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Hubble Expansion and Speed of Galaxies
Name: Daniel
Status: educator
Grade: 9-12
Location: AL
Country: N/A
Date: 10/18/2005
Question:
I have a question that has been gnawing at me
for sometime, and I can not find the answer on the Internet or in the
library. Please help me.
Hubble compared the distances to the velocity with which the galaxies
were speeding away, and explained that the farther away the galaxy, the
more rapidly it moved. This relation, known as Hubble's Law, was proof
that the universe was expanding.
Now consider, Einstein's Law of Relativity which states that
light in a vacuum travels at a constant speed of 299,792,458 meters per
second.
For instance, if we viewed a galaxy 10 billion light years away; we are in fact seeing the galaxy as it appeared 10 billion years ago. Are we
not seeing the speed of this galaxy racing away 10 billion years ago?
This does not mean that it is traveling faster now, only in the distant
past. This could be a result of being nearer the time of the Big Bang. In
fact, this could cast doubt on the Big Rip Theory if it is traveling
slower now. Has this been taken into account, or have I missed something?
Replies:
The story of the cosmology of the Universe is fascinating, but far too
long to address here. In addition, it becomes mathematically messy when
looked at in "detail". There are some misconceptions that arise because
that story is counter-intuitive. For example, "being nearer the time of
the Big Bang" seems to imply that we have a watch and can say when the
"Big Bang" occurred. That is not quite accurate because prior to the "Big
Bang" there was no such thing as time (so far as we know). The "Big Bang"
creates time and space. The finite speed of light does mean that there is
an event "horizon". that is there may be some galaxies whose light has not
yet reached us (we have only been observing distant galaxies for about 50
years). Presumably, there may also be galaxies that we have not observed
because they have "burned out" before we ever got a chance to observe
them. Distant galaxies do not appear to be slowing down. In fact the speed
of recession from our view (Earth) appears to be accelerating, and all are
moving away from us and one another, so that the average density of the
Universe appears to be continually decreasing. Recent measurements and
analysis show that the distant objects in the Universe are "younger" than
the Universe itself (which is measure by cosmic background microwave
radiation first observed by Penzias and Wilson), so there can be an event
horizon i.e. galaxies "out there" whose light has not yet reached "us".
I recommend a very readable book, "Big Bang: The Origin of the Universe" by
Simon Singh who has a talent for making difficult ideas approachable.
Vince Calder
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Update: February 2012
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