Einstein’s Equivalence Principle verified for the first time in quasars

Credit: NASA, ESA and J. Olmsted (STScI)

Scientists from the University of Granada and the Instituto de Astrophysical de Canarias have been able to measure the gravitational redshift in quasars and extend the test to very distant regions, whose light was emitted when our Universe was very young

The study has been published in The Astrophysical Journal and has recently been highlighted by the American Astronomical Society.

According to Einstein’s theory of general relativity, light feels the influence of gravity just like matter. A consequence of this theory, based on the Equivalence Principle, is that light that escapes from a region with strong gravity loses energy on its way, so that its wavelength becomes redder. This phenomenon is known as gravitational redshift. The measurement of this effect is a fundamental test of Einstein’s theory of gravitation. Until now the test had been applied exclusively in regions of the Universe very close to us. Thanks to the use of a new experimental procedure, scientists from the University of Granada (UGR) and the Institute de Astrophysics de Canarias (IAC) have been able to measure the gravitational redshift in quasars and extend the test to very distant regions, whose light was issued when our Universe was very young.

Einstein’s Equivalence Principle is the cornerstone of Einstein’s General Theory of Relativity that constitutes our current description of gravity and is one of the fundamental pillars of modern physics. This principle establishes that there are no experiments that allow distinguishing between the existence of a gravitational field or an accelerated motion of the observer and predicts, among other things, that the light emitted from an intense gravitational field must experience a measurable shift towards the less energetic part ( redder) of the spectrum, an effect known as redshift.

Confirmation outside our galaxy has been elusive and has barely been proven, with complex inaccurate measurements, in neighboring galactic clusters located at relatively close distances on a cosmological scale.

The cause of this absence of checks in the more distant Universe is the difficulty of measuring the redshift since, in most situations, the effect of gravity on light is very small. This is why giant black holes, which generate extraordinary gravitational fields, provide one of the most promising scenarios for measuring gravitational redshift. These supermassive black holes inhabit the centers of galaxies and, in particular, the extraordinarily luminous and distant quasars.

A quasar is a celestial object with a deceptive stellar appearance, but which is located at enormous distances, so the light we receive was emitted when the Universe was much younger. This implies that they are exceptionally bright. The source of its extraordinary brightness is a disk of hot material that is being engulfed by a huge supermassive black hole so that, in a very small region, just a few light days in size, an enormous amount of energy is generated.

Bibliographic reference:

Mediavilla, E .; Jiménez-Vicente, J. (2021): “Testing Einstein’s Equivalence Principle and Its Cosmological Evolution from Quasar Gravitational Redshifts”. The Astrophysical Journal. 914: 112. DOI: 10.3847 / 1538-4357 / abfb70. arXiv: 2106.11699