The search for gravitational waves: physical motivations and experimental perspectives

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The search for gravitational wavesphysical
motivations and experimental perspectives

• Michele Maggiore
• Département de physique théorique - UniGe
• CHIPP meeting, Lausanne, 12/04/06

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• Why GW physics is interesting
• astrophysics
• cosmology
• Experiments
• present status
• mid-term perspectives (2010 ?)
• ? see talks by Mours, Penn, Vitale

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• Motivations
• testing a prediction of G.R.?
• not really. We already know that GWs exist

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Hulse-Taylor binary pulsar (PSR191316)
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Timing residuals affected by variouseffects due
to special and general relativity(Roemer,
Einstein and Shapiro time delays)

• Fitting the timing formula (with 27 yrs of
  data!)
• all Keplerian parameters are known very precisely
• a1 sin ? 2.341774(1) s , e
  0.6171338(4)
• T0 (MJD) 46443.99588317(3) , ? 226.57518(4)
  deg
• P27906.9807807(9) s
• 3 post-Keplerian parameters
• d?/dt 4.226607(7) deg/yr, ? 0.004294(1)
• dP/dt -2.4211(14) 10-12

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• d?/dt , ? fixed by GR in terms of mp and
  mc
• dP/dt fixed by the quadrupole formula for
• GW emission, once mp and mc are known
• ? mp 1.4408(3) M? , mc 1.3873(3)
  M?
• (dP/dt)exp / (dP/dt)GR 1.002 0.005

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Hulse-Taylor, Nobel Prize 1993
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• double pulsar PSR J0737-3039
• orbital period 2.4 hr !
• after one year of data, test of GW emission at
  a comparable precision
• We know that GW exist !

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The real motivation

• opening a new window on the Universe
• both in astrophysics and in cosmology

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E.m. waves vs. GWs in astrophysics

• e.m. waves
• arise from incoherent superposition from many
  emitters
• interact strongly with matter. Dense regions
  are opaque
• ? ltlt size of the source (image)
• deep imaging of small fields of view

• GWs
• arise from coherent superposition arising from
  bulk dynamics
• interact very weekly.
• Even the interior of a
• NS is transparent
• ? gt size of the source (sound)
• 4 p detectors

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What can we hope to learn about fundamental
physics from the observation of GWs.Three
examples (for advanced detectors)

• Neutron stars and the ground state of QCD
• Coalescing binaries and dark energy
• Stochastic backgrounds and the Big-Bang

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What is the true ground state of QCD?

• TltTc quarks and gluons confined into hadrons
• seems obvious we only see confined matter around
  us
• ? hadrons bound into nuclei
• ? why so little 56Fe ? Coulomb barrier
• strange quark matter hypothesis
• at high density, deconfined u,d,s mixture.
• The price payed to deconfinement and to ms is
  compensated by the opening of a third Fermi sea
  

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The energy barrier could be overcome in the core
of NS
10 km
Crust lattice of heavy nuclei
8 km
n, p, e, µ superfluid
QCD regime quark-hadron mixed phase ?
2 km
? 1 GeV/fm3 ? high-density QCD ? hybrid
stars, quark stars ?
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• astrophysical processes can induce non-radial
  oscillations of NS (e.g. SN?NS)
• the normal modes decay by emission of GWs
• their frequency depends on the internal
  structure of the NS
• ? GWs can probe the core of a NS

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Benhar, Ferrari and Gualtieri, 2004
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Coalescing binaries

• First experimental proof of the existence of GWs
  and precise confirmation of
• General Relativity
• Cosmological standard candles
• ? dark energy

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Binaries as standard candles
(Gc 1)
?We get the distance r !
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for sources at cosmological distances

• luminosity
  distance
• For a spatially flat Universe

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• having dL(z) (and with a measure of z) we get
  a
• gravitational standard candle
• out to what distance ? At the level of
  Advanced VIRGO/LIGO
• NS/NS coalescences, at least O(40)/yr, out to 2
  Gpc
• BH/BH coalescences, 10 M , out to z ? 2-3
• At LISA, supermassive BHs with 106 M , would
  be visible out to z 10

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• With type Ia Supernovae, measure of dL(z) up to
  z 1.7
• p w(z) ? ,
• relativistic matter, w1/3
• non-relativistic matter, w0,
• cosmological cosntant , w -1
• From SN Ia, acceleration of the expansion
  of the Universe, and w(z) lt - 0.55
• Gravitational standard candles can extend to
  higher z, and have different systematics
• ? cosmological constant/ dark energy

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GWs are a unique probe of the early Universe

• Decoupling from the primordial plasma
• two scales ? (interaction rate)
• H (Hubble expansion
  rate)
• ? gtgt H ? equilibrium
• ? lt H ? decoupling
• A stochastic background carries a snapshot of
• the Universe at time of decoupling

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• ? n s v n T3 v 1
• HT2/MPL (during radiation
  dominance)
• e.m. interaction
• s drops at (re)combination
• T eV, t370 000 yr
• neutrinos
• T MeV , t 1 s
• gravitons
• T MPL 1019 GeV, t tPL
  10-44 s !!
• A picture of the Big-Bang
  itself ?

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Slow-roll inflation
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Pre-big-bang cosmology (Brustein, Gasperini and V
eneziano)
f3
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Electroweak phase transition in the NMSSM
Apreda, Nicolis, MM, Riotto
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• Predictions of cosmological backgrounds of GWs
  are difficult
• physics beyond the Standard Model
• early Universe cosmology
• at first generation detectors it is unlikely to
  see something
• advanced interferometers can penetrate deeply
  into an unknow region, where a number of examples
  indicate that it is plausible to expect some
  signals

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a few words on the experimental situation

• resonant-mass detectors
• ALLEGRO (Louisiana), AURIGA (Padua),
  NAUTILUS(Rome) , EXPLORER (Cern)
• MiniGRAIL(Leiden)
• small-scale experiments
• 6-20 people
• costs O(1) MEuro

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• large-scale interferometers
• ground-based LIGO , VIRGO
• (smaller detectors GEO, TAMA)
• Space-borne LISA
• Big Science