Fundamental Physics with LISA

1
Fundamental Physics with LISA

• Bernard Schutz
• Albert Einstein Institute, Golm
• School of Physics and Astronomy, Cardiff

1
LISA
2

• Verifying gravitational waves
• Testing GR in strong field domain
• Cosmogony
• New fundamental physics

3
Testing GRs GW model

• Do GWs exist?
• Verification binaries, good SNR. Given
  Hulse-Taylor binary pulsar, hard to imagine that
  LISA will see no GWs. But observations will test
  quadrupole radiation formula, look for extra
  polarizations.
• Do GWs travel at the speed of light?
• If LISA finds an edge-on CWDB that can be
  observed optically for eclipses, then phasing of
  GW should match phasing of light.
• Not as strong as ground-based observations of
  ?-bursts from NS-NS coalescence.
• Are there other associated gravitational
  radiation fields (eg scalar-tensor theories),
  polarization states?
• By observing thousands of resolved CWDBs over 5
  years, LISA can look for residuals from standard
  GR fits to waveforms.
• Dispersion?
• Fitting inspiral signal to PN model, with high
  SNR, may reveal unexpected phasing if higher
  frequencies travel faster than lower (graviton
  mass).
• For inspirals or EMRIs, orbital plane might show
  anomalous precession due to parity failure
  (right- and left-hand polarizations propagate
  differently in some string theory models).

4

• Verifying gravitational waves
• Testing GR in strong field domain
• SMBH binaries
• EMRI as Probes of the Metric
• EMRIs in Binary SMBH Systems
• EMRIs as Tests of Gravitation Theory
• Cosmogony
• New fundamental physics

5
SMBH Mergers Strong-field Dynamics

• Verifying that black holes exist
• Probably the observation that will create the
  biggest impression in physics and astronomy
  listening to their merger and ringdown will be
  our only direct test of black holes.
  (Ground-based detectors may do this earlier than
  LISA, but with lower SNR.)
• Test Cosmic Censorship Hypothesis
• Remember still a hypothesis, so there is
  potential for something new!
• Look for compact object with a/m 1.
• Might come from comparing mergers with numerical
  simulations, or from EMRI observations. Since
  accretion-fed holes may have a/m 0.98, EMRIs
  with high SNR will be needed to be sure.
• Test Hawking Area Theorem
• Genuine theorem, but deeply linked to
  thermodynamics, quantum theory, probably quantum
  gravity strong motivation for testing it.
• Equal-mass mergers generate a lot of entropy, but
  in simulations they are very far from limiting
  case of Afinal SAinitial.
• EMRI and IMRI mergers do not seem promising
  either.

6
Probing Strong Curvature with EMRIs

• EMRIs are one of LISAs strongest tools for
  studying fundamental physics, and they set the
  LISA noise requirement at mid-range frequencies.
• Very sensitive because of large number of cycles
  chirp time
• Null test of uniqueness of Kerr metric fit EMRI
  waveforms to signal, determine if errors are
  consistent with noise/confusion background.
• Testing for non-Kerr metric existing studies
  (Glampedakis Babak 2005, Barak Cutler 2007,
  Barausse et al 2007) examine how EMRIs could test
  if metric is non-Kerr but still GR eg due to
  accretion disk or tidally distorting nearby body.
  
• They do not look for evidence for non-GR
  theories, because they assume GR to generate
  waveforms in the distorted metric.

7
EMRIs from Binary SMBH Systems

• An EMRI might come from a compact object falling
  onto a SMBH that is itself in a binary. Would
  notice a gradual drift in f due to acceleration
  of central SMBH.
• In 3 yr observation, ?f 10-8 Hz. At f 1 mHz,
  LISA could resolve ?v/c 10-5, or an
  acceleration a 3x10-5 ms-2.
• If both SMBHs were 106 M?, then they should be
  closer than 0.3 pc to one another.
• The acceleration parameter should be part of the
  signal fit.

8
Using EMRIs to Test Gravity Theory

• To compare GR with an alternative theory, need to
  compute EMRI waveforms self-consistently in the
  other theory, including EOM.
• For Hulse-Taylor Binary Pulsar, the limits on
  Brans-Dicke ? come from a calculation that
  includes scalar radiation and its back-reaction
  (Will).
• In Hulse-Taylor system, scalar effects are
  anomalously small (test anomalously weak) because
  stars have nearly equal mass, reducing scalar
  dipole radiation.
• Black holes radiate away massless fields when
  formed, so in Brans-Dicke, BHs are the same as in
  GR.
• EMRI signals from stellar-mass BHs falling into
  SMBHs will not test such theories. Weaker EMRI
  signals from NSs or WD cores of giant stars will
  provide tests.
• We lack a Parametrized Post Kerr framework that
  includes other theories hard to quantify the
  meaning of a null result when looking for
  violations of GR.

