UCL Mathematical & Physical Sciences


Einstein's general relativity passes another stringent test

11 July 2016

In this image — one of the deepest ever taken by the Hubble Space Telescope — most of the luminous sources are distant galaxies. The expansion of the Universe causes a cosmological redshift, so that the most distant sources appear reddest. The work …

In a recent paper published today in the Astrophysical Journal, Ignacio Ferreras (UCL Space & Climate Physics/Mullard Space Science Laboratory) and Ignacio Trujillo (IAC) presented the most detailed test of the so-called cosmological redshift.

The theory of General Relativity -- which celebrated its 100 years in 2015, and received its latest confirmation through the discovery of gravitational waves this year -- predicts a general expansion of space-time in our Universe, leading to the Big Bang model.

This expansion produces changes in the observed wavelength of light from distant sources, so that distant galaxies appear redder than their nearby counterparts. This effect can be described by a simple scaling relation, where the observed wavelength is the wavelength in the emission source multiplied by a factor (1+z), where z is the redshift term. z is a number close to zero for nearby galaxies, increasing with the distance to the source.

This effect has been routinely observed in galaxy spectra for over 100 years, with pioneering studies from Slipher and Hubble. By dispersing the light from distant galaxies, it is possible to compare features in the spectra - produced by atomic level processes in the atmospheres of the stars in the galaxies - with the equivalent ones in nearby galaxies, where the redshift effect is smaller.

In this paper, the authors explored in a minute level of detail possible departures from this simple redshift law. Such changes could be expected, for instance, if gravity were not perfectly explained by General Relativity, or if the photons from distant sources experimented interactions with the intergalactic plasma along its path to the

By combining up to half a million optical spectra from the Sloan Digital Sky Survey, the authors could constrain variations down to 1 part per million. Following many tests that looked for potential systematic effects in the results, the authors confirmed that no significant departure from the standard (1+z) law is found, providing yet another stringent test on the predictions of General Relativity.