In 1998, two groups of astronomers, set out to determine the deceleration of the universe by measuring the recession speeds of type la supernovæ (SN1a), came to an unexpected conclusion: its expansion rate has been speeding up. To justify this acceleration, they suggested that the universe does have a mysterious dark energy and they have proposed a positive cosmological constant consistent with the image of an inflationary universe. They have bet on their distance revised upwards to explain the observed dimming of high-redshift SN1a. We consider that an accelerated expansion leads right to a «dark energy catastrophe» and we suppose rather that the universe knows a slowdown expansion under the positive pressure of a dark energy, otherwise called a variable cosmological constant. The dark luminosity of the latter would be that of a «tired light» which has lost energy with distance. We propose that the SN1a luminosity continually fades by interactions with cosmic magnetic fields, like the earthly PVLAS experiment which loses much more laser photons than expected by crossing a magnetic field. It goes in the sense of a «tired light» which has lost energy with distance and, therefore, a decelerated expansion of the universe.