A research conducted by a team led by astronomer Peter A. Milne of the University of Arizona published in two articles in the “Astrophysical Journal” shows that Type Ia supernovae can be divided into two groups with different characteristics. For years, astronomers had thought that their brightness depended almost exclusively on their distance. This can have consequences on our knowledge of the universe expansion, also calculated based on this type of supernovae.

Type Ia supernovae originate from the explosion of a white dwarf. Typically, these dying stars have a mass too small to cause a supernova but if it grows by subtracting mass to another star that is its companion and therefore very near, it can become massive enough to explode.

These supernovae have been considered for a long time as space “beacons” with very consistent brightness. For this reason, they were used to measure the distances of their galaxies, based on their brightness, which was considered proportional to their distance. Their observation were also used as a basis for establishing that the expansion of the universe was accelerating.

The research carried out at the University of Arizona shows that things are different from what we thought. Peter A. Milne and his colleagues Ryan J. Foley of the University of Illinois at Urbana-Champaign, Peter J. Brown at Texas A&M University and Gautham Narayan of the National Optical Astronomy Observatory, or NOAO, in Tucson examined a large sample of Type Ia supernovae at visible and ultraviolet light by combining observations of the Hubble Space Telescope and NASA’s Swift space observatory.

The Swift satellite, launched on November 20, 2004, is equipped with three telescopes for observations at very different wavelengths. Its primary purpose is to study gamma-ray bursts but another scientific goal is the observation of phenomena such as supernovae with its Ultraviolet/Optical Telescope (UVOT).

The observations made by Swift were crucial because the differences between the two groups of type Ia supernovae are slim to visible light while are more easily detectable at ultraviolet light. A groups is a bit brighter at blue and ultraviolet frequencies and another is a bit brighter at red and infrared frequencies. This means that the most distant supernovae are less bright than expected because they have characteristics different from what was predicted.

The consequence is that the calculations of the universe expansion based on Type Ia supernovae were incorrect. The acceleration of the expansion could be lower than that calculated. According to the authors of this research that could mean that there’s less dark energy than previously thought.

However, the estimates of dark energy were also carried out by other methods. The analysis of the cosmic background radiation (CMB) is one of the sources of that estimate. Another is given by the phenomenon called Baryon Acoustic Oscillations (BAO), which influences the distribution of matter in the universe.

The agreement is on the fact that we need more data that can allow us to understand the nature of dark energy before its quantity. There are several projects to carry out studies related to this problem and each one can bring new information to solve one of the greatest mysteries of astrophysics of the beginning of the third millennium.

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