Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: John Bermingham on 14/01/2010 22:30:03
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John Bermingham asked the Naked Scientists:
Hi,
I understand that when they say the universe is expanding, they mean that new space is continuously coming into being at every point in the universe.
If this creation of new space were at a constant rate, galaxies would be accelerating away from each other, which they are.
I'm curious to know if this is what they mean when they say the universe is expanding at an accelerated/increasing rate, or do they mean the rate at which new space is being created is increasing (accelerating)?
This is maybe not suitable for the podcast, but I would be grateful if you could answer anyway.
Thanks, love all your podcasts (http://www.thenakedscientists.com/HTML/podcasts/)
John from Dublin
What do you think?
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To make this one reasonable we will have to look at how Astronomers ever succeeded in deciding that there was an expansion. "The evidence for an accelerating expansion comes from observations of the brightness of distant supernovae. We observe the redshift of a supernova which tells us by what the factor the Universe has expanded since the supernova exploded"
But how do they know how far those 'supernovas' are from us? There they use statistics. Looking at how stars evolve, regarding what we know, similar stars will have a similar solar output of energy or 'known intrinsic luminosity' like searching for 100W light-bulbs in space. They call those 'Standard Candles'. We use those to measure the relative distances in space, and knowing what light they should have we can then also measure their redshift.
"A prime candidate for such a standard candle is a certain type of stellar explosions. Known to astronomers as Type Ia Supernovae these explosions occur at the end stages of stellar evolution, when low-mass stars exhaust their fuel and start to contract and cool down. The resulting white dwarfs are so compact and dense that they are supported by electron pressure....
...The Chandrasekhar mass is a natural limit and makes it conceivable that all Type Ia Supernovae are similar. Such a uniform constellation assures that there are only small differences between individual explosions, a prime condition for a standard candle. "
So we have certain objects that we can find, that we expect to have the same luminosity. Those objects exist in all galaxies. Now to find their distances from us. "Hubble and his contemporaries assumed that the intrinsic brightness of the galaxies is more or less the same. Those appear to be brighter are closer to us whereas the fainter ones are farther away. But one problem follows. Galaxies of different types and sizes have different intrinsic brightness."
And there is where our 'Standard Candles' come in and help us define the distance as well as the expansion. Their luminosity are presumed to be the same everywhere and that together with the galaxies own luminosity, as well as form etc helps us define the distances. If a 'standard candle' is further away its light will be weaker to us and that with the redshift observed from it help us define both distance and 'speed' relative us. And looking at it this way the universe's expansion seems to be accelerating.
how?
Well, the further away that 'standard candle' is the older that light reaching us must be, right? And the redshift measured from those 'older' objects seems to be less than the redshift we get from stars closer to us in distance and time. And the way to make sense of that? Acceleration.
So it seems as we have an accelerating expansion. What it seems to mean is that 'space' is growing. expansion (http://www.phy.cuhk.edu.hk/phyworld/articles/univexpand/univexpand_e.html)
One explanation for this acceleration comes from what's called Einstein's cosmological constant cosmological constant (http://www.astro.ucla.edu/~wright/cosmo_constant.html)
It has to do with distance to those stars too. "If the universe is decelerating, it was faster in the past and the distance NOW is smaller" but as we seen the opposite seems to be true and there the cosmological constant comes in as possible explanation.
But it's still theories, as far as I know it haven't been proven as a fact. It's just what we know now seems to point too.
If it is space expanding it, as far as I understand, have to make more 'points' in space (uniformly as a guess). And so you can say that all points are doing more of themselves now than they did before. It's all measured in time (times arrow) of course :) And it presumes that this arrow have been the same since the big Bang, not considering the inflation.
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On the Universe from the history channel I was watching a program discussing the theories about how the universe could possibly be expanding at an accelerated rate. Gravity, after all, should be reversing the effects of expansion over time, and eventually suck the universe back in. However, many mainstream scientists believe that when the universe expands, dark matter is generated. This dark matter, after a fashion, reverses the effects of gravity and speeds up the expansion rate. As the universe continues to expand, more dark matter is created. Therefore the universe will continue to expand at an endlessly accelerating rate and all matter in the universe will drift off and far away from anything else.
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Stefanb, it is importent not to confuse dark matter with dark energy. Dark energy is what is postulated as being "created" as the universe expands and it is this that results in the universe to be expanding at an increasing rate. It can be thought of as an increasing volume containing a negative pressure. The gravitation from ordinary matter (and dark matter) opposes this but evidence suggests that this is insufficient to create a reversal of the expansion.
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If there is such a thing as dark energy then it has the capacity to 'grow' space? Which is a strange property. How do you grow it? How do one make a already 'decided' nothingness larger in 'distances' by this 'energy'. It seems as our type of matter possibly might be five percent of the universe, twenty five percent is expected to be what's called dark matter, and seventy percent? would then be dark energy.
