Expanding Universe and the Red Shift

ynot said:
There are two machine guns. One fires 100 rounds per second and the other fires 200 rounds per second. If both are fired at a target for one second, would one gun fire twice as many bullets in to the target than the other?.
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The analogy doesn't line up to light, though.

A machine gun firing twice as many rounds per second would be like a star giving off twice as many photons/second. Which isn't what you're suggesting. You're suggesting that one wavelength would travel faster than another.

You could phrase the question, "If there were two machine guns, both firing at the same rate, but one which fired bullets that traveled at speed X, and the other fired bullets at speed 2X, would the targets have the same number of impacts, or more?"
The answer is that while the first impact would come from the gun whose bullets travelled faster, after that the rate of impact would match up perfectly. So basically, no. The second guns bullets would be more spread out (in space, not time) on the way to the target, but you would have the same number of impacts/second. That number would be dependant not upon the speed of the bullets, but on the rate of fire.

Don't take my word for this, though, I think I'm right, but I just thought about this in my head, and could have made an error. Work through it yourself.

But as PixyMisa points out, I don't see what this has to do with redshift.
 
ynot said:
There are two machine guns. One fires 100 rounds per second and the other fires 200 rounds per second. If both are fired at a target for one second, would one gun fire twice as many bullets in to the target than the other?.
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Well, yes, because you've stated in the question that one fires twice as many bullets in a given time than the other does.

But how is this at all relevant to red-shifted light?

ETA: Beaten to it...
 
Here's the thing, ynot. From what I know (which isn't much) redshift is perfectly explained by the Doppler affect.
No one has suggested any other mechanism that would shift the entire spectrum of light emitted from a distant body. But this is exactly what we would expect to see happening if that body is moving away from us.

You can't explain it by some of the light being left behind along the way, because we're not receiving less light than we would expect to(well, unless I misunderstand), it's just different wavelengths. But the weird thing is that those wavelengths all have a similar pattern to what we'd expect from that body- just slightly shifted toward red.

Of course there could be another explanation other than movement. But considering how well expansion explains the observations, it would have to be a very good one. Preferably one which made new predictions.
 
ynot said:
Obviously the universe is never totally static. I would have thought that a blue shift would be more difficult to explain in an expanding universe than a relatively static one. Especially given that an expanding universe creates the opposite of a blue shift (red shift). Surely whatever causes a blue shift in an expanding universe could also apply to relatively static one. If not, please why not.
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Well take a rotating galaxy for example. The part moving away from us would be red shifted,, the part moving towards us is blue shifted and the parts moving laterally to us wouldn't have either. This has been observed and shows that the spectrum shifts are caused by movement.

Just because a galaxy as a whole is moving away from us does not mean that every portion of it is.
 
ynot said:
If something is constantly travelling away for me, I expect that it will end up being so far away that it will end up being out of my field of vision. Likewise, if the universe is constantly expanding and constantly travelling away, I would expect distant parts of it to travel out of my field of vision (not naked eye vision of course). Surely if the expansion is uniform and constant, very distant objects would be travelling away from my relative position at an ever accelerating and extremely rapid rate.



I didn’t particularly like this one either.



It is not a matter of one eventually catching up with the other, it is the quantity delivered. I am suggesting that if the infrared was travelling faster than the ultraviolet and a greater quantity of infrared was reaching the retina, that the object emitting the light would appear more red in colour. If two conveyor belts that shared the same start and finish points were delivering a substance, wouldn’t one that was travelling twice the speed of the other deliver twice the amount of substance?
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Your conveyor belt analogy isn't right. In order to see more lower spectrum (red) light, the star would have to emit more of it. Also, if the lower spectrum light traveled faster, it would appear disproportionately dimmer compared to the higher spectrum.

Why dimmer? Hmm...how do I explain this.

Ok you have two belts. One moving at rate X and the other moving at 2X. At the start of each, you have a guy who places a red photon on it at the rate of one per second. Now imagine you are at the end of each belt, what do you see?

The X belt's photons are coming of more densely packed than the 2X belt. Densely packed photons means more energy is striking your eye at any given moment, and therefore appears brighter. In order to appear just as bright as the X belt, the 2X belt guy would have to double his rate.

