# Universe expansion

1. Jan 17, 2005

### woz

I have read (I don't remember where) that the expansion of the universe is not as an explosion, what happens is that the space itself is expanding.

Can someone please make this clear to me and explain exactly who this expansion works?

thanks
art

2. Jan 17, 2005

### DB

That's right, space itself is expanding leading to the galaxies expanding futher away from eachother aswell. The Doppler effect and Hubble's Law are key to understanding the universe expansion. As we measure (through time) the wavelenghts of light being emmited from distant galaxies, we notice that they are constantly redshifted. We also realized that the galaxies were receding at a velocity proportional to their distance. Basically, The futher distant the galaxy, the greater its redshift, and therefore the higher the velocity. This is how we apply Hubble's Law
$$v=H*D$$
v is the redshift
H is Hubble's constant
D is the distance (at that point in time)

*One would normally think that for instance the Andromeda Galaxie would be receding from us, but at the "close" distance between Andromeda and the Milky Way, gravitational forces pull us closer together. But we are still receding through space, it's expansion. Therfore we are futher away from where we were 10 000 years ago and so on.

3. Jan 17, 2005

### Chronos

Expansion is merely things moving farther apart as a function of time. You cannot see space expand, merely the change in distance between stuff you can see. You also cannot view the universe as a whole and compare to something else to arrive at a measurement of size. There is nothing else to compare it to. Even a spatially infinite universe can still expand by making stuff move farther apart over time.

4. Jan 18, 2005

### cosmoboy

this is right that the expansion of the universe is not an explosion. explosion is always localized to somewhere, that is
not true for the universe.

5. Jan 19, 2005

### Phobos

Staff Emeritus
Probably not. I'm not sure the human mind can visualize an expanding infinity with no external reference frame.

But what we do see is the galaxies in the universe getting farther apart from each other....not in any preferred direction and not just because of their own proper motion. Instead, more space is appearing in between these galaxies (which is why its said space is stretching/expanding...even though that wording has the incorrect connotation of an expansion into something...our common experience just doesn't cover the weird situation of the universe).

There are some analogies which can help. They're imperfect, but they help.

One is the "balloon analogy". Imagine yourself as a 2D (flat) creature on the surface of a balloon. As a 2D creature, you can't see up or down....only left or right in space. You can travel in any direction and never find a center or edge to the balloon. The balloon material represents 3D space of the universe. As the balloon expands, you see points on the balloon getting farther apart...and no particular point is at the center. One of the imperfections in this analogy is that you immediately picture a 3D balloon expanding into the air of the larger room its in. But you need to force yourself to understand there is no outside or inside to the balloon.

The other is the "raison bread analogy". Imagine a rising loaf of raison bread. The bread represents 3D space and the raisons represent galaxies. As it rises, the raisons move further apart from each other with no preferred center or edge. This analogy has a similar imperfection as the other. Here, you need to imagine that the bread is infinitely large (with nothing exterior to it).

Where is all this extra space coming from? Good question. Maybe someone else can take a stab at that. But since the universe is made of "spacetime" (not just "space")...it may be that the full extent is already in existence in spacetime and we're just observing it's current phase as we move through spacetime one second at a time (although we can move freely in 3D space, we're limited to a one-way timeline).

Of course, there are speculations about other dimensionalities to the cosmos which does allow for our universe to expand relative to something else....but there's no well supported theory based on the available evidence.

6. Jan 19, 2005

### RandallB

On the expansion issue - an item of interest.
Does anyone know of a site that may summarize the current thinking of how much mass has been accounted for; And what may be suspected (or wished for)

Galaxies:
-Stars
-planets
-Black-holes
- other galaxy mass

Other mass
- dust not in galaxies
- Dark matter
- Other?

As I understand, enough mass has not been accounted for to indicate that expansion might reverse.

7. Jan 19, 2005

### DB

Yes, today as we know it, I believe there is not enough mass to space for the universe to collapse. This is because the universe is below the critical density. I found this link: http://www.opencourse.info/astronomy/introduction/35.universe_structure/
I think most people today beleive that the Universe is hyperbolic.

Last edited: Jan 19, 2005