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Gravitational mass is made up of energy density and pressure. If the pressure is sufficiently negative, the gravitational mass can be negative. Yes, I think 73% has to be dark energy to tie in with current observations and the FLRW metric.
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It's a funny idea, a 'nothing'. For it to work you will also either have to set a limit to 'virtual particles' or you will have to define them as 'limitless' as they are expected to be in all points of SpaceTime, well, as I understands it?
You might argue that they have a relation to f.ex gravity and electromagnetism. But electromagnetism is only seen in its interaction and 'clean space:)' should be a 'nothing', that is not 'polluted' by matter of any kind, be it dark, or dark energy. Gravity on the other hand seems to be everywhere.
You will also have a problem with defining the amount of 'dark energy' there might be, if one expect SpaceTime to be a closed system energy wise :)
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On many of these subjects I am agnostic about which is correct or even which is the right way to look at the issues. I don't know the truth about this and there are vast amounts I don't understand (I mean bits that others do rather than the obviously infinite amount yet to be discovered). When I answer a question I try to give what I think is the most accepted view rather than some theory I have or one of the less accepted ideas (though they may be right). Of course I get things wrong sometimes :-) I would not like to dispose of the FLRW metric as there are few reasonable alternatives that describe the universe consistent with GR.
One thing that nobody seems to have fathomed about dark energy is the matter of the 10^120 error factor if it is supposed to be due to quantum fluctuations (zero point energy). Some people have suggested state equations that "adjust" this, but it seems like a fiddle factor to me.
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Graham don't take it as I'm questioning you specifically :)
It's just the concepts I'm wondering about.
I'm doing my best to understand them but sometimes they seem contradictory.
And I also know that you constantly give the best answers you have. And that you are clear on what is your thoughts and what is main stream, and that is a trait I really like.
Just as me, about once in a blue moon :)
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I wasn't aiming any criticism at you at all, yor_on. I'm just explaining my ignorance :-) I do worry a little about some others who give very definite answers based on rather non-mainstream viewpoints. But I guess that is the nature of a forum such as this. It is a problem for people who "believe" to promote their views, as many mainstream journals are rather conservative in what they accept as papers.
My problem is that I don't have the time to study and work through the maths or even to read the more detailed work of others. I was surprised when someone suggested that subjects such as GR and QM could be explained in words only and that the maths was not necessary for understanding. That is entirely contrary to my view. Many of the concepts are not able to be visualised, except by analogy to simplistic scenarios. The maths is the only route available, though even then the only way to predict anything but very simple events is by computer simulation.
To get back to the original question. I think it is hard to say whether "Space" is being created. I think we all have preconceptions about the meanings of words and that these are not sufficiently well defined, so what one person means by a phrase is not necessarily the same as that interpreted by someone else. If you think of space-time as a hypersphere (an equivalent to a sphere in 4d), which is the basis of the FLRW metric, then this hypersphere is expanding and the rate of expansion is increasing. You can think of this as all of space-time stretching or you can think of it as mass-energy having some repulsive field in opposition to gravity. I don't think either views are exactly wrong but different ways of interpreting the maths by associating the terms with the forces of nature with which we are familiar. I don't really think we have a familiar concept for the cosmological constant in Einstein's field equations.
In a familiar 3d universe we observe that the velocity of distant objects is approximately proportional to distance (ignoring the recently discovered accelerating factor). This means v = dx/dt = Hx so that x = exp(Ht). This implies the universe is already increasing at an exponential rate and that the "accelerating" factor is just that the exponent (H) is increasing. This is a false picture if 4d space-time is considered though, as the 4d dimension does not behave in such a fashion. The metric has 4d "distances" (intervals) as x^2 + y^2 + z^2+ -(c^2)(t^2). This interval is not required to grow exponentially to give the required observations (until recently). These observations suggest it does, in fact, seem to be growing at an accelerating rate and it is this that was not expected. This is not easy to visualise and the popular view is sufficiently simplistic as to be misleading.
Re-reading this shows that I give a false picture here. Although it is correct to say x = exp(Ht) the growth is not exponential because H (the Hubble "constant") is not constant but also a function of t.
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Are you thinking of the way you treat time mathematically in a Minkowski spacetime. (http://en.wikipedia.org/wiki/Minkowski_space)
As a mathematician you might also treat time as a 'spatial dimension', same as all other dimensions inside 'SpaceTime' Spatial dimensions (http://en.wikipedia.org/wiki/Fourth_dimension#Fourth_spatial_dimension)
As I understands it there is a subtle difference there?
