The Potential for Very High Frequency Gravitational Wave Science Mike Cruise University of Birmingham GPhyS 2010 [In memory of P.Tourrenc]
LISA, LIGO and VIRGO The obvious sources of gravitational waves lie in the millihertz 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 20 th 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 10 15 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: P = 3 1 c 32π G 2 ω h 2 So for a given power the amplitude goes as: h 1 ω Hence going from ω= 10-3 to ω=10 9 we may expect h to go from 10-22 to 10-34
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= 10 15 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 10 10 Hz to 10 15 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 F µα Very strong Masers pointed exactly at us could deliver x F τ ν α 2 2 µν = c 1 2 1 4 t 2 2 F π F = h µα B µν F ν α + 16πG = τ 4 c 1 4 η µν F αβ µν F E cos( ωt) αβ = B 2 + E 2 cos 2 ( ωt) + BEcos( ω t ) h=10-26
Cosmological Backgrounds The stochastic background is usually specified in terms of the relative energy density Ω gw The standard model of inflation predicts a monotonically decreasing spectrum of h with frequency, caused by parametric amplification of quantum fluctuationsthese must exist at some level Ω h = gw = 3x10 1 d ρ gw ρ d ln f c 21 100 υ Ω gw
Energy Density
Many Possibilities
Other Inflation Theories Garcia -Bellido
New Physics? At λ~1cm, source is in the Planck regionunknown physics? Nucleosynthesis 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 h = 9x10 21 1 1 0. 1 M p 0.5 Rkpc M mm l e ( d 5l) / 2l
Spectra
This is a Source which exists! But maybe in a universe which doesn t
Target sensitivity for detectors Stochastic Background at Ω gw ~ 10-10 and this means h~10-31, Sn 1/2 =10-39 Brane oscillations at ν=10 9-10 15 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, Sn 1/2 =10-41 Frequency ranges from 10 8 to 10 15 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 Strain sensitivity: 6.4-8.5 10-17 /rhz Upper limit for h 02 Ω GW : < 6 10 25 Integration for 1000 sec Sensitivity h 02 Ω GW < 6 10 25
Detector choices Laser interferometers sensitivity becomes worse with increasing f S=10-23 @f=100hz S=10-17 @f=100mhz Whereas the ratio of: Minimum detectable EM signal ~10-20 Available EM power 10 +5 ~10-25 S 1 2 f = 1 8FL 4πhλc ηp 1 2 1 + f f p 2
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 h 2 x Field Energy Density This is a graviton to photon conversion process GW EM Field
Conversion Physics P emw = h 2 2 2 B L K 2µ 0 2 c
Optical Detector
Current sensitivity
Developing Issues We can probably reach h=10-25 in a years observing at ν=10 15 Hz, and h=10-21 in a years observing at ν=10 8 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 = = h cos( ω t ) h 2 B 2 K 2 L 2 BKL { hcos( ω t) BKL+ B cos( t) } 2 seed ω = 2h cos 2 ( ωt) BB seed KL
Properties of the cross term Proportional to h not h 2 ( large advantage) Proportional to B seed,i.e. proportional to P seed 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 Sn 1/2 =10-34 per root Hz if you could modulate the current magnetic fields
Aperture synthesis
Transparent Ferromagnets Very high fields exist inside some Ferromagnets. If such a material were transparent at the observing frequency the gravitonphoton conversion could take place in the bulk material. For materials with a Curie temperature T, the field inside would be ~ KT µ This could be as high as 1000 T b
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 B EMW s ω g ω g h 2 B 2 ω g Seed Field Bs B ω g h 2 B 2 hbb s B s B s ω g Bcos(ω s t) ω g -ω s h 2 B 2 Seed Field Bs ω g hbb s B s B s
Modulated seeding Current Sensitivity BUT SEEDING HAS NOT YET BEEN DEMONSTRATED Current apparatus + seeding at P=1W
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 Cryogenic amplifier 40 T Large waveguide
Conclusions-Why try to do it? Study the very early universeobservations 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