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Searches for very high frequency
gravitational waves
Mike Cruise
University of Birmingham
Signals from Neutron Stars
Signals from SMBH’s
LIGO , Virgo, LISA or pulsar timing
are most likely to make the first detections
Why go to higher frequencies?
• Other branches of astronomy developed
through mullti-frequency observations
• For frequencies n>1 MHz
– Cosmological signals from the Planck era.
– KK mode oscillations in higher dimensions
– EMW-plasma instabilities generate GW’s
– Radiation from non-minimal coupling of the
EM and G fields
Very High Frequency detectors?
• The best upper limit at 100 MHz by Akutsu
and Kawamura of ~10-17 was with an
interferometer
1
• But Interferometer sensitivity worsens as n 2
• Minimum detectable EM signal in a 1 Hz
bandwidth ~ 10-20 W
• Maximum EMW power ~ 10’s of MW
• ratio of min detectable signal ~ 10-27
max available energy
E or B
Pathways for coupling EM and
Gravity
 2 1   n
G n
   2 2  h  16 4 T
c t 
c

2
May contain EM Fields
Fn  g gn F
Contain G potentials

•
How does a GW interact with a
static
EM
Field?
De Logi and
Mickelson (1977)
Photon
Graviton
Virtual Photon
( Static Magnetic Field )
For one incoming
graviton per second
8GB L Spin states of g, B and n


3
c
2 2
What are the fluxes?
• Flux of photons is 
times the flux of
2
c
gravitons
Flux  
1
 h
16G

• EM Signal Power is
P
1
8 0
2
2
B L K h cSin  
2 2
2
2
2
Early Universe
Brane Oscillations
• Seahra and Clarkson have calculated the GW
emission in 5-D gravity when stellar mass black
holes fall into a black hole
• Different from
the LF radiation from such
a system, there is also an
excitation of the
brane separation
itself
This is a Source which exists!
But maybe in a universe which doesn’t
Plasma-EMW instabilities
Tidal forces
affect
charge
density
Increased
charge
density
contributes to
stress energy
tensor
• Linearised field
equations in terms of
a small metric
 2
1  2  n
16G n
perturbation, hn  x 2  c 2 t 2 h   c 4 


• Interaction of EM
Fields and EM Waves

n
1

4
  n 1 n
 
F
F


F
F




4


Current Detectors
Nucleosynthesis limit
Cosmological Models
Higher Energy Density
Current Detectors
Galactic centre
Shadow Brane
Galactic Centre
Visible Brane
Two element interferometer
Nucleosynthesis limit
Cosmological Models
Statement
• The work presented here derives from
many papers on EM-GW interactions
published in properly peer reviewed
journals since the 1970’s
• This work has no connection with (and
does not support or endorse) ideas
published by the HFGW group
Conclusion
• Very high frequency
gravitational waves may allow
us to observe the very early
universe, violent astrophysical
events or exciting areas of
new physics
• Current detectors are now
beginning observations of the
Galactic Centre at GHz and
Optical Frequencies
• A two element interferometer
is being designed jointly by
Birmingham and Jodrell Bank
Higher sensitivity
Gravitational Waves in
Space
• measure the
separation between
three spacecraft using
laser beams.
• Use a long baseline
so that the movement
is larger.
• Measure separation
to 10 pm 10-11 m over
5 million km.
Other mechanisms
Bubble collisions
Decay of Cosmic Strings
“ Conversion” process
•
•
•
•
Inverse Gertsenshtein Effect
EMW signal ~ h2L2K2B2
Same frequency and direction as GW
Must ensure phase coherence of EMW
and GW
L
Lens+CCD
h
B
EMW
•
The Universe- and how we
study
it
Everything we know about
the Universe comes from
studying electromagnetic
waves ( Infra-red, X-rays,
radio waves, etc) of
different frequencies
• Different frequencies tell us
about different temperature
regimes
• But many of the problems
in astrophysics are to do
with mass, not temperature.
What can
Gravity tell us?
Measuring the waves:
Interferometer
Mass
M
a
s
s
Laser
Photodiode
The Largest Instrument Ever!
• Three spacecraft with
laser beams between
them in a solar orbit.
• The pattern rotates
each year to scan the
sky.
How does a GW affect an
EMW?
Amplitude
•
• Direction
• Frequency
• Polarisation state
GW
Esig ~ hEquiescent
2
EnergySig ~ h EMField
h  9 x10
 21
1
1 0.1mm ( d 5l ) / 2l
Mp
e
0.5
Rkpc M
l
Other Inflation Theories
• Garcia -Bellido
What kind of Instrument?
• Interferometer sensitivity worsens as
hmin
1  4hc 



8FL  P 
1
2
n
1  n  
2
• Current best Upper Limit is by Akutsu,
Kawamura et al
– 10-17 at 100 MHz
• So h ~ 10-23 at 1000Hz will be 10-20 at 1
GHz
1
2