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It is accepted that space-time can be allowed to expand faster than the speed of light during the Big Bang theory's inflation phase. How can this be? I would like to know how relativity allows this since space and time are intrinsically connected together within any frame of references light cones. Surely any hypothetical observer within the initial frame of reference at point zero waiting for universe to start could not observe space moving faster than light as rate of time of the event moving away would be zero, ie rate of time at speed of light is zero.Any hypothetical observer placed outside light cone observing event of universe start is outside universe and therefore observation is meaningless.

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The limit on massive objects not travelling faster than the speed of light is a theory from special relativity where we have a static flat space. This limit does not hold in our expanding universe.

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A question...... How do we know it is still happening today if we are looking at light that is 13 billion years old?

Strictly we don't know - but then there are very few if any things we can see at 13 billion LYs, so most of the measurements are done out to redshift 1 or so, and the few we can measure further out fit the curve.

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When it refers to expansion, is it In fact talking about the expansion of space time itself, therefore explaining how we can see light that was emitted by something moving faster than c in the opposite direction.

Or does it mean that the galaxies are simply getting further away from each other?

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When it refers to expansion, is it In fact talking about the expansion of space time itself, therefore explaining how we can see light that was emitted by something moving faster than c in the opposite direction.

Or does it mean that the galaxies are simply getting further away from each other?

Sort of both, space is expanding - so things are getting further apart - but only on big scales. On smaller scales, the force of gravity, and the electromagnetic force is enough to hold things together. So out local group of galaxies will forever stay together (and merge on occasions), but distantly separated groups will get further separated.

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A question...... How do we know it is still happening today if we are looking at light that is 13 billion years old?

Stu

It has been observed and calculated that the space expands at a rate of approx. 70 kms per secon per megaparsec of space (and accelerating, due to dark energy (vaccum energy?)).

Add the distances in observable universe in megaparsecs together and you will simply get to the point of one point in space, that is sufficient distance away from another point drifts away from that point in space at a rate greater than the speed of light.

Or alternatively, add the speeds of 70 km per seconds together to match and even exceed the speed of light (300 000 km per second) and extrapolate the distance that the two points have to be apart (to drift away at the speed of light or greater) and you will discover that the distance between the two points (to observe faster than light expansion) turns out to be approx. 4800 Mpc or greater and I believe that this number fits the distances in the observable universe (observable from our point of view) with room to spare.

This is however not that the object is travelling through the grid of space at a speed of light (and thus not violaiting the relativity), but it means that the space between the two points expands at such a rate that the two points sufficiently distant from each other drift apart at such a rate.

And you would think that if an object is drifting away from me at the speed greater that the speed of light, I cannot percieve it at all, because the its light will not ever get to me. Well that is the thing. We can (from our point in the universe) see a significantly distant object A on one side and similarly distant object B on the completely other side. While the light from both objects can reach us, they are not drifting away from us faster that the speed of light.

However, the point A can be so far away from the point B that the expansion of the space between causes them to drift away from each other faster than the speed of light, thus the two points cannot observe each other at that point. But we, standing between them, can see both objects. I am given to understand that this is really the argument supporting the inflation theory in fact, because if the two objects (or parts of space) at the completely opposite edges of the observable universe (observable from our perspective) manage to look similar despite being able to communicate information between each other, there must have been a period in universe's evolution to allow this exchange of information to happen, and the theory of inflation explains all this nicely.

Furthermore, this is why cosmologists believe that the universe we can actually see counts for something like 5% of the total universe. And what is more, due to acceleration of universe's expansion, the "bubble" of the space that we will be able to see will gradually shrink and eventually, we might not be able to observe more that the local group of galaxies that we are gravitationaly bound together.

I hope I get it right however, such a difficult topic for discussion, the mind boggles :) Apparently, this is not from my head. Try and hunt down a couple of Astronomy Cast episodes, they manage to explain it well there, better than I can

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It has been observed and calculated that the space expands at a rate of approx. 70 kms per secon per megaparsec of space (and accelerating, due to dark energy (vaccum energy?)).

Actually, the 70 kms per second per megaparsec is decreasing right now, even though the expansion of the universe is accelerating.

Add the distances in observable universe in megaparsecs together and you will simply get to the point of one point in space, that is sufficient distance away from another point drifts away from that point in space at a rate greater than the speed of light.

Or alternatively, add the speeds of 70 km per seconds together to match and even exceed the speed of light (300 000 km per second) and extrapolate the distance that the two points have to be apart (to drift away at the speed of light or greater) and you will discover that the distance between the two points (to observe faster than light expansion) turns out to be approx. 4800 Mpc or greater and I believe that this number fits the distances in the observable universe (observable from our point of view) with room to spare.

This is however not that the object is travelling through the grid of space at a speed of light (and thus not violaiting the relativity), but it means that the space between the two points expands at such a rate that the two points sufficiently distant from each other drift apart at such a rate.

And you would think that if an object is drifting away from me at the speed greater that the speed of light, I cannot percieve it at all, because the its light will not ever get to me. Well that is the thing. We can (from our point in the universe) see a significantly distant object A on one side and similarly distant object B on the completely other side. While the light from both objects can reach us, they are not drifting away from us faster that the speed of light.

However, the point A can be so far away from the point B that the expansion of the space between causes them to drift away from each other faster than the speed of light, thus the two points cannot observe each other at that point.

This isn't necessarily the case.

But we, standing between them, can see both objects. I am given to understand that this is really the argument supporting the inflation theory in fact, because if the two objects (or parts of space) at the completely opposite edges of the observable universe (observable from our perspective) manage to look similar despite being able to communicate information between each other, there must have been a period in universe's evolution to allow this exchange of information to happen, and the theory of inflation explains all this nicely.

Furthermore, this is why cosmologists believe that the universe we can actually see counts for something like 5% of the total universe. And what is more, due to acceleration of universe's expansion, the "bubble" of the space that we will be able to see will gradually shrink

No, in theory, anything that we can see now, we will always be able to see, even when it is receding from us at greater than the speed of light. In practice, we lose sight of stuff because it becomes dimmer and dimmer.

and eventually, we might not be able to observe more that the local group of galaxies that we are gravitationaly bound together.

I hope I get it right however, such a difficult topic for discussion, the mind boggles :) Apparently, this is not from my head. Try and hunt down a couple of Astronomy Cast episodes, they manage to explain it well there, better than I can

Sorry, I should explain my comments, but I am very hard-pressed for time right now. Maybe I'll be able to expand on my comments sometime in the next few days, but maybe not.

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Sorry, I should explain my comments, but I am very hard-pressed for time right now. Maybe I'll be able to expand on my comments sometime in the next few days, but maybe not.

Oh I see where I went wrong. Firstly, the "bubble" of the observable universe will not actually be shrinking, only most of what we can see now will eventually drift away from view. And its not actually drifting away. The objects that are now drifting away at a rate greater than the speed of light currently emmit light that will not ever get to us and the light they have already released will get so red-shifted, due to expansion of the universe, that they will become dimmer and dimmer until eventually, they will fade away completely.

My point was that the future (billions or trillions of years) cosmologists would only see our gravitationally bound local group and anything beyond that will expand so far to fade away and become unobservable. Please, ignore my math in the previous post, I am not gifted in this respect, but I tried :)

Only thing I do not get is if the expansion of the universe accelerates, how can the expansion rate per megaparsec slow down?

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It's still happening today - galaxies more than 13 billion light years are receding faster than the speed of light. There is quite a full discussion of some of these common misconceptions here.

http://arxiv.org/pdf/astro-ph/0310808v2.pdf

This is using the particle theory of light that detaches light from the fabric of space-time. The fabric of space-time is composed of wavelengths of light. Light is woven into the fabric of space time, it can not be separated. Increasing speed of space-time increases speed of light. Surely it clear from general relativity that light is constant and it is the time component that is changing. Surely the large scale universe is better described using rules of general relativity as though it were a black hole.

Why is it that since we live in the oldest part of the universe that there are no quasars around here?

Inflation was devised because no one was clever enough to figure out how an expanding universe was created 13.7billion years ago yet still have a smooth CBR radiation signature. Observation evidence for expansion of universe correct... Inflation theory is rubbish.

Vacuum energy! Why is it not considered an alternative to Big Bang. Sorry to mention it again.. But it works. Accumulate mass as space grows.. (Alternative to Universe thread)

http://en.m.wikipedia.org/wiki/Vacuum_energy

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This is fascinating stuff and despite the brain power demonstrated, I'm still in the "I just don't get it" camp. The question I would ask, or rather the point I would make is that there seems to be an assumption in the comments above that objects are moving away from us at more than the speed of light.

Does that not make a key assumption error, in that we are assuming everything is moving away from us and that we are therefore assumed to be stationary (or is it stationery....?). I mean, there are some very technical points made and all seem sound, but almost every comment above talks about stuff moving away or expanding at some speed.

Could it not be the case that we are expanding in the opposite direction and we therefore have two objects moving away from each other, in opposite directions with each one moving at less than the speed of light but the combined effect is a seemingly faster than light speed thing happening? It's the combined speed of both objects which causes this to be the case?

Sorry for being such a layman here but if you don't ask....

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The idea is that we are not stationary, it just appears to us we are. Like sitting on the Earth thinking the sun and planets are moving, but we're sitting still.

The usual analogy is a balloon blowing up - no matter what point you pick on the balloons surface, all other points seem to be moving away from you, but actually its just the balloon expanding, and all points on the balloon if you sat on them would appear stationary. Also if there is a speed limit on the balloon, and you blow it up fast enough, some parts can move apart faster than the speed limit. Therefore you could visit the nearby places, but the further away ones are moving away faster than you can get to them.

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Inflation theory is rubbish.

And yet it matches the WMAP black-body data with an accuracy that is unmatched by any other scientific theory in the history of human thought.

http://map.gsfc.nasa.gov/media/ContentMedia/990015b.jpg

The error bars on the original plot were so small that they couldn't be plotted on the same scale, and the match with the theory of Inflation was so good that it received a spontaneous standing ovation when it was first presented.

Seriously. Humanity has never ever come up with a theory that matches the experimental evidence so well. Never. Your claim that it is "rubbish" is a little unconvincing, and needlessly rude.

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And yet it matches the WMAP black-body data with an accuracy that is unmatched by any other scientific theory in the history of human thought.

http://map.gsfc.nasa.gov/media/ContentMedia/990015b.jpg

The error bars on the original plot were so small that they couldn't be plotted on the same scale, and the match with the theory of Inflation was so good that it received a spontaneous standing ovation when it was first presented.

Seriously. Humanity has never ever come up with a theory that matches the experimental evidence so well. Never. Your claim that it is "rubbish" is a little unconvincing, and needlessly rude.

Well to be fair, very little of this is down to inflation. You need something to produce the inhomogenities, but most of what is seen from the CMB is after effects of the big bang, and doesn't rely directly on inflation. There are other theories that can produce the CMB too - as most of what makes up the structure in the CMB is produced from between around 100 seconds and 300,000 years post big bang.

Inflation happens at about 10^-32 after the BB, and although it sets the scene for the CMB, little of it depends directly on inflation.

What can be found from the CMB is a temperature, which relates directly to the BB time, and anisotropies (variations) which are down to a number of effects which I could bore you about.

As you say the error bars make theory and observation fit EXTREMELY well.

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Thanks for clarifying Julian0.

My understanding is that the black-body curve of the CMB is directly attributable to inflation, since (for one) it explained why the universe appears to have thermalised at a point in time where it should not be possible due to speed of light limitations. The black-body curve measured by WMAP is a direct prediction of Guth's theory.

Not only does inflation theory explains this very nicely, but it also provides a mechanism (spontaneous symmetry breaking) that fits nicely with current HEP theories.

Mostly I was just irritated by someone so casually dismissing a theory that is widely regarded as a triumph of modern physics. Apologies to all for my tetchy response.

