A question about Cosmic expansion.

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A question about Cosmic expansion.

Postby curiosity on November 3rd, 2019, 11:28 pm 

Could someone tell me where I might uncover some information regarding cosmic expansion (In plain English please.)
I understand the principle of metric expansion and that its effect is cumulative, (so the more distant an object is the greater its redshift will be) but I cannot find any information relating to how the conclusion that the expansion is accelerating was reached.
Proponents of an accelerating expansion, seem to mention the Doppler effect way to often for my liking, (as cosmic redshift is not actually caused by the doppler effect.)
There must surely be a good reason for so many people believing that cosmic expansion is accelerating and I would be most grateful If one of those proponents could enlighten me .

Thank you in advance... Graham.
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Re: A question about Cosmic expansion.

Postby BurtJordaan on November 4th, 2019, 1:08 pm 

Hi Graham. The short answer is that when we used Hubble to look and measure up the distant 'standard candles' (supernovae type IA), we found them to be fainter than we thought they should have been for the measured redshift.

The only explanation that makes sense according to Einstein's relativity is that the expansion rate is increasing lately. This causes the distant galaxies to be farther away than what the older cosmological model (always decelerating expansion) would have placed them.
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Re: A question about Cosmic expansion.

Postby curiosity on November 5th, 2019, 5:17 pm 

Hi Graham. The short answer is that when we used Hubble to look and measure up the distant 'standard candles' (supernovae type IA), we found them to be fainter than we thought they should have been for the measured redshift.

The only explanation that makes sense according to Einstein's relativity is that the expansion rate is increasing lately. This causes the distant galaxies to be farther away than what the older cosmological model (always decelerating expansion) would have placed them.


Hmmm, The standard candle method of using the luminosity of type Ia supernovae then linking that luminosity to the values of redshift as derived from the Hubble curve, would be fine, if it wasn't for the occurrence of non-standard luminosity among these events. The standard candle method is a useful guide, but it is far from precise, due to the luminosity factor, which can be influenced in particular by any intervening dust/gases between the event and the observer.

The most distant type 1A supernovae events (which we occasionally get to observe,) occurred eons ago, when the universe was less influenced by entropy than it is now, so the conditions at that time, may well have differed from their currentl values.

Indeed our understanding of space-time and particularly why it is expanding at all, is far from comprehensive, so I think it would be prudent to reserve judgement until our understanding is improved...

I among many others am less than convinced by the validity of this theorem, but I sincerely appreciate your input.

Regards, Graham.
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Re: A question about Cosmic expansion.

Postby BurtJordaan on November 6th, 2019, 3:49 am 

curiosity » 05 Nov 2019, 23:17 wrote:The most distant type 1A supernovae events (which we occasionally get to observe,) occurred eons ago, when the universe was less influenced by entropy than it is now, so the conditions at that time, may well have differed from their current values.

The fortunate thing is that we have various different types of observations that corroborate the luminosity and distance measurement processes, as is discussed in this Wikipedia article.

There are surely uncertainties, but the modern techniques have narrowed the error bars to a point where the acceleration of expansion is not disputed, just (maybe) the magnitude of it.

Always challenged, but no confirmation of any of those challenges has been forthcoming.
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Re: A question about Cosmic expansion.

Postby curiosity on November 6th, 2019, 8:14 am 

There is no disputing the fact that the more distant galaxies are accelerating faster than closer ones, as this is an effect caused by the metric expansion of space-time itself, I am just skeptical of our ability to accurately measure any possible increase in that rate of acceleration. Metric expansion is incremental in nature, so unless we know precisely how far away the supernovae being observed is, (relative to ourselves) then measuring the redshift is a pointless exercise.

I will read the Wiki article, but I doubt very much that it will change my mind.

thanks again for your help.

Regards Graham
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Re: A question about Cosmic expansion.

Postby curiosity on November 6th, 2019, 9:08 am 

As I suspected, there are way too many variables and even unknowns for there to be any guarantee of absolute accuracy with the distance calculations, these problems are exacerbated by the sheer distance between ourselves and the most distant supernovae.

I think I'm destined to remain a skeptic unless more reliable methodology can be developed, which support these preliminary findings.

Bye the way, I've just noticed that you hail from SA... "Congratulations on your country's success in the rugby world cup, your team were awesome !!!"
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Re: A question about Cosmic expansion.

