Gravitational waves

David J. Weir - University of Helsinki

History of gravitational waves

  • 1915: General relativity (Einstein)
  • 1916: Prediction of gravitational waves
    • Are they real?
  • 1936: Einstein 'proves' they don't exist (wrong!)
  • 1957: Persuasive "Sticky bead argument"
  • 1960s: Searches start, initially with resonant bars
  • 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


Two polarisations

- just like light

First evidence: Hulse-Taylor pulsar

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 curve - predicton of relativity; red dots - measurements

Early efforts: resonant bars

Joseph Weber with his resonant bar in the 1960s

Major ongoing projects:

  • LISA
  • Pulsar timing arrays

LIGO at the Hanford Site

About LIGO

  • Cost: 1 billion dollars
  • Interferometers with 4km arms, $1064 \, \mathrm{nm}$ laser
  • Each arm: Fabry-Pérot cavity (path length ~280 trips)
  • Sister project in Europe: VIRGO

Two black holes merging

Prediction and measurement

Two neutron stars merging


LIGO almost didn't see it...

Neutron star merger and cosmology

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

LISA mission

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

  • Three arms (six lasers), 2.5 M km separation
  • Launch 2034

LISA's orbit

LISA pathfinder

LISA: astrophysical signals

  • Massive black hole mergers
    (nearly anywhere in the universe)
  • Small objects
    (stars, black holes)
    falling into big black holes
  • Very compact pairs of stars
    (orbital periods < 10 minutes)
    in the Milky Way

Pulsar timing arrays

About pulsar timing arrays

  • Millisecond pulsars (MSPs): very accurate clocks
  • Check if they are 'ticking' like we expect
    • Irregularities ⟷ gravitational waves
  • Compare multiple pulsars at the same time
    • Improve the signal
    • Remove effects of 'physics' in the pulsars
  • Applications:
    • Very low frequency background
    • Supermassive black holes

Pulsar timing arrays

Use radio telescopes like this (Arecibo)

Gravitational waves
and the early universe

How did the Higgs get that way?

Thank you!