The cosmological Big Rip and a shrinking hubble radius

[fet2105]

The cosmological "Big Rip" and a shrinking Hubble radius

Doesn't an expanding universe forecast such outcomes as a faded and unobservable cmb, galaxies moving away from each other greater than the speed of light, a gravitationally unbound solar system, and eventually an observable universe which is microscopic in size resulting in ultimately a "Big Rip" ? If so, does this necessarily imply a shrinking Hubble radius?

 

[Mordred]

The big rip was one proposal but its no longer considered as a fate of the universe. We are rather headed towards heat death if expansion stays the same. Galaxies are not moving anywhere near the speed of light and will never move at the speed of light. What pop media articles are referring to by the statement this galaxy is moving at 3c. what they are really referring to is the recessive velocity to distance relation.

Hubbles law states the greater the distance the greater the recessive velocity. This is an observer dependent relation. the article on this thread will explain that for you

https://www.physicsforums.com/showpost.php?p=4437226&postcount=2

 

[Naty1]

All of this is model dependent on the cosmological model...reigned in [parameters constrained] where necessary by observational evidence:

Doesn't an expanding universe forecast such outcomes as a faded and unobservable cmb, galaxies moving away from each other greater than the speed of light,QUOTE]

ok for an unending accelerating expansion...which is a bit of a different concept than distance..

a gravitationally unbound solar system

If by this you mean cosmological constant [energy] becomes dominant as distances between mass [like galaxies] continues to expand and therefore gravitational attraction diminishes, yes, that has been underway since roughly 7 billion years ago. So it appears we are now in an 'energy dominated universe'.

and eventually an observable universe which is microscopic in size resulting in ultimately a "Big Rip"

?

oops, never heard that..any source you to which you can link?? That's not mainstream cosmology today.

If so, does this necessarily imply a shrinking Hubble radius?

In brief, the Hubble parameter [H] is believed to be decreasing, but the radius continues to expand.

but it's a bit tricky explaining speed, distance, time is the expanding cosmological spacetime...
Here are two solid explanations from others in these forums which I found helpful:

[The context of the first is that at the Hubble radius, objects are moving away at the speed of light, so momentarily light seems to be at a fixed distance from us.]

Our universe has an expansion rate (Hubble parameter H) that is slowing down. So, after some amount of time, after the light ray had traversed some distance, eventually the expansion rate slowed enough that the light ray started to make headway against the expansion, finally reaching us billions of years later. But the galaxy that emitted that light was further away still: it wasn't traveling away from us at the speed of light; it was just sitting where it always was among its local galaxies. So even though the expansion rate slowed enough that the light ray could eventually get to us, it didn't need to slow enough [along the route of transmission] for that galaxy to stop receding at faster than the speed of light.

[This next post is relative to the Hubble recession velocity v = HD. D is understood to be the distance “now” (at some moment) and v the current rate that distance is expanding.

Current evidence suggests the expansion [rate] of the universe is accelerating, meaning that for any given galaxy, the recession velocity dD/dt is increasing over time as the galaxy moves to greater and greater distances; however, the Hubble rate [parameter] H is actually thought to be decreasing with time, meaning that if we were to look at some fixed distance D and watch a series of different galaxies pass that distance, later galaxies would pass that distance at a smaller velocity than earlier ones.

[On second thought, I think I got the second explanation from Wikipedia..]Lots of good Q & A at Ned Wright's site:

http://www.astro.ucla.edu/~wright/cosmology_faq.html

 

[phinds]

Doesn't an expanding universe forecast such outcomes as a faded and unobservable cmb, galaxies moving away from each other greater than the speed of light, a gravitationally unbound solar system, and eventually an observable universe which is microscopic in size resulting in ultimately a "Big Rip" ? If so, does this necessarily imply a shrinking Hubble radius?

Uh ... "universe which is microscopic in size" ? Oh ... I guess you mean there will be nothing left bigger than microscopic particle. No ... entire galactic clusters may hang together. Certainly there is no evidence that anything as "small" (on cosmological scales) as a galaxy will be affected, but you are certainly right about the unobservable CMB and in fact the total lack of ANYTHING observable outside of our galaxy since all others (except perhaps the local group) will have eventually receded out of detectable range.

 

[Naty1]

A note on the Hubble parameter that is not always made clear...

The Hubble parameter is currently, right now, a 'constant'. That means it is the same in all directions based on the assumptions of the favorite cosmological model...the FLRW or FRW model.

But it varies over time: As noted in the explanations I posted, it is decreasing currently...

And you can calculate much of the sort of stuff...You can probably find such in the link Mordred posted.

Marcus of these forums explained elsewhere:

The Hubble law v = HD says that on large scale galaxies are currently increasing [proper] distance at a percentage rate of 1/140 of a percent every million years. That [increase] is expected to slow to about 1/160 of a percent every million years.

elsewhere, somebody posted the Hubble constant is asymptotically approaching a constant value...which leads approximately to the 1/160 increase.

 

[Naty1]

pHinds

No ... entire galactic clusters may hang together.

