originally posted by: bigfatfurrytexan
a reply to: artistpoet
That is kinda my point. It seems just as plausible that we really can't rely on redshift due to various factors such as inconsistent speed of light,
gravitational lensing effects, and the fact that we just don't know if gravity is even constant.
There are multiple checks and balances when measurements such as these are taken, the way you describe your criticism of the appears not to take into
account actually how these measurements are typically taken and tries to vastly simplify the process as through astronomers whip out a little
telescope, eyeball something and scribble down a few numbers.
Truth is that redshift and blueshift are used in order to determine movements of material at these kinds of distances.
What will happen for a target galaxy is that spectrographs will be taken at many points within a galaxy's observable shape. The global red/blue shift
will be determined by looking at specific adsorption and emission lines. This determines the background radial velocity which is subtracted.
Now, the part when people say "Oh but the speed of light can be variable" i find it hard for that to be a justifiable criticism because the physics we
know and understand as determined by constants such as the speed of light appear to work here just as well as they work remotely. That is, if the
local speed of light at a target galaxy is different, then you would expect atomic emission lines to not only be in the wrong place, but doppler
shifts such as redshifts to have total inconsistencies when observing galaxies that are the same distance but, different locations in the sky... truth
is, its not, everything appears quite nicely consistent.
So what is observed? well for a normal galaxy after the bulk doppler shift has been removed, you determine the geometric rotation of the target
object. if it is a disk like the milkyway, and is roughly side on, you will see one side with a redshift and the other with a blue shift. You take all
of this data, put it into a model of a disk galaxy and you figure out what the rotation curve looks like for matter positioned at different distances
from the galactic centre.
Now, what we find is that the rotation curves for ALL disk galaxies are roughly flat out to very large distances, theoretically if basically what we
can see is all the matter there is, the rotational velocity should fall with a roughly 1/r^2 relation.
Now thus 'FUDGE' factor as people so often want to call it, is a theoretical correction. we ask the question
"Observation and theory are so different something has to be happening... but what?" We come with several possibilities
1) Unobserved material
Dust, blackholes, rogue planets.
- We put these into the models and still it doesn't work, such objects would have a similar distribution as the observable material! We would have to
think up some weird and exotic things to happen (in every galaxy) to get what we see.
2) Unobserved Exotic material
WIMPs , Axions... etc
- If a particle that only interacted gravitationally but not strongly in any other way was present, it wouldn't have to clump in the same way a galaxy
does, and could, take far longer to form dense clumps, sounds like this could for clouds around regular material and give us exactly the rotation
curves we observe with almost NO effort and fine tuning magic. Pretty much put it in the model as a sphere that a galaxy sits in, boom it works
3) Modified Newtonian dynamics.
Gravity doesn't work the same everywhere and that on larger scales it doesn't have a 1/r^2 drop off.
- This is an ok idea except it quickly turns into a fine tuning mess of fudge factors that do not appear to have any physical meaning. That is, every
galaxy has different tuning factors, and two galaxies at the same distance from the earth, with roughly the same gravitational environment around them
will have tuning factors that are completely different... This sounds like a complicated and none physical idea
4) Speed of light is different
See above. The thing about physics is that we see repeatable patterns and the ultimate goal is to devise experiments and tests that will give you
repeatable results in order to test theories. Theories can then be thrown out dependant upon these results.
If the speed of light is truly different at different locations, then the universe as we observe it just wouldn't work. Think about it... nuclear
processes are dependant upon the speed of light... why? well the old none relativistic form...E=mc^2
There would be a paradox in something as simple as positron emission energies. why? well, you have an inverse beta decay that gives you a positron,
now lets assume that a positron here is the same as a positron there... you collide that with an electron and boom, you get 2 gammas equal to their
rest mass energy... only, if the speed of light is different in their local, then the energy produced by that WONT be the same everywhere... it will
be more, or less depending on if the speed of light is higher or lower. We have zero... i repeat ZERO evidence that this occurs and yet we have lots
of evidence that we do actually observe 511keV gammas originating all over the galaxy... sooooo yeah speed of light does appear to be constant.
So which seems a better theory? one that works and is simple? oooor one what is terribly complicated and probably doesnt work even with all the kinks
Not that we shouldn't try to study these things with our current understandings. Only that we seem to be risking a pigeon hole now, as we have flat
out invented something that has never been observed simply to balance an equation: dark matter.
And perhaps continuing studying it will help us identify novel dimensions and such. Who knows. But at the same time alternate theories should be
funded to find a way to balance the equation without the dark matter kludge.
Again, you make it sound like it is a horrendus kludge... when it really isn't, if you studied the field fully you would see for yourself that
alternative theories are funded and there are more theories than i can even count...
It wouldnt be the first time that physics 'invented' something to fix a problem, that actually turned out to be true... neutrinos are a fine example.
Back then, the very notion of conservation of energy and momentum... something that worked perfectly in every observable instance, EXCEPT in beta
radiation... why? why and how could that be?
Well they couldn't see the neutrinos being produced and the invention (as they saw it back then) of this third particle of the interaction that was
unobservable was also a terrible travesty/cop out/kludge.... science proved its existence though... so... not a kludge after all.
Could Dark Matter prove to be the same? Dont know... but I and a whole host of physicists intend to find out one way or another
edit on 2-10-2016 by ErosA433 because: (no reason given)
edit on 2-10-2016 by ErosA433 because: (no reason given)