We extend analytic formulas for the gravitational-wave (GW) spectrum from first-order phase transitions to include cosmic expansion under the thin-wall and envelope approximations. We demonstrate that even for strongly supercooled transitions the GW amplitude is bounded from above. This conclusion is explicitly verified for several representative nucleation histories, including delta-function, power-law, and power-exponential types. Moreover, the spectral shape, amplitude, and peak frequency remain largely unaffected by the details of the nucleation rate once expressed in terms of the comformal variables evaluated at an appropriately defined characteristic collision time.

Japan

Masaki Yamada

Maximal GW amplitude from bubble collisions in supercooled phase transitions

Recent cosmological observations have revealed growing tensions with the standard ΛCDM model, including indications of isotropic cosmic birefringence and deviations from <math altimg='si1.svg'><mi>w</mi><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><mo linebreak='badbreak' linebreakstyle='after'>&#8722;</mo><mn>1</mn></math> in the dark energy equation of state, as suggested by DESI and supernova measurements. In this paper, we point out that such deviations can arise even from a subdominant energy density component. We then propose a unified framework based on a dynamical axion field that simultaneously accounts for both anomalies, providing a simple and natural extension of the standard &#923;CDM model. In our scenario, the axion field with <math altimg='si2.svg'><mn>2</mn><msub><mrow><mi>H</mi></mrow><mrow><mn>0</mn></mrow></msub><mo>&#8818;</mo><mi>m</mi><mo>&#8818;</mo><mn>7</mn><msub><mrow><mi>H</mi></mrow><mrow><mn>0</mn></mrow></msub></math>, where <math altimg='si3.svg'><msub><mrow><mi>H</mi></mrow><mrow><mn>0</mn></mrow></msub></math> is the current Hubble constant, induces a nonzero rotation of the CMB polarization plane and modifies the present-day dark energy equation of state. This framework accommodates recent observational data with natural parameter choices, even for a string axion with a decay constant of order 10 <sup>17</sup> <hsp sp='0.20' />GeV.

China

Shota Nakagawa

Yuichiro Nakai

Yu-Cheng Qiu

Interpreting cosmic birefringence and DESI data with evolving axion in ΛCDM

Axion rotations can simultaneously explain the dark matter abundance and the baryon asymmetry of the Universe by kinetic misalignment and axiogenesis. We consider a scenario in which the Peccei-Quinn symmetry breaking field is as large as the Planck scale during inflation and the axion rotation is initiated by the inflaton-induced potential immediately after the end of inflation. This is a realization of the cogenesis scenario that is free of problems with domain walls and isocurvature perturbations thanks to large explicit Peccei-Quinn symmetry breaking at the Planck scale during inflation. The baryon asymmetry can be more efficiently produced by leptoaxiogenesis, in which case the axion mass is predicted to be larger than <math><mi>O</mi><mo>(</mo><mn>0.1</mn><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>meV</mi></math>. We also discuss a UV complete model in supersymmetric theories.

United States

Raymond T. Co

Axion cogenesis without isocurvature perturbations

We revisit the hybrid inflation model within the framework of the Grand Unified Theory (GUT), focusing on cases where the waterfall phase transition extends over several e-foldings to dilute monopoles. Considering the stochastic effects of quantum fluctuations, we demonstrate that the waterfall fields (i.e., GUT Higgs) maintain a nonzero vacuum expectation value around the waterfall phase transition. By accurately accounting for the number of degrees of freedom of the GUT Higgs field, we establish that these fluctuations can produce observable gravitational waves without leading to an overproduction of primordial black holes. The amplitude of these gravitational waves is inversely proportional to the degrees of freedom of the waterfall fields, thereby providing a unique method to probe the representation of the GUT Higgs.

Yuichiro Tada

Multifield stochastic dynamics in GUT hybrid inflation without monopole problem and with gravitational wave signatures of GUT Higgs representation

We investigate the thermalization of high-energy particles injected from the perturbative decay of inflaton during the pre-thermal phase of reheating in detail. In general, thermalization takes a relatively long time in a low-temperature plasma; therefore, the instantaneous thermalization approximation is not justified, even for the reheating of the Standard Model (SM) sector. We consider a pure Yang-Mills (YM) theory as an approximation of the SM sector or a possible dark sector, considering the Landau-Pomeranchuk-Migdal effect, a quantum interference effect in a finite temperature plasma. We perform the first numerical calculation to solve the time evolution of the system, including the redshift due to the expansion of the Universe, and show the details of the temperature evolution near the maximum and the behavior of the quasi-attractors at later times. The maximal temperature and time scale are determined quantitatively, such as ≃ 0.05 × $$ {\left({\Gamma}_I{M}_{\textrm{PI}}^2/{m}_I^3\right)}^{2/5}{m}_I $$ and ≃ 2 × 10 × $$ {\left({\Gamma}_I{M}_{\textrm{PI}}^2/{m}_I^3\right)}^{-3/5}{m}_I^{-1} $$ in the SM-like system, where and Γ are the mass and decay rate of inflaton. We also provide a similar formula for pure SU( ) and SO( ) YM theories for general values of and coupling constant , including ∝ and ∝ behaviors and their numerical coefficients. The thermalization occurs in a finite time scale, resulting in a lower maximal temperature of the Universe after inflation than that under the instantaneous thermalization approximation.

