Indeed if the source, the deflector and the observer are not perfectly aligned, the optical paths taken by the rays from the different images differ. As a consequence, if the flux emitted by the source varies over the course of time, these variations will be marked by a certain time delay in the different images. Part of this delay is due to the geometric difference in the trajectory of the rays. But this is not the full story. Whenever one of the rays travels a longer distance than the other, the universe continues to expand, and the second ray  propagates over a longer distance due to this expansion. This is just the same as when two people run after a moving bus. The first, having a distance to run that is shorter than the second by two meters will reach the bus first, but the second person will run more than two extra meters before catching the bus.

The measurement of temporal delays that exist between the different images makes it possible to estimate the value of the Hubble constant, the measurement of the speed at which our universe is expanding.

From an overall point of view, all these observations will make it possible to create a statistical sample making it possible to test the model of the universe.  Indeed, the number of sources for which there are multiple images due to the gravitational lens effect among a population of quasars detected, is largely dependent on the model of the universe, or more precisely, on the cosmological mass density (the fraction of matter in the the universe). Therefore, once we have detected the quasars with the ILMT and, among these, the gravitational lenses present, we will be able to find the value for the cosmological mass density which reproduces the observed fraction of multiply imaged sources. By this means, we will thus be able to constrain the model of our universe.

What’s next?

For four years, François Finet contributed to the development of the telescope and the theoretical framework for the use of the data that will be acquired. Today, he is bound for India, where finally, the ILMT will be able to show what it is capable of. The young astrophysicist will be present for its assembly, its calibration and its first observations. “We are already hoping to detect potential quasar candidates in the first of observation, so that we can quickly obtain some first scientific results”, says the enthusiastic astrophysicist.

At the present time, liquid mirror telescope technology is gaining more and more ground with scientists. Feasibility studies are underway to send a liquid mirror telescope into space on board of a satellite. Another project that is even more ambitious is to send a telescope with a diameter of 100 meters to the moon. The mercury could also be replaced by other metals that are reactive to a magnetic field to influence their rotation and generate spherical mirrors instead of parabolic ones. Finally, the use of a liquid mirror telescope using a secondary mobile mirror is also being studied. This would make it possible to point a liquid mirror telescope at a distant object. The liquid mirror telescope would therefore no longer be a passive spectator of our universe and all this would be available at a much lower cost than the competition. However, while technology allows us to dream, all these projects are still at a theoretically embryonic stage. In the meantime, the ILMT has gone beyond this stage and is preparing to restlessly image the sky passing overhead.