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Guru
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Chandra Confirms the Hubble Constant

08/09/2006 3:55 PM

Nearly every single astronomical measurement depends on the Hubble constant, a number that calculates the expansion of the Universe. NASA's Chandra X-Ray Observatory recently measured this value independently, and came up with 77 km per second per megaparsec, give or take 12 (3.26 million light-years to the megaparsec). The previous 'best-buy' was 71 km/s/Mpc, give or take 8.

This result confirms that the Universe is still between 12 and 14 billion years old. The interesting part is that just about a week ago, a result from Bonanos et al, see arxiv, claimed a Hubble constant of 61 km/s/Mpc. This was not very well received in the Cosmology community, since it was apparently based upon measurements of a single binary star system. Read a fuller account of the Chandra result in Universe Today.

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The Engineer
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#1

Almost posted the same story

08/09/2006 4:23 PM

This does appear to be much more thorough than the other Hubble Constant value the other researchers found. Jorrie, how does dark energy fit into this picture. As I understand it, the Hubble Constant says how fast an object will move away from us based on it's distance from us. Does dark energy skew this? I'm sorry if my question is vague, I'm just not sure how to phrase it.

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#3
In reply to #1

Dark Energy

08/10/2006 4:08 AM

Dark energy influences the age of the universe for a given Hubble constant Ho. If the expansion curve was always parabolic (flat, matter dominated) since the BB, then with Ho = 71 km/s/Mpc, it gives an age of 9.2 billion years, which is apparently shorter than the age of some stars!

Dark energy lessens the deceleration of the expansion initially and apparently is now accelerating the expansion. With everything else the same, but with a dark energy density of 73% of the critical density, the age of the universe comes out at 13.7+-0.2 billion years, assuming Ho = 71 km/s/Mpc precisely. With the Ho error bar at about +-11%, it means the age could be anywhere between 12 and 15 billion years. A download from web page The Expanding Universe – an Engineers View sheds some more light on how these calculations work.

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#2

Hubbles constant

08/09/2006 11:43 PM

It was my undrstanding that it is no longer considered a constant
but is regarded as a factor changing over time.
Google "Big Rip`
"Things just keep getting more complicated.
Just when I figured out how to get the toothpaste back in the tube
they came up with this stripped stuff!"

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#4
In reply to #2

Re:Hubbles constant

08/10/2006 4:28 AM

Quote: "It was my understanding that it is no longer considered a constant but is regarded as a factor changing over time."
The Hubble constant Ho (pron. H-naught) means the value of the variable H at the present moment. H very slowly changes over time, depending on the model that the universal expansion follows. It is also not completely ruled out that H may actually be constant, due to dark energy.

The Big Rip scenario is postulated as 'runaway' expansion due to dark energy, where H gets very large.

On the lighter side, some calls Ho the "Hubble variable" because its value is revised so often. Due to the inherent errors in cosmological measurements, we are still a long way off from measuring whether H is actually changing over time.

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#5
In reply to #4

Re:Hubbles constant

08/10/2006 2:48 PM

After having read all the comments in this thread I still find myself unconvinced of the truth of any of the conclusions written. How can one really conclude anything when there are so many ambiguous presuppositions used as a starting point for resesrch and observation? I find it difficult to comprehend that one could come to any definitive conclusions about something which we have been observing for such a proportionally short time span(assuming that the "confirmation" is actually even correct). Am I completely out in left field here or am I just missing something that's obvious to everyone else? Please, can someone explain this to me? I know it may take more time than it may be worth, but I'm sure I would benefit from the knowledge gained.

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#6
In reply to #5

Re:Hubbles constant

08/10/2006 3:16 PM

Mostly yes W.B.,
But remember, when we look outward, we also look backward.
Hubble distances were origionally predicated on the premise that
the expansion was slowing over time due to gravitational decelleration.
The origional numbers, (IIRC), were based on 'standard candle`
measurements from relatively nearby Cephied variables.
Now, new long distance estimates based on more distant 'standard candle`
supernovae dissagree with previous assumptions.
The mass estimates on which this decelleration was based kept changing.
We are stuck with the seemingly absurd 'dark matter` because it has
been observed to lense.
We are equally stuck with the seemingly even more absurd dark energy to
make the numbers agree with observations.
But at the distances we are now observing, the time scale of our observations
is on the order of 10 Billion Years.

