ar X iv : a st ro - p h / 99 11 29 4 v 1 1 6 N ov 1 99 9 What is the Universe made of ? How old is it ?

For the past 15 years most astronomers have assumed that 95% of the Universe was in some mysterious form of cold dark matter. They also assumed that the cosmological constant, ΩΛ, was Einstein’s biggest blunder and could be ignored. However, recent measurements of the cosmic microwave background combined with other cosmological observations strongly suggest that 75% of the Universe is made of cosmological constant (vacuum energy), while only 20% is made of non-baryonic cold dark matter. Normal baryonic matter, the stuff most physicists study, makes up about 5% of the Universe. If these results are correct, an unknown 75% of the Universe has been identified. Estimates of the age of the Universe depend upon what it is made of. Thus, our new inventory gives us a new age for the Universe: 13.4± 1.6 Gyr. “The history of cosmology shows us that in every age devout people believe that they have at last discovered the true nature of the Universe.” (E. Harrison in Cosmology: The Science of the Universe 1981)