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Hi, My question
concerns the change in the rate of expansion of objects in the universe
being a function of their distance from us. I believe the facts
are that the greater the distance from us the faster they are receding.
If the universe were to be expanding all over at the same rate,
like the atoms in a bar of metal when heated uniformly, then pick
any star or atom and view a close neighbour and then compare its
rate of recession with a distant star or atom, we would logically
see the distant object moving faster. My question is, is the change
of the rate of expansion directly proportional to the distance of
the object or atom, relative to the point of observation, or is
it greater? Thanks, in anticipation.
Reply
The rate of expansion of the universe is governed by Hubble's Law,
which states that the velocity is proportional to the distance away
from us, so as you say the more distant objects are receding at
higher rates than closer ones. The constant of proportionality in
this case is called the Hubble Constant, and astronomers have been
trying for years to pin down it's value. I think the current consensus
is that it lies around 50 -75 km per second per Megaparsec, but
is has proven very elusive. The rate of expansion seems to be increasing
(i.e. the universe is now expanding faster than it was in the past),
although the greatest expansion rates occurred almost immediately
after the Big Bang (~13 billion years ago) in what astronomers called
the period of "inflation", so clearly the expansion has not been
uniform since the Big Bang. Exactly what is causing the rate of
expansion to now be increasing is still undetermined, but is thought
to relate to exotic things like Dark Energy. So, the rate of expansion
is directly proportional to the distance, but there does seem to
be some time-dependence to the value of the rate (so it does seem
to change over long time scales, billions of years).
Paul Roche
Final Frontier
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