During the 1920s, Hubble and his colleagues observed the galaxy's light spectrum while estimating their distance. In 1929, Hubble concluded that the more distant the galaxies are, the more their light is shifted from blue to red. This law of Hubble led Georges Lemaître to propose that the Universe is expanding and to sketch the theory of the big bang.
Light from distant objects has taken time to reach us. During this time, the expansion of the Universe has lengthened the wavelength of this light. Short wavelengths like blue shift to larger lengths, such as red.
The explosion of stars, supernovas of type IA, are phenomena very bright. Type IA supernovas all produce about the same amount of light. In the 1990s, two teams took advantage of it to measure the distance of very distant galaxies, and they also measured the shift towards red. Both teams publish their results in 1999.
They found that the farthest galaxies are less shifted to red than expected. That means that at the time when the light was emitted, the expansion was weaker than now ... and it is accelerating! This discovery will be crowned by the 2011 Nobel Prize in Physics.
All the matter of the Universe, the visible galaxies as the invisible black matter, produces a gravitational attraction that tends to oppose expansion. These researchers had discovered a phenomenon that compensated for this attraction of matter, and even accelerated the expansion. This black energy constitutes two thirds of the energy density of the Universe and its properties remain very intriguing.
By Fabrice Feinstein