The radiation-dominated universe is a key ingredient of the standard Big Bang cosmology. Radiation comprises numerous quantum elementary particles, and the macroscopic behavior of radiation is described by taking the quantum thermal average of its constituents. While the interactions between individual particles and gravitational waves are often neglected in this context, it raises the question of whether these elementary particles interact with gravitational waves in the framework of quantum field theory. To address this question, this paper aims to explore the quantum mechanical aspects of gravitational waves in a universe dominated by a massless scalar field, whose averaged energy-momentum tensor plays the role of background radiation. We establish the equivalence between the classical Einstein equation and the mean-field approximation of the Heisenberg equation in a local thermal state. Beyond the mean-field approximation, we analyze the quantum corrections to gravitational waves, particularly focusing on the thermal radiation loop corrections. Interestingly, we find the 1-loop correction surpasses the tree-level spectrum of primordial gravitational waves, which is ( ) where = is the ratio of the inflationary Hubble parameter to the Planck mass. This indicates break down of the perturbative analysis. Then, to see if this result persists even if we take into account the higher order loop corrections, we schematically discuss two-loop diagrams that may give ( ) contributions. We leave explicit computations of these diagrams for future studies. Thus, although we cannot claim that the whole loop corrections exceed the tree-level spectrum at the moment, our findings highlight the significance of considering quantum effects when studying the interaction between radiation and gravitational waves in the cosmological context.

China

Atsuhisa Ota

Taiwan

Japan

Misao Sasaki

Yi Wang

One-loop thermal radiation exchange in gravitational wave power spectrum

We propose a new probe of primordial non-Gaussianities (NGs) through the observation of gravitational waves (GWs) induced by ultra-light (<math><msub><mrow><mi>M</mi></mrow><mrow><mtext>PBH</mtext></mrow></msub><mo>&lt;</mo><msup><mrow><mn>10</mn></mrow><mrow><mn>9</mn></mrow></msup><mi>g</mi></math>) primordial black holes (PBHs). Interestingly enough, the existence of primordial NG can leave imprints on the clustering properties of PBHs and the spectral shape of induced GW signals. Focusing on a scale-dependent local-type NG, we identify a distinct double-peaked GW energy spectrum that, contingent upon <math><msub><mrow><mi>M</mi></mrow><mrow><mtext>PBH</mtext></mrow></msub></math> and the abundance of PBHs at the time of formation, denoted as <math><msub><mrow><mi>Ω</mi></mrow><mrow><mi>PBH</mi><mo>,</mo><mi>f</mi></mrow></msub></math>, may fall into the frequency bands of upcoming GW observatories, including LISA, ET, SKA, and BBO. Thus, such a signal can serve as a novel portal for probing primordial NGs. Intriguingly, combining BBN bounds on the GW amplitude, we find for the first time the joint limit on the product of the effective non-linearity parameter for the primordial tri-spectrum, denoted by <math><msub><mrow><mover><mrow><mi>τ</mi></mrow><mrow><mo>¯</mo></mrow></mover></mrow><mrow><mi>NL</mi></mrow></msub></math>, and the primordial curvature perturbation power spectrum <math><msub><mrow><mi>P</mi></mrow><mrow><mi>R</mi></mrow></msub><mo>(</mo><mi>k</mi><mo>)</mo></math>, which reads as <math><msub><mrow><mover><mrow><mi>τ</mi></mrow><mrow><mo>¯</mo></mrow></mover></mrow><mrow><mi>NL</mi></mrow></msub><msub><mrow><mi>P</mi></mrow><mrow><mi>R</mi></mrow></msub><mo>(</mo><mi>k</mi><mo>)</mo><mo>&lt;</mo><mn>4</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>20</mn></mrow></msup><msubsup><mrow><mi>Ω</mi></mrow><mrow><mi>PBH</mi><mo>,</mo><mi>f</mi></mrow><mrow><mo>−</mo><mn>17</mn><mo>/</mo><mn>9</mn></mrow></msubsup><msup><mrow><mo>(</mo><mfrac><mrow><msub><mrow><mi>M</mi></mrow><mrow><mi>PBH</mi></mrow></msub></mrow><mrow><msup><mrow><mn>10</mn></mrow><mrow><mn>4</mn></mrow></msup><mi>g</mi></mrow></mfrac><mo>)</mo></mrow><mrow><mo>−</mo><mn>17</mn><mo>/</mo><mn>9</mn></mrow></msup></math>.

