Expansion vs contraction

[Mild mannered]

If I'm buzzing along through deep space with no frame of ref and you come sailing past me - assuming neiter of us is accelerating - we have no way of knowing who is actually "moving" and who is "still" - in fact the question becomes non-sensical - all that matters is our relative motion.

So to distant Galaxies looking back at us we are accelerating away from them just as they appear to be to us.

I am fine with this on the galaxies painted on an inflating sphere model but if the Universe is like a ball of energy with a centre how do we know it's not contracting?

If it's contracting and we are accelerating towards the centre it would appear from our frame of ref that distant Galaxies are accelerating away (expansion) but this would be an illusion of course one day we would start to see galxies coming toward us with blue shifted light - then ouch!

I know this isn't entirely correct as I've phrased it but do we know 100% that we are expanding not contracting?

[Timing]

ever heard of the phrase: "....as above, so below..."?

Adding to your questions, what if our Universe and other Universes are 'Breathing'?
Meaning that we are expanding for 100M's years, and then contract... and start expanding and contracting again and again? What if this is the theory... could it be the reason why Scientists still do not know how old our Universe is?

[John L]

If the universe were contracting we would see the blue shift in both close and distant galaxies. The only blue shifted galaxies we see are those that are gravitationally bound to us in the local group. All other nearby galaxies are red shifted due to the expansion.

Timing,

There are theories that state just that. They are sometimes called the Cyclical Models as they cycle between expansion and contraction. Based on WMAP data, though, it appears that the universe is about 13.7 billion years old and that it is expanding at an accelerating rate. We're still waiting for the next round of data to be released from WMAP, but what has been released seems pretty firm on those facts.

[antoniseb]

Originally posted by Timing@Apr 14 2005, 12:26 PM
What if this is the theory... could it be the reason why Scientists still do not know how old our Universe is?

Hi Timing, I'd like to welcome you and Procyon to the forum. You both joined about the same time.

It is easy to make statements like Scientists still do not know how old our Universe is, but in fact, we may never know certain things with great precision, but we do have a strong convergence of observations that tell us a lot about how old the universe is. It seems very likely that it is between 13.5 and 13.8 billion years old.

We do NOT know how it started. No one pretends that we do. If you want to say scientists still don't know something, I'd trying picking on that.

[Nereid]

I know this isn't entirely correct as I've phrased it but do we know 100% that we are expanding not contracting?

We don't. :huh:

But then, except for rather limited (philosophical) statements about one's own existence, none of us 'know 1005" anything! :P

If your question mild concerns science, may I suggest that a more relevant one might be "are there any ideas - self-consistent, consistent with all good observational and experimental results within their domains of applicability, and consistent with good, modern scientific theories in the regions where their domains of applicability overlap - that have the universe contracting not expanding?" The answer to that question is "no, there are no such ideas - at least wrt being published in peer-reviewed astronomy/cosmology/astrophysics journals".

[Mild mannered]

John L

If the universe were contracting we would see the blue shift in both close and distant galaxies.

I assumed this as it seems the logical reverse - yet Nereid says simply

We don't

!

If the Universe were contracting and experiencing an accelerating contraction then would the light be blue or red shifted?

I don't want to get to deep with this - it was just a passing thought I thought I'd pass on! - I have no real doubt that the Universe is expanding not contracting and it is just a matter of whether this will continue indefinitely or eventually stop and contract.

I was just musing over relative motion and how strange a concept this is to really pin down in your brain when you have no absolute frame of reference.

But thanks all for your replies.

[Nereid]

If the Universe were contracting and experiencing an accelerating contraction then would the light be blue or red shifted?

It all depends .... IF the two states you want to compare are identical, except that in one there is expansion, and in the other contraction, then the difference would be that our spectroscopes attached to our telescopes would register wavelengths of well-known atomic transitions shifted to the blue (cf red, which we observe).

HOWEVER, there is some intriguing, relatively recent theoretical work (the holographic principle) in which the distinction between 'expansion' and 'contraction' becomes much, much more subtle (and, observationally, undetectable?).

I don't want to get to deep with this - it was just a passing thought I thought I'd pass on! - I have no real doubt that the Universe is expanding not contracting and it is just a matter of whether this will continue indefinitely or eventually stop and contract.

Who knows? How could you tell??

Homo sap. has been around for only ~1 million years; we have evidence that our ancestors could &#39;do&#39; abstract thinking ~50k years ago; recorded history is <10k years old; &#39;science&#39; in a modern guise is ~<500 years old; cosmology as a science <100 years old .... our current view is that the universe is >10 billion years old.

The best results we have today suggest that the universe is dominated by something we know essentially nothing about: &#39;dark energy&#39; ... yet we have come to this conclusion in ~<a decade&#33;

The consensus scientific view of what the universe will be doing &#39;in the distant future&#39; has been evolving at at rate ~10^9 times faster than the universe itself&#33;

I was just musing over relative motion and how strange a concept this is to really pin down in your brain when you have no absolute frame of reference.

If you find this twists your brain, then may I give a friendly suggestion? Do not even think of diving into Aspect, Bell, the EPR paradox, "local reality" or other aspects of quantum weirdness&#33; :P