razeontherock
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Ask a physicist anything. (6)
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hisgrace26
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So can the redshift be trusted? What are other theories concerning this redshift. Can you explain more, thanks!
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Chalnoth
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Yes, the redshift can be trusted.
Basically, when you generate a redshift based upon spectral lines, there are only two possible causes for shifts in those spectral lines: gravity and relative velocity. Other things that can cause a reddening, such as dust between us and the far-away stuff, at most broaden spectral lines, but do not shift them in frequency.
And if you change the physics at the source in an attempt to change the redshift, by, for example, seeing what might happen if you changed the speed of light, then you don't actually get a change in the redshift: you get a different pattern of spectral lines entirely.
So no, in the end, there simply aren't any reasonable alternative explanations. It's either gravity or relative velocity, period.
Edit: I should mention, however, that it is not only redshift that is important for measuring the expansion of the universe. You also need some measurement of distance. For this we have two basic concepts: standard candles and standard rulers. Standard candles are things that are always about the same brightness (it doesn't have to be exact, just approximate). Type IA supernovae are one example. These are supernovae where you have a white dwarf and another star orbiting one another close enough that the white dwarf can steal mass from the other star. Once the mass of the white dwarf gets to be about 1.4 times the mass of our Sun, it can no longer support its own weight and it collapses into a neutron star, releasing a supernova explosion. Because this particular type of supernova always happens from a star of the same mass, they all have about the same amount of brightness, so we can use them to determine distance.
Another method of determining distance is by using standard rulers. A standard ruler is something whose real size we know, and so when we look at how big it appears, we can determine how far away it is. One way this idea is used is by looking at the average separation of galaxies at each redshift. We know that the universe is basically the same everywhere, so a nearby redshift is, basically, an expanded version of a far-away redshift. So we know that the typical distances between galaxies at different redshifts should remain roughly the same.
When we compare these two types of measurement, or combine them with other ways of measuring the expansion, we get the same answer to within experimental errors.
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Wiccan_Child
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Redshift is the observed 'shift' in absorption spectra. Different chemicals absorb certain wavelengths of light, and we can look for these tell-tale absorption lines as indicators that a certain chemical exist:
This is a sample of the EM spectrum, from 385nm to 765nm, showing the points along that spectrum where there are dips in the intensity of light given off by the Sun at those wavelengths. These dips correspond to various different chemicals; for instance, the line marked 'K' at corresponds to light absorbed by Calcium ions, b[sub]1[/sub] to b[sub]4[/sub] correspond to light absorbed by Magnesium and Iron atoms, etc.
So, we can look at the spectrum of a particular star, look at the absorption lines, and deduce what elements and chemicals exist in that star.
However, what we see is these tell-tale patterns of lines, but they're all shifted towards the 'red' end of the spectrum by the same amount:
So we have the expected pattern, but it's all redshifted. This is what we'd expect to see if the star was moving away from us, creating a sort of Doppler effect. We can quantify the amount of redshift using the equation:
Where z is a unitless measure of redshift we observe, λ[sub]obsv[/sub] is the observed wavelength, and λ[sub]emit[/sub] is the 'normal', un-redshifted wavelength. A negative z means the object is moving towards us, a positive z means it's moving away from us - thus, we can measure velocity using redshift. We also have techniques for working out how far these objects are from us.
What we can create is a map of the universe, deducing where objects are in space relative to each other (and to us), and how fast they are moving away from or towards us. What we see is that, the further away objects are, the faster they're moving away from us. Importantly, the further away any stellar object is from any other object, the faster they receded. Everything's moving away from everything else, like dots on an expanding balloon.
That's a general overview of what redshift is, and its major implication for cosmology. Redshift has other causes, like light moving out of gravity wells, but that's another story
Images courtesy of Wikipedia.
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Naraoia
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Chalnoth: fascinating stuff. Keep educating us!
What do you mean by "a different pattern of spectral lines entirely"? Or would it be too complicated to explain here?
What do you mean by "a different pattern of spectral lines entirely"? Or would it be too complicated to explain here?
How do we know it's a Type IA supernova? Is it from watching the pair before the white dwarf blows up, or is it the properties of the explosion itself?
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Chalnoth
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ThanksChalnoth: fascinating stuff. Keep educating us!
Well, the images that Wiccan_Child posted above are really useful here. If you change the physics at the source, such as changing the strength of the electromagnetic force, instead of just shifting the spectral lines you actually change the distances between the spectral lines in different ways. You can think of this as being due to the electromagnetic force not only determining the emission/absorption of photons, but also the structure of atoms and molecules. Change that structure, and you change the spectrum.
They're identified by their spectra. I don't know exactly what they look for, but type IA supernovae all have similar (though not identical) chemical compositions, and they don't explode through lots of other material, so they have pretty distinct spectra.