In one sense this is true, but I would explain it differently...eventually if expansion continues long enough, all the galaxies matter will be absorbed by black holes. Those BH may all coalesce or may not, but as the universe cools [Mordred calls it 'heat death expansion' above those black holes will eventually evaporate...in a disappointing 'poof' as Chalnos of these forums expressed it.

 

[fet2105]

Ok, so what am I missing here? . . . .
1) The Hubble radius is defined as the distance at which objects are moving away from us at the speed of light.
2) The rate at which the universe is expanding is accelerating causing objects objects to move faster as time goes on.
3) In my mind, as a consequence of points 1 and 2, I imagine the Hubble radius contracting.

 

[phinds]

pHindsIn one sense this is true, but I would explain it differently...eventually if expansion continues long enough, all the galaxies matter will be absorbed by black holes. Those BH may all coalesce or may not, but as the universe cools [Mordred calls it 'heat death expansion' above those black holes will eventually evaporate...in a disappointing 'poof' as Chalnos of these forums expressed it.

Ah ... I was overlooking that, possibly because I was not aware that it was established fact that the black holes WILL eat the galaxies. Has this really been confirmed as a probable outcome? I had thought that they wouldn't.

 

[marcus]

Ok, so what am I missing here? . . . .
1) The Hubble radius is defined as the distance at which objects are moving away from us at the speed of light.
2) The rate at which the universe is expanding is accelerating causing objects objects to move faster as time goes on.
3) In my mind, as a consequence of points 1 and 2, I imagine the Hubble radius contracting.

Don't let the words confuse you, the word "acceleration" can give people the wrong idea. When they say expansion is accelerating cosmologists are talking about the standard cosmic model in which the Hubble radius is NOT expected to ever get smaller.

R is currently 14.4 Gly (billion light years) meaning large-scale distances expand 1/144 of a percent per million years
R is expected to continue increasing towards a limit of 17.3 Gly meaning in far distant future distances will be growing at a rate of 1/173 % per million years. (no "big rip" stuff)

The percentage growth rate is actually DECLINING. So why do cosmologists say that expansion is accelerating?

they say that because the decline in percentage growth rate is so slow that if you watch a particular distance between two widely separated objects it will be showing nearly exponential growth at a nearly constant percentage rate. As the distance grows in size, the actual speed it is expanding grows.

It's like you put money in a savings account at some fixed percentage interest. Your principal keeps increasing so the yearly DOLLAR gain keeps increasing. That's a kind of acceleration in terms of yearly dollar gain. Now imagine that the bank very very slowly reduces the percentage interest rate that it pays. Your account could still be "accelerating" because the principal is growing, and the percentage rate is almost constant.

To see what the standard cosmic model says about change of Hubble radius over time, go here:
http://www.einsteins-theory-of-relativity-4engineers.com/LightCone7/LightCone.html

and click on "set sample chart range" and then click "calculate"

T is the year and R is the Hubble distance. The present is year 13.8 Gy
The table will start at around 0.067 Gy, that is at year 67 million when R is quite small (distances are growing a full 1% per million years)
The table will go up to the present T=13.8 Gy and then continue on a few billion years into the future and show R beginning to converge to its eventual limit of 17.3 Gly. (eventual growth rate 1/173 % per million years.)

 

[Naty1]

R is expected to continue increasing towards a limit of 17.3 Gly meaning in far distant future distances will be growing at a rate of 1/173 % per million years. (no "big rip" stuff)

That must be the update to the prior [older] 1/60 % I posted earlier...darn scientists keep changing the numbers!

me:

all the galaxies matter will be absorbed by black holes.

and go 'poof'...

pHinds

Has this really been confirmed as a probable outcome?

Absolutely, positively, without any doubt...NOT confirmed...[LOL]

I checked Wiki:
Wiki says heat death is 'most commonly accepted'...here
Heat death of the universeand here, Wiki implies black holes will be the last to go...

http://en.wikipedia.org/wiki/Heat_death_of_the_universe#Time_frame_for_heat_death

How 'mainstream'? I have no idea...I have a knack for infuriating a few 'Gods of the
forums" so maybe one will appear here to pummel me...If Marcus did not comment adversely, likely such an idea is at least in the ballpark...[I have to 'cool it' until some infractions expire...hopefully that will be before the end of the universe!]

 

[timmdeeg]

Ok, so what am I missing here? . . . .
1) The Hubble radius is defined as the distance at which objects are moving away from us at the speed of light.
2) The rate at which the universe is expanding is accelerating causing objects objects to move faster as time goes on.
3) In my mind, as a consequence of points 1 and 2, I imagine the Hubble radius contracting.

You are perhaps missing the consequences of Hubble's law, which others already mentioned. The recession velocity v of a co-moving object is proportional to it's proper distance r, v = Hr, with H the Hubble parameter. In the case of a photon the proper distance coincides with the radius R of the Hubble sphere, R = c/H. As H decreases in an expanding (and even in an accelerated expanding) universe, R increases. Should the universe expand exponentially, then H and R will asyptotically approach constant values. So, R will not shrink. This is true, if the the w-parameter > -1.

With unlimited growth of antigravitating energy (w < -1, big rip case) the horizon becomes zero, as you said. That's unlikely, but to my knowledge not yet definitely excluded.