitter ( dS4 ) into exterior and interior sub-regions. We also consider the initial choice of vacuum to be Bunch–Davies state. First we derive the solution of the wave function of the axion in a hyperbolic open chart by constructing a suitable basis for Bunch–Davies vacuum state using Bogoliubov transformation. We then derive the expression for density matrix by tracing over the exterior region. This allows us to compute the entanglement entropy and Rényi entropy in 3+1 dimension. Furthermore, we quantify the UV-finite contribution of the entanglement entropy which contain the physics of long range quantum correlations of our expanding universe. Finally, our analysis complements the necessary condition for generating non-vanishing entanglement entropy in primordial cosmology due to the axion.

Sudhakar Panda

Entangled de Sitter from stringy axionic Bell pair I: an analysis using Bunch–Davies vacuum