Greece

Italy

Theodoros Papanikolaou

Xin-Chen He

Xiao-Han Ma

Yi-Fu Cai

Chile

Emmanuel N. Saridakis

New probe of non-Gaussianities with primordial black hole induced gravitational waves

We reexamine production of primordial black holes in a supercooled phase transition. While a mere overdensity associated with a surviving false-vacuum patch does not imply formation of a black hole, it is possible for such a patch to evolve and create a black hole, thanks to the gradient energy stored in the bubble wall.

United States

France

Marcos M. Flores

Alexander Kusenko

Revisiting formation of primordial black holes in a supercooled first-order phase transition

We report a novel prediction from single-field inflation that even a tiny step in the inflaton potential can change our perception of primordial non-Gaussianities of the curvature perturbation. Our analysis focuses on the tail of probability distribution generated by an upward step transition between two stages of slow-roll evolution. The nontrivial background dynamics with off-attractor behavior is identified. By using a non-perturbative <italic>&#948;N</italic> analysis, we explicitly show that a highly non-Gaussian tail can be generated by a tiny upward step, even when the conventional nonlinearity parameters <math altimg='si1.svg'><msubsup><mrow><mi>f</mi></mrow><mrow><mtext>NL</mtext></mrow><mrow><mtext>local</mtext></mrow></msubsup></math>, <math altimg='si2.svg'><msubsup><mrow><mi>g</mi></mrow><mrow><mtext>NL</mtext></mrow><mrow><mtext>local</mtext></mrow></msubsup></math>, etc. remain small. With this example, we demonstrate for the first time the sensitive dependence of non-perturbative effects on the tail of probability distribution. Our scenario has an inconceivable application to primordial black holes by either significantly boosting their abundance or completely forbidding their appearance.

United Kingdom

Dong-Gang Wang

Zihan Zhou

One small step for an inflaton, one giant leap for inflation: <italic>A novel non-Gaussian tail and primordial black holes</italic>

We present for the first time a Friedmann-like construction in the framework of an osculating Finsler–Randers–Sasaki (F–R–S) geometry. In particular, we consider a vector field in the metric on a Lorentz tangent bundle, and thus the curvatures of horizontal and vertical spaces, as well as the extra contributions of torsion and non-linear connection, provide an intrinsic richer geometrical structure, with additional degrees of freedom, that lead to extra terms in the field equations. Applying these modified field equations at a cosmological setup we extract the generalized Friedmann equations for the horizontal and vertical space, showing that we obtain an effective dark energy sector arising from the richer underlying structure of the tangent bundle. Additionally, as it is common in Finsler-like constructions, we obtain an effective interaction between matter and geometry. Finally, we consider a specific model and we show that it can describe the sequence of matter and dark-energy epochs, and that the dark-energy equation of state can lie in the quintessence or phantom regimes, or cross the phantom divide.

E. Kapsabelis

Emmanuel Saridakis

P. Stavrinos

Finsler–Randers–Sasaki gravity and cosmology

Causal soliton formation (e.g. oscillons, Q-balls) in the primordial Universe is expected to give rise to a universal gravitational wave (GW) background, at frequencies smaller than scales of nonlinearity. We show that modifications of the soliton density field, driven by soliton interactions or initial conditions, can significantly enhance universal GWs. Gravitational clustering of solitons naturally leads to generation of correlations in the large-scale soliton density field. As we demonstrate for axion-like particle (ALP) oscillons, the growing power spectrum amplifies universal GW signals, opening new avenues for probing the physics of the early Universe with upcoming GW experiments. Our results are applicable to variety of scenarios, such as solitons interacting through a long range Yukawa-like fifth force.

