Historical research guide · 0664 · 2021
Potential of Radio Telescopes as High-Frequency Gravitational Wave Detectors
Valerie Domcke · Camilo Garcia-Cely
In one minute
Valerie Domcke and Camilo Garcia-Cely point out that the universe already contains a gravitational-wave detector far larger than any we could build. In a magnetic field a gravitational wave turns into a photon, and a photon back into a gravitational wave — the Gertsenshtein effect, a direct consequence of general relativity plus classical electromagnetism. Each conversion is faint, but stretch the process across cosmological distances threaded with magnetic fields and it stops being negligible. Their target is the dark ages, the quiet stretch between the release of the microwave background and the first stars, when so few free electrons remain that photons carry almost no effective mass and the conversion runs at its most efficient. Gravitational waves crossing that era would leave extra brightness in the low-frequency tail of the microwave background — so measurements already taken by the ARCADE 2 balloon and the EDGES 21-centimetre experiment become bounds on gravitational waves at megahertz to gigahertz frequencies, for the strongest allowed cosmic magnetic fields some seven orders of magnitude beyond what laboratory detectors reach there.
Connection
The conversion of a gravitational wave into a photon in a magnetic field, and back, is the settled physics behind chapter 10’s claim that electromagnetism and gravity are two ends of one lever — here it is used as an instrument, in a Physical Review Letter, with numbers attached. It also opens the megahertz-to-gigahertz band of chapter 4, the band where small, engineered sources would sit rather than merging black holes, and it names the telescope that would improve the reach.
From the source
In the presence of magnetic fields, gravitational waves are converted into photons and vice versa — the Gertsenshtein effect — and as an immediate consequence of general relativity and classical electromagnetism this is a purely Standard Model process.Abstract; section ‘The Gertsenshtein effect’, Equation (1)
The gravitational-wave to photon conversion probability oscillates with distance travelled, with an oscillation length fixed by the photon plasma frequency and the magnetic field; because cosmic magnetic fields and electron densities are patchy, the wave crosses many independent regions and the result is an average conversion rate rather than coherent build-up — and any additional inhomogeneity further enhances that rate.Equations (4) and (5); discussion of coherence loss
During the dark ages, between recombination at redshift about 1100 and reionization at redshift about 10, the small fraction of free electrons suppresses the effective plasma mass of photons and so increases the conversion probability, which makes the Rayleigh-Jeans tail of the cosmic microwave background a detector for gravitational waves active in that era.Introduction; Equations (6) and (7)
Existing radio measurements can be cast as bounds on the gravitational wave amplitude: EDGES gives a characteristic strain below 10 to the minus 21 at 78 megahertz for the strongest cosmic magnetic fields allowed, and ARCADE 2 gives below 10 to the minus 24 at 3 to 30 gigahertz — for the strongest fields exceeding current laboratory constraints by about seven orders of magnitude.Abstract; section ‘Probing the Rayleigh-Jeans tail of the CMB spectrum’; Figure 2, right panel
Because the frequency of a cosmological gravitational-wave background tracks the comoving Hubble horizon at the time it was made, processes in the very early universe at energies far beyond any collider generically produce gravitational waves in the megahertz and gigahertz regime today, well above the reach of LIGO, Virgo and KAGRA.Introduction; section ‘Discussion’
The Square Kilometre Array, assuming an effective area per antenna temperature of at least 100 square metres per kelvin in the 0.1 to 10 gigahertz range, would reach microjansky sensitivity in a few hours of observation against cosmic microwave background fluxes of at least 1000 jansky — very promising, though sufficient foreground subtraction will be extremely challenging.Section ‘Discussion’, final paragraph
The way in
https://doi.org/10.1103/PhysRevLett.126.021104How to cite it
Valerie Domcke, Camilo Garcia-Cely (2021) Potential of Radio Telescopes as High-Frequency Gravitational Wave Detectors doi:10.1103/PhysRevLett.126.021104