I should mention that we have seen at least one object that looked like a Type IA supernova, but was far, far brighter compared to its redshift. I think the suspicion there is that it was due to a merger between two white dwarf stars, so that you get the same spectral properties, but the mass available is no longer limited by the Chandrasekhar limit. The vast majority of supernovae that are spectrally-identified as Type IA, however, match the same redshift/brightness relationship as one another.
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Naraoia
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I assume the changes are predictable? So if, say, the speed of light was actually different in some region of the universe, could we look at the light coming from there and calculate the difference?
(Oh, I just realised this is relevant to the old YEC argument about why we can see further than 6000 lightyears
)Cool, thanks!
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hisgrace26
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Redshift is the observed 'shift' in absorption spectra. Different chemicals absorb certain wavelengths of light, and we can look for these tell-tale absorption lines as indicators that a certain chemical exist:
This is a sample of the EM spectrum, from 385nm to 765nm, showing the points along that spectrum where there are dips in the intensity of light given off by the Sun at those wavelengths. These dips correspond to various different chemicals; for instance, the line marked 'K' at corresponds to light absorbed by Calcium ions, b[sub]1[/sub] to b[sub]4[/sub] correspond to light absorbed by Magnesium and Iron atoms, etc.
So, we can look at the spectrum of a particular star, look at the absorption lines, and deduce what elements and chemicals exist in that star.
However, what we see is these tell-tale patterns of lines, but they're all shifted towards the 'red' end of the spectrum by the same amount:
So we have the expected pattern, but it's all redshifted. This is what we'd expect to see if the star was moving away from us, creating a sort of Doppler effect. We can quantify the amount of redshift using the equation:
Where z is a unitless measure of redshift we observe, λ[sub]obsv[/sub] is the observed wavelength, and λ[sub]emit[/sub] is the 'normal', un-redshifted wavelength. A negative z means the object is moving towards us, a positive z means it's moving away from us - thus, we can measure velocity using redshift. We also have techniques for working out how far these objects are from us.
What we can create is a map of the universe, deducing where objects are in space relative to each other (and to us), and how fast they are moving away from or towards us. What we see is that, the further away objects are, the faster they're moving away from us. Importantly, the further away any stellar object is from any other object, the faster they receded. Everything's moving away from everything else, like dots on an expanding balloon.
That's a general overview of what redshift is, and its major implication for cosmology. Redshift has other causes, like light moving out of gravity wells, but that's another story
Images courtesy of Wikipedia.
OK, so redshift is one way. What are other ways? Can you please tell me about light moving away out of gravity? Thanks, that's very informative.
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hisgrace26
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What if God stretched out the heavens as mention in the book of Isaiah 45:1, and other places in the bible, would that indicate the redshift? It's possible. However, the disagreement is when or how long it's been expanding? Because... God could have just made it "yesterday" or few thousand years ago. LOL.That's possible too.
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Chalnoth
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Oh, yes. There is one group that claims to have actually detected this, in fact. However, their results aren't consistent, and haven't been replicated, so I am highly skeptical.
Well, sure, but we're talking less than a percent change over the history of the universe, not something that can explain a million-fold error. And besides, we have geometrical measurements that show the universe must be much older tan 6,000 years: SN1987A and the Age of the Universe(Oh, I just realised this is relevant to the old YEC argument about why we can see further than 6000 lightyears
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Chalnoth
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You mean, are you asking if your god could have made a young universe look as if it were old? Well, in principle, sure, why not? Your god is claimed to be all-powerful, after all. In principle, an all-powerful god could do anything at all. But the problem with that is that it presupposes that your god is deceptive. And if you believe there is an all-powerful deceiver god out there, then you can't trust anything at all.
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Chalnoth
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Well, there are two ways in which gravity can affect the redshift. One is if the light is coming from very, very close to an extremely dense object, such as a black hole. The other is due to intervening space-time curvature.
As far as the light coming from very close to an extremely dense object is concerned, well, this generally isn't a problem, because the vast majority of things we observe are not matter falling into black holes. And of those that are matter falling into black holes, typically we see not only matter that is just about to enter the black hole but also matter that is further away, so we can easily correct for the effect.
As far as intervening space-time curvature is concerned, well, that's the same as relative velocity. That may sound a little bit strange, but in General Relativity, whether a far-away object is moving away from us or not depends entirely upon what coordinate you use. I can describe a far-away object as moving away and the intervening space-time curvature as being flat, or I can describe a far-away object as being stationary and the intervening space-time curvature as being highly curved. In either case, I get the same redshift. It just depends on what coordinates I use. Whether the expansion is due to space-time curvature or velocity are just two different ways to talk about the exact same physical effect.
