Gravitational wave cosmology

David J. Weir [they/he]
david.weir@helsinki.fi - davidjamesweir

Wave physics and imaging applications workshop, 2022

This talk: saoghal.net/slides/waves

Hello

  • Academy Research fellow and Associate Professor (tenure track) in gravitational wave cosmology
  • From Scotland, Finnish citizen since 2019
  • Our Computational Field Theory group has approximately 3 faculty, 2 postdocs, 5 graduate students and 5 research assistants
  • Member of the LISA gravitational wave mission consortium; holds Finland's seat on Executive Board

History of gravitational waves

  • 1915: General relativity
  • 1916: Prediction of gravitational waves
  • 1936: 'Proof' they don't exist (wrong!)
  • 1957: Persuasive "Sticky bead argument"
  • 1960s: Searches start
  • 1975: Inferred from Hulse-Taylor pulsar
    (1993 Nobel Prize)
  • 2015: Directly detected by LIGO
    (2017 Nobel Prize)

What is a gravitational wave?

- Stretches and squeezes a ring of matter

$\Leftrightarrow$
Sources: Wikimedia; ESA

Two polarisations

- just like light

First evidence: Hulse-Taylor pulsar


Source: Shane L. Larson

Hulse-Taylor pulsar

  • Two neutron stars, of which at least one pulsar
  • Orbital diameter: 3 light seconds
  • Orbital period: 7.75 hours
  • Orbit slowly contracting (3.5 metres per year)
  • Energy must be going somewhere...
    gravitational waves
  • Gravitational wave power output: $7\times 10^{24} \, \mathrm{W}$
    (about 2% of the Sun's EM radiation).
Orbital decay of Hulse-Taylor pulsar

Solid line - GR prediction; red dots - measurements

Source: NASA

Direct detection of GWs

Measure time-dependent strain with an interferometer:

LIGO at the Hanford Site

Source: (CC-BY-NC-ND) Prachatai

LIGO design

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

Two black holes merging

Two neutron stars merging

Light and gravitational waves from neutron stars

Source:
(CC-BY) ApJ 848 L12 (2017)

Neutron star merger and cosmology

  1. Photons arrived 1.7s later, after travelling 100 M ly
    ⇒ gravitational waves travel at the speed of light
  2. Independent measurement of universe's expansion:
    • Luminosity of gravitational waves → distance
    • Telescopes observe host galaxy → velocity

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

Source: arXiv:1205.2451

LISA mission

Need to study longer wavelengths, need to go to space!

  • Three arms (six lasers), 2.5 M km separation
  • Launch mid-2030s as ESA L-class mission

Source: LISA Consortium.

LISA's orbit

LISA: "Astrophysics" signals

Source: arXiv:1702.00786

LISA: Stochastic background?

[qualitative curve, sketched on]

Higgs boson

How did the Higgs get that way?

Source: Anna Kormu

How did the Higgs get that way?

Colliding bubbles + aftermath ➟ gravitational waves

To conclude: key points

  • Scale of the problem?
    • Strains around $10^{-21}$
    • Frequencies around 100 Hz (LIGO), 1 mHz (LISA)
  • Measurement devices?
    • Michelson interferometers (LIGO etc.)
    • Time delay interferometers (LISA etc.)
  • Unresolved questions
    • Existence and amplitude of stochastic background
    • Whether it can be detected, given foregrounds