Postby BurtJordaan on November 6th, 2019, 9:21 am 

curiosity » 06 Nov 2019, 14:14 wrote:There is no disputing the fact that the more distant galaxies are accelerating faster than closer ones, as this is an effect caused by the metric expansion of space-time itself, ...

Actually the more correct term would be "the more distant galaxies are receding faster than closer ones, ..."

The accelerated expansion observations are based on that they are receding even faster than a matter dominated metric would allow. Add to this that all the matter (normal and dark) that can be inferred from galaxy and cluster observations is only 30% of what is needed to make space 'flat' - it should be open, with negative spatial curvature. The CMB observations, especially acoustic density waves in the plasma, indicate that the spatial curvature is extremely close to zero, far too 'flat' for a 30% matter universe.

Hope it helps - Jorrie

PS: Thanks! Yes, the Springboks surprised even ourselves...
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Re: A question about Cosmic expansion.

Postby curiosity on November 16th, 2019, 10:59 pm 

Thanks for taking the time to answer BurtJordaan'
I have been extremely busy with a project I've taken on, so I apologise for this late response...
Well... It seems I'm still no closer to getting my question answered.
I have just read yet another article about cosmic expansion and although it mentions accelerating expansion, it didn't give any details about it, or offer a possible explanation for the acceleration.

As I have already stated, I understand the concept of cosmic expansion. Which metaphorically speaking is...
If we existed within a universe, where at any point in time there existed a sequences of galaxies spaced convenientl at intervals of maybe ten light-years distance from both us and one-another, (then for simplicities sake,) I just pluck a figure of say ten miles a secocd out of the air and use that number as the amount of expansion occuring between each of the galaxies during each second of time, the distance between us the observers and the nearest galaxy would increase by ten miles in one second The distance between the nearest galaxy and the second nearest galaxy would also have expanded by ten miles during the same second, as would the distance between the second and third nearest galaxies, This means the nearest galaxy receded by ten miles in one second, the second galaxy receded by twenty mies during that same second, and the third receded by 30 miles, This concept explains why more distant galaxies are receding faster than closer ones.

There are too many problems associated with using the observation of standard candle supernovae to ascertain the distance between ourselves and the very distant galaxies it is such an imprecise method and to add to our woes any attempt to calculate the distance to those most distant galaxies by using parallax, is impossible.

I noted that in your response you said...
The accelerated expansion observations are based on that they are receding even faster than a matter dominated metric would allow
I presume that you are referring to the gravitational effects of matter,
which in fact only adds more worms to an already crowded can.
In reality cosmic expansion is likely uniform and relativity is responsible for the percieved accelerating expansion. If it were possible to observe from a location midway between our planets current location and any distant galaxy we happened to be observing the galaxy being observed would only appear to be receding at half the speed an earth bound observer was recording. (So how fast is it actually receding?) There are also way too many fudge factors for my liking, in the maths used to estimate recession rates. So... I remain a sceptic.
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Re: A question about Cosmic expansion.

Postby BurtJordaan on November 17th, 2019, 3:14 am 

curiosity » 17 Nov 2019, 04:59 wrote:I noted that in your response you said...
Burt wrote:The accelerated expansion observations are based on that they are receding even faster than a matter dominated metric would allow

I presume that you are referring to the gravitational effects of matter, which in fact only adds more worms to an already crowded can.


Yes, but Einstein's GR helps us to take the present observations and roll time back to predict what we the observable universe should 'look like' (in terms of brightness, redshifts, cluster and filament structures etc. when it was mush younger. More than a decade of observations forced us to consider an accelerating expansion.

Let us define what is meant by accelerating expansion: it is not the fact that more distant galaxies recede faster - we convert redshifts to a scale factor (usually labelled 'a') and then determine changes in the slope of the scale factor over time.

expansion double.png
Scale factor comparison for accelerating and non-accelerating expansion


The darker blue indicates the accelerating expansion curve observed and the lighter curve what we thought is should have been (pre-1998). With 20 years of scrutiny, cosmologists are pretty confident that the broad picture is correct. Here and there "tensions" surface, but they are so carefully analyzed and sometimes are only resolved when new data comes along.

Scientists are generally skeptical about other peoples (team's) findings, so there is a good self-regulating mechanism in place. If we do not believe the (best) established data and interpretations, we really have nothing (but philosophy...;-))
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