WARNING: NOT a cosmologist. I am but a mere chemist. I really don't know anything useful. I am willing to be corrected.

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Another thing to think about:

I plan on beating the rotating galaxy example to death, so bear with me.

Say we have two identical stars (sample spectum emitted, same size, etc) in the same galaxy that are both the same distance away from us. The only difference ... one rotate towards us and one rotates away. What would we expect to see?

In your theory, they would appear identical because they are both the same distance away.

In the Big Bang Theory, we would observe one red shifted and the other blue shifted.

We see the latter.
 
ynot said:

I don't mean to be confrontational in saying this but sometimes I wonder if some of you respond to threads from an almost dogmatic belief position. “I know this, and this doesn’t agree with that, so that is wrong”. It’s always easier to accept a new idea if it supports a currently held idea. This applies equally to myself as well of course and is in no way mean to relate to replies to my post. In fact I have been impressed with the quality of response and will learn heaps from it - Thanks again.
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I can see that some of the terse responses you have received may have seemed dogmatic to you. That really doesn't matter. All that matters is the evidence. Are they right or not?

Not to be confrontational (:D), but you seem overly worried that because the Big Bang seems to agree with one particular interpretation of the Bible. If it makes you feel any better, Genesis doesn't really fit with the BB all that well. The universe is too old, and the order of Creation is all screwed up.
 
Just as a point of order;

The vast, vast majority of stars that we can observe are within our own Milky Way Galaxy, and their positions and velocities relative to us are unaffected by the Universal expansion. Their redshifts or blueshifts are due to their orbits around the Galactic centre.

The few individual stars we can see outside the Milky way are almost all either Cepheid variables, in Local Group galaxies (such as Andromeda) and nearby galaxies, or novae or supernovae in other galaxies. These objects are all gravitationally bound to their parent galaxies and are therefore subject to the same redshift as their parent galaxy.

Talking about the redshift/blueshift of stars as regards Universal expansion is thus redundant.

Doppler shift (ie velocity) is the only workable explanation for galactic redshifts that I know of (see PixyMisa's excellent first post for an elegant description of why that is), and having spent several years working in an astrophysics research institute staffed largely by cosmologists and extragalactic astronomers I feel fairly certain that I would have heard of any viable alternatives.

The remarkably well constrained relationship between the redshifts and distances of galaxies leads to the almost inescapable conclusion that the Universe is expanding, and that this expansion is slowing down.
 
I hate to point this out, but the "Tired Light" hypothesis is actually a subset of the "Expanding Space" hypothesis. What, you thought the light started out red-shifted? What, you thought those distant objects were actually moving?

Guess again.
 
Melendwyr said:
I hate to point this out, but the "Tired Light" hypothesis is actually a subset of the "Expanding Space" hypothesis. What, you thought the light started out red-shifted? What, you thought those distant objects were actually moving?

Guess again.
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Well "moving" is just the easiest way to picture an increase in distance over a period of time.
 
A couple of things:

1. The speed of light is predicted as a constant regardless of relative motion or wavelength by Maxwell's equations of Electromagnetism.

2. The expansion of the Universe is predicted by Einstein's General Relativity theory (although strictly speaking it predicts that the Universe is either contracting or expanding but cannot be static). Einstein then added a cosmological constant to keep the Universe static (which is what was generally believed at the time). Georges LeMaitre proved that Einstein's Cosmological constant leads to a highly unstable equilibrium, which in turn would produce either a quick contraction or runaway expansion.

So we interpret the red shift of most galaxies as a Universal expansion. The Cosmic Microwave Background Radiation is the extremely red-shifted afterglow of the hot early Universe when it was ~300,000 years old (whatever that means).
 
KingMerv00 said:
Really? I've heard numerous times that the expansion is accelerating.
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Yeah, it's complicated by several factors, but for the vast majority of the Universe's history the expansion has been decelerating, hence the Hubble constant being pretty, well, constant at ~70km/s/Mpc (ie the further away something is the faster it's going and the further back in time it's light was emitted).

There is evidence to suggest that the rate of deceleration has dropped and that the expansion may actually be beginning to accelerate, but it's far from conclusive.
 
