Expanding Universe: Mass or Space-Time?

[Moore1879]

Okay, I understand that the universe is expanding. I also understand that in 1998 Cosmologist discovered that the Universe is accelerating rather than decelerating. My question is: "By expanding, do they mean that the mass in the universe is becoming more spread out, or is space time actually stretching out?" The idea that space-time itself is stretching out doesn't seem right to me. I'm just wondering.

Thank You

 

[Garth]

Space. not space-time is expanding with time within space-time. You have to foliate (slice) space-time into hyper-surfaces of space each separated by a time duration.

You need a special frame of reference to make that foliation and such is provided by the cosmological principle of homogeneity and isotropy.

In one special frame of reference, the one co-moving with the cosmological representative fluid, a homogeneous universe will look isotropic.

In the standard GR model, as time progresses, that hyper-surface of events 'simultanteous' with the observer, expands.

This means as you say,

the mass in the universe is becoming more spread out.

I hope this helps,

Garth

 

[hurk4]

I suppose that in a hole, with a event horizon beeing constant (for a very long while), there is/was a kind of singularity which is expanding as far as we can observe (in our observable universe).
Starting at some time the singularity changed from radiation behaved to baryon behaved. In the mean time our hole is changing from a white hole into a black hole. In this hole, specialness from high to low, Blackhole are forming en clustering, entropy is going from low to high. Entropy will reach a value dictated by the Bekenstein-Hawking formula Sbh=A/4 . (Planck units 10^123). (see Penrose book "The road to reality"). That will not be the end of the story because black holes will evaporate in the spacetime of a multiverse. I suppose that in a multiverse a kind of concentrated radiation singularities will form serving as new bigbangs. (In such formation process entropy goes down). This way of thinking makes it possible for me see reality as a autonomous process without creation, begin or end. Of course this is pure speculation, but creation for me is even more speculative.