Gravitational waves from strong first-order phase transitions

David J. Weir [they/he] - Helsinki - davidjamesweir

This talk: saoghal.net/slides/qmul2022

Queen Mary, 6.7.2022

Aim of today's talk:

Tour of the 'pipeline' from particle physics model to dynamical processes in strong phase transitions

Particle physics model
$\Downarrow \mathcal{L}_{4\mathrm{d}}$
Dimensional reduction
$\Downarrow \mathcal{L}_{3\mathrm{d}}$
Phase transition parameters from lattice simulations
$\Downarrow \alpha, \beta, T_N, \ldots$
Real time cosmological simulations
$\Downarrow \Omega_\text{gw}(f)$
Cosmological GW background

A "pipeline"

Particle physics model
$\Downarrow \mathcal{L}_{4\mathrm{d}}$
Dimensional reduction
$\Downarrow \mathcal{L}_{3\mathrm{d}}$
Phase transition parameters from lattice simulations

$\Downarrow \alpha, \beta, T_N, \ldots$
Real time cosmological simulations
$\Downarrow \Omega_\text{gw}(f)$
Cosmological GW background

My focus: extensions of the Standard Model

$$ \mathcal{L}_{4\mathrm{d}} = \mathcal{L}_\text{SM}[\text{SM fields}] \color{red}{+ \mathcal{L}_\text{BSM}[\text{SM fields},\ldots ?]} $$

SM electroweak phase transition

  • Process by which the Higgs 'switched on'
  • In the Standard Model it is a crossover
  • Possible in extensions that it would be first order
    ➥ colliding bubbles then make gravitational waves


Using dimensional reduction

  • At high $T$, system looks 3D at distances $\Delta x \gg 1/T$
  • Match Green's functions at each step to desired order
  • Handles the infrared problem, light fields can be studied on lattice arXiv:hep-ph/9508379

The electroweak phase transition

  • Simulate DR'ed 3D theory on lattice arXiv:hep-let/9510020
  • With DR, can integrate out heavy new physics and study simpler model

How to get strong transitions?

Theories that look SM-like in the IR ⇒ not observable!

arXiv:1903.11604

Lattice Monte Carlo benchmarks

$\Sigma$SM (triplet) example

Perturbation theory doesn't see the phase transition!

arXiv:2005.11332

Key points so far

  • Dimensional reduction + lattice simulations a well-proven method for studying BSM theories
  • Higher dimensional operators or light new physics needed for a strong phase transition
  • Should benchmark perturbation theory with DR + lattice, particularly for strong transitions

Particle physics model
$\Downarrow \mathcal{L}_{4\mathrm{d}}$
Dimensional reduction ✅
$\Downarrow \mathcal{L}_{3\mathrm{d}}$
Phase transition parameters from lattice simulations ✅
$\Downarrow \alpha, \beta, T_N, \ldots$
Real time cosmological simulations
$\Downarrow \Omega_\text{gw}(f)$
Cosmological GW background

Particle physics model
$\Downarrow \mathcal{L}_{4\mathrm{d}}$
Dimensional reduction ✅
$\Downarrow \mathcal{L}_{3\mathrm{d}}$
Phase transition parameters from lattice simulations ✅
$\Downarrow \alpha, \beta, T_N, \ldots$
Real time cosmological simulations
$\Downarrow \Omega_\text{gw}(f)$
Cosmological GW background

Model-independent parameters bridge the gap

Including:

  • $\alpha$, the phase transition strength
  • $\beta$, the inverse phase transition duration
  • $T_N$, the temperature at which bubbles nucleate
  • $v_\mathrm{w}$, the speed at which bubbles expand

Out of equilibrium physics

  1. Bubbles nucleate and grow
  2. Expand in a plasma - create reaction fronts
  3. Bubbles + fronts collide
  4. Sound waves left behind in plasma
  5. Shocks [$\rightarrow$ turbulence] $\rightarrow$ damping

Explore $\Omega_\text{gw}(f)$ with PTPlot.org

Model ⟶ ($\alpha$, $\beta$, $T_N$, $v_\mathrm{w}$ ) ⟶ this plot

arXiv:1910.13125

Explore $\Omega_\text{gw}(f)$ with PTPlot.org

Assumes GW emission stops when nonlinearities form.

arXiv:1910.13125

Nonlinearities?

  • Nonlinearities during the transition:
    • Generation of vorticity
    • Droplets
  • Nonlinearities after the transition:
    • Shocks
    • Turbulence (and acoustic turbulence)
  • Let's take a look at droplets and acoustic turbulence

Strong deflagrations ⇒ droplets

[$\alpha_{T_*} = 0.34$, $v_\mathrm{w} = 0.24$ (deflag.)], velocity $\mathbf{v}$

Droplets form ➤ walls slow down

At large $\alpha_{T_*}$ reheated droplets form in front of the walls

Droplets may suppress GWs

Suppression compared to sound waves (redder = worse)

arXiv:1906.00480

Sound waves ➤ acoustic turbulence

  • Thermal phase transitions produce sound waves
  • Over time, sound waves steepen into shocks
  • Overlapping field of shocks = 'acoustic turbulence'
  • Distinct from, but related to Kolmogorov turbulence

arXiv:2112.12013

2d acoustic turbulence

Acoustic turbulence: GWs

Spectral shape $S$ as function of $k$ and integral scale $L_0$:

Different from sound waves and Kolmogorov turbulence!
⇒ all must be taken into consideration.

Thanks

  • Students:
    Jani Dahl, Anna Kormu, Lauri Niemi, Satumaaria Sukuvaara, Essi Vilhonen
  • Postdocs:
    Daniel Cutting, Oliver Gould
  • Collaborators:
    Jonathan Kozaczuk, Mark Hindmarsh, Stephan Huber, Hiren Patel, Michael Ramsey-Musolf, Kari Rummukainen, Tuomas Tenkanen

Key point: strong transitions ⇒ nonlinearities

  • Nonlinearities include:
    • Turbulence (Kolmogorov-type and acoustic)
    • 'Hot droplets'
  • Consequences for
    • Observables [e.g. gravitational waves]
    • Processes [e.g. baryogenesis]

What I want you to remember

  • Dimensional reduction is a valuable field theory tool
    $\Rightarrow$ lattice Monte Carlo simulations of phase transitions
  • Nonlinearities matter when studying phase transitions
    $\Rightarrow$ large-scale real-time cosmological simulations

More questions you can ask me

  • How accurate are bubble nucleation calculations?
  • What are the consequences of droplet formation?
  • What about other types of turbulence?

When new physics is heavy

Benchmark: ● 4d PT vs ● 3d PT vs ● NP (lattice)

arXiv:1903.11604

Simulating weak transitions: $\alpha \ll 1$

Isolated spherical droplets

In the spherical case, we can get a self-similar droplet. We see the same wall velocity slowdown: