Celestial Sphere

Mike Hewson
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Birds Of A

Birds Of A Feather

Cepheids are one example of the general idea of a standard candle. Let's do a gedanken experiment or thought experiment to make firm this idea.

Quote:
It's often quite impractical to do such experiments, but one can do them in one's mind providing you don't break any laws of physics. It's not like you're ever likely to do one either. It's more an exercise in seeing whether some set of rules/laws when applied to some circumstance yield sensible results, or at least results that don't contradict known experience and actual experiments performed. However sometimes it can even lead the way in suggesting real investigations to be done, so as to test the validity of some theory and/or interpretation thereof.


Let's make a standard candle of our own. Make it an electric light of some suitable type. I like the newer LED systems - my workplace is in the process of installing these - that dramatically reduce power losses ie. you get more shine for less watts. So that we'll have some chance of seeing it from a long way off, attach it to some large ( say fusion nuclear reactor ! ) power source. Yottawatts for preference. Suppose we travel with both together in the same craft. Let this light source be absolutely constant in ( average ) photon output - which it can be made to be - plus or minus some photons ( quantum mechanical fluctuation around some mean ). So we can then consider this 'candle' to always produce the same number of photons each second. Assume it sprays them isotropically - the same number per second evenly over all surrounding directions. If you like make the photons of some close frequency/energy range.

Quote:
In the following when I say 'go to some place in the Universe' : either assume you're willing to wait for me to get there, or if you're impatient then assume I left Earth a sufficiently long time ago in order to arrive and do whatever. See how much fun gedanken can be? :-)


Now set up some detector of your choice, preferably of a design that reliably intercepts a high fraction of the photons ( of the given frequency as above ) that hit it. Now repeat the logic we have earlier described ( this is a re-working ) : measure the flux over the area 'en face' to the source, divide this by the known standard candle intensity giving the fraction of photons we have intercepted. This fraction also then represents the fraction of the total surface area of a sphere centred on the source. That total area may then be deduced, because we know our detector's area, and thus we get the sphere's radius.

So let me roam all over the known Universe, having first told you all about my candle ( even letting you calibrate your detector against it ), then providing you can reliably distinguish my candle from all the other light about the place THEN you have a 'light meter' that measures the distance to the standard candle. Light intensity - with the given assumptions - has now become a proxy for ( otherwise unachievable ) distance measurements.

Now the real kicker is : if you have ANY phenomenon from which can be reliably deduced distance then you have a standard candle. It doesn't actually have to be light. For gravitational waves - yet to be confirmed mind you - you could have a standard siren. It's the same type of idea but using gravity, or equivalently spacetime deformation, to deduce distance to source ( in the knowledge of source intensity, law of flux decrease with distance, assumption of isotropy ... ).

In fact if I can construct a proxy for a proxy then I also have a proxy too. Mind you there will be more assumptions and work ( error! ) to do the more I dereference away from the original intent of measuring distance. We sort of do this alot everyday anyway. Take the speedometer of a car. We generally don't get an exact timing over a given distance to yield speed. Typically the rate of the wheels turning gives a speed, but this assumes a few things not least of which is the diameter of the tyre ( so the car will appear to go 'faster' as the tyre wears ). The police used to use amphometers : two triggering strips on the road a set distance apart thus timing a car's transit. Nowadays it's often a radar so the speed is translated from a frequency shift. You get the idea.

Quote:
Some forty-ish years ago my Dad successfully disputed a speeding citation ( and he wasn't actually speeding ) because he showed that the police officer hadn't, and didn't know how to, set up the amphometer correctly. There was quite a queue in the court that day of other motorists who were likewise stung.


To be painfully exact all information that is not directly sensed by us is of proxy type, thus any instrument we use has a causal chain of events to be reckoned with in evaluating the reliability of data. As a neophyte experimenter gleefully mucking around in university physics labs in the late 70's ( the gadgets, the gadgets ! ) I was initially rather annoyed by the insistence of our lab supervisors to attend quite closely to error estimations and the like. I hadn't yet twigged that the key to differentiating competing theories was to compare quantitative outcomes of investigations* in order to rule possible explanations in or out of physical bounds. Later when I did medicine I was suitably horrified that medical research, as an inquiring discipline, had only recently ( mid 1950's ) acquired the double blind method of clinical trial. Prior to that treatments were chosen on the basis of 'reputation' ie. it was good solely because Professor Bob said so. Pleas to authority cut nil frozen water with me .... :-)

Thus if I have a group of objects in the sky that I reckon are 'nearby one another', and at least one of them has standard candle or proxy behaviour, then the evaluated distance to the standard candle becomes an approximation for the entire group including the non-standard-candle objects. The subtlety here is the additional assumption of 'nearby one another'. Maybe you could see some mutual orbiting going on, or a shockwave of material ejected by one hitting another, or a velocity distribution that implies gravitational binding and perhaps a common direction of nett motion, or anything that reasonably implies 'togetherness'. Next up :

Mr Hubble

Cheers, Mike.

* To this day I regret not being able to complete a particular experiment - hardware failures alas - that of determining the 'cross-section' for a specific nuclear transition. I used to sit and watch the apparatus well into the late evening. Just me and some particles hitting the detectors : imagining the tiny particles going in to the target, doing something, and then some products firing out. Transmutation ! All the while being confused by quantum mechanics ( no change there .... ) and wondering why the heck a miller/baker called Green several centuries ago had any business defining convolution kernels ! :-)

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Maximilian Mieth
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I have to admit that I did

I have to admit that I did not read most of this thread (mostly due to a lack of time, not interest) I would like to correct your german ;)

Quote:
Cepheids are one example of the general idea of a standard candle. Let's do a gedanken experiment or thought experiment to make firm this idea.

There are two rules that are important in this case:

1. Compound nouns are always written in one word.
2. Nouns are always written with a capital letter.

Hence, the correct notation is: Gedankenexperiment

Btw: If you are interested in the German language you can read this very interesting and funny text about it. It is written by Mark Twain.

The Awful German Language (It starts at page nine)

Mike Hewson
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RE: I have to admit that I

Quote:

I have to admit that I did not read most of this thread (mostly due to a lack of time, not interest) I would like to correct your german ;)

Quote:
Cepheids are one example of the general idea of a standard candle. Let's do a gedanken experiment or thought experiment to make firm this idea.

There are two rules that are important in this case:

1. Compound nouns are always written in one word.
2. Nouns are always written with a capital letter.

Hence, the correct notation is: Gedankenexperiment

Btw: If you are interested in the German language you can read this very interesting and funny text about it. It is written by Mark Twain.

The Awful German Language (It starts at page nine)


Mea culpa, I should have used Wikipedia ! Thank you kindly for the correction, and thanks for reading at all! :-) :-)

Cheers, Mike.

( edit ) Max hilarious an article !! I feel rather relieved now, reckoning my error could be dismissed by a member of language judiciary as a mere trifling matter, with no penalty to be recorded. :-)

Seriously : I did briefly flirt with a 'quick course in German' - ahem - in 2011 before I did go to Germany for the E@H event. I was quite happy when I apparently pleased many there with my brief vocal bursts. Fortunately just pointing at menu items and then adding 'riesling' covered most needs. The classic was attempting to buy an entry ticket for those lovely gardens in Hannover - I quickly lost track of the flow of the conversation and so I declared 'nein sprechen sie Deutsch'. The sharp rejoinder - I think - was that I actually was sprechening sie Deutsch but simply was crap at it .... :-) :-)

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Mike Hewson
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Hubble He was a champion

Hubble

He was a champion boxer before he did serious astronomy. This was at a time when it was none too clear as to what 'nebulae' were. The word implies a fuzzy blob of light. Nowadays we would be describing either luminous gas clouds or entire galaxies. Charles Messier had earlier cataloged many that were visible to the naked eye and that were a nuisance, if you like, to those who were hunting for comets - which can also look fuzzy and gas-ball like.

Hubble got access to basically the best telescope in the world at the time, and thus was able to get good images of these nebula things. He continued the work of Shapley, Slipher, Humason and others who were measuring the frequency shift of the light received from these objects.

Imagine you are on a boat at sea, with the waves coming from some direction at a certain wavelength when you are stationary. Now turn the boat and start heading into the waves. Your bow will hit more wave crests, per some time interval, than before. Turn the boat to run away from the source of the waves and your stern will be hit by fewer wave crests, per some time interval, than before. Call this the 'seaborne Doppler effect' if you like.

Much the same thing happens when sources of light move ( relatively ) toward, away, or at some intermediate angle to our position in the Universe. In fact the cosmic microwave background ( CMB ) as we have measured here on/nearby Earth has a mild trend toward higher frequencies in one direction of the sky and a corresponding mild trend toward lower frequencies in the opposite direction. This measures our motion - magnitude and direction - with respect to the body of the CMB radiation. That's about as close to an 'absolute' movement ( in the Newtonian sense ) that you'll get.

Anyhows what was revealed was that most of the nebulae ( that were also beginning to be called 'galaxies' or 'island universes', with the implication that they were much like the one we are sitting in too ) have a red shift. What this means is that if you study the set of light frequencies emitted by, say, hydrogen atoms here on Earth then : you can see the same pattern of frequencies coming from a given nebula, but each one shifted by the same frequency amount. Toward lower frequencies, hence the phrase 'red' meaning toward the red end of the visible spectrum. A small number of nebulae are blue shifted. Also for a given source thus studied with different frequency sets from different atomic types you get the very same frequency shift applying. So rather than say that atomic physics is different over there compared to Earth here, so as too produce consistently shifted frequencies on emission, it is far simpler to accept that the distant atoms are behaving the same as on Earth but merely all moving toward/away from us as a bulk body within that nebular object.

[ I have explained this somewhat excruciatingly here because (a) there is another type of redshift - gravitational - that has another mechanism entirely ( General Relativity ), and (b) there had been vigorous discussion about what 'moving toward/away' really means ( also resolved by GR, but in a separate sense to (a) ). For the moment let's accept the classical Doppler sense/interpretation. ]

Now even though in today's terms he had a woefully small data set, he did get the trend right. For each nebula : (a) measure the distance to it via say Leavitt's techniques and (b) measure the frequency shift and deduce a relative velocity then (c) the first two correlate more or less linearly giving rise to the key conclusion :

the further away a nebula is the faster it is moving away from us

I can't overestimate the profundity of this. It dominates anything you may care to say about the Universe.

[ In what follows try to distinguish in your mind between what I call 'cosmo-kinetics' ( describing what is happening ) vs what I call 'cosmo-dynamics' ( describing why it is happening ). Because we are entering an area of study which is very 'model-dependent' ..... ]

Next up :

Backwards In Time

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Mike Hewson
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Backwards In Time So I

Backwards In Time

So I hope by now you may have already deduced that because it takes time for light to travel - the speed is fast but still finite - then we always get a delayed snapshot of what we see. Roughly the speed of light is one ( Imperial ) foot per nanosecond ( billionth of a second ). So as you are walking down the street watching stuff happen there, your witness of events is ever so slightly delayed by photon transit. Thus some event fifty feet away ( say a car's brake lights going from off to on ) has also happened 50 nanoseconds ago ie. the light had to leave that event 50 nanoseconds before you can record it as having happened. Plus the more distant things are, the longer time ago that will be. Now one needs to be careful with language here, especially of whose clock we are talking, and in this case I mean the one that you carry with you. So if you note some distant change in the brake lights when your clock said exactly midday, then photons must have left those brake lights when your clock said 50 nanoseconds before noon.

If we extend the scale of this by going from the trivial delays we experience locally to the astronomical realm then we have to really take notice. I don't just mean the moons of Jupiter now, but way beyond. Suppose you knew that the Alpha Centauri system of stars ( they are bound to each other, so let's treat them as a group ) was heading in a certain direction and speed, as measured at Earth. Suppose also that you wanted to travel ( admittedly a long time! ) to that system. Would you point your rocket at the direction where we see Alpha Centauri now and fire off that way ? [ Note we have good evidence that it takes ~ 4.3 years for light to travel b/w AC and Earth. ]

You would miss AC altogether if you tried that, as it would have moved during your voyage. To succeed you'd need to predict a future course for AC and work out an intercept of your path with it's. Now let's imagine that we are merely sending a light signal to AC, via a fairly narrow beam. The same applies. If we want someone at AC to see our signal we'd have to aim it where AC will be in 4.3 years time. Otherwise the photons will slide on past and an AC observer won't see our signal.

Back to Hubble. The nebulae he studied - now called galaxies - were quite a distance away. The closest galaxy to ours is in Andromeda, and light takes two million years to get here. So if tonite you look and see something amazing happening to it - say the stars rearrange themselves to show a portrait of Elvis - then those photons must have left some ~ 100,000 human generations ago. The current record for the furthest definable galaxy is ~ 13.3 billion years. The Hubble Telescope imaged it. What might have happened to that galaxy since the photons left there to come here to form the image, that we don't know about yet ? Don't know 'bout you but my mind just boggles .... :-)

So this is the curious thing ( from an everyday point of view I mean ) :

the further away we look in distance the further in the past we are witnessing

recall a prior conclusion ( last post ) :

the further away a nebula is the faster it is moving away from us

Next time we ponder further on those two statements, and ask :

How did it get that way?

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Mike Hewson
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How did it get that

How did it get that way?

Here's a thing : if we were at 'the centre' of an expanding universe we would see everything rushing away from us. So ( apart from some nearby to & fro local movements ) we would see all objects as redshifted, and interpret that shift in a Doppler sense. But we have more than that from our evidence, as we have the speed of recession in proportion to the distance from us, and that has an extra implication.

Arrange three people in a line, initially stationary and some 10m apart from one to the next. Tell the left hand one to stay put ( call them the 'base' ), tell the middle one to walk for one second at 1m/sec to the right with respect to base, plus tell the right hand person to move to the right at 1m/sec with respect to the middle one. After one second the base is unmoved, the middle one is 11m to the right of base and the right hand person is 22m to the right of base. So from the viewpoint of base : the middle one has done 1m/s, and the right hand one has done 2m/s. What have we demonstrated ? We have shown that using a constant expansion instruction - move away from your nearest neighbour by 1m/s for every 10m of separation from said neighbour - leads to a recession speed in proportion to distance.

You could repeat the logic for, say, the middle one by keeping them stationary ( = base ) and letting the left and right hand person each move at 1m in that second : you still get the same relative changes. The left hand person will move a further 1m to the left of base, and the right hand person will displace a further 1m to the right. It would still be a 2m increase in separation b/w the left hand and right hand person for that second of travel. Ditto if you consider the right hand person the 'stationary' one.

But this is symmetric for all players : everybody deems everyone else moving away, the recession speed each measures is proportional to the distance to the thing they are measuring the speed of, and everybody will agree on the amount of proportionality ( 1m/s for each 10m of separation ).

Most importantly no-one can claim to be 'at the centre'.

I've described the case for one-dimension of travel ( the line of three people ) but the argument extends simply to three dimensions and the conclusions remain. Hubble deduced the proportionality from his data as about ten times what we would claim now. We have more precise observations than he. Roughly : for every megaparsec ( Mpc ) of separation the velocity is 70 kilometres per second ( depending upon who you ask, the Planck mission recently reported their figure as 67.80 ± 0.77 ). So if we measure the distance to some galaxy as 10 Mpc from us, then it will be moving at 700 km/s away from us. Well, not exactly but that it will be close to that. This behaviour is generically called the Hubble Flow.

You already know the punchline : wind the clock back, everything gets closer and closer. Everyone must have been all together some finite time in the past. Everything crammed into the same place at the same time. With the observed numbers that's about 14 billion years ago, give or take.

