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Post by eamonnshute on Nov 20, 2010 13:46:42 GMT 1
Fine again, but what error of observation or calculation would there be? One example:Observations show that the rate of expansion of the universe was decreasing until about 5 billion years ago, but since then it has been accelerating. No force could produce this behaviour. |
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Post by eamonnshute on Nov 20, 2010 13:48:37 GMT 1
If it was evenly distributed then what was its shape? The universe seems to be infinite and without a boundary, which makes it rather meaningless to talk about its shape. |
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Post by Progenitor A on Nov 20, 2010 14:20:25 GMT 1
If it was evenly distributed then what was its shape? The universe seems to be infinite and without a boundary, which makes it rather meaningless to talk about its shape. Was that also the case at time t=0 Also what does 'expansion' mean when we have an infinite space anyway? Surely an infinite space occupies all possible space and expansion of that space is a contradiction? |
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Post by Progenitor A on Nov 20, 2010 14:26:29 GMT 1
Fine again, but what error of observation or calculation would there be? One example:Observations show that the rate of expansion of the universe was decreasing until about 5 billion years ago, but since then it has been accelerating. No force could produce this behaviour. That does not seem like a convincingly overwhelming case for expanding space. If a force is required to change that velocity, then surely this increased expansion is put down to the presence of Dark Matter? (God knows what it was doing before the acceleration.) |
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Post by speakertoanimals on Nov 20, 2010 14:36:18 GMT 1
As regards mutual gravitu somehow overcoming expansion, that is not quite the case.
First off, the space uniformly expanding cosmological solution is, if you like, an approximation, based on the largest-scale picture, where matter is taken as being uniformly distributed throughout space. Then we get uniform expansion.
Except at the next level of approximation, matter isn't quite uniform. Because gravity attracts, these differences are amplified over time -- in short, where matter starts off a little more dense, gravity pulls it together into clumps, and we get a greater degree of non-uniformity.
So, what calculation you use depends on what you are interested in. Given computer simulations, you can include both effects, largest-scale expansion, and local clumping (which gives clusters of galaxies, galaxies, stars and solar systems).
Take our solar system. The point about expansion is that it gives an apparent recesisonal speed that depends on distance -- given length scales of our solar system, the related speed is actually very small. You can compute what difference adding the expansion in would make to planetary orbits over the lifetime of the solar system, and the effect is really tiny. So for all practical purposes, you can forget about expansion on the scale of the solar system.
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Post by abacus9900 on Nov 20, 2010 16:32:48 GMT 1
So, the further away galaxies are to you the faster the rate of expansion relative to you and vice-versa?
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Post by Progenitor A on Nov 20, 2010 17:47:34 GMT 1
So, the further away galaxies are to you the faster the rate of expansion relative to you and vice-versa? That is what we are told However what is the difference between expansion of space and stars/galaxies simply hurtling away from each other? |
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Post by abacus9900 on Nov 20, 2010 18:52:31 GMT 1
Well, from what I understand galaxies do move within their local space but according to STA expanding space is only significant on large scales, so that, for example, the Andromeda galaxy and our Milky Way will collide some distant time in the future because their mutual gravitational attraction will overcome the smaller local effect of expanding space. But I could be wrong, because STA seems to like setting traps for people.  |
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Post by Progenitor A on Nov 21, 2010 17:30:40 GMT 1
Well despite the best efforts of Eammon and Abacus, ther is still no satisfactory explanation of why space expansion is necessary. It seems that all the effect that we observe can adequately be explained if we consider the stars/galaxies to be rushing apart rather than the space between them expanding.
It seem to be a point of dogma, a recital of a catechism, that space expands, just as, as a child,I would question some of the catechismal dogma, and answers there were none.
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Post by speakertoanimals on Nov 22, 2010 1:15:39 GMT 1
Why expansion rather than recession? Well, locally the two descriptions are equivalent, you can look at things either in a coordinate frame that describes it as one or the other.
But this only works locally. For larger redshifts, when computing the redshift of light, you have to account for the fact that the rate of expansion changes over the journey of the light.
So, the point is that the receding from us explanation only works for relatively small recession speeds and redshifts -- for larger redshifts, we need to emply the space is expanding viewpoint, because the two are not equivalent, and especially not in cases where the rate of expansion changes whilst the light is in inflight.
There really isn't a simple way to show this, because the simple explanations that we can visualise specifically don't really distinguish between the twp scenarios. Just when you do the full computation, the two viewpoints are not the same, and that's just it. Hence in many popular science acounts, you can find the two treated as if they are the same, and just by reading those, you won't discover why they are not, or believe me when I say they are not. So, the only proper answer I can give is -- go study general relativity, and then you will discover that they are not. But if you don't, don't complain to me that y aren't convinced, who said everything important was capable of being explained on a message board without the use of equations....................
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Post by Progenitor A on Nov 22, 2010 7:50:18 GMT 1
Still no explanation of why expanding space and recession speed cannot be treated in the same way. Just repetition of dogma.
I assume that someone, somewhere, is capable of explaining the difference.
It is rather, I fear, like speaking to ordinary non-intellectual priests who believe in their dogma, but do not comprehend the intellectual underpinnings of the dogma.
I have certainly met enough such priests in my time
Myself , I am not even a priest, just a curious onlooker, who generally hears the dogma, and in questioning it, am called a fool
Never mind eh?
We will have to leave this one on the back burner unless someone has some insight.
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Post by speakertoanimals on Nov 22, 2010 13:51:32 GMT 1
Yes, and you'dn have to go and do an advanced course in relativity to find out why. Just because it is not possible to give a full explanation of why on here, doesn't mean such an explanation does not exist.
But for starters, you might like to think about how to define recession speed in relativity. Because what do we mean by distance for starters -- obvious, you might say, we just mean how far away a thing is. But how far away NOW means defining what is the same instant here and at a distance galaxy, which is far from trivial in relativity, where what NOW means depends on your point of view, so events which occur at the same time for one observer, occur at different times for another observer.
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Post by speakertoanimals on Nov 22, 2010 15:30:08 GMT 1
Except we can't forget that as far as experiments go, we have to explain gravity using general relativity, which explains what happens to light passing near the sun, the precession of the orbit of mercury, as well as stuff like gravitational lensing. So we also have to look at what GR predicts for a spacetime uniformly full of matter -- and what it predicts is that spacetime either contracts or expands.
Hence we have to consider the question -- does an expanding model for spacetime (which comes from GR, which is the best explanation for garvitational effects that we have) fit in with what we see on cosmological scales? And the answer is yes, we do see the exact relation between distance and redshift that GR associates with an expanding universe. Calling this dogma is just ignoring the usual scientific process, and ignoring the evidence we have supporting GR on many scales.
If we wanted to say -- it could just be recession, then we have to explain why we are saying GR is wrong on cosmological scales, since all the evidence we do have agrees that GR seems to work.
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Post by abacus9900 on Nov 22, 2010 15:49:48 GMT 1
STA, I notice you have not mentioned the repulsive force that is contributing to the galaxies flying further apart from one another in addition to the inertial force provided by the BB.
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Post by speakertoanimals on Nov 22, 2010 16:55:02 GMT 1
No, because the existence of dark energy is a further complication, you have to start with the simplest cosmological models (i.e., basic uniform expansion), before adding in more complicated stuff.
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