What would happen if the universe stopped expanding?

[astro geek]

The Universe is constantly expanding and as a consequence some very distant stars are moving away from us at speeds greater than the speed of light. This means that these stars will never be seen by us. If the Universe stopped expanding, or worse, started shrinking, the radiation from those distant stars would reach us. This is the case for every point in the Universe. Would the Universe be destroyed along with the Earth and all of humanity?

[Martin]

The Universe is constantly expanding and as a consequence some very distant stars are moving away from us at speeds greater than the speed of light. This means that these stars will never be seen by us...

 

What you say here contains a misunderstanding. One thing you could do is read a popular article by two Aussie scientists, Charles Lineweaver and Tamara Davis, that was originally published in the Sci. Am. of March 2005.

 

I have a link in my signature at the end of the post. Anyone who calls him or herself "astro geek" should certainly know everything in that article. It is basic stuff about the standard cosmo model, with good pictures and diagrams.

 

If you have trouble with the link, or any questions, please let us know.

 

MOST OF THE GALAXIES THAT WE SEE were receding from us faster than the speed of light when they emitted the light that we are now receiving from them.

 

If you don't see how that works, read the Lineweaver article. If you still don't understand, ask here.

 

Also most of the galaxies that we see and photograph with telescopes etc are NOW receding at rates greater than c.

 

=================

 

I have to go out, I'll check back later.

 

There are models where the universe expands, stops expanding, and starts contracting more or less at the same rate. It takes tens or hundreds of billions of years for anything uncomfortable to develop. Finally there is a crunch. Those models are not anything to worry about.

 

The main thing is to get straight on the first part of your post, about recession rates.

==================

 

I'm only back for a moment. Have to leave for the day. Things to understand about the standard cosmo model:

 

1. Acceleration does not mean the Hubble rate H(t) is increasing. It has been decreasing and is predicted (in the std. model) to continue decreasing. What accelerates is the scale factor. Learn about this. The Friedman model is based on the scale factor.

 

2. Because the Hubble rate is decreasing, the Hubble distance is increasing. It is the reciprocal c/H.

 

3. The Hubble distance is the distance that is currently increasing at rate c. It is the distance to a galaxy which is receding at rate c. IF A PHOTON CAN GET WITHIN HUBBLE DISTANCE OF US, IT WILL EVENTUALLY REACH US.

 

4. Because the Hubble distance has been increasing rapidly it has kind of "reached out" to photons that were trying to get to us but were being swept back by the expansion. Thus we have received and are now receiving a whole lot of light which was initially being swept back, getting farther away rather than nearer.

 

5. As Lineweaver etc explain we currently see with our telescopes many galaxies which were receding faster than c when they emitted the light, and which are still to this day receding faster than c. Indeed that is typical.

As I recall, anything with a redshift z > 1.6.

 

You can check this using the online cosmos calculator. Google "cosmos calculator" and get someone to explain what numbers to put in, to get started.

Edited January 17, 2010 by Martin

[astro geek]

Thanks for the article and your explanation. I appreciate it very much.

P.S. I'm still only a junior astro geek.

[ironfrown]

Hi Martin,

 

I am not sure if my interpretation of my reading on the subject is right. However, in the decelerating model of the Universe, the Hubble volume indeed is expanding and so the photons outside its past boundary will eventually fall within the Hubble limit and be seen by us. In the accelerating model, this will not be the case and once the stars fall outside the Hubble limit they are gone forever. It may be worth noting that in the accelerating universe model the stars that we see today may 'now' be well outside the Hubble volume. Note also that I have quoted 'now' as this discussion does not explicitly take into consideration the observer and distant clocks.

 

The original question is still interesting, i.e. what would be the observable effects if the Universe suddenly (i.e. roughly 14 billion years ago) stopped expanding.

 

Also see the previous discussion by budcamp et al.

