The inevitable youthfulness of known high-redshift radio galaxies

Some galaxies are very luminous in the radio part of the spectrum. These ‘radio galaxies’ have extensive (hundreds of kiloparsecs) lobes of emission powered by plasma jets originating at a central black hole1. Some radio galaxies can be seen at very high redshifts2, where in principle they can serve as probes of the early evolution of the Universe. Here we show that, for any model of radio-galaxy evolution in which the luminosity decreases with time after an initial rapid increase (that is, essentially all reasonable models3), all observable high-redshift radio galaxies must be seen when the lobes are less than 107 years old. This means that high-redshift radio galaxies can be used as a high-time-resolution probe of evolution in the early Universe. Moreover, this result explains many observed trends of radio-galaxy properties with redshift4,5,6,7,8,9, without needing to invoke explanations based on cosmology10 or strong evolution of the surrounding intergalactic medium with cosmic time6, thereby avoiding conflict with current theories of structure formation11.