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Our estimate of the age and size of the Universe has changed during the past century. Cosmic Times reflects this through the "Age of the Universe" and "Size of the Universe" boxes in the upper left and upper right of each edition. The Size refers to the diameter of the known universe.
Here we describe these estimates.
At the time, some astronomers thought that the Milky Way comprised everything in the Universe. As described in the article Mt. Wilson Astronomer Estimates Milky Way Ten Times Bigger than Thought, Harlow Shapley studied the distances to globular clusters to determine the size of the Milky Way Galaxy to be 300,000 light years across.
In 1924, Edwin Hubble determined the distance to the Andromeda Nebula to be 900,000 light years. By 1929, he had measured the distances to 24 additional spiral nebulae in his study to determine distances to the galaxies for which Slipher had previously determined redshifts. The farthest was 140 million light years away, making the universe 280 million light years across.
One result from Hubble's discovery of the relationship between the recessional velocity and distance to distant galaxies is that the constant which defines that relationship is also related to the age of the universe. If the universe has been expanding, and Hubble's constant gives the expansion rate, then its inverse gives the amount of time that the expansion has been going on. Hubble's initial value of this constant gives an age of the universe of 2 billion years. Interestingly, at this same time geologists had determined the age of the Earth to be 3 billion years.
As a result of the recalibration of the Cepheid distance scale and of the new results from the 200-inch telescope at Mt. Palomar, the size of the Universe increased to 4 billion light years by the mid-1950's.
In 1952, Walter Baade redetermined the value of Hubble's constant to be much lower than what Hubble had estimated. As a result, the Universe was found to be about 6 billion years old.
The farthest objects in 1965 were the quasars. The most distant known quasar, named 3C9, was found to be about 12 billion light years away. This gives a size for the universe of about 25 billion light years.
In 1958, Alan Sandage again lowered the value of Hubble's constant, but ended up with a range of ages for the Universe between 15 and 25 billion years. As of 1965, this uncertainty remained, since subsequent studies by a variety of astronomers found different values within this range.
Quasars continue to define the size of the universe into the early 1990's. Quasars had been found with recessional velocities nearly 90% the speed of light, giving distances of 15 billion light years. This gives a size of the universe of 30 billion light years across.
The value of Hubble's constant remained uncertain, giving a range in age for the universe of 12-20 billion years.
The most distant objects in the Universe are 47 billion light years away, making the size of the observable Universe 94 billion light years across. How can the observable universe be larger than the time it takes light to travel over the age of the Universe? This is because the universe has been expanding during this time. This causes very distant objects to be further away from us than their light travel time. For additional information, see Ned Wright's Cosmology FAQ.
The Hubble Key Project, conducted by the Hubble Space Telescope from 1991 to 2000, nailed down the value of the Hubble Constant and hence the age of the Universe. Results from the WMAP satellite further confirmed and refined the age of the Universe to be 13.7 billion years.
Cosmic Times is a product of the Imagine the Universe! website. Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan P. Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.
The Cosmic Times Team