# Thread: Acceleration of the expansion of the Universe and open Universe?

1. Member
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## Acceleration of the expansion of the Universe and open Universe?

Wouldn't acceleration of the expansion be a natural phenomenon in an open Universe?

2. Order of Kilopi
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No

Open means that gravity cant pull it back together, not that it is flying apart faster. A high enough expansion speed and/or a low enough mass density is enough to determine open or closed.

3. Could the expansion be due to the big bang ? For example, instead of dark energy, could it be that it is still expanding and accelerating from the
initial "bang"?

4. Order of Kilopi
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Originally Posted by kevin1981
Could the expansion be due to the big bang ? For example, instead of dark energy, could it be that it is still expanding and accelerating from the
initial "bang"?
No, models based on observations showed that the expansion was slowing until about z = .5. At that time, current interpretation of the observations, show the expansion accelerating.

5. Ok thanks Tensor. I had wondered that for a while so glad I asked. I don't understand what z = .5 means, in years, when did it start to accelerate ?

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Originally Posted by kevin1981
Ok thanks Tensor. I had wondered that for a while so glad I asked. I don't understand what z = .5 means, in years, when did it start to accelerate ?
The z value, in years, is different for different models of the universe. If you change the amount(and ratios) of matter, dark matter, and dark energy, you will get different lookback times. Currently, based on the current lambda CDM model, z = .5 is a bit over 5 billion light years. Here is Ned Wright's Cosmological Calculator. Here is his Tutorial.

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Originally Posted by kevin1981
Could the expansion be due to the big bang ? For example, instead of dark energy, could it be that it is still expanding and accelerating from the
initial "bang"?
Interesting question. It depends a bit on what you specifically mean.

If you mean by the "initial bang" the singularity at the start, then the question cannot be answered. Nothing is ever, causally speaking, "due to" a singularity. The reason for that is that while we can take our universe, turn the clock backwards, and solve for a future singularity (which corresponds to our past singularity), we can't do it the other way around. You can't take a singularity and solve for our universe, you have to introduce our universe "by hand".

If you however mean by the "initial bang" the state our universe was in very early in its history, a small hot "blob" of energy that's rapidly expanding, then the answer may be yes. But only in the sense that all the "ingredients" were already there. Though, as Tensor points out, it would take another 8 billion years for dark energy to win the "struggle" and reverse the slowing down of expansion. So in a sense you could say that the acceleration is due to the initial bang, if you mean that the stuff responsible for it was already contained in that stage.

Note that stuff like the early universe is always a bit speculation, but it is widely expected that dark energy was there already. Experiment puts a lower bound of its appearance at 9 billion years ago, and there's nothing to suggest it hasn't always been there, with "always" meaning post-inflation.

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Originally Posted by Tensor
models based on observations showed that the expansion was
slowing until about z = .5. At that time, current interpretation of
the observations show the expansion accelerating.
Are observations of supernova before z = .5 in the data?

-- Jeff, in Minneapolis

9. We have observations of Type Ia supernovae out to at least z~1.7, which was nearly 10 billion years ago.

http://lanl.arxiv.org/abs/astro-ph/0104455
http://lanl.arxiv.org/abs/0901.2476

10. Originally Posted by kevin1981
Could the expansion be due to the big bang ? For example, instead of dark energy, could it be that it is still expanding and accelerating from the
initial "bang"?
The problem is the rate has changed ... increasing at first, then slowing down then again slowly speeding up.

11. Originally Posted by Jeff Root
Are observations of supernova before z = .5 in the data?