Astronomers were able to obtain for the first time more precise distances, and a more accurate value of Ho. So in 1993, the orbiting Hubble Space Telescope began a "key project" to obtain distances to the Cepheids in 18 galaxies. Up until the 1990's, the best estimates for Ho were between 50 km/s/Mpc and 90 km/s/Mpc, giving a range on the age of the universe between 7 and 20 billion years. However, the accuracy of the distance measurement was hampered by how faint ground based telescopes could see. The distance to the galaxy can be determined using observations of certain types of pulsating stars, called Cepheids, whose instrinsic brightness is related to the period of their brightness variation. The velocity could be determined via the redshift in the spectrum. Astronomers further determined that the age of the universe is related to Hubble's constant, and that it is between 1/Ho and 2/3Ho depending on cosmological models adopted. By independently measuring the velocity and distances to galaxies, the value of Ho could be determined. He found that galaxies which are further away are moving at a higher speed following the law, v=Hod, where v is the velocity in km/s, d is the distance in Mpc, and Ho is the Hubble constant in km/s/Mpc. In the 1920's Edwin Hubble discovered the expansion of the universe. How did this come to be? Early estimates of the Age of the Universe But with advances in technology and the development of new techniques we now know the age of the universe is 13.7 billion years, with an uncertainty of only 200 million years. Before 1999, astronomers had estimated that the age of the universe was between 7 and 20 billion years.
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