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The potential difference $\Delta V \equiv V_+ - V_-$, also known as $\epsilon$, is the temperature-independent vacuum energy that is released in the phase transition.
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The potential difference $\Delta V \equiv V_+ - V_-$, also known as $\epsilon$,
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is the temperature-independent vacuum energy that is released in the phase transition.
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The potentials are usually shifted so that $V_b = 0$.
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The name of the bag model originates from quantum chromodynamics (QCD), where it's used to describe the proton as a bag of quarks.
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The name of the bag model originates from quantum chromodynamics (QCD),
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where it's used to describe the proton as a bag of quarks.
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\cite{giese_2020}
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Using eq.\eqref{eq:entropy_density} we get the entropy density
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Using eq.~\eqref{eq:entropy_density} we get the entropy density
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\begin{equation}
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s_\pm = 4 a_\pm T^3,
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\end{equation}
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and with eq.\eqref{eq:enthalpy_entropy} the enthalpy density
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and with eq.~\eqref{eq:enthalpy_entropy} the enthalpy density
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\begin{equation}
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w_\pm = 4 a_\pm T^4.
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\end{equation}
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Finally with eq.\eqref{eq:enthalpy_sum} we get the energy density
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Finally with eq.~\eqref{eq:enthalpy_sum} we get the energy density
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\begin{equation}
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e_\pm = 3 a_\pm T^4 + V_\pm
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e_\pm = 3 a_\pm T^4 + V_\pm.
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\end{equation}
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The speed of sound from eq.\eqref{eq:cs2_explicit} simplifies to
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The speed of sound from eq.~\eqref{eq:cs2_explicit} simplifies to
As in section \ref{general_eq}, this corresponds to assuming both of the phases to be ultrarelativistic.
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It also happens to be the same as the $\tilde{v}_-$ that corresponds to eq. \eqref{eq:v_tilde_plus_limit}, and therefore the Chapman-Jouguet speed $v_{CJ}$ of the bag model is given by eq. \eqref{eq:v_tilde_plus_limit}.
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The trace anomaly of eq. \eqref{eq:theta} simplifies to
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As in section~\ref{general_eq}, this corresponds to assuming both of the phases to be ultrarelativistic.
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It also happens to be the same as the $\tilde{v}_-$ that corresponds to eq.~\eqref{eq:v_tilde_plus_limit},
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and therefore the Chapman-Jouguet speed $v_{CJ}$ of the bag model is given by eq.~\eqref{eq:v_tilde_plus_limit}.
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The trace anomaly of eq.~\eqref{eq:theta} simplifies to
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\begin{align}
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\theta_\pm = V_\pm.
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\end{align}
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Therefore the transition strength of\eqref{eq:alpha_plus} simplifies to
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Therefore, the transition strength of~\eqref{eq:alpha_plus} simplifies to
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\begin{equation}
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\alpha_{+,\text{bag}} = \frac{4 \Delta V}{3 w_+},
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\label{eq:alpha_plus_bag}
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\end{equation}
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and the transition strength at nucleation temperature from\eqref{eq:alpha_n} simplifies to
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and the transition strength at nucleation temperature from~\eqref{eq:alpha_n} simplifies to
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and their gravitational wave spectra based on the Sound Shell Model,
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including the conversion to the gravitational spectra today.
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PTtools was tested with the constant sound speed model due to the availability of reference data by Giese et al.\cite{giese_2021},
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PTtools was tested with the constant sound speed model due to the availability of reference data by Giese et al.~\cite{giese_2021},
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and it was shown to work reliably for a broad range of the combinations of the sound speeds $c_{s,s}$ and $c_{s,b}$, the phase transition strength $\alpha_n$ and the wall speed $v_\text{wall}$.
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The PTtools fluid velocity profile solver has been updated beyond that of the available references in such a way
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that it supports arbitrary particle physics models with a temperature-dependent sound speed $c_s(T,\phi)$,
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as long as they provide $V_s(T,\phi), V_b(T,\phi)$ and two of $g_p(T,\phi), g_e(T,\phi), g_s(T,\phi)$ or two of $p(T,\phi), e(T,\phi), s(T,\phi)$, but preferably all three for numerical precision.
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This enables the easy integration of models developed by other researchers,
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and therefore the comparison of the gravitational wave spectra of these models.
