Expert: Jayendra Upadhye - 4/8/2004

Question
I am looking for an explanation of how distance and time to the furthest away known galaxies can be estimated, e.g. how the new Hubble ultra-deep images can be determined to reach to 12 billion years ago.  I have good explanations about red and blue shift, and parallax to measure distance to relatively close stars, and then comparison of absolute luminosity (determined through spectral analysis) and apparent luminosity (through inverse square-of-distance relation) to measure further distances.  But I don't have information about how far can be measured by this method, or if there are other methods.  I am also looking for more information about what is known about the observation that the further away objects are from us, the faster they are moving away.

Any information or references will be greatly appreciated.

Thanks,
Karen

Answer
Hi Karen,
Frankly speaking, I wouldnt be able to give you specifics of the photometry involved (it is as esoteric to me as to you!), but I do know that a sort of inverse square relationship holds over distance for luminosity. Mainly due to the fact that all objects - no matter how they are shaped and how large they are - behave as idealised point sources when viewed over great distances. The wavefronts then are spherical and hence the inverse square relationship. Moving to your question, there should be really no limit on how far can be measured using the red shift method. The limiting factor being the "observable dimness" to coin a phrase. For even ultra violet light can be red shifted to infra-red and further down to radio wave regions! The thing is that by the time that happens, the object would have "luminosity-wise" winked out!
Actually that brings me to the topic of the "light barrier". I mean there is a distance beyond which you cannot see simply because the light from the region has not yet reached you! (after 15 billion years that is!). That is another obvious limit. Not to mention that again after a point the recession speed will approximate that of light! (meaning the red shifting will enter unmeasurably low frequencies..i wonder if i am in quicksand territory now..better to stop!)
Regarding more info about recession speeds, the whole www is at your disposal! Details about why that happens and space-time curvatures and 4 dimensional space-time structure
are abundantly available on the web . use them.
By the way hope you have gone into the "stellar standard candles" topic about the cepheid variables and why they are considered "standard".
Jayen