do you want the simple or complex answer?
simple:
1. The theoretical physicists answer:
Intuitively, in our span of time, the few millions of years for which we have existed, the rate at which the universe expands will not change a
significant or even measurable rate. (hence the value is constant). Hubble made his discoveries about 80 years ago. How much has the Hubble constant
changed over the course of 80 years, when you are looking at a universe spanning tens (atleast one) of billions of years? To a degree for which we do
not posses the accuracy to measure.
2. The experimental physicists answer:
Perhaps a more mathematical way of putting it, is that the hubble constant is the measure of recessional velocity (the speed of stuff moving away from
us) divided by the distance that stuff actually is, or v / d. Well since we are talking such large distances, -even though its moving away (so its
distance is perpetually changing)- it is speeding up as its doing so. The ratio v / d remains the same, for all objects moving away from us.
Its just one of those quirky facts. That if you take a star/galaxy/quasar say 10 mega parsecs away, measure its speed as its going away from us, and
divide it by its distance from us, that ratio of speed / distance, will be the same for say another object 100 mega parsecs away, which is obviously
moving much faster. (I won't confuse you by talking about factoring in the speed of light etc, I'll just say that the distance to the object does
not change significantly in the time we are making the measurement of its speed, or in our entire lifetimes for that fact
).
to give you an idea of speeds, the expansion
rate is about 74 kilometers per second, per megaparsec (which happens to be about 3.1 x 10 ^ 20
km). which means its getting faster by 74km per second for every mega parsec that it is from us, that value is constant for all objects moving
away.
The simple way to look at it, is to say that we can see, from plotting a curve of speeds vs distances for a lot of objects far away, that you end up
with pretty much a straight line. This means that the gradient of that curve is constant, and that gradient just so happens to be the: hubble
constant. Its called a constant because its measured ratio of: speed / distance is the same for many objects at different distances away from us in
the universe. Constant expansion. Not everything is moving away at the same speed. But everything is getting faster by 74km per second per megaparsec,
as its moving away from us at its own individual speed. Thats (in simple non-quantum mechanics- i can go there later for you lol) a universal
FACT.
so when you look at the hubble time, know that its only thanks to a nice little mathematical cancellation that we can use it to estimate the age of
the universe. It is called a constant because it is measuring the rate (speed per distance) of the universe flying apart.
more complex response, but you should do research into this yourself- as its quite an eye opener:
Time is a tricky thing, its all relative you see. Can we measure the expansion of the universe if we are actually expanding with it ourselves? Are we
not moving away with it at the same rate?
(it turns out we are not expanding at all, but only the space between galaxies/stuff/matter, but its a
trickier concept to get your head around).
the hubble constant is the best evidence we have for the expansive rate of the universe. Within our limited view, that rate
is changing. If we
knew more about the structure of the universe itself and the net force -out- on it causing it to expand (pseudo-particles, quarks, antimatter, stuff
smaller than atoms (electron/positron) --- all possible explanations of the so far elusive dark matter) which is overcoming the attractive force of
gravity and all mass -in- causing the expansion to occur. it's mostly all theoretical, and is very hard to prove. Some conclusions are based on
assumptions, which have already been proven wrong.
I suggest you pick up a nice introductory book to quantum physics, eg something by Stephen Hawking, who writes at a beginner introductory level. And
discover the wonderful, if somewhat pedantic world of physics.
a few edits to make it a little clearer, feel free to post any more questions you have, or if that confused you lol im sorry
[edit on 18/7/05 by painkiller]
[edit on 18/7/05 by painkiller]