the potential for very high frequency gravitational wave sciencegphys.obspm.fr/paris2010/mercredi...
TRANSCRIPT
The Potential for Very High Frequency Gravitational Wave
Science
Mike CruiseUniversity of Birmingham
GPhyS 2010[In memory of P.Tourrenc]
LISA, LIGO and VIRGO
• The obvious sources of gravitational waves lie in themillihertz to kilohertz regions
• The science cases for LISA, LIGO and VIRGO are exceptionally strong• These are exceptional instruments of exquisite sensitivity and deserve to be successful in opening up new science
Lessons from History
• The huge expansion in our knowledge of the Universe in the 20th Century came from studying different wavebands
• Different frequencies tell us about different temperature regimes, different objects different physical processes
• When gravitational wave astronomy is established we may benefit from looking outside the mHz-kHz range that we focus on today.
• What might we observe?• How might we observe?
Outline
• Discrete sources• Stochastic, cosmological sources• “New Physics” sources• Detectors
– What are the options?
– Where have we currently reached?– What are the future challenges?
Definition
• Very High Frequency in this talk means above 1 Mega Hertz and extending to 1015 Hz at least
• So this is equivalent to a talk on– Radio Astronomy– Infra-red Astronomy– Optical Astronomy– ( And probably UV, X-Ray and Gamma Ray
Astronomy, too)
• We may need to agree on the same nomenclature as the Electromagnetic people have.
Warning!
• We haven’t detected 10-22
yet!!• The power flux in a
gravitational wave is given by:
• So for a given power the amplitude goes as:
• Hence going from ω= 10-3 to ω=109 we may expect h to go from 10-22 to 10-34
223
32
1h
G
cP ω
π=
ω1∝h
Discrete Sources
• Excellent Review by Bisnovatyi-Kogan and Rudenko (CQG 21 (2004) 3347-3359)
• The Sun• Gravitational Bremsstrahlung- radiation
from electrons and protons accelerated by Coulomb collisions in the hot plasma
• Peak Frequency ν= 1015 Hz• Peak amplitude h=10-33
• Spectral density ~10-41 Hz-1/2
Primordial Black Holes
• Formed in the early universe• Decaying by Hawking radiation which has
a gravitational sector• Expected frequency
– From 1010 Hz to 1015 Hz
• Expected amplitude– From 10-32 to 10-36
• Spectral density ~10-37 -10-44 Hz-1/2
“Grasers”
• Linearised gravity
• EM Stress energy tensor
• Maser action in the ISM leads to strong EMW’s in regions of strong static B fields
• Field products in the stress energy tensor have terms like
• Very strong Masers pointed exactly at us could deliver
µνµν τπ42
2
22
2 161
c
Gh
tcx−=
∂∂−
∂∂
−= αβαβ
µννα
µαµν ηπ
τ FFFF4
1
4
1
)cos( tEBF ω+=
)cos()(cos222 tBEtEBFF ωωνα
µα ++=
h=10-26
Cosmological Backgrounds
• The stochastic background is usually specified in terms of the relative energy density Ωgw
• The standard model of inflation predicts a monotonically decreasing spectrum of h with frequency, caused by parametric amplification of quantum fluctuations-these must exist at some level
fd
d gw
cgw ln
1 ρρ
=Ω
gwxh Ω= −
υ100
103 21
Energy Density
Many Possibilities
Other Inflation Theories
• Garcia -Bellido
New Physics?
• At λ~1cm, source is in the Planck region-unknown physics?
• Nucleo-synthesis limit is not new physics
New Physics Sources
• Seahra and Clarkson have calculated the GW emission in 5-D gravity when stellar mass black holes fall into a black hole
• The normal LF radiation from such a system is emitted plus an excitation of the brane separation itself
Waveforms
lldp e
l
mm
MRkpcMxh 2/)5(
5.021 1.011
109 −−−=
Spectra
This is a Source which exists!
But maybe in a universe which doesn’t
Target sensitivity for detectors
• Stochastic Background at Ωgw ~ 10-10 and this means h~10-31 , Sn1/2=10-39
• Brane oscillations at ν=109-1015 Hz and h~10-18 This is speculative science ( 5-D gravity ) but then the actual source is probably more dependable
• The Sun at h=10-33, Sn1/2=10-41
• Frequency ranges from 108 to 1015 Hz
Detectors
• At the lower part of the frequency range there are two main possibilities:– Optical interferometers– Electromagnetic devices
• As the frequency increases it seems that only the electromagnetic detectors stand a chance of reaching the desired sensitivity
• However, compared to LIGO or LISA only a few staff years of development has been focussed on these detectors so far…..
