1. Feb 19, 2004

### Sko

My english teacher (we talk about everything in english class) said that the universe, without a doubt, would continue expanding forever. I remember reading in a book tho that there were several theories about this and none was for certain and they depended on the density of the universe which hasn't been found.

I also read in another place that the universe wasn't really expanding, the distances were just stretching.So my question is how does gravity affect this if galaxies aren't actually moving away.

2. Feb 19, 2004

### mathman

There are many theories about the ultimate fate of the universe. For some of the latest ideas see Feb. 2004 issue of Scientific American. The princial observations, leading to the current models, seem to indicate that the expansion of the universe is speeding up.

3. Feb 20, 2004

### Phobos

Staff Emeritus
Welcome to Physics Forums, Sko!

Science is never "without a doubt". But there are different levels of certainty. Based a lot of good astronomical evidence, it's fairly certain that the universe will expand forever. There just isn't enough matter in the universe (at least based on our current data) to stop the expansion and bring it all back into a Big Crunch. There are many competing theories about the fate of the universe, but the consensus is on eternal expansion.

The difference between "expanding" vs. "stretching" is a little subtle and probably not worth worrying about. Sufficed to say, the galaxies of the universe are getting farther and farther apart because of the expansion/stretching of space. Perhaps the discussion you saw was someone trying to explain that "expansion" doesn't mean that the universe is expanding into something else?

Galaxies have 2 kinds of movement (aside from their rotation)...(1) motion through space and (2) movement away from each other due to the expansion of space - - which as you suggest, is not the galaxies moving through space, but rather being carried away by space itself.

The actual motion through space is affected by gravity just like the planets around the sun. Galaxies gather into groups, orbit/collide, etc. Our Milky Way Galaxy is 1 of the 2 large galaxies in a "local group" of 30 or so galaxies (the other large galaxy in this group is the Andromeda Galaxy). The galaxies within our local group interact with each other gravitationally (orbiting/colliding). Our whole local group interacts with more distant groups of galaxies (acceleration toward those other groups).

As far as gravity & the expansion of space...gravity acts to slow the expansion.

4. Feb 28, 2004

Hi, this is my first post and this is what I have always believed the universe to be:

There are 3 types of universes 1, 0 or -1.

In the case where the universe= 1 it means that there is more than enough matter in the universe for it to stop expanding, and actually implode on itself.

If the universe= -1 then, there is not enough matter in the universe and it will expand forever.

But, if the universe= 0 then that means there is just enough matter for the universe to stop expanding and not collapse on itself so it just stays at a constant volume.

I beleive that that our universe= 0 so eventually it will stop expanding and stay at a constant volume.

5. Feb 28, 2004

### clicky

If the unverse expands forever and ends in nothingness, then why did it come to being at all?

6. Feb 28, 2004

### Nereid

Staff Emeritus

Your summary is a concise one, and reflects the cosmological alternatives until some interesting astronomical observations pushed cosmologists to develop the concept of 'dark energy' (or quintessence, or the cosmological constant, or ...). Basically, analysis of distant Type 1a supernovae suggests that the rate at which the universe is expanding is increasing, and started to do so several billion years after the Big Bang.

This now means (if these ideas can be supported by much more detailed observations) that the fate of the universe is quite uncertain!

This Astronomy Picture of the Day gives a nice summary, as well as lots of good links so you can explore the ideas further.
Excellent question! Would you like to give us some ideas how we, as scientists, might one day be able to answer that question? Or maybe this is just the kind of question which an application of the scientific method could never address?

7. Feb 29, 2004

### clicky

The answer is a oscillating "firework universe" that stays always
finite according to Eugene Savov theory of interaction.

Last edited by a moderator: May 1, 2017
8. Feb 29, 2004

### Nereid

Staff Emeritus
Has Savov's idea been published in a peer-reviewed scientific journal? If so, would you mind giving us some references (and links too, if they are available).

I read all five pages of the "Some Predictions" section, but could only see one prediction: that binary stars will (may?) have planets. That was accompanied by a link to a New Scientist article reporting the possible discovery of a planet in a wide orbit around a tight binary. IMHO, this is far from a compelling prediction by Savov; IIRC, such orbits have been discussed in the astrophyical literature for some time, with the earliest possibly before Savov was born (despite the breathless prose of the New Scientist article).

Perhaps Phobos would consider a suggestion that discussion of Savov's ideas be undertaken in Theory Development?

9. Mar 1, 2004

### Phobos

Staff Emeritus
I agree.
Start a new topic there or, if we're hijacking Sko's topic, then I'll move this one.

10. Mar 1, 2004

### Phobos

Staff Emeritus
Welcome Tassadar. Like Nereid said, this is a lot like modern cosmology's Omega. A difference is that with the balance of matter & expansion (Omega = 1 = "flat" universe = 0 in your description), the universe doesn't slow to a complete stop, but instead slows to a stop at Time = infinity. (i.e., never)

The current evidence points to a "flat" universe (like you believe) but as Nereid pointed out, Dark Energy is accelerating the expansion instead of allowing it to slow down.