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There are often some very interesting threads in this section and I enjoy reading them. Most of it goes way over my head but I learn little bits.

So at the expense of sounding very dim I dare to ask a question.

The universe is expanding and I can go with that it makes sense enough for me. But, is there any reason why all matter could not have been constantly shrinking within a fixed size of space that was initially created by The Big Bang. Therefore effectively giving the appearance that everything is expanding. So I suppose I am saying that all matter could possibly be continuously under compression... (maybe by the dark matter :)).

Just a thought any how..:)

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Thanks for clarifying Julian0.

My understanding is that the black-body curve of the CMB is directly attributable to inflation, since (for one) it explained why the universe appears to have thermalised at a point in time where it should not be possible due to speed of light limitations. The black-body curve measured by WMAP is a direct prediction of Guth's theory.

Certainly thermalisation is one thing that inflation fixes. There are other schemes that could give the same effect though, initial conditions or something like the ekpyrotic model (which is just another way of getting to the same conditions). So thermalisation is required to have an isotropic (the same in every direction) signature, and something needs to give it little lumps of structure - both of which inflation gives you (and more).

So I'm not sure the black body signature is a direct result of inflation even so. Maybe you could explain your reasoning.

Not only does inflation theory explains this very nicely, but it also provides a mechanism (spontaneous symmetry breaking) that fits nicely with current HEP theories.

Yes - inflation fixes a number of issues (horizon, flatness, monopoles...) - and the data is consistent with it.

Mostly I was just irritated by someone so casually dismissing a theory that is widely regarded as a triumph of modern physics. Apologies to all for my tetchy response.

Indeed!

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There are often some very interesting threads in this section and I enjoy reading them. Most of it goes way over my head but I learn little bits.

So at the expense of sounding very dim I dare to ask a question.

The universe is expanding and I can go with that it makes sense enough for me. But, is there any reason why all matter could not have been constantly shrinking within a fixed size of space that was initially created by The Big Bang. Therefore effectively giving the appearance that everything is expanding. So I suppose I am saying that all matter could possibly be continuously under compression... (maybe by the dark matter :)).

Just a thought any how..:)

I think it comes to the same thing, more or less. However we are use to things expanding - blowing up balloons, stretching rubber sheets. Making things shrink seems to give you much the same thing, but somehow seems more complex. Expanding space stretches light, but I suppose diminishing size of detectors would do the same thing. If they amount to the same thing, stick with something more familiar I think, unless one model makes predictions the other doesnt and we can check it out.

Dark matter has very specific functions in the current models, and squashing everything isn't one of them. You'd have to come up with something new. Dark energy might be nearer.

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So I'm not sure the black body signature is a direct result of inflation even so. Maybe you could explain your reasoning.

Perhaps I wrote was a little strong; let me try to be more precise. My understanding was that, taking GR at face value, it was not possible to explain the homogeneity and isotropy of the universe. Alan Guth proposed the theory of inflation, along with a possible mechanism, to explain the thermalisation of the universe at very early timescales.

This theory predicts a perfect black body curve, which was what WMAP measured, thereby confirming the theory. I use the phrase "confirming the theory" in the sense that in science we never really prove things (that's for the mathematicians), but rather data either provides support for a theory, or proves it is wrong.

You are right of course that initial conditions could provide a flat universe without the need for an inflationary phase, but wouldn't Occam's razor get rid of that stunningly unlikely scenario?

I'm not familiar with the ekpyrotic model -- as I'm sure you can tell, this discussion isn't really my area of expertise :)

I genuinely thought that the WMAP data was a strong confirmation of Guth's theory -- he even mentions it in his book! -- but perhaps I'm a little off?

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Making things shrink seems to give you much the same thing...

Wouldn't you have to come up with an explanation for why the proposed shrinking of matter is only happening on very large scales, and not on scales the size of galaxies (or even galactic clusters)?

This non-uniform motion is neatly explained by gravity counteracting the expansion due to dark energy. I think you'd have to come up with some serious mental gymnastics (like epicycles) to fit that into a "shrinking" theory.

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