Kaloian D. Lozanov

Volodymyr Takhistov

Universal gravitational waves from interacting and clustered solitons

We show that the general vacuum states that respect the de Sitter symmetry, known as the α-vacua, can introduce non-vanishing parity-violating tensor non-Gaussianities. This is due to the mixing by the Bogoliubov transformation of the positive and negative frequency modes of the Bunch-Davies vacuum. We calculate explicitly the bispectra of tensor perturbations and show that the amplitude can be exponentially enhanced for certain choices of the squeezing parameter α and the phase ϕ of the α-vacua. We find a new shape for the parity-violating tensor bispectrum which peaks in the flattened configuration.

South Korea

Jinn-Ouk Gong

Maria Mylova

New shape of parity-violating graviton non-Gaussianity

Cosmological stochastic gravitational waves (GWs) induced by a spectator field are usually expected to have an amplitude very small compared with those generated by the curvature perturbation, or equivalently by a field dominating the universe. On the contrary to this expectation, we show that a spectator field that provides a tensor perturbation, on top of the metric tensor perturbation, can generate a significant amount of GWs. The amplitude and frequency of the generated GWs may lie within the sensitivity range of future GW detectors. In particular, if the sound velocities of the two tensor perturbations coincide, the induced GW amplitude may become very large due to resonance by forced oscillation, even in the limit of small coupling between them. A distinct feature of this scenario is that, since tensor modes can hardly lead to the formation of primordial black holes (PBHs), we expect no presence of PBHs, in contrast to the usual scalar-induced case, in which the detection of strong enough induced GWs suggests the existence of PBHs.

Spain

Mohammad Ali Gorji

Primordial-tensor-induced stochastic gravitational waves

Recently, pulsar timing array (PTA) collaborations announced evidence for an isotropic stochastic gravitational wave (GW) background. The origin of the PTA signal can be astrophysical or cosmological. In the latter case, the so-called secondary scalar-induced GW scenario is one of the viable explanations, but it has a potentially serious issue of the overproduction of primordial black holes (PBHs) due to the enhanced curvature perturbation. In this letter, we present a new interpretation of the PTA signal. Namely, it is originated from an extra spectator tensor field that exists on top of the metric tensor perturbation. As the energy density of the extra tensor field is always subdominant, it cannot lead to the formation of PBHs. Thus our primordial-tensor-induced scenario is free from the PBH overproduction issue.

Teruaki Suyama

Extra-tensor-induced origin for the PTA signal: No primordial black hole production

We show that there are certain perturbatively stable non-supersymmetric AdS vacua which are also brane-jet stable. Also we extend the analysis of brane-jets to the AdS vacua from curved domain walls like Janus solutions. First, we apply the brane-jet analysis to the non-supersymmetric Janus solutions of type IIB supergravity found by Bak, Gutperle and Hirano. Second, we study the brane-jet of AdS 4 vacua from eleven-dimensional supergravity on Sasaki-Einstein manifolds: the supersymmetric and the skew-whiffed Freund-Rubin, the Pope-Warner, and the Englert solutions. Third, we examine the non-supersymmetric AdS 4 vacua from Q 1,1,1 and M 1,1,1 manifolds discovered by Cassani, Koerber and Varela. It turns out that all the AdS vacua we consider in this work are brane-jet stable. Especially, the Janus, the skew-whipped Freund-Rubin, and the AdS 4 vacua from Q 1,1,1 and M 1,1,1 are perturbatively stable within known subsectors of truncations and also brane-jet stable.

Minwoo Suh

Brane-jet stabilities from Janus and Sasaki-Einstein