[thedoc (OP)]

steve williams asked the Naked Scientists:
   
It's accepted that the Universe expanded from a single point during the big bang, into the ever expanding Universe we observe today; that the further away a Galaxy is the further back in time we are observing it due to the vast distance light has to travel to reach our eye. Edwin Hubble also suggested that Galaxies are moving away from us faster over distance as space expands, due to measurable red-shift.

However, if a Galaxy several billion light-years away (observed as it was several billion years ago) has a greater red-shift than a Galaxy one billion light-years away (observed as it was only a billion years ago); why do we conclude that the expansion of the Universe is speeding up over time?

Why do we not conclude that as red-shift has decreased in the closer Galaxy, that the expansion of the Universe isn't slowing down over time?

Many thanks

What do you think?

[zx16]

The "Big Bang" theory is the currently fashionable view.
But views go in swings and cycles. There's no real evidence for any "Accelerating Expansion".

Much less the ridiculous idea that "The Entire Universe Originated From A Microscopic Pin-Point". I mean really - how absurd can you get!

I confidently predict that by say, 2030, a "Steady-State" theory of the Universe will be the fashion. And much more reasonable.

[Janus]

steve williams asked the Naked Scientists:
   
It's accepted that the Universe expanded from a single point during the big bang, into the ever expanding Universe we observe today; that the further away a Galaxy is the further back in time we are observing it due to the vast distance light has to travel to reach our eye. Edwin Hubble also suggested that Galaxies are moving away from us faster over distance as space expands, due to measurable red-shift.

However, if a Galaxy several billion light-years away (observed as it was several billion years ago) has a greater red-shift than a Galaxy one billion light-years away (observed as it was only a billion years ago); why do we conclude that the expansion of the Universe is speeding up over time?

Why do we not conclude that as red-shift has decreased in the closer Galaxy, that the expansion of the Universe isn't slowing down over time?

Many thanks

What do you think?

It's not the fact that we see an increase in red-shift with distance that indicates an expanding universe, we would see that with a universe expending at a constant rate or even one expanding at an decelerating rate.  This observation was made long before anyone suspected that the expansion was accelerating. What tells us that the universe's expansion is accelerating is how the red-shift changes with distance.

If the universe was exapandiing at a constant rate, we would see a linear relationship between distance and red-shift. at double the distance, we would measure twice the red-shift, for example.  If you plotted the relationship between distance and red-shift, you would get a straight line.  If the expansion is accelerating or decelerating, the same plot would not come out as a straight one, but one that curves away from that straight line. If the universe is decelerating it curves in one direction, and if accelerating, the other direction.   What we have found is that when we make such a plot from the measured red-shifts and distances, we get the curve that points in the direction of an accelerating expansion.

[zx16]

Janus, although you've presented a good case, I think "Big Bang" is due to be replaced soon by "Steady State".

[yor_on]

Janus made a very good explanation, although I think we need to correct the idea of that the universe started in 'One single pinpoint'. The universe is homogeneous and isotropic, meaning that it look exactly the same no matter from where you are, observing it (over a greater magnitude). The same can be said for all physical laws we have. They are the same 'everywhere' as far as physics can prove.

But if you can stand everywhere, finding that the Big Bang is just some 13 billions year away, then that Big Bang was a very 'local' phenomena, measured as if being at the same distance from you,  'everywhere' you might be.
=

Hmm, light years that should be, not years. It's a measure of the distance from you, as well as a temporal definition of 'time'.