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:

• 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

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: astrophysical signals

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?