The anti-intuitive part is : there is no centre, or if there is one it is everywhere. Here 'common sense' collides with observation and everyday analogy just collapses inadequately. The especial difficulty being that we are within the thing we are describing. As far as is known there is no 'outside' that the Universe is expanding into, or if there is it is superfluous to our explanation ( and with logical positivism that equates to the same theoretical position ).

That is what makes the label 'Big Bang' so misleading, which is why I from here on will avoid using it. That phrase naturally invokes the idea of an explosion at a point within some pre-existing space, and subsequently expanding into that space. But what we have - to the best of our knowledge - is an expansion of space itself. Time for that matter too, in that neither space nor time existed prior to that ( or at least that is the stance 'as measured' ).

Now pulling back from the verge of what was/not there beforehand, what's with looking back in time with greater distances ? Well, you'd get that effect with any Universe having a finite signal speed. Though adding that idea in to our expanding scenario as above, we get a measure/view of objects as they were closer to that Point Of Confluence ( yep POC, I'm avoiding using 'Big Bang' alright ). But we have to consider here another issue, that of whether the Universe has always behaved the way we see it now. That time from now to the POC assumes more or less a recession rate of 70 km/s/Mpc, and remember that we haven't actually got time and distance measures mapped deliberately by us for all time and space. We have assumed ( even if we didn't realise that we did ) that the Universe is explicable and has some 'thread of consistency'. As that deserves it's own discussion, then we have next up :

Why Believe Cosmologists Anyway ??

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Mike Hewson
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Why Believe Cosmologists

Why Believe Cosmologists Anyway ??

[ By this I don't mean that they are untrustworthy people, but that we should uncover the assumptions they use in their work ]

Ok, to date I have avoided discussing some elephants in the room. They are called supernovae. Stars suddenly blowing up and making quite a mess nearby. We use supernovae ( some of them at least ) as standard candles. That's because you can see them from a long way off and I really mean a long way. Literally from the other side of the Universe. For a relatively short time in the life of some stars ( their deaths actually ) they will outshine their previous emissions and that of the galaxies that they are within.

So for use as standard candles we need to examine their intrinsic brightness, or if you like what they would appear to be if they were all at the same ( unobstructed ) distance away. Then if for some of them we have a distance measurement independent of their brightness, we can use the apparent brightness of other similiar supernovae to deduce their distances via inverse square etc.

Why do some stars explode ? Mostly it's the astronomical equivalent of 'what goes up must come down'. Gravity always wins in the end. What stops our Sun contracting today is that it is generating heat - from fusion reactions in the centre - and the hot gas pressure pushes outwards to balance the attractive gravity. The trouble is that the Sun radiates heat energy into the much cooler surrounding space, and so to maintain the temperature - hence the outward pressure - that energy loss must be replaced. By 'burning' more hydrogen to helium. Yes, you've guessed it, the hydrogen will run out. But even when that happens, you can still convert the helium to, say, carbon and produce excess heat. When the helium runs out, you can now burn the carbon to oxygen, then the oxygen to .... etc.

But eventually when you get up to near iron, fusion no longer releases excess heat and you have to put energy in by some other mechanism to obtain further fusion. If at all. For stars that have proceeded this far ( and not all do, especially if they are lighter ) this brings on a crisis. Quite suddenly the star cannot generate the pressure required to combat gravity's inward pull, and it collapses catastrophically in a fraction of a second. So you have all this stuff rushing inwards - sort of a death dive - and it meets in the middle and 'bounces'. Splat goes the neighbourhood as a relativistic fireball of material, a blizzard of neutrinos and a gazillion gammas pour outwards.