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Sorry, had to correct the spelling somewhat here :)
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I was thinking more of the FLRW metric which is a reasonable (I think) model of the universe consistant with GR and observation. Minkowski space-time is really a special case where there is no gravitation and can be applied in local areas of (flat) space. I should not have confused the two by giving Minkowski coordinates. I was really just trying to show that time can be considered another dimension, as you say. The FLRW metric is not flat, of course, so the cordinates are not independent of each other. They are also usually expressed in 4d polar form - as hyperspherical coordinates.
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Yes that was how I inferred it too.
But it's better to be sure than ah? Wrong :)
But if you look at it that way how does time influence distance?
Hypothetically can you treat time without it being bound to the other qualities we have in SpaceTime. That is, can you 'isolate it' mathematically.
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I don't think you can isolate time from the spacial dimensions. There are plenty of references to Friedmann, Lemaitre, Robertson, Walker that explain the maths and the observational assumptions used. I note that none of it is easy to follow though!
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But then you have string theory that treats one dimensional strings? If one on one side mathematically treat time as any other dimension?
Why I'm asking is because I suddenly got some weird idea about time and redshift from reading your thoughts here :)
Otherwise I've always had the same idea as you, I presume that at least :) That time is a property not able to treat 'isolated', but when thinking of those strings and then of the vague possibility of treating time as those other dimensions I started to wonder.
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I don't know much about string theory, but most QM concepts do not relate well with GR ideas, as far as I can see. Most of the recent theories have left me way behind in understanding. And I don't think it is at all practical to keep up, even at a superficial level, in all fields, especially while working full time.
I note many recent papers in cosmology talk about 5D space-time, with the nature of the 5th dimension not wholly specified (as far as I can see).
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Yes Graham, you're perfectly correct. It seems that if we use a dimension more, in one way or another, as a brane or as something 'infinitely small' there becomes new approaches to what SpaceTime might be. FIVE-D (http://www.newscientist.com/article/mg20327231.900-beyond-space-and-time-5d--into-the-unseen.html)
I don't like the idea(s) myself though :)
That as I do think what we see is what we get. (WYSIWYG)
A simple principle that suits me. The world we exist in is 'real'.
Matter do exist, SpaceTime have certain 'forces' and 'laws' macroscopically. Others at a QM level.
Times arrow is what generates the change macroscopically, and has nothing to do with us 'conning ourselves'.
And then I'm not as sure as most on what 'dimensions' are either :)
Most seem to see SpaceTime as some different 'pieces/dimensions' wedged together by 'forces' into a SpaceTime. I prefer a concept where what we got was a 'symmetry' already from the beginning, expressed 'materializing' through 'emergences' where each one creates new properties that we finally observe as SpaceTime.
And in that motto I don't expect us to be able to prove for example 'two dimensionality' existing in our 3D world (and times arrow). Just ponder how such a system would express itself in a 3D world. It would be visible from some angles, disappear from other. I've never seen any experiment proofing that concept (2D in a 3D + times arrow) and I don't expect to see one either.
But if dimensions was something 'singular' this experiment should exist, and I'm not talking about mathematical models here. It should be proof able here and now.
So, without it I'm still wondering what 3D really is. The best approximation I got too is as 'emergence's', and the thing I see as defining it would then be the 'arrow of time'. And then it comes down to geometry and how the plasticity of our Space-Time creates just different geometry's simultaneously, depending on your frame of reference for its definition.
And if that universe would exist you can't pick it apart. Although it doesn't exclude different 'dimensionality's' as you as easily could have a two, three, four or a fifty six dimensioned universe i would expect each one of them to be 'self sufficient and whole' in themselves. No loose dimensions to pickle together that is :) or 'pressing in' our '3D + times arrow' in that fifty sixth universe f.ex.
And that's also why I'm doubting 'distances' too. And times arrow will have to be the answer to 'distances' and it have a relation to mass and acceleration, and space and 'matter' will then have to be 'emergences' from ? And there I don't know anymore, or rather I don't want to guess anymore:)
But I differ between time and its arrow. I think time is a different concept from what we see as the arrow.
But it's quite easy to prove it wrong. Just introduce a real 2D matter object into SpaceTime and show me how it disappear from certain angles as it will contain f.ex. width and length but no depth (height).
If dimensions are 'pick able' it seems to me that such objects should exist already. And if they're not existing?
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If there is such a thing as dark energy then it has the capacity to 'grow' space? Which is a strange property. How do you grow it? How do one make a already 'decided' nothingness larger in 'distances' by this 'energy'.
The vacuum of space contains fields consisting of all wavelengths up to a limit. That gives it a negative pressure, which the First Law of Thermodynamics turns into negative work for expansion. See Caltech physicist Sean Carroll's website (not the biologist) for a coherent explanation. Googling "sean carroll cosmological constant" should do it. The basic idea is sound, the 10^120 stuff is a misunderstanding, but I have not been able to convince a physicist of it.
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Thanks, I'll have to look at that