A new method to measure the effect of gravity on the propagation of light in the Universe is proposed in a project led by cosmologists from UNIGE that was performed in collaboration with members of the HIRAX telescope from the University of KwaZulu-Natal, thanks to a bilateral Joint Research Project between Switzerland and South Africa. This innovative method combines radio intensity surveys and observations of distant galaxies. It represents one more step to test Einstein's predictions on cosmic scales.

 

One of the spectacular historical predictions of Einstein's General Relativity is that light is affected by gravity, in spite of being massless. This light deflection was confirmed in 1919 by the observations of the solar eclipse by Arthur Eddington. It is now at the heart of testing alternative theories of gravity through cosmological observations by mapping the mass distribution in the Universe.

 

This deflection is also useful for astronomers: "A massive object will deflect the light in its surroundings and will play the role of a lens, allowing us to have a deeper vision of the universe. It provides a kind of natural telescope which increases the power of our observations" explains Fabien Lacasa, postdoctoral fellow at the Institut d'Astrophysique Spatiale d'Orsay France, and co-author with the UNIGE team. "Additionally, this lensing effect affects the angles under which you see far-away sources. For radio intensity measurements, these two effects compensate each other, and the lensing does not induce a signal" continues Camille Bonvin, professor in the Department of Theoretical physics of the Faculty of Sciences of UNIGE. "On the other hand, for the detection of galaxies it is different. The lensing allows us to detect less luminous galaxies, which are generally much more numerous. Angular dilution and increase of the signal do not compensate each other."

 

It is on this difference in behavior that the cosmologists have focused to isolate the gravitational lensing, by disentangling it from the other contributions to the observed signal. According to Mona Jalivand, doctoral student in the Department of Theoretical Physics and lead author of the article published in Physical Review Letters: "Variations in the density in the Universe also lead to variations in the light intensity observed in radio observations, and in the number of galaxies detected. With a survey of galaxies alone, it is difficult to disentangle these two contributions, density and lensing. By using radio surveys as second source of information, we have built a measurement tool which makes it possible to very effectively remove the contribution of density fluctuations, to reveal the contribution of lensing. We have shown that our methodology will allow measuring lensing in the near future, where traditional methods using only galaxies would fail."

 

What are the prospects for this work? "We are now working to predict how well this methodology will allow us to test General Relativity and alternative theories of gravity. We have to prepare now to be able to apply this method to future surveys in which our group is involved: the HIRAX radio telescope in South Africa and the European satellite Euclid.” concludes Martin Kunz, professor in the Department of Theoretical Physics.