Re: boundary of the universe... for dummies.
- From: PD <TheDraperFamily@xxxxxxxxx>
- Date: Thu, 16 Oct 2008 06:12:37 -0700 (PDT)
On Oct 16, 7:02 am, ronybc3 <rony...@xxxxxxxxx> wrote:
Hello sci.phy people... i'm not a scientist but have keen interest to
watch science stuff. I do have a question that puzzles me since my
childhood:
what will be there beyond the boundary of the universe..?
Let's stop right there. [rest based on this, snipped]
There isn't a boundary to the universe.
Now your next question will be, how can something that started at a
point not have a finite size?
To understand this, I'll propose an analogy.
Suppose you came upon an infinitely long ribbon. No endpoints to the
ribbon, no boundaries.
You see there are marks on the ribbon, one labeled A, another labeled
B, another C, and so on. You are standing on a mark labeled OO.
The distance from you to A is 2 meters.
The distance from you to B is 3 meters.
The distance from you to C is 7 meters.
The distance from you to D is 11 meters, and so on.
Then you notice that the ribbon is stretching, so that the mark A is
receding from you at a rate of 0.2 meters/year. This means that a year
from now, mark A will be 2.2 meters away from you.
Then you notice that mark B is receding from you at the rate of 0.2
meters/year, so that a year from now it will be 3.3 meters away from
you.
Then you notice that mark C is receding from you at the rate of 0.7
meters/year, so that a year from now it will be 7.7 meters away from
you. It occurs to you that today mark C is 4 meters from mark B, and
that C is receding from B at a rate of 0.4 meters/year (because next
year mark C will be 4.4 meters from mark B).
And it dawns on you that all the marks are receding away from each
other at a rate of 0.1 meters per year per meter of separation. The
further the mark is from your point of observation, the faster it is
receding from you.
And sure enough, you look at mark D, and it is receding at 1.1 meters/
year from you, at 0.4 meters/year from mark C, at 0.8 meters/year from
mark B, and so on.
In a flash of insight, it also dawns on you that the spot you're
standing on (OO) is not special. From ANY mark on the ribbon, the
other marks will be seen to be receding at exactly the same rate of
0.1 meters per year per meter of separation. Every place on the ribbon
is just like any other point. There is no center to this infinitely
long ribbon.
Finally, in another flash of insight, it dawns on you that you could
run the clock backwards. Presently mark A is 2 meters away from you
and it is receding at a rate of 0.2 meters/year, which means that 10
years ago (if that rate were constant) mark A would have been right on
top of mark OO. Presently mark C is 7 meters away and receding at a
rate of 0.7 meters/year, so that 10 years ago mark C would have also
been right on top of mark OO. And looking way out at mark M, which is
presently at 395 meters away from you, you see that it is receding at
39.5 meters/year, which means that 10 years ago, mark M was *also*
sitting right on top of mark OO. And in fact, because this rule
applies to any mark on the infinitely long ribbon, no matter how far
away, you can reasonably conclude that ten years ago, the all the
points on the entire *infinite* ribbon were on top of each other.
Chew on that for a while.
PD
.
- Follow-Ups:
- Our Horizon is currently 45 giga light years away and receding.
- From: Jeff▲Relf
- Our Horizon is currently 45 giga light years away and receding.
- References:
- boundary of the universe... for dummies.
- From: ronybc3
- boundary of the universe... for dummies.
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