Thread: Red shift

Red shifting light is a feature of special relativity.

In classical mechanics, when light is emitted from an object that is moving away from you, it will appear to be moving towards you slower than when emitted from a stationary observer.

This does not fit with observed facts.

In Special Relativity, the speed of light is constant for all reference frames. In relativistic mechanics, the wave length of the light shifts. For an emitter moving away from you, the light will appear red and for an emitter moving towards you, the light will appear blue. This is just Doppler shifting. This does fit empirical facts.

So the biggest supporting factor for it is the doppler shifting of everything outside our 'local cluster' and the fact that special relativity tells us that this means that every point in the universe is moving away from us.

Another huge supporting bit of evidence is the Cosmic Microwave Background Radiation. This is coming at us from all direction in the night sky and we interpret it as a record of the big bang.

Another bit of evidence is that the ratios of elements in the universe are consistent with a universe being around 13 billion years old. We can count the generations of stars that could occur in that timespan and since they manufacture the elements, the number of generations of stars affects the ratio of hydrogen to everything else.

The big shortcoming in it is that, as the theory stands today, one has to have a non-zero gravitational constant in Einsteins field equations for it to work (this is general relativity now). So the question is, "where does this come from?" In Quantum Field Theories, one gets a vacuum effect where one constantly has particle/anti-particle virtual pairs popping into existence and annihilating each other. This is one popular candidate but all the QFD's give a value for it of around 1 and our observations indicate that it should be slightly greater than 0. Another way of going about it is to postulate the existence of a 'quintessence' field. This is not well understood.

At any rate, postulating a non-zero graviational constant is supported by the evidence and clears up a couple of other questions, for example "why is the expansion of the universe accelerating as opposed to slowing down as GR would have us expect?"

Another possible solution is to propose that gravity tapers off to mM/r at great distances as opposed to mM/r^2. This is supposed to fit the facts almost perfectly but nobody's figured out how to make it work.

All in all, it's a pretty good theory with some problems.

The LHC is a large project specifically designed to examine the moments after the big bang. Brian Cox also elaborates on the Big Bang within this video. Give it a watch.

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Wow, thanks guys.

One question though. Philosipher, what does "M" and "r" stand for? I take it "m" stands for mass.

I'm sure I've seen M used somewhere. And I'm guessing "r" stands for radius, but I'm not sure.

m is the mass of one object and M is the mass of the other

Originally Posted by PhilosopherStoned

m is the mass of one object and M is the mass of the other

oh realllly.

I would have put mamb

And r is the distance between their centres of gravity.

You can generalise this to the force between any two point particles. It's basically proportional to the force-generating aspect of the two particles (for example their charge in the case of electrostatic attraction or their mass in the case of gravitational attraction), and inversely proportional to the square of the distance between them;

F = k*x1*x2/r^2

Ok, I hardly understood that Xei lol.

I am only looking for something that a class of idiots will understand. Not that I am saying the class are all idiots. I am just saying it needs to be that simple. Which is annoying, cause for some things, that just isn't possible.

Originally Posted by PhilosopherStoned

Another possible solution is to propose that gravity tapers off to mM/r at great distances as opposed to mM/r^2.

Originally Posted by slash112

I am only looking for something that a class of idiots will understand. Not that I am saying the class are all idiots. I am just saying it needs to be that simple. Which is annoying, cause for some things, that just isn't possible.

Better leave that part that I quoted out. I'm not sure about it and I've been doing some research. It got published in a major journal but the general consensus seems to be that it's crackpot. On the surface, it looks like it would make gravity stronger but then negative pressure comes in (not sure how yet) and makes it expand faster.

Idiots generally have a hard time with the BB.

Sorry.

Basically if x is proportional to y then you write y = k*x where k is constant. If you double x then y doubles.

Force is proportional to the thing that causes force. In the case of two particles applying a force to each other, it's proportional to the force-causing aspect of both of them, so F = k*x1*x2; if you double either force-causing aspects, the force doubles. Also it's proportional to 1/r^2, so if you double the distance between the particles, the force divides by 2^2 = 4.

Makes more sense..?

Originally Posted by Xei

Sorry.

Basically if x is proportional to y then you write y = k*x where k is constant. If you double x then y doubles.

Force is proportional to the thing that causes force. In the case of two particles applying a force to each other, it's proportional to the force-causing aspect of both of them, so F = k*x1*x2; if you double either force-causing aspects, the force doubles. Also it's proportional to 1/r^2, so if you double the distance between the particles, the force divides by 2^2 = 4.

Makes more sense..?

Yea, it makes a bit more sense. I just don't see where this comes into red shift or big bang.

It doesn't really, I was just responding to post 4.

The easiest way to think about red shift is that, as a light emmiting body moves away from you, the distance between the peaks of the light waves increases compared to what they would be if the body was still, and therefore the light has a longer wavelenth; i.e. it is shifted towards the red end of the spectrum.

Oh right, cool.

And thanks, that makes sense.

Wait a minute.

I just realised something. Where the heck does the energy come from for the acceleration to happen? I know this is what Philosopher said kinda, but are there any theories on that?

Or... is that to do with negative pressure?

Originally Posted by slash112

Wait a minute.

I just realised something. Where the heck does the energy come from for the acceleration to happen? I know this is what Philosopher said kinda, but are there any theories on that?

Or... is that to do with negative pressure?

What acceleration? The acceleration that we see today is the reason that we postulate dark energy. The change to gravity that I was talking about is all about making dark energy disappear since it's never been observed. The reason that this ties into the big bang is that various ways of explaining dark energy also explain cosmic inflation that occurred after the big bang.

read here for stuff about the big bang.

Originally Posted by PhilosopherStoned

What acceleration?

Originally Posted by PhilosopherStoned

The acceleration that we see today.

Ok, how come you didn't know what acceleration i was talking about, then go on about the acceleration.

(just to be clear, I was talking about the acceleration of the expansion)

And you answered my question, so thanks.