Kyohei Mukaida

Perturbative reheating and thermalization of pure Yang-Mills plasma

We discuss the unique phenomenology of first-order phase transitions catalyzed by primordial black holes (BHs). If the number of BHs within one Hubble volume is smaller than unity at the time of bubble nucleation, each bubble catalyzed around them can expand to the Hubble size, and the universe is eventually filled with true vacuum much after nucleation. This super-slow transition predicts enhanced gravitational wave signals from bubble collisions and can be tested in future observations. Moreover, the remaining rare false vacuum patches give birth to baby BHs, which can account for the abundance of dark matter in our universe.

Ryusuke Jinno

Italy

Jun'ya Kume

Super-slow phase transition catalyzed by BHs and the birth of baby BHs

In the presence of an anomalous <math><mrow><mi>C</mi><mi>P</mi></mrow></math> phase in a U(1) gauge theory, a monopole becomes a dyon via the Witten effect. When the anomalous <math><mi>C</mi><mi>P</mi></math> phase is promoted to a dynamical field, the axion, the electric charge of the dyon changes according to the coherent motion of the axion oscillation. Once the electric charge exceeds a certain threshold, the Schwinger pair production of charged particles becomes efficient near the surface of the dyon. These nonperturbative effects lead to the backreaction of the axion dynamics by causing the dissipation of the axion oscillation energy and the change of the effective potential due to the Witten effect. Taking these effects into account, we consider the dynamics of the whole system, including the axion, monopole, and charged heavy vector bosons, and discuss to what extent the axion abundance is modified. We also discuss the electric dipole radiation from a bound state of a monopole-antimonopole pair due to the axion coherent oscillations.

South Korea

Kwang Sik Jeong

Fuminobu Takahashi

Dissipation of axion energy via the Schwinger and Witten effects

We explore a dynamical mechanism to realize the emergence of a global <math><mi>U</mi><msub><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mi>PQ</mi></mrow></msub></math> symmetry and its spontaneous breaking at an intermediate scale for an axion solution to the strong CP problem. Such a dynamics is provided by a new supersymmetric QCD near the middle of conformal window that couples to fields spontaneously breaking the <math><mi>U</mi><msub><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mi>PQ</mi></mrow></msub></math> symmetry. A large anomalous dimension of the <math><mi>U</mi><msub><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mi>PQ</mi></mrow></msub></math> breaking fields leads to the suppression of explicit <math><mi>U</mi><msub><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mi>PQ</mi></mrow></msub></math>-violating higher dimensional operators. The <math><mi>U</mi><msub><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mi>PQ</mi></mrow></msub></math> breaking vacuum is generated at a scale hierarchically smaller than the Planck scale by a non-perturbative effect. The <math><mi>U</mi><msub><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mi>PQ</mi></mrow></msub></math> breaking drives the conformal breaking, and all the new quarks become massive. The axion potential is generated by the ordinary color <math><mi>S</mi><mi>U</mi><msub><mrow><mo>(</mo><mn>3</mn><mo>)</mo></mrow><mrow><mi>C</mi></mrow></msub></math> effect as the <math><mi>U</mi><msub><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mi>PQ</mi></mrow></msub></math> symmetry is only anomalous under the <math><mi>S</mi><mi>U</mi><msub><mrow><mo>(</mo><mn>3</mn><mo>)</mo></mrow><mrow><mi>C</mi></mrow></msub></math>. The saxion direction is stabilized by supersymmetry breaking and cosmologically harmless.

Yufei Zhang

Dynamics of superconformal axion: Quality and scalegenesis

We point out that SO(2N) pure Yang-Mills theory provides a candidate for dark matter (DM) without the explicit need to impose any additional symmetry. The DM candidate is a particular type of glueball, which we refer to as a baryonic glueball, that is naturally stable and produced by a novel production mechanism for a moderately large N. In this case, the intercommutation probability of cosmic strings (or macroscopic color flux tubes) is quite low, which offers characteristic gravitational wave signals to test our model. In particular, our model can simultaneously account for both abundance of DM and the recently reported gravitational wave signals detected in pulsar timing array experiments, including NANOGrav.

Kazuya Yonekura

Dark baryon from pure Yang-Mills theory and its GW signature from cosmic strings

A chiral chemical potential present in the early Universe can source helical hypermagnetic fields through the chiral plasma instability. If these hypermagnetic fields survive until the electroweak phase transition, they source a contribution to the baryon asymmetry of the Universe. In this Letter, we demonstrate that lepton flavor asymmetries above <math><mo>|</mo><mi>μ</mi><mo>|</mo><mo>/</mo><mi>T</mi><mo>∼</mo><mn>9</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>3</mn></mrow></msup></math> trigger this mechanism even for vanishing total lepton number. This excludes the possibility of such large lepton flavor asymmetries present at temperatures above <math><mrow><msup><mrow><mn>10</mn></mrow><mrow><mn>6</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math>, setting a constraint which is about 2 orders of magnitude stronger than the current CMB and BBN limits.