The discussion revolves around the concept of a universe that may re-collapse and the implications this has for the Hubble constant and the nature of time. Participants explore theoretical aspects of cosmology, including the relationship between the Hubble constant, the age of the universe, and the dynamics of expansion and contraction in a cosmological context.
There is no clear consensus among participants. Multiple competing views remain regarding the implications of a re-collapsing universe, the interpretation of the Hubble constant, and the nature of space and time in this context.
Participants highlight limitations in understanding the relationship between the Hubble constant and the age of the universe, as well as the assumptions underlying different interpretations of cosmological observations.
What do you mean by 'go negative' - as in having a negative sign?Orodruin said:In a recollapse the Hubble constant would first decrease to eventually go negative.
Yes. The further away things are in a collapsing universe, the faster they move towards us.Ranku said:What do you mean by 'go negative' - as in having a negative sign?
But isn't that counterintuitive, since gravitational attraction, and therefore acceleration only increases as objects approach each other.Ibix said:Yes. The further away things are in a collapsing universe, the faster they move towards us.
No. It is a direct consequence of homogeneous expansion/contraction.Ranku said:But isn't that counterintuitive, since gravitational attraction, and therefore acceleration only increases as objects approach each other.
You're making the mistake of thinking of Hubble's law as if it were describing the motion of a single point across a range of distances, whereas it describes the state of motion of different points at a moment in time.Ranku said:But isn't that counterintuitive, since gravitational attraction, and therefore acceleration only increases as objects approach each other.
To be clear, negative Hubble constant will increase in value over time in a contracting universe?Bandersnatch said:You're making the mistake of thinking of Hubble's law as if it were describing the motion of a single point across a range of distances, whereas it describes the state of motion of different points at a moment in time.
Start with the moment when an initially expanding universe is decelerated so that it's neither expanding nor contracting (it's at the inflection point, just before the contraction begins).
At this moment, every recession velocity, of any distant point, is zero, and the Hubble parameter is zero.
Then the points start approaching. The point A at distance d from the observer is accelerated inward by all the mass that is contained within a sphere of radius d. Point B at distance 2d is accelerated by all the mass within a sphere of radius 2d. I.e. points further away are accelerated more, so the velocity they gain is greater. Just as Hubble's law describes, only with the Hubble parameter now going negative (which only means the direction being reversed from expansion to contraction).
After some time, those same points will have moved closer to the observer, and will have gained speed. But they still obey Hubble's law, with points twice more distant having twice more speed. It's the changing Hubble parameter (in this case, becoming more negative), that reflects the increasing approach velocity of points A and B as they get closer together.
edit: I should clarify - it's how H is changing, since it'd be changing even in a coasting universe, where all velocities remain constant for all time.
Depends what you mean by "increase". It would be 0 at some time, -1 at a later time, -2 at a still later time, which I would call a decrease. Its magnitude increases, though, if that's what you mean.Ranku said:To be clear, negative Hubble constant will increase in value over time in a contracting universe?
Yes, that's what I meant.Ibix said:Depends what you mean by "increase". It would be 0 at some time, -1 at a later time, -2 at a still later time, which I would call a decrease. Its magnitude increases, though, if that's what you mean.
(Yes there should be units, but it doesn't matter which ones they are here.)
If there is enough matter in the universe for it to re-collapse, the universe would be spatially closed like a sphere. So the re-collapsing spacetime universe will become smaller along with the matter in it growing closer.Imager said:Just clarify for me, is it the "things" in universe grow closer together or is it space that is that is getting smaller?
Should I should said space-time?
That isn't either-or. "Things get further apart/closer together" is a direct observation you can make by bouncing radar pulses off distant galaxies (in principle anyway - there are a few practical challenges). The question is how you interpret that - as "space expanding/contracting between stationary objects" or as "objects moving through space". There isn't a right answer to that. The usual interpretation is the first one and it has a lot of advantages, notably that it directly reflects the "everything's the same everywhere" cosmological principle. But the other isn't wrong.Imager said:Just clarify for me, is it the "things" in universe grow closer together or is it space that is that is getting smaller?
Definitely not. Spacetime includes space now, space in the past, and space in the future. It doesn't change, it just is. In fact, the choice of interpretation above is a choice of different ways to slice ("foliate" is the technical term) 4d spacetime into a stack of 3d "space at one time" slices.Imager said:Should I should said space-time?