Gravitational wave cosmology for LISA

David Weir [he/him/his] - davidjamesweir

University of Helsinki

Jyväskylä, 24.1.2020

saoghal.net/slides/jyvaskyla

What happened in the early universe? when the universe was optically opaque? in dark sectors?


Credit: Stephan Paul, arXiv:1205.2451

Credit: Stephan Paul, arXiv:1205.2451

Credit: Stephan Paul, arXiv:1205.2451

How could
gravitational waves help?

What is a gravitational wave?

Stretches and squeezes a ring of matter

$\Leftrightarrow$
Sources: [CC-BY-SA] Nico 0692 on Wikimedia Commons; ESA / C. Carreau

Q: How are they made?

A: By moving mass and energy around quickly.
[cf. electromagnetic waves, made by moving electrons]

Q: How are they measured?

A: They change the proper length $L$ between test masses, so producing a strain $\Delta L/L$.

[in fact gravitational waves obey a form of Hooke's law]

First sign: Hulse-Taylor pulsar

1993 Nobel Prize


Credit: Shane Larson
Orbital decay of Hulse-Taylor pulsar

Solid line - prediction; red dots - measurements
Source: [PD] Wikimedia Commons

Source: [PD] NASA
Source: (CC-BY) Andrea Nguyen on Flickr

LIGO at the Hanford Site

Sources: [PD] LIGO Lab/Caltech/MIT; Umptanum on Wikimedia Commons

LIGO design

Source: (CC-BY) Phys. Rev. Lett. 116, 061102

Two black holes merging

Source: (CC-BY) Phys. Rev. Lett. 116, 061102

Two neutron stars merging


Credit: Christopher Evans/Georgia Tech

Start of GW astrophysics era

Source: [CC-BY]
ApJL, 848:L12, 2017

Neutron star merger
and cosmology

  1. Photons arrived 1.7s later, after travelling 140 M ly
    ⇒ gravitational waves travel at the speed of light

  2. Independent measurement of universe's expansion:
    • Known luminosity of gravitational waves → distance
    • Telescopes observe host galaxy → redshift; velocity

To look at longer wavelengths,
need to go into space!

The LISA mission

  • Three laser arms, 2.5 M km separation
  • ESA led, launching before 2034
  • Mission adopted 2017 arXiv:1702.00786

Source: [PD] NASA via Wikimedia Commons

LISA's orbit


Source: [CC-BY-SA] Nicolas Douillet on Wikimedia Commons

LISA: astrophysical signals

Credit: LISA proposal

White dwarf binaries


Source: [PD] Dana Berry on Wikimedia Commons

White dwarf binaries

Source: arXiv:1907.00014

So what about the GW background?

LISA: GW background

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


Credit: [FDL] GRAN via Wikimedia Commons; Morrissey and Ramsey-Musolf, arXiv:1206.2942

Tiny walls to giant bubbles

⬇︎ ?

Fitting everything in is hard

Simulation-based ansätze: PTPlot.org

[Here: $Z_2$-symmetric xSM points from arXiv:1910.13125]

At the cutting edge:
strong transitions

(i.e. most of the energy of the universe
is released as latent heat)

Credit: Daniel Cutting

The team

  • Mark Hindmarsh, Kari Rummukainen
  • Oli Gould, Asier Lopez-Eiguren, Tuomas Tenkanen
  • Daniel Cutting, Jani Dahl, Lauri Niemi
  • Anna Kormu
  • Satumaaria Sukuvaara, Essi Vilhonen

Research goals

  • Next few years: GW Cosmology for LISA
    • Cutting-edge simulations of the early universe
    • Microscale (Monte Carlo) to macroscale (hydro)
    • Dark and visible sectors

  • After that: The Energy and Gravity Frontiers
    • Electroweak phase transition and beyond:
      CEPC (2030-), ILC (2035-)
    • Finnish role in European GW missions:
      Einstein Telescope (2030-), LISA (2034-)
    • Complementarity between GW and colliders