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