FINDING OUR PLACE IN THE UNIVERSE

By Ronald D. Ferdie, Tim Hunter, and James McGaha

Where are we in the Universe?

Observational results from the Hubble Space Telescope (HST) and advances in other satellite and ground-based astronomy have greatly enhanced our ability for determining distances in the Universe compared to prior times when the largest functional telescope in the world was the Mount Palomar 200-inch. One hundred years ago, the known Universe was the Milky Way. Thirty years age, the known Universe stretched around us some five billion light years with cautious suspicion it stretched perhaps thirteen to fifteen billion light years in space as well as time. Twenty years ago, quasars were considered enigmatic super energy structures. Today, we feel they formed early in the history of the Universe and are energetic galaxy nuclei.

This article summarizes how we determine our place in the Universe by building upon different overlapping yardsticks to measure distances. However, these yardsticks are still built upon a "house of cards" wherein parallax methods used to directly and precisely determine “close” distances to earth are then in turn used to support other increasingly less precise yardsticks for determining distances to far away Milky Way stars and nearby galaxies (Cepheid variable stars, supernova explosions, and planetary nebulae brightness) which in turn are used to support other yardsticks (spiral galaxy surface brightness fluctuations, elliptical galaxy fundamental plane and red shift determinations) for determining distances to remote galaxy clusters and quasars (Sky & Telescope December 1983, pages 516; Sky &Telescope February 2002, pages 18-19).

This house of cards technique for overlapping distance scales allows us to literally take a ruler to the Universe. However, it is fraught with uncertainty and must constantly be re-evaluated. If you change or modify a parameter anywhere in one method of determining distances, then the downstream distance scales are changed, and our view of the Universe can radically change, particularly at its distant fringes.

Also, each method is limited to a certain scale; for example, if the stars of a particular galaxy cannot be individually resolved, the technique for measuring distance by using the spectral classification or absolute magnitude of selected stars in the galaxy cannot be used for this galaxy, and its distance must be inferred by the next, more indirect and less precise method in the chain of distance scales.

The methods for finding our place in the Universe are summarized in an overlapping set of scales: ( see http://www.astro.virginia.edu/~jh8h/glossary/distanceladder.htm)

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