Stretching space
Unclevodka
I have a question, I hope it is not too basic.
I understand that the universe is expanding - red shifts of distant
galaxies tell us this.
I understand this is not that they are just moving away, but that
space itself is expanding.
Am I right in this?
If so:
Is space only expanding on an intergalactic level (or inter galaxy
cluster level?) - ie is this new space only appearing between galaxy
clusters?
Is "Dark Energy" supposed to be responsible for this?
Roberto Trotta
You are absolutely right about the expansion of space between us and
distant galaxies. The analogy that is often given is the one of an
inflating balloon with pennies glued on it, representing galaxies.
Although the galaxies themselves do not expand (because the
gravitational attraction of their mass prevents space from expanding),
space does stretch between one galaxy and the next. What is more, any
observer in any one galaxy will see all other galaxies receding from
her/him.
It is not really "new space appearing", rather then the fabric of
spacetime stretching. Now the expansion itself has been set in motion
with the Big Bang, and if the Universe contained only matter (both
ordinary and dark) then the expansion should be slowing down, because
gravitational attraction between galaxies would act aginst the
expansion. But if dark energy is around (and we believe that actually
70% of the energy density in the Universe is made of dark energy) then
the expansion does not slow down, but it actually accelerates,
inflating the distance between galaxies faster and faster. So dark
energy is not responsible by itself for the expansion, but is is
responsible for the acceleration in the expansion that we are
presently witnessing through a variety of cosmological observations.
Unclevodka
Hi again. I thought I replied, but I guess the message ended up in a
parallel universe or something....
Just to get this straight in my head, I assume that space stretches
between galaxy clusters rather than actually galaxies?
Secondly I want to get my head round the space/time stretching
concept. Do we know the physics involved in this? What I mean is that
if we think of galaxies moving/accelerating away as if they were in
space, then we have to evoke dark energy to explain this.
But actually they are not moving away in space, they are, as we say,
stretching space. That is not the same thing, and do we know how much
energy is involved in stretching space as opposed just to moving in
space?
Roberto Trotta
> between galaxy clusters rather than actually galaxies?
gravitationally bound with each other.
> Secondly I want to get my head round the space/time stretching
> concept. Do we know the physics involved in this? What I mean is that
> if we think of galaxies moving/accelerating away as if they were in
> space, then we have to evoke dark energy to explain this.
> But actually they are not moving away in space, they are, as we say,
> stretching space. That is not the same thing, and do we know how much
> energy is involved in stretching space as opposed just to moving in
> space?
Energy conservation is a tricky concept in General Relativity. Due to
the expansion of space, object that stay at the same
"coordinate" (think of this as if pennies were glued at a given
coordinate on the surface of the expanding balloon) see their mutual
physical distance increase. One could compute the energy necessary to
keep the objects at the same physical distance despite the expansion,
ie the energy you need to counteract the expansion (ie, compensate the
stretching of space by actual movement in space). I suppose this could
be used to define an "equivalent energy" due to the expansion. I dont
think however that this can be used in any way to harvest energy, for
example.