9

• Verifying gravitational waves
• Testing GR in strong field domain
• Cosmogony
• Dark energy measurement
• Redshift-distance relation
• Dark energy mission
• New fundamental physics

10
LISA and Dark Energy

• We have known for 22 years that binary inspirals
  are standard candles (standard sirens).
• Ground-based detectors will use them to perform
  an independent measurement of the Hubble
  constant.
• LISAs SMBHs permit it to measure the
  acceleration of the universe.
• The so-called Dark Energy is possibly the biggest
  challenge to fundamental theoretical physics
  today.
• The Dark Energy Task Force (DOE/NSF 2006) did not
  treat LISAs capability seriously
• A year later, the BEPAC committee (NAS 2007) took
  a different view

Other techniques , such as using gravitational
waves from coalescing binaries as standard
candles, merit further investigation. At this
time, they have not yet been practically
implemented, so it is difficult to predict how
they might be part of a dark energy program. We
do note that if dark energy dominance is a recent
cosmological phenomenon, very high-redshift (z ?
1) probes will be of limited utility.
LISA also has the potential to measure the dark
energy equation of state, along with the Hubble
constant and other cosmological parameters.
Through gravitational wave form measurements LISA
can determine the luminosity distance of sources
directly. If any of these sources can be detected
and identified as infrared, optical or x-ray
transients and if their redshift can be measured,
this would revolutionize cosmography by
determining the distance scale of the universe in
a precise, calibration-free measurement.
11
Measuring Redshift-Distance Relation

• Any binary system that chirps during observation
  has intrinsic distance information in signal.
  Chirp time measures chirp mass M
  (m1m2)3/5/(m1m2)1/5. Amplitude depends just on
  M/DL, where DL is the luminosity distance, so
  measuring it gives DL.
• Converting detector response into signal
  amplitude requires measurement of polarization,
  sky position. Strong covariance of errors among
  these and the chirp mass.
• Getting the redshift normally requires
  identifying the host galaxy or cluster and
  obtaining an optical redshift. Small error box is
  key to this.
• Since 2006 LISA community has made better
  estimates of errors by implementing full TDI in
  data analysis and using better signal models.
• Talks by Cutler, Cornish, van den Broeck, Porter,
  Babak, Husa address this.
• NB identification reduces error in DL.
• Weak lensing produces random errors in DL. Not
  clear how much can be removed by lensing studies
  of signal field.
• Using EMRI spirals, Hogan McLeod (2007) show
  that LISA can measure H0 to 1 accuracy (needs 20
  events to z 0.5).

12
LISA as a Dark-Energy Mission

• LISA has 3 capabilities that complement existing
  dark energy proposals
• Calibration-free. No distance ladder, binary
  sources are clean systems. Even G is not
  involved DL is measured in seconds.
• Not statistical each event gives a measurement
  of DL.
• Potentially long range LISA will see
  coalescences out to z 20, although
  identifications are probably not likely beyond z
  3. Most dark-energy methods, even with
  dedicated missions, stop around z 1.
• If we take the dark energy EOS as the
  conventional P w?, and look for evolution in w
  by defining w -1 w1z O(z2), then one can
  show that in the recent past, if our universe is
  flat,
• Then if there is no evolution (w1 0), dark
  energy is 25 of H2 at z 1, 7 at z 2, and 3
  at z 3. (Recall DETF comment on high-z
  behavior.) If w1 0.2, then at z 2 dark energy
  is 10 of H2. LISA may well be able to make this
  measurement, but it will need errors of order 1
  to distinguish w1 0.2 from w1 0.

13

• Verifying gravitational waves
• Testing GR in strong field domain
• Cosmogony
• New fundamental physics
• Fundamental physics and gravity
• New physics
• Brane physics

14
Fundamental Physics and Gravity

• Physics beyond the standard model should provide
  ways of
• unifying the strong and electroweak interactions
  (GUTs),
• showing why there is a non-zero baryon number (CP
  violation),
• explaining why the universe is so well fine-tuned
  for life to exist (multiverse, Everitt-Wheeler,
  )
• Because we expect GR to give way to a quantum
  theory, we expect corrections. Naïvely they
  should be at the Planck scale, but new ideas
  suggest other ways of looking for new physics.
• Branes. Since string theory is renormalizable
  only if the full theory is written in 11
  dimensions, there is plenty of freedom for new
  physical ideas. Instead of compactifying the
  extra dimensions, Nature could allow them to be
  large and just confine us to the 31 brane. Only
  gravity leaks out into the bulk. But LISA
  observes gravity, so LISA can touch the bulk.
• Emergent gravity. Gravity may not be the
  fundamental interaction we think it is. It might
  be an effective theory based in some other kind
  of microphysics, emerging at low energies the way
  superfluidity emerges from molecular dynamics.
• Loops and topology. Gravity may be the long
  length-scale limit of a theory that is not based
  on a continuous manifold but rather on some
  topological structure. Already loop quantum
  gravity has found a way through the Big Bang to
  an earlier epoch. Some topological theories
  predict that ? maintains a constant proportion to
  the matter energy density at all times.

15
Where is the new physics?

• Cosmic strings (see talk by Siemens) distinctive
  signature in GWs. Ground-based detectors already
  searching for them. May be a consequence of
  superstring theory (Damour Vilenkin 2001)
• Stochastic GW background LISA sensitive at level
  Ogw 10-10. Too strong for standard slow-roll
  inflation. But f 0.1 mHz is in the band of
  radiation emitted during the electroweak
  transition, when T 1 TeVThe EW transition
  is normally thought of as second-order (no
  density perturbations), but if baryon-antibaryon
  asymmetry arose there, then it could have
  resulted in strong density perturbations
  (Megewand Astorga 2006)
• String theory could make many modifications in
  gravity, including adding extra fields (talk by
  Yunes) and f(R) action terms. In principle, the
  best chance for observing these fields may be in
  EMRIs where R is large and dR comes from
  inspiral. (But recall that massless fields get
  radiated away in collapse.)

16
Physics on branes

• The brane paradigm (Maartens, Living Reviews)
  offers plenty of scope for new gravitational
  effects
• Larger amounts of stochastic gravitational
  radiation (eg Randall Servant 2006)
• Radiation in our universe from shadow matter on
  a nearby brane connected to us by a black string
  that looks to us like a black hole (Maeda Wands
  2000, Clarkson Searha 2006)
• No stochastic background radiation the Ekpyrotic
  Universe (Steinhardt Turok)