Upper limit for stochastic GW
Integrationfor 1000 sec
h02ΩΩΩΩGW < 6××××1025
Sensitivity
Strain sensitivity: 6.4-8.5 ××××10-17 /rHz Upper limit for h 0
2ΩΩΩΩGW: < 6××××1025
Detector choices
• Laser interferometers sensitivity becomes worse with increasing fS=10-23 @f=100HzS=10-17@f=100MHz
• Whereas the ratio of:– Minimum detectable EM signal ~10-20
Available EM power 10+5
~10-25
22
1
21
14
8
1
+
=p
ff
f
P
ch
FLS
ηλπ
EM Detector concepts #1
• “Geometric” Detectors– In principle GW can
affect form of an EM Wave :
– Amplitude– Frequency– Polarisation
• Field change hE
• Energy change (hE)2
Transmitter
Receiver
Two Detectors in Correlation
Note : Detectors are mobile to allow change in overlap function
Microwave Power Source
At the moment P=0.25 W, T=300k
Data at 100 MHz
Consistent with thermal noise limit
Result of correlation
Low noise correlation
EM Detector concepts #2
• Set up a static E or B Field in the Lab
• A passing GW wave will generate modes in the E or B field at the frequency of the GW
• Over one GW λ– Field change = hB– Energy change = (hB) 2
• Note that the energy change is – h2 x Field Energy Density
• This is a graviton to photon “conversion” process
EM Field
GW
Conversion Physics
0
2222
2µcKLBh
Pemw =
Optical Detector
Current sensitivity
Developing Issues
• We can probably reach– h=10-25 in a years observing at νννν=1015 Hz, and – h=10-21 in a years observing at νννν=108 Hz
• This is not good enough!• We need stronger fields, better designs, better
ideas• Brief comments on technological advances
currently being explored– Using “Seed” photons– Aperture Synthesis– Transparent Ferromagnets
Detectors with “seed” fields
• In normal conversion detectors– Field change– Energy in photons
generated at ω
• If a seed field is
added at ω then– Energy change – Energy change has a
cross term
BKLth )cos( ω=
2222 LKBh=
2)cos()cos( tBBKLth seed ωω +=
KLBBth seed)(cos2 2 ω=
Properties of the cross term
• Proportional to h not h2 ( large advantage)• Proportional to Bseed ,i.e. proportional to • But the larger number of photons have to be
detected against the photons of the seed field that have increased shot noise
• Possibilities ( the only possibilities )– Amplitude- defeated by noise increase– Direction- difficult because of momentum conservation– Polarisation- might achieve a factor 10-4
– Frequency- worth considering-modulate B field• With a 1W seed field a detector might reach
Sn1/2=10-34 per root Hz if you could modulate the current magnetic fields
seedP
Aperture synthesis
Transparent Ferromagnets
• Very high fields exist inside some Ferromagnets.
• If such a material were transparent at the observing frequency the graviton-photon conversion could take place in the bulk material.
• For materials with a Curie temperature T, the field inside would be ~
• This could be as high as 1000 T b
KT
µ
Development Path- no seeding
Current
Magnet upgrade
1 yr observing
Cryogenic Amplifier
Large WG, 40T
Large WG, 1000 T
Seeding-how would it work?GW EMW’s
Bωωωωg ωωωωgh2B2
ωωωωgBcos( ωωωωst) ωωωωg -ωωωωs
h2B2
hBB sωωωωgBsBs
Bωωωωgωωωωg
hBB s
h2B2
BsBsSeed Field Bs
Seed Field Bs
Modulated seeding
Current Sensitivity
Current apparatus + seeding at P=1W
BUT SEEDING HAS NOT YET BEEN DEMONSTRATED
Noise Spectral Density
Current
Cryogenic amplifiers
Large waveguide
40 T
Dimensionless Amplitude
Current
Cryogenic amplifier
Large waveguide
40 T
Modulated Seeding plus….
current
Modulated seeding
Large waveguide
Cryogenic amplifier
40 T
Conclusions-Why try to do it?
• Study the very early universe-observations of inflation and the Planck Scale
• Accessing strong gravity from the bulk in higher dimensions
• The possibility of a Hertz experiment
• There are huge opportunities for new ideas– Using well developed EM techniques
to focus signals and reduce detector noise
– Using EM “optical” configurations– Using new materials