Inconsequence in Lawrence Krauss lecture

[Gloyn]

Hi!

I just found that vide on Youtube:

http://www.youtube.com/watch?v=veU6hK3jMH4&feature=watch-vrec

And something is not quite ok for me. In ~1:00 Krauss says, that in flat universe model universe would expand slower and slower but would never quite stop expanding. On the other hand, in last 4 minutes he says that energy of the vacuum would would cause accelerating expansion of the universe (which is a feature of open universe, like he says in the begining), which is flat at the same moment. How is that consistent?

 

[George Jones]

Early in the video, Krauss talks about universes for which dark energy/cosmological constant is zero. In this case, a "flat universe model universe would expand slower and slower but would never quite stop expanding."

Later in the video, Krauss talks about universes for which dark energy/cosmological constant is non-zero. In this case, at late times, a flat universe has "accelerating expansion".

 

[Gloyn]

Thanks for answering my question!

Ok, I get this one now, even though I think that he should do some clear remark on that.

Now i come up with another question. Maybe it's lame, but it's bothering me a bit. There is a 'deal' between physicists, that graviataional potential at infinite distance is zero. Let us consider the Universe that is ever expanding without a limit. At some point, if we wait for an infinite period of time, all lumps of mater would be at infinite distances from each other. Therefore overall negative energy of gravitational interaction would 'leak out' of the Universe. How is that?

 

[Chronos]

Inserting infinities into the mix is a good way to get invalid results.

 

[Gloyn]

I mentioned the infinity, but in fact if there is an overall expansion and in general things are getting further and further from each other then energy of gravitational interaction is decreasing even in finite distances. If Univerese was made of three big lumps and the distance between them was increasing, then overall energy of sustem of those three lumps of mater would be decreasing, wouldn't it? I'm aware of the fact that locally sometimes density increases, but all in all Universe becomes less dense if it's meant to expand. So how about it, even without the infinities?