Study dark energy with active galactic nuclei

The precise determination of distances is one of the pillars on which stood astronomy and astrophysics. A group of Japanese researchers has found a new elegant and simple method to determine the active nuclei of galaxies and indirectly the value of the Hubble constant. It therefore provides a new tool to study the nature of dark energy.

As shown in astronomy courses and astrophysics line of the Paris Observatory, “Windows on the Universe”, all research in these disciplines begin with measurements of distance, time, mass and temperature. The cosmology is of course no exception to this rule, and this is why it is given for the task not only to map the observable cosmos across galaxies, but also determine the speed of expansion of the universe. In this regard, Hubble and especially Lemaître learned that there was a relationship between the distance sufficiently distant galaxies and their apparent speed of removal, specifically their spectral redshift.

The measurement of the Hubble constant gives us an estimate of the age of the universe. We therefore sought to measure with the greatest possible precision for decades. The company has proved difficult. For a long time, one could hardly do better than conclude that the universe must have an age between 10 and 20 billion years. Commissioning of the telescope Hubble has enabled significant progress. With Planck, we now know that the age of the universe is about 13.8 billion years.

The Hubble constant, the SN Ia to AGN

Along the way, we also discovered that the expansion of the universe is accelerating past few billion years. We do not yet know the cause of this phenomenon, which is connected to the existence of a mysterious dark energy. As we had long, the Hubble constant is not really, it changes the course of time. New method of its determination, including long distance and therefore in the more distant past, can help the light on the nature of dark energy.

The principle of measurement of the Hubble constant using the variability of the light emissions of an active galactic nucleus (AGN also called, active galactic nucleus in English) is shown in this diagram. A torus of dust (dusty torus) emits light near infrared with peaks of brightness related to those in the UV produced by a central super massive black hole. A time lag between the peaks is related to the distance d between the central holes of the toroid black dust.

Much has been used SN Ia supernovae for this purpose and continues to do so, as we had explained the Nobel Prize Saul Perlmutter. We think their brightness intrinsic, which is very large, varies little enough that measures magnitude apparent enable us to derive distances with some confidence. Measuring their redshift then allows us to derive the value of the Hubble constant.

A group of astronomers recently filed on arxiv an article in which they show that one can also use the active galactic nuclei ( active galactic nuclei or AGN English) for determining distances and ultimately constant Hubble. As Ia SN, AGN are particularly bright, which means we can detect billions of light-years. But they have the advantage of being more numerous than the supernovae when making comments on the stars to large spectral redshift. We know for example that there are lots of quasars to redshift of about z = 7.

Intrinsic brightness related to the size of the toroid dust

The idea implemented by researchers is simple. The AGN are black holes supermassive surrounded by a torus of dust. Radiation ultraviolet they emit intense swallowing the material is partially absorbed by the torus, which retransmits a part of this energy in the field infrared. AGN A behaves as a source with variable intensity peaks. When a product is in ultraviolet, it does occur in the infrared but late. This time lag is easily explained by the fact that the radiation emitted at the center of the AGN must travel to the torus that surrounds it. One can show that the distance between the central black hole torus varies as the square root of the intrinsic luminosity of the AGN. Its fluctuations to determine this distance, one can know that brightness and compare the apparent magnitude of the AGN. As for the SN Ia, more the AGN will be far, over its apparent brightness is low compared to its intrinsic brightness.

Astronomers have tested their method with 17 AGN with a small redshift and compared the estimates obtained with those derived from the study of Cepheids hosted by galaxies. Measurement accuracy was found to be comparable to those achieved using the telescope Hubble. There is therefore every reason to think that we have many new complementary tool SN Ia to measure the value of the Hubble constant over long distances. If it could highlight a variation in time of the cosmological constant, we would at the same time that dark energy is not a simple manifestation of the quantum vacuum fluctuations.