Gravitational waves
David Weir [he/him/his] - davidjamesweir
University of Nottingham
This talk: saoghal.net/slides/future-colliders
IFT, Madrid, 13 June 2019
What happened in the early universe? when the universe was optically opaque? in dark sectors?
Start of the GW astrophysics era
(CC-BY) arXiv:1710.05833
After LIGO comes LISA
- Three laser arms, 2.5 M km separation
- ESA-NASA mission, launch by 2034
- Mission adopted 2017 arXiv:1702.00786
LISA: "Astrophysics" signals
Source: arXiv:1702.00786
LISA: Early Universe Cosmology
Science Investigation 7.2: Measure, or set upper limits on, the spectral shape of the cosmological stochastic GW background.
Operational Requirement 7.2: Probe a broken power-law stochastic background from the early Universe as predicted, for example, by first order phase transitions ...
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 - Can also be probed with experimental particle physics
Thermal phase transitions
- Bubbles nucleate and grow
- Expand in a plasma - create reaction fronts
- Bubbles + fronts collide - violent process
- Sound waves left behind in plasma
- Turbulence; damping
Key parameters for GW production
4 numbers parametrise the transition:
- $T_*$, temperature
- $\alpha_{T_*}$, vacuum energy fraction
- $v_\mathrm{w}$, bubble wall speed
- $\beta/H_*$:
- $\beta$, inverse phase transition duration
- $H_*$, Hubble rate at transition
How the wall moves
- In EWPT: equation of motion is (schematically)
Liu, McLerran and Turok; Prokopec and Moore; Konstandin, Nardini and Rues; ... $$ \partial^2 \phi + V_\text{eff}'(\phi,T) + \sum_{i} \frac{d m_i^2}{d \phi} \int \frac{\mathrm{d}^3 k}{(2\pi)^3 \, 2 E_i} \delta f_i(\mathbf{k},\mathbf{x}) = 0$$- $V_\text{eff}'(\phi)$: gradient of finite-$T$ effective potential
- $\delta f_i(\mathbf{k},\mathbf{x})$: deviation from equilibrium phase space density of $i$th species
- $m_i$: effective mass of $i$th species
Force interpretation
$$ \overbrace{\partial_\mu T^{\mu\nu}}^\text{Force on $\phi$} - \overbrace{\int \frac{d^3 k}{(2\pi)^3} f(\mathbf{k}) F^\nu }^\text{Force on particles}= 0 $$This equation is the realisation of this idea:
Field-fluid system
Using a flow ansatz for the wall-plasma system:
$$ \overbrace{\partial_\mu T^{\mu\nu}}^\text{Field part} - \overbrace{\int \frac{d^3 k}{(2\pi)^3} f(\mathbf{k}) F^\nu }^\text{Fluid part}= 0 $$i.e.:
$$ \partial_\mu T^{\mu\nu}_\phi + \partial_\mu T^{\mu\nu}_\text{fluid} = 0 $$Can simulate as effective model of field $\phi$ + fluid $u^\mu$
astro-ph/9309059What sources GWs?
- Bubbles nucleate + expand, reaction fronts form, then:
- $h^2 \Omega_\phi$: Bubbles + shocks collide - 'envelope phase'
- $h^2 \Omega_\text{sw}$: Sound waves set up - 'acoustic phase'
- $h^2 \Omega_\text{turb}$: [MHD] turbulence - 'turbulent phase'
- Sources add together to give observed GW power: $$ h^2 \Omega_\text{GW} + h^2 \Omega_\text{sw} + h^2 \Omega_\text{turb}$$
- Bubbles nucleate + expand, reaction fronts form, then:
$h^2 \Omega_\phi$: Bubbles + shocks collide - 'envelope phase'- $h^2 \Omega_\text{sw}$: Sound waves set up - 'acoustic phase'
$h^2 \Omega_\text{turb}$: [MHD] turbulence - 'turbulent phase'
- Sources add together to give observed GW power: $$ h^2 \Omega_\text{GW} + h^2 \Omega_\text{sw} + h^2 \Omega_\text{turb}$$
Why focus on sound waves?
- "Envelope phase" is short-lived, one-off
- Relevant only in exotic models where $\alpha_{T_*} \gg 1$ or vacuum transitions...
- ... but then dynamics aren't so simple arXiv:1802.05712
- "Turbulent phase" may never occur if Hubble damping
happens on a shorter timescale
- Turbulence: bubble radius/average fluid velocity
- Hubble damping: Hubble time
➤ Focus on the hydrodynamics of the
transition
and immediate aftermath.
Weak transitions
$\alpha_{T_*} \ll 1$
arXiv:1704.05871 ∙ arXiv:1512.06239 ∙ arXiv:1504.03291 ∙ arXiv:1304.2433
Velocity profile development: detonation vs deflagration
Deflagration $v_\mathrm{w} < c_\mathrm{s}$
Detonation $v_\mathrm{w} > c_\mathrm{s}$
Simulation slice example
[$\alpha_{T_*} = 0.01$, $v_\mathrm{w} = 0.68$ (detonation)]
Velocity power spectra
$v_\mathrm{w} < c_\mathrm{s}$
$v_\mathrm{w} > c_\mathrm{s}$
GW power spectra
[curves scaled by $t$]
$v_\mathrm{w} < c_\mathrm{s}$
$v_\mathrm{w} > c_\mathrm{s}$
Putting it all together
- For any theory, can get $T_*$, $\alpha_{T_*}$, $\beta/H_*$, $v_\mathrm{w}$ arXiv:1004.4187
- It's then easy to predict the signal...
(example, $T_* = 94.7~\mathrm{GeV}$, $\alpha_{T_*} =
0.066$, $v_\mathrm{w} =0.95$, $\beta/H_* = 105.9$)
$\mathrm{SNR} = 95$ ☺️
From ptplot.org (beta!)
PTPlot.org
Model ⟶ ($T_*$, $\alpha_{T_*}$, $v_\mathrm{w}$, $\beta$) ⟶ this plot
[Here: dark photon points from arXiv:1811.11175]
Aside: GW reach of SM EFT
arXiv:1903.11604 + Tuomas Tenkanen's talk
Fluid profile around the wall
Deflagration $v_\mathrm{w} < c_\mathrm{s}$
Detonation $v_\mathrm{w} > c_\mathrm{s}$
Most notable change:
Deflagration front ends at a shock ($\approx c_\mathrm{s}$)
Strong simulation slice 1
[$\alpha_{T_*} = 0.5$, $v_\mathrm{w} = 0.92$ (detonation)], velocity $\mathbf{v}$
Strong simulation slice 2
[$\alpha_{T_*} = 0.5$, $v_\mathrm{w} = 0.44$ (deflag.)], velocity $\mathbf{v}$
Strong deflagrations: walls slow
At large $\alpha_{T_*}$ reheated droplets form in front of the walls
Strong simulation slice 3
[$\alpha_{T_*} = 0.34$, $v_\mathrm{w} = 0.24$ (deflag.)], vorticity $\nabla \times \mathbf{v}$
Strong deflagrations: vortical modes
GW production can be suppressed
Suppression relative to LISA CosWG ansatz
Conclusions
- Weak transitions: good estimates of
power spectrum
☛ ptplot.org - Strong transitions still an emerging topic:
☛ acoustic gravitational wave production suppressed - Causes: vortical mode production and slower walls
- Worst for large $\alpha_{T_*}$, small $v_\mathrm{w}$
- Turbulence still a challenge, work ongoing
- In any case: LISA provides a model-independent probe of
first-order phase transitions around
$100~\mathrm{GeV}$
☛ ... but new physics must be dynamical