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Wiccan_Child
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Of determining the age of the universe? Redshift gave us the Big Bang theory, and from that theory we can make other predictions about what we should see. The most famous triumph of the theory was its correct prediction of the Cosmic Microwave Background Radiation: according to the theory, we should see a near-uniform radiation from the background of space. The theory can even predict the spectrum of this light - lo and behold, prediction matches theory:
This provides very strong evidence indeed that the Big Bang theory, as currently understood, is correct. This includes the age of the universe.
If you stand on the lip of a gravitational well with a clock, and throw another clock down, you'll see that second clock tick slower and slower as it goes deeper into the well. What this means for the light shone up at you from the bottom of the well, is that it appears to have been redshifted - as well as time dilating (getting longer), you get length contraction (getting shorter). You can think of it as a photon struggling to move upwards away from a planet, star, black hole, etc, and becoming stretched like a slinky. This gives it a longer wavelength, and, thus, is redshifted.
The effect is minute on Earth, but it can be detected with very accurate equipment. This is useful, as it tells us that light from distant stars has been ever so slightly blueshifted as it falls towards Earth - just as light is redshifted when it leaves a gravitational well, it blueshifts when it falls back down.
It's certainly possible, but I'm always dubious of attempts to shove modern science into ancient texts and then saying "Look! The Bible/Qur'an/Bhagavad Gita had it right all alone!". In all probability, the stretching out of the Heavens was a poetic flourish to convey the great swathe of sky the Hebrews observed at night.
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Chalnoth
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A little while ago, just for kicks, I made my own version of this plot using the FIRAS data at LAMBDA - Legacy Archive for Microwave Background Data in order to show the errors in the measurement:Of determining the age of the universe? Redshift gave us the Big Bang theory, and from that theory we can make other predictions about what we should see. The most famous triumph of the theory was its correct prediction of the Cosmic Microwave Background Radiation: according to the theory, we should see a near-uniform radiation from the background of space. The theory can even predict the spectrum of this light - lo and behold, prediction matches theory:
Those error bars that, for most of the plot, you can barely see at all? Those are 50 standard deviations. To show how absurdly accurate this measurement is, at 50 standard deviations, the error is less than one part in 10^500. That means that there is less than one part in 10^500 chance that the true value of the CMB spectrum lies outside the error bars on the plot. And yet, for the most part, those error bars are about as close together as the thickness of the line.
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GrowingSmaller
Muslm Humanist
Please:
If I balance a triangle on a larger sphere, and shine a torch such that the shadow of the triangle falls on the spehere, what is the relation between the original shadow and the triangle's shadow - is it symmetrical, transformed or what?
If I balance a triangle on a larger sphere, and shine a torch such that the shadow of the triangle falls on the spehere, what is the relation between the original shadow and the triangle's shadow - is it symmetrical, transformed or what?
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GrowingSmaller
Muslm Humanist
Oops I suppose I mean the not the original shadow but the relation of original shape of the triangle and the shadow it would have on a plane, as opposed to the shadow it has on the sphere.
A triangle's shadow on a plane may be symmetric, but on a sphere it is....?
Also a relation between a triangle and it's shadow is... (I guess "transformation"). So if I am right would there be different names for the transformation onto a plane surface and a sperical one?
I am asking this because I ultimately also want to know what the relation between a retinal image of a triangle and a processesd image of that triangle (assuming it might not completely preserve it's original form or proportions) in the visual cortex would be classed as.
But I am obviously interested in the original question too.
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Chalnoth
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Ah, well, that would all depend upon how it is projected onto the sphere. You can visualize sort of how this would work if you drew a straight line from the light source through one of the points of the triangle and onto the sphere. How symmetrical it would be would all depend upon precisely where the sphere is, the angle of the triangle, and the location of the light source.
For example, if this is an equilateral triangle, and the light source is directly above the sphere, then by balancing it you ensure that the center of the triangle is precisely at the point of the sphere closest the light source. This would indeed make a symmetrical shadow. But if you moved the light source so that it was falling on the system at an angle, it would no longer be symmetrical.
As for the precise shape, that's just a matter of drawing straight lines from the light source past the edges of the triangle and seeing where they hit the sphere.
Well, that's a somewhat different problem, actually, because we don't see shadows in that way. There is a lens involved, and that makes things a fair bit more difficult. But suffice it to say there is a lot of processing that goes on in the brain to interpret what we see. It can be kind of fun to look at various optical illusions to get a hint of what some of that processing is.
For example, when something is in shadow, our brains don't interpret it as actually being darker, but instead just being in shadow. This means that you can construct an image where you have one object in shadow and another in the light, and if these two objects are painted with the exact same color in the image, it will look to us like the one in shadow is much brighter.
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GrowingSmaller
Muslm Humanist
Ok thank you...but is it right to say then that the relationship between the object and it's shadow is one of projection?
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J
Jazer
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Consideroing the spin down rate of the earth. How long was the first day?
How much sun light and how how long was the darkness?
How much sun light and how how long was the darkness?
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