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

$\Leftrightarrow$

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:

  • LIGO/VIRGO
  • 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

Multi-messenger

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!