An international team of researchers led by the Instituto de Astrofísica de
Canarias (IAC) has measured, with unprecedented accuracy, the gravitational
redshift of the Sun, a change in frequency of the lines in the solar
spectrum which is produced when the light escapes from the gravitational
field of the Sun on its way to Earth. This work, which verifies one of the
predictions of Einstein’s General Relativity, is to be published in the
journal Astronomy & Astrophysics.
The General Theory of Relativity, published by Albert Einstein between 1911
and 1916, introduced a new concept of space and time, by showing that
massive objects cause a distortion in space-time which is felt as gravity.
In this way, Einstein’s theory predicts, for example, that light travels in
curved paths near massive objects, and one consequence is the observation of
the Einstein Cross, four different images of a distant galaxy which lies
behind a nearer massive object, and whose light is distorted by it.
Other well known effects of General Relativity are the observed gradual
change in Mercury’s orbit due to space-time curvature around the “massive”
Sun, or the gravitational redshift, the displacement to the red of lines in
the spectrum of the Sun due to its gravitational field.
The gravitational redshift is an important effect for satellite navigation
systems such as GPS, which would not work if General Relativity was not put
into the equations. This effect depends on the mass and the radius of an
astronomical object, so that even though it is bigger for the Sun than for
the Earth, it is still difficult to measure in the solar spectrum.
In 1920, Einstein wrote: “For the Sun, the theoretical redshift predicted is
approximately two millionths of the wavelength. Whether this effect really
exists is an open question, and astronomers are currently working hard to
resolve it. For the Sun, its existence is difficult to judge because the
effect is so small”.
To measure it, the scientists have used observations of the solar spectrum
reflected from the Moon, obtained with the HARPS (High Accuracy
Radial-velocity Planet Searcher) instrument using the new technology of the
laser frequency comb.
“Combining the precision of the HARPS instrument with the laser frequency
comb, we have been able to measure with high accuracy the position of the
iron lines in the solar spectrum”, explains Jonay González Hernández, a
Ramón y Cajal researcher at the IAC and first author of the article. “This
has enabled us to verify one of the predictions of Einstein’s Theory of
General Relativity, the gravitational redshift, to a precision of just a few
metres per second”.
“New measurements with the laser frequency comb attached to the ESPRESSO
spectrograph, on the 8.2 m VLT telescopes, would allow us to improve these
measurements”, adds Rafael Rebolo, a researcher and the Director of the IAC
and a coauthor of the article.
Other researchers at the IAC who have participated in this work are
Alejandro Suárez Mascareño and Borja Toledo-Padrón, who used the LFC (Laser
Frequency Comb) attached to the HARPS instrument on the 3.6m telescope of
the European Southern Observatory (ESO), at the La Silla Observatory
(Chile).
Reference:
J.I. Gonzalez Hernandez, R. Rebolo, L. Pasquini, G. Lo Curto, P. Molaro, E.
Caffau, H.-G. Ludwig. The solar gravitational redshift from HARPS-LFC Moon
spectra. A test of the general theory of relativity. Astronomy &
Astrophysics, 2020;
DOI: 10.1051/0004-6361/202038937