[ This is the stock description for one supernovae mechanism. There are other supernovae types on the table and I won't go into them. ]

However that outgoing material does have stuff heavier than iron though and that is because the kinetic energy during the inrush did provide the energy to fuse up to those heavier elements. Some of those will now decay by fission and in doing so release light of a specific signature that we can detect and characterise. Now there is ( believed to be ) a good relationship between the absolute brightness and the rate of it's subsequent decline. So if I see a supernova like this and can map it's 'light curve' over the ensuing days, weeks and months then I can ( closely ) estimate the peak absolute brightness from the pattern of light reduction. Comparing the absolute peak brightness thus calculated with the observed/apparent peak brightness I can use inverse square to give a distance. Phew !! :-)

So this is very much a proxy for a proxy for distance manner of argument. It's not invalid per se, just that it contains alot of qualifications and assumptions. The main 'weakness' if you like is that it depends on how well we understand a supernova explosion. We are fortunate in that the putative mode of collapse also involves the production of radioactive material ( eg. Cobalt ) that we can understand and validate/calibrate here on Earth. Hence we know how that glows after the star goes bang.

What I've outlined here is the really deep distance method. A galaxy very far away is going to have Cepheid variables for sure. But we will never distinguish any given Cepheid from the other stars within the same galaxy at that distance. Supernovae shine out from the pack though, indeed for a short time they outshine their host galaxy. We are also fortunate in that the supernovae type described here are common enough in the Universe ( but not nearby us or else we would not survive to tell ) that if we look frequently enough at the sky in detail we will see them. Having seen one we can alert about that and then follow the light curves ... etc.

Finally back to ".... the Universe is explicable and has some 'thread of consistency' ..." : if I saw a really distant supernova then that also is one that happened a long time ago. I'm seeing it now because it took most of the age of the Universe for the light to reach me. So even if I reckon I understand the supernova collapse/rebound mechanism well, I still need one further assumption being that supernovae now behave like supernovae then. Or if you like : are the sort of stars created shortly after the Point Of Confluence sufficiently like the sort of stars created a much longer time after the POC ? Because maybe they aren't and if so then they might explode differently, thus screwing up my proxy based upon post-bang supernova light curves. Next up :

What's All This Dark Stuff About ??

Cheers, Mike.

( edit ) I should emphasise that the further along in this thread that you go, the more possible contention could occur about the statements, the findings and the logic. So someone could step in and challenge somewhere, a 'point of order' so to speak. Indeed something like that is increasingly needed as cosmology has reached a bit of an impasse, as we will shortly describe. I remember a chap called Rocky Kolb ( cosmologist in the USA ) who about ten years ago said that "we don't need a 'consensus model', we need the right one". What he was alluding to was that it is agreement with observation that is required to validate a theory, and not especially agreement amongst the knowledgeable practitioners. :-)

( edit ) I remember a sci-fi story, but not it's title, which had explorers from Earth leaving the solar system to find that they quickly reached a barrier or membrane. Some aliens then invited them in/out so to speak, because the Universe was really only a bit bigger than the solar system. The aliens had been projecting stuff onto the enclosing surface of such content as to make us think there was a larger Universe behaving in a certain way. The aliens didn't allow the explorers to travel back to Earth, thus revealing the secret scam. The novel didn't reveal what the aliens were sitting in ie. what was the larger entity containing them. But it was a cute emphasis upon the point that, to date, it is only light that informs us in detail of what's out there. I guess when we get our GW detectors firing then maybe the aliens could fake such responses too ..... :-)

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

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What's All This Dark Stuff

What's All This Dark Stuff About ??

We have the Hubble Flow. This is the streaming of the universe's large scale constituents ie. galaxies and groups thereof, away from us. Expansion for sure, but no particular centre as all observers will see that trend happening, because the relationship b/w distance from to speed from is the same for all. Meaning that if we do a plot of, say, recession speed on the y-axis and distance on the x-axis you'd have a straight line.

Except that you don't. Or at least you get it mostly for the closer stuff. But for really long distances away ( = long times ago ) the speeds ( redshifts ) are more than expected. Where 'expected' means assuming the expansion rate that we observe 'nearby' ( = recent times ) holds there/then too. Thus because the redshifts are greater for those older objects that means we are moving away faster now than we were then ( yes, that may seem a paradoxical way to say it, but remember we are recording what we receive here on Earth now, and things may have changed since the light was emitted then ). We get the phrase/idea that the 'expansion is accelerating with time'. That no doubt conjures up all manner of further ideas, not the least of which is why ??

