From: roberts@cmr.ncsl.nist.gov (John Roberts)
Subject: Re: sticky questions
X-Added: Forwarded by Space Digest
Organization: National Institute of Standards and Technology formerly National Bureau of Standards
Original-Sender: isu@VACATION.VENARI.CS.CMU.EDU
Distribution: sci
Date: Tue, 17 Nov 1992 04:34:33 GMT


-From: ednclark@kraken.itc.gu.edu.au (Jeffrey Clark)
-Subject: Two sticky questions on astrophysics
-Date: 16 Nov 92 12:21:34 GMT
-Organization: ITC, Griffith University, Brisbane, Australia

-This may display my ignorance but:

-1. Nothing can travel faster then the speed of light. Therefore
-gravitational influence takes time to travel. Therefore the influence of
-objects on the other side of the galaxy are being felt in our solar system
-as those far flung objects were some 80,000 years ago, yes? More to the
-point the massive centre of our galaxy (possibly contains a mega-black hole)
-will not influence us from it's current position for another 30,000 years.
-Now this (according to my naive musings) should not present a problem if we
-are orbiting the centre of our galaxy in a near perfect circular orbit, but
-I would surmise that our solar system would have some eccentricity in it's
-orbit. According to me the solar system is falling toward a non-existant
-centre and has been doing this (as all galactic objects do) since the
-beginning of galactic history.  Should this not cause orbital deviations
-that are measurable? Can someone help me out here please am I missing some
-obvious relativistic point?

Ignoring things like spin, the gravitational field of a black hole (as seen
from outside the black hole) should be virtually indistinguishable from that
of an equal but uncollapsed mass.

-2. An object is detected 15 billion light years away, pushing the beginning
-of time to at least that many years ago. But surely it pushes that time to
-double 15 billion years (ie 30 billion years). Nothing can travel faster
-than light. The object that generated that radiation did so 15 billion years
-ago from 15 billion light years away. But first we had to get 15 billion
-light years away from this object. Both the object and the particles that we
-consist of must have been together at the Big bang. In order for the light
-to have taken 15 billion years to reach us, the object must have been 15
-billion light years away from our current position 15 billion years ago. In
-other words the earth and the object relative to each other must have been
-travelling for some 15 billion years (at least) to get that far apart before
-the light was emmitted from far-flung object. Once again am I missing some
-obvious relativistic point or have I just doubled the age of the universe?

George Smoot, well-known researcher on the COBE team that measured the cosmic 
background radiation of the universe, gave a lecture on the findings this 
spring in Washington, D.C. After the lecture, I asked him about the "drift" 
of our galaxy with respect to the "uniform" (isotropic?) glow. If I understood
his answer correctly, he said that shortly after the big bang, the universe
was much larger "across" than the age of the universe times the speed of
light. (In other words, space expanded along with everything else?)
So there may well be parts of the universe that are more light years away
now than the age of the universe in years, though we can never directly
know of their existence. If that's right, then it's not unreasonable to
have a young galaxy just fresh from the big bang, but still 10-15 billion
light years from our galaxy.

John Roberts
roberts@cmr.ncsl.nist.gov