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Expansion of the Universe

Threads - Expansion of the Universe
On 9/12/2003, Kevin Phyland wrote:

Every so often I get to thinking about something and the niggling gets so bad I have to ask for clarification...

The topic is the Expansion of the Universe.

As I understand it (clearly quite poorly) the evidence is based on red-shifting of light spectra...i.e. the furthest
objects we can *see* are more red-shifted than those closer (in a cosmological span of distance - I understand that within a local supercluster it's possible to get blue-shifts..)

My problem is this: If we say that the light from an object 5 billion light years away has a yada-yada red-shift I'm
assuming that the light we're basing this on is 5 billion years old already?

(If I'm incorrect at this point the rest of my missive is not only pointless but embarrassing...)

So if it IS 5 billion years old, would we actually know if the Universe had slowed its expansion say 2 billion years ago? i.e. would we have to wait 2 billion years to find that the red-shift had decreased?

Sorry...it's been bugging me for years...

Ray replied:

"Red shift "provides evidence for the velocity at which an EMR producing object is moving away.  The further away it is the further the red shift in its visible light spectrum and the faster it appears to be moving.

There are objects with a blue shift moving towards us, in sections of the cosmos where galaxies are accreting.

In time Kevin, the red shift would be further that it is now as the tail end of the ray of light being emitted now reaches us in however many light years it is distant.

Paul Williams responded:

My own very limited understanding matches this view.  I would add that reasonably well understood cosmic events - in particular certain Supernovae and their intrinsic brightness give us further evidence that we are judging distance pretty well correctly. (I think)

> My problem is this: If we say that the light from an object 5
> billion light years away has a yada-yada red-shift I'm
> assuming that the light we're basing this on is 5 billion
> years old already?

Yes. I believe that this is the accepted case - taking "c" to be constant throughout the history of the Universe.

> (If I'm incorrect at this point the rest of my missive is not
> only pointless but embarrassing...)
>
> So if it IS 5 billion years old, would we actually know if the
> Universe had slowed its expansion say 2 billion years ago?
> i.e. would we have to wait 2 billion years to find that the
> red-shift had decreased?

If we take "c' as being the constant it is defined to be and taking cosmic events as following the physics of the Universe as is evidenced now, it seems that we can accept that the Universe is still expanding. Most startling is the very recent, now verified, evidence that the Universe is expanding at an accelerated rate! No one understands how this could be. I obviously have no idea how this is happening.

Best (wild) guess - based on very little more than speculation:

Gravity weakens with time?

Even more wild:
As particles have their anti-particles.
And as there appears to be a certain symmetry about our Universe.
And as I like to play around with thoughts...

A 'shadow Universe' (not anti-matter) - time direction is reversed and 'gravity' is repulsive - there is no interaction between the two - bar gravity and it's symmetric shadow force partner (best to call it "levity") :-)

> Sorry...it's been bugging me for years...

I'm sorry that I can't really help - idle speculation is my go. :-)

John Winckle commented:

The same thought has me confused also.
The nearer galaxies are travelling slower. so does this just mean the information about the speeds in the universe is just more up to date there?

Tim Daly added:

    Not only could it ?? have slowed, but is it at all possible that the Universe may have been contracting too for some amazingly inconceivable period of time ??
Paul Williams answered:


The Universe is expanding and accererating in its expansion.

The speed of light is constant.

"Doppler shift":
When light sources move away from us their hydrogen absorbtion lines appear to be moved to longer wavelengths - red shifted.
When light sources move towards us these bands are blue shifted (shorter wavelengths).

Only some light sources within our local group are blue shifted - they are moving towards us.
The rest of the Universe is moving away from us. The light we see is red shifted.
The further a light source is away, the more red shifted is its light.

"Standard Candles":
Are used to confirm the distance of a light source.  (These are objects whose intrinsic brightness/size is very well known).
These "Standard Candles" agree pretty well with Doppler shift distance calculations.

Accelerated expansion of the Universe:
This has now been confirmed in various ways by various instruments.  
One certain type of supernova discovered in 1997 began to confirm what astronomers had seen indications of but really needed more data to accept.  
This supernova exploded about 10 billion years ago when the Universe's expansion was actually slowing down.
It was dimmer than it *should* have been if expansion was invariable.
Since then many more observations have confirmed that the expansion of the Universe started to accelerate about half way through it's present age.

Sounds like science fiction but the Universe is expanding at an increasing rate and the things that we see - all the visible matter in the Universe makes up only about 5 percent of what there is. 30 percent is "Dark Matter" and 65 percent is *unknown* "Dark Energy". This is not speculation, it is what our instruments and calculations are telling us.

Once again, no one has a clue as to what "Dark Energy" may be - all we know is that it is exerting a force which is pushing galactic groups further and further apart at an ever incresing rate...

We are indeed made of star stuff - star stuff is not too important though.