SpaceFluffer said:
It is - we have pretty strong evidence for a positive cosmological constant.
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Ooh, nice plot, where's that from?

I have to admit, being a nearby stellar observer, I've not been keeping fully up to date on this stuff. I stand corrected.

Of course, evolution of type 1A supernova properties could throw a spanner in those works!

edited to correct speeling
 
Understandable! The evidence for a positive cosmological constant comes from Type-1a Supernova in the range z=0.2-1.2 or thereabouts, which didn't come into it's own as a technique until the past 5-8 years.

However, the CMB provides an important cross-check on this, and there's enough evidence to support a positive cosmological constant without the SNe 1a evidence, since we know the matter density and total density to good precision on the basis of WMAP data alone.

The plot is from a paper by Saul Perlmutter, but doesn't show experimental data. It just shows the different histories that result when one changes the cosmological constant from negative to positive.
 
SpaceFluffer said:
The plot is from a paper by Saul Perlmutter, but doesn't show experimental data. It just shows the different histories that result when one changes the cosmological constant from negative to positive.
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I thought those little black dots, with error bars, were the experimental data. No?
 
Just to beat a dead horse. :D

Your explanations are interesting; however, they are built on a flawed interpretation of what red shift is. Any explanations geared toward explaining a false representation of the red shift will be easily and quickly explained away.

Red shift doesn’t mean we are getting more red light, or that even the stars look redder. In fact, if you follow what red shift means we get (at least see) less red light, the light that started as red light has been stretched to a slightly longer wavelength and is now infrared and beyond our visible spectrum. Orange light has been stretched to red, yellow to orange, blue to green, and so on. Also, the ultraviolet, which was slightly too small to be seen before, has now been stretched a bit and fills the violet light spectrum we’re used to seeing and completes the full spectrum white light we see the stars as.

The next logical question is; if the light is still white, how do we tell there has been a red shift. For this we have to use spectroscopy. Every element (such as hydrogen, oxygen, iron, gold, etc) gives specific bands of light when heated to the glowing point, or in the case of stars, absorb specific bands of light when pure white light is shown thru it. This is very consistent, and we are very good at lining up the lines to identify the element. The red shift was kind of discovered by accident. When they went to identify the elements of different stars the lines didn’t line up, but if they moved the expected lines towards the red side of the spectrum they lined up again. This combined with the idea of the Doppler Effect, suggests that the distance between the source of the light and us is increasing.

Few links:
http://en.wikipedia.org/wiki/Redshift
http://en.wikipedia.org/wiki/Spectroscopy
http://www.astro.ucla.edu/~wright/doppler.htm
http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/redshift.html
 
I simply don’t have time to respond to all the replies but thanks for them all (very helpful). The “penny has dropped” (like it when that happens) regarding exactly what the red shift is so can now better understand the error of my ways. The dead horse can now be buried.

I still have some Big Bang itches to scratch though - If the universe is constantly expanding what area is it expanding in to? Is it occupying a space that was formally nothing? If so, what is nothing. It would have to be absolute nothing, not relative nothing. It is difficult for me to accept the concept that the universe has an outer edge let alone that it is expanding in to a space that was formerly absolute nothing. How can absolute nothing exists?
 
ynot said:
I simply don’t have time to respond to all the replies but thanks for them all (very helpful). The “penny has dropped” (like it when that happens) regarding exactly what the red shift is so can now better understand the error of my ways. The dead horse can now be buried.
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:dig:

I still have some Big Bang itches to scratch though - If the universe is constantly expanding what area is it expanding in to? Is it occupying a space that was formally nothing? If so, what is nothing. It would have to be absolute nothing, not relative nothing. It is difficult for me to accept the concept that the universe has an outer edge let alone that it is expanding in to a space that was formerly absolute nothing. How can absolute nothing exists?
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Well, I'll try my best to answer some of these later.

Here is a question for you though: Why would it be so hard to imagine an edge to the universe? Why is an infinite universe any easier?

(The universe doesn't have an edge as far as we know. I'm only asking because it is important to remember that unbelievable does not mean untrue.)
 
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