---

Merged post follows:

Consecutive posts merged

By the way, intuition tells me that apart from the blue shift of galaxies, any distant event observed in the contracting Universe would be seen as accelerated.

Edited January 18, 2010 by ironfrown
Acknowledging previous postings

[Martin]

Hi Martin,

 

I am not sure if my interpretation of my reading on the subject is right. However, in the decelerating model of the Universe, the Hubble volume indeed is expanding and so the photons outside its past boundary will eventually fall within the Hubble limit and be seen by us...

 

I agree with this part of your post. In a non-accelerating expansion there is no event which we would not eventually get to see. There is no horizon.

 

But I'm not sure I understand this, and probably don't agree:

In the accelerating model, this will not be the case and once the stars fall outside the Hubble limit they are gone forever.

 

The Hubble distance is estimated some 13.8 billion LY. The cosmogical event horizon is estimated at 15-16 billion LY. There are plenty of stars which are today outside the Hubble sphere (the radius c/H sphere) but not outside the event horizon.

If something happens, like a supernova event, TODAY, at a star which is say 14.5 billion LY from here, WE WOULD SEE IT. If our civilzation continued to point good telescopes at the sky. It is not "gone forever" even though it is outside the Hubble sphere.

 

All that means is that the photons from that supernova would eventually be gradually swept back and would not get closer to us for some time. But they would stay nearly the same distance from us and eventually the Hubble radius c/H would extend far enough out to include them.

 

This is according to the standard model with the typical parameters people use. If you have specific questions please ask. I'll try to help.

[crownedconquern]

Its the world stop,EVOLVING,then it would DISSOLVE.Common sense....

[Aitor]

I'm having a problem with a photon not getting any closer or farther from us. A photon cannot ever be stationary, can it? Wouldn't this have repercussions on the bending of spacetime to make that photon travel at the speed c from or towards us?

[Martin]

I'm having a problem with a photon not getting any closer or farther from us. A photon cannot ever be stationary, can it? Wouldn't this have repercussions on the bending of spacetime to make that photon travel at the speed c from or towards us?

 

In its local frame the photon is always traveling at normal speed c.

 

You should know that most galaxies we see with, say, the Hubble telescope are receding at rates greater than c. That means their current distance to us (if you could freeze expansion and take the time to measure it) is increasing at a rate faster than c.

 

You should learn how to use the simple online calculator (google "cosmos calculator") that converts redshift into recession rate.

 

These galaxies are not moving significantly in any conventional sense. They are not going anywhere. Just the distance to them is increasing, as allowed by General Rel, and specified by the Hubble law. v = Hd

(recession rate = H times distance)

 

So think of a galaxy which is sitting still in its local frame, and receding from us at rate c. So the distance from us to it is increasing at exactly the rate that light travels in the local frame. If that galaxy emits a photon in our direction, that photon will stay at the same distance from us, while the galaxy's distance from us grows at the rate c. The photon's local speed just cancels the recession rate.

 

If you google "cosmos calculator" and find Morgan's online redshift-to-distance-and-recession-rate converter, and if you want help using it, just ask.

 

Most of the galaxies we can see have redshifts greater than 2---that gives an idea how much their light has been stretched. The symbol for redshift is z. To use the converter you need to plug in 3 standard numbers that describe our universe's expansion

.27 for the matter fraction

.73 for the cosmological constant

71 for the current Hubble rate.

 

Once those 3 numbers are in the proper boxes, you just type in any z, any redshift you please, and it will tell you the distance to the galaxy that we see with that redshift, and its recession rate (which is a rate of distance increase, not a speed of ordinary conventional motion).

 

If you don't learn how to use that simple calculator your soul is forever doomed to darkness. Ask questions. Have fun.

[Airbrush]

Nice explanation Martin.

 

"What would happen if the universe stopped expanding?"

 

Taking another look at the question, we would notice expansion SLOWING tens of Billions of years before it stopped expanding.