Here's especially where explanations get squirrelly - IMHO - as one's theoretical ratification of circumstances now depends upon what factors ( that led to this deduction of accelerating expansion ) you want to keep, to change, or to throw away. Or add for that matter. I'll outline a couple of ways to proceed, based upon what I think I know the best, which of course means I may have well misunderstood things .... :-)

One resolution is to claim that earlier supernovae, from a 'young' universe, came from a set of stars with a different character than those we see now. If you take the composition of the elements in the universe just after the Point Of Confluence ( well, at least more than a handful of minutes after ) then you will really have alot of Hydrogen, a good slab of Helium, and little else. Maybe Deuterium, and perhaps some Lithium, but that's pretty well it. They were on average probably much bigger stars too than we have forming today. The upshot is that maybe these early types evolve differently ( but still basically it's gravity vs nuclear fusion energy ) and thus go bang differently, and thus their light curves post-supernova are different. Thus throwing out our light-curve to absolute-intensity to distance calibrations. You could reverse engineer the characteristics of said early stars in order to gain a fit with what we see. Nothing per se wrong with that exercise, but can one corroborate with other orthogonal data?

Another resolution is to say that if I plug stuff into Einstein's GR equations and see what must be true for GR to predict what we see, then you wind up with a mathematical term that in effect states that empty space has a 'negative pressure'. Or put another way : empty space has a constant amount of energy content, a small but fixed amount for each element of volume. The Dark Energy*. But do take care to note that this really doesn't equate to an everyday concept very well, meaning that's it's not like we have a sample of it to play with. This recycles a concept brought up by Einstein himself, the so-called 'cosmological constant'. Back then Albert was trying to achieve something rather different though. He wanted to make a universe which wouldn't change much in size, a static flavour. This was his self-confessed 'biggest blunder', but to be kind to him the evidence ( pre-Hubble was when he said that ) did point to that. My personal opinion on Dark Energy is that it is an intellectual placeholder.

Maybe this is an opportune time to mention a theory which once competed with the 'Big Bang'. Indeed it was a theorist who came up with an alternate explanation called 'Steady State' that actually coined the term 'Big Bang' to describe the opposing theory. But he'd intended that to be a derogatory term! It was Hermann Bondi, Tom Gold and Fred Hoyle who championed this idea, that of continuous creation of matter and space in between that which already exists. The idea is alleged to have been first thought up by some earlier astronomers, and added to by the influence of a suspense movie plot! If you like the Steady State theory has a tendency to assume what it proves, in the sense that : things are the way they are because they always have been. Now it's OK up to a point to have self-consistent circularity in theory, but it's predictions conflicted with observations regardless. In particular it completely falls over in the presence of the cosmic microwave background.

In any case if you Google, say, "expansion of universe accelerating" you'll touch upon all sorts of discussions. Model dependent discussions that is. Here's a recent one, quite cute if you like, alleging a conspiracy of mass variations. And so it goes. Next up:

Dark Matter

Cheers, Mike.

* Some sort of new anti-hero for Batman to have a crack at perhaps ?? :-)

( edit ) BTW - doesn't anyone have any questions ? :-)

[ forum stats indicate about two dozen of you are reading along .... either that or someone is re-viewing alot ! :-) ]

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

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[ forum stats indicate about


[ forum stats indicate about two dozen of you are reading along .... either that or someone is re-viewing alot ! :-) ]

Carry on Mike. I enjoy your articles.
Richard

Richard

Holmis
Joined: 4 Jan 05
Posts: 1,007
Credit: 712,756,983
RAC: 616,695

RE: BTW - doesn't anyone

Quote:
BTW - doesn't anyone have any questions ? :-)


Probably a lot of questions just can't think of any right now... =)
I do enjoy reading and learning about this so please do keep up the good work!

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