Expanding universe question

[Vay]

So the current knowledge is that our universe is expanding at an accelerating rate; the farther the detected universe, the faster its velocity away from a center.

 

My question is, when we look at distant galaxies, we are also looking into the past. So the greater doppler shift to red, or the faster movement away from a center of the galaxy, the farther into the past we also look, because of the fact that a greater doppler shift to red also means that the galaxy is farther away from us.

 

So can't the reason for the greater velocity at greater the distance be due to the fact that galaxies in the past moved at a greater speed due to the momentum given to them during the big bang? Such that when we look at distant galaxies, we looked at a period in time from the past, when these galaxies still maintained a majority of the momentum from the big bang, such that the farther we look the more the momentum of a galaxy.

Edited May 2, 2012 by Vay

[Spyman]

The whole Universe is not thought to have a center, but the observable universe is a sphere centered around us because we can only see so far.

 

The cosmological redshift is not a doppler effect due to speed through space, it is caused by space expanding between us and the objects we see.

 

The Big Bang is not a theory of one giant explosion in space violently displacing matter, it is the distances between matter that is expanding.

 

You are correct that when we look far away, we also look far back in time, which means that space was expanding faster further back in time.

[Iggy]

So the current knowledge is that our universe is expanding at an accelerating rate; the farther the detected universe, the faster its velocity away from a center.

 

...

 

So can't the reason for the greater velocity at greater the distance be due to the fact that galaxies in the past moved at a greater speed due to the momentum given to them during the big bang?

It looks like you mixed up accelerating expansion with ordinary expansion.

 

The observation that further away things have a greater radial velocity is what Hubble discovered in 1929. It is known as Hubble's law. The further away a galaxy is from us, the faster it recedes from us. That isn't the acceleration of expansion -- just expansion.

 

The 1998 discovery of acceleration indicates that expansion will speed up over time, so that the rate of expansion at any and every distance will increase over time. Things that are relatively close to us today recede from us relatively slowly. In the far future things that are relatively close will recede quite a bit faster. Just like today things that are relatively far recede relatively fast and in the distant future things that are relatively far will recede even faster.

 

Acceleration of expansion would mean that the rate of expansion of the whole universe increases over time... not that the rate of recession increases over distance.

Edited May 2, 2012 by Iggy

[Airbrush]

Is the universe generally accepted as having expanded at its' highest speed shortly after the moment of the Big Bang (inflation)? When inflation ended, expansion slowed down. Then gravity slowed down expansion even more? Finally, several Billion years ago the universe began to accelerate in expansion?

[pantheory]

Is the universe generally accepted as having expanded at its' highest speed shortly after the moment of the Big Bang (inflation)? When inflation ended, expansion slowed down. Then gravity slowed down expansion even more? Finally, several Billion years ago the universe began to accelerate in expansion?

Yes, this is one of the more common interpretations. Most interpret that as a result of Riemann geometry of GR applied to Inflation theory closes the universe like a 4D spheroid with no center to it.

 

One problem with the directional motion of galaxies is that many believe Special Relativity would be violated (exceeding the speed of light), so the presently accepted model is instead that space itself is expanding. Some, or maybe most theorists presently think that accelerated expansion is related to the expansion of space itself. Presently there seems to be no accepted mainstream theoretical separation between the cause of the hypothetical expansion of space, and the cause for its accelerated expansion, other than the names given to it such as "quintessence," and lambda, for instance.

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Edited May 2, 2012 by pantheory

[Iggy]

Is the universe generally accepted as having expanded at its' highest speed shortly after the moment of the Big Bang (inflation)?

I agree.

 

When inflation ended, expansion slowed down. Then gravity slowed down expansion even more? Finally, several Billion years ago the universe began to accelerate in expansion?

According to most sources I've seen we just recently entered the epoch of accelerated expansion. For example, this diagram has the onset of acceleration happening about 800 million years ago: http://www.astro.virginia.edu/class/whittle/astr553/Topic16/t16_three_distances_6.gif

 

For the majority of the history of the universe the expansion has been decelerating. It wasn't until matter spread out enough that the mass density became less dominant than the vacuum energy density and the universe began expanding exponentially with time again

 

EDIT

 

I'm sorry, this was the diagram... I misread...

 

 

and haven't kept informed. It looks like we entered the present accelerating epoch 6 - 8 billion years ago... from the diagram above and the couple sources I've just read. You would be absolutely correct then.

 

At that point the second derivative of the scale factor would have turned positive, the first derivative would have reached a minimum value, and lambda's density would have overtaken the mass density.

 

Very embarrassing slip.

Edited May 2, 2012 by Iggy

[pantheory]

Iggy,

 

If I'm reading the chart correctly it looks like the beginning of accelerated expansion was at a redshift of about .77 according to the pointing arrow. This would have been about 7 billion years ago, which according to the present BB model age, would have been when the universe was half its present age. Is this the way you interpret the graph?

 

Here's a handy calculator of redshifts vs. time past (distance in light years).

 

http://hyperphysics....tro/hubble.html

 

Most dark energy articles and papers that I have read put this transition time a little sooner than that at a redshift of about .6 which accordingly would have been 6 billion years ago. Of course there is not enough clear supernova data at present to come up with an exact transition time concerning the dark energy hypothesis.

Edited May 2, 2012 by pantheory

[Iggy]

Yes, I believe you are quite right. I had edited my post when the thought hit me that omega-lambda is certainly larger than omega-M in the concordance model and surely has been for some time.

 

For added accuracy with higher redshifts you might like, http://www.astro.ucla.edu/~wright/CosmoCalc.html

Edited May 3, 2012 by Iggy

[pantheory]

Iggy,

 

Got your note on that, and thanks for the link. As you may know I am not a fan of present theories/ hypothesis in cosmology (nor have I ever been). I wrote a paper on type 1a supernovas back in '08' that came up with a different conclusion than the dark energy hypothesis, but I used my own formulations for distances based upon my own cosmological model, which is like comparing apples with oranges. My own analysis comes very close to a change in the data occurring at a redshift of about .6, but my conclusion instead was that they were using a somewhat incorrect formulation (the Hubble formula), to come to their dark energy conclusion. Whether right or wrong, they got the Nobel Prize for it And what did I get for all my hard work? Zilch! (a little tongue in cheek humor). It is rare that a mainstream journal would ever publish a non-mainstream paper I was just hoping

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Edited May 3, 2012 by pantheory

[wucko]

perhapse spacetime is made of discrete space points, ad the expansion is expansion of time. http://www.scienceforums.net/topic/66209-the-speed-of-light-and-discrete-notions-of-space-and-time

[swansont]

perhapse spacetime is made of discrete space points, ad the expansion is expansion of time. http://www.scienceforums.net/topic/66209-the-speed-of-light-and-discrete-notions-of-space-and-time

 

!

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