With the discovery of Cepheid variable stars in the distant Virgo Cluster of galaxies, astronomers using the Canada-France-Hawaii Telescope have settled a long-standing debate as to the distance scale of the universe, a debate which has been raging for decades. The results establish that the Virgo Cluster is at a distance of 50 million Light Years from Earth and that remote objects in the universe are at as little as half the distance previously believed. Virtually all astronomers agree that the Cepheids represent the key to ending the controversy over the distance of remote objects. A variety of other methods have yielded estimated distances to individual galaxies which vary by as much as a factor of two. The Cepheid results strongly favor the closer distances and appear to have settled the controversy. The work is described in the September 29 issue of Nature and is the result of an international effort. This is the first time that these stars have been found at a sufficiently large distance to establish directly the size of the universe.
The discovery is important because the Virgo Cluster is the nearest large concentration of the many different types of galaxies we see throughout the rest of the universe. "For some time astronomers have compared the properties of galaxies in the Virgo Cluster with those found in even more distant clusters of galaxies in order to determine how much further away these clusters are than Virgo", explained Dr. Pierce. "Since we all agree on whether a particular cluster may be, say, three or five times the distance of Virgo the debate has been focused upon the distance to the Virgo Cluster itself. We find the distance of the Virgo cluster to be 50 million Light Years with an uncertainty of only about 8%. Now that we have established the distance to Virgo accurately, the distance to any other cluster and size of the universe follows. We can now establish other properties of the universe, such as its rate of expansion, and place limits on its age."
The newly revised distance to the Virgo Cluster implies that the universe is currently expanding at a rate of 27 kilometers per second for each million Light Years in distance. The current rate of expansion, also called the Hubble Constant, is a key parameter in defining the evolution of the universe over time. One of the more curious results of these measurements is an apparent paradox in the age of the universe. "The age of the universe ends up being between 7 and 11 billion years, depending on the details of the model for its expansion. The best age estimate for the oldest stars is thought to be about 16 billion years, so we have a problem", explains Dr. Pierce. "Either we are missing something in our understanding of the evolution and age of the oldest stars, or we are missing something in our understanding of how the universe has been evolving since the Big Bang. It's going to be very interesting in the next few years while we struggle to unravel this mystery."
"One of the possible interpretations is that the equations developed by Einstein which describe the `Big Bang' may require modification", explained Dr. van den Bergh. "The modification would be to insert a `Cosmological Constant' which Einstein had originally considered and then left out of the final form of General Relativity. It is, perhaps, slightly ironic that Einstein once said that introducing the `Cosmological Constant' had been the greatest blunder of his scientific career since we may have to include it after all."
Cepheids are stars which pulsate in a regular fashion and whose true brightnesses can be accurately determined once their pulsation characteristics are established. Cepheids can be found relatively nearby in our own galaxy, the Milky Way. They have a long history of use in estimating distances and are generally accepted as the most reliable tool used by astronomers for this purpose. "The true brightness of a Cepheid variable star is directly related to the length of time it takes to go through its pulsation, or brightness cycle", explained Dr. Welch. "If we find a Cepheid in a distant galaxy we can measure its brightness over time. Once we determine its true brightness, from its cyclical variations, we can estimate its distance."
The detection of the Cepheid variables in the Virgo Cluster was made possible due to the excellent images produced by a special instrument called the High Resolution Camera (HRCam) on the Canada-France-Hawaii Telescope on Mauna Kea, Hawaii. "This camera produces images which are about three times sharper than most other ground-based telescopes," Dr. McClure explains. "It accomplishes this by correcting for some of the image distortion due to turbulence in the atmosphere above the telescope. By monitoring the position of the image of a nearby star which has been focused onto a light-sensitive detector, a computer can direct a fast-moving mirror inside HRCam to compensate for the `shimmering' of the atmosphere. Pictures of galaxies in the Virgo Cluster obtained with conventional cameras just aren't sharp enough to show the Cepheids."
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