19 - Cosmological SETI Frequency Standards

Summary

Introduction

In 1973 Drake and Sagan proposed a SETI frequency standard of V₀ ∼ 56 GHz tied to the observed cosmic microwave background hv₀ = kT₀, where T₀ is the current temperature of the cosmic microwave background. They noted that a transmitting civilization in a distant galaxy will, however, have transmitted its signal to the Earth at an earlier cosmological epoch when T was larger than is measured today, tending to increase the ‘natural’ frequency, but that the cosmological Doppler effect will tend to decrease the frequency. Not knowing of their work, I proposed this same frequency standard (hv₀ = kT₀: Gott, 1982) in the first edition of this book. I had noticed that the two effects mentioned above in fact cancel each other out exactly (which was a new result), so that this frequency standard was indeed universal. If a transmitting civilization is at a redshift z, it will observe a microwave background temperature of T₁ = T₀ (1 + z) and will emit signals at a frequency of hv_e = kT₁ = kT₀ (1 + z), but because of the cosmological Doppler shift we will observe these transmitted photons at a frequency hv₀ = hv_e (1 + z)^–1 so that we observe hv₀ = kT₀ (1 + z) (1 + z)^–1 = kT₀ ∼ 56 GHz independent of the redshift of the emitting civilization.