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This is also to the author's best knowledge the first time that phase transitions in the early universe have been simulated with temperature-dependent speeds of sound without resorting to time-consuming 3D simulations.
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This is also to the author's best knowledge the first time
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that phase transitions in the early universe have been simulated with temperature-dependent speeds of sound
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without resorting to time-consuming 3D simulations.
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The numerical results of the fluid shell solver have been demonstrated to be consistent with a reference
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for a broad range of parameters.
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the effects of the phase transition parameters on the resulting gravitational wave spectrum.
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This master's thesis lays the groundwork for the author's PhD thesis on the topic.
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Now there is a framework for determining the gravitational wave spectrum from the phase transition parameters, including temperature-dependent speed of sound.
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Determining the parameters of a phase transition from LISA data will require solving the inverse problem: what are the phase transition parameters based on the gravitational wave spectrum?
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Now there is a framework for determining the gravitational wave spectrum from the phase transition parameters,
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including temperature-dependent speed of sound.
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Determining the parameters of a phase transition from LISA data will require solving the inverse problem:
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what are the phase transition parameters based on the gravitational wave spectrum?
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This will require novel tools such as machine learning or Markov chain Monte Carlo simulations.
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There is also potential for integrating PTtools with the existing web-based simulation utility PTPlot \cites{ptplot}{hindmarsh_shape_2017}{caprini_detecting_2020} to create a comprehensive but easy-to-use web-based utility for researchers to simulate phase transitions with various parameters.
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Further possibilities for extending PTtools itself include integrating the low frequency spectral density function by Giombi et al. \cite[eq. 3.6]{giombi_cs_2024}.
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There is also potential for integrating PTtools with the existing web-based simulation utility
Even though we have used $T_0 = T_c$ in the intermediate steps, this restriction is general.
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\fi
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Approximating $T_+ \approx T_-$ results in eq.\eqref{eq:cs2_compact} being approximated as
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Approximating $T_+ \approx T_-$ results in eq.~\eqref{eq:cs2_compact} being approximated as
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\begin{equation}
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c_{s,b}^2
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\equiv\frac{dp_b}{de_b}
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= \frac{dp_b/dT}{de_b/dT} \big|_{T_+ \approx T_-}
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\approx\frac{\delta p_-}{\delta e_-}.
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\label{eq:const_cs_approx}
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\end{equation}
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In the case of the constant sound speed model $c_{s}$ is independent of temperature, and therefore this approximation is exact.
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With this approximation and the use of eq.\eqref{eq:delta_relation} the second junction condition of eq.\eqref{eq:junction_conditions_simplified} becomes
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In the case of the constant sound speed model,$c_{s}$ is independent of temperature, and therefore this approximation is exact.
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With this approximation and the use of eq.~\eqref{eq:delta_relation} the second junction condition of eq.~\eqref{eq:junction_conditions_simplified} becomes
Using this, eq.\eqref{eq:delta_relation} and eq.\eqref{eq:enthalpy_sum} on the first junction condition of\eqref{eq:junction_conditions_simplified} results in
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Using this, eq.~\eqref{eq:delta_relation} and eq.~\eqref{eq:enthalpy_sum} on the first junction condition of~\eqref{eq:junction_conditions_simplified} results in
which is a basic second-order equation for $\tilde{v}_- ( \tilde{v}_+, \alpha_{\bar{\Theta}_+} )$.
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Since $\tilde{v}_+ = v_w$ for all detonations, and $\alpha_{\bar{\Theta}_+} = \alpha_{\bar{\Theta}_n}$ for all detonations in the constant sound speed model, this gives $\tilde{v}_- ( v_w, \alpha_{\bar{\Theta}_n} )$.
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% And since\eqref{eq:alpha_n_const_cs} gives $\alpha_n(\mu_+, \mu_-, \alpha_{n,\text{bag}})$, we have $\tilde{v}_- (v_w, ...)$
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Eq.\eqref{eq:const_cs_vp_vm} can also be rearranged to
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% And since~\eqref{eq:alpha_n_const_cs} gives $\alpha_n(\mu_+, \mu_-, \alpha_{n,\text{bag}})$, we have $\tilde{v}_- (v_w, ...)$
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Eq.~\eqref{eq:const_cs_vp_vm} can also be rearranged to
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