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#7
In reply to #6

Re:Hubbles constant

08/10/2006 3:55 PM

Here again we are working off of a 'premise'. Assuming certain conditions necessary to making an observation. Doesn't that make the whole exercise subjective. And then when we can't even reconcile the raw observations we must resort to even more subjective factors such as 'dark matter' and 'dark energy' to explain why things are not as we assume them to be. I would submit that there are things that we can see but do not have the capacity to explain or even fully comprehend. That does not mean by any means that we should give up trying. I think there is always something to be learned by asking questions, but don't limit your further understanding by making concluding statements that are not based on complete understanding. If you don't know say "I don't know." and leave it at that until you do know. I have never thought of ignorance as a bad thing unless it is due to arrogance.

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#8
In reply to #7

Re:Hubbles constant

08/10/2006 4:08 PM

I agree with your assessment of Dark Matter. I was always struck by the fact that Dark Matter was never actually observed (thus dark), it just was a trick to make the numbers work out.

Dark energy is different. I think the name is an unfortunate choice because it gives it a science fiction feel, but it is very real. The fact is that astronomical observation can not be explained by our current list of forces, there is another force at work that is accelerating matter that is large distances from each other. As of right now, no one knows what this force is, so they have labled the phenomenon "dark energy".

This is as classic as science gets, an unknown force is observed, physicists develope theories to explain the force, theories are tested until one is proven correct, our understanding of the universe increases. Don' lump dark matter and energy together, they are two different things.

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#9
In reply to #8

The Dark Stuff

08/11/2006 10:38 AM

Hi Roger, I don't quite agree with your dark stuff assessment. Dark matter is more probable than dark energy. Unless Newton and Einstein had gravitation theory grossly wrong, there must be 10 times more dark matter than visible stuff. Otherwise galaxies could not rotate like they do and they could not gravitational lens like they do. Also, not all dark matter is exotic - there must be some part that is ordinary stuff, but just not visible to us.

Dark energy on the other hand, has only one piece of evidence - the accelerating spatial expansion. The chances that a very minor modification to gravitation theory, that explains it all may be discovered is small, but significant, IMO.

So if I were a betting man, my bets would have been on dark matter being more likely.

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#10
In reply to #9

Re:The Dark Stuff

08/12/2006 2:14 PM

Hi Jorrie,

I wrote a detailed response but lost it before I could post. I hope you'll forgive the terseness of this response as I only have time to highlight the points of my lost response.

My problems with dark matter are:

1. Dark matter doesn't emit
2. Dark matter doesn't absorb
3. Dark matter is 10x more abundant yet we don't know of a particle that has it's properties
4. It's effects are only visible on the large scale.

Given the following observations:

1. Expansion is quicker than expected
2. Rotation of galaxies faster than expected
3. Lensing larger than expected
4. Space flatter than expected

I think there is a good chance there is a force significantly weaker than the gravitation force that interacts with mass and only has measurable effects in the large scale. An anology would be the magnetic force which interacts with charged particles but is much weaker than the electric force. Perhaps just as the magnetic field is generated by moving charge, this force is generated by moving mass, or spining mass, who knows.

I had a question. I know big bang requires a period of inflation to work. Is there any current forces that explain this inflation? I'm not aware of any but your understanding of relativity is much better than mine.

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Guru
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#11
In reply to #10

Re:The Dark Stuff

08/13/2006 6:32 AM

Hi Roger, I mostly agree, but for a few points: Quote: "4. It's effects are only visible on the large scale."
Depends how we define large scale. Dark matter is required on galactic scales - now that is small by comparison to the scales dark energy becomes 'noticeable' at. Dark energy becomes a factor in structures larges that super-clusters, e.g. voids and walls, where super-clusters move away from each other in a presumably accelerating fashion.

About inflation, no, I don't think there are current forces that explain it - that is unless dark energy is taken as a current force! As I briefly explained in my website chapter on inflation, dark energy (or rather vacuum energy) could have been there since the beginning - 'out of control' during inflation, but 'meek and mild' afterwards. That is, until the vacuum has expanded to such an extent that even the 'mild' vacuum energy becomes the dominating 'force' – it grows with the expansion in a sort of positive feedback loop! Eventually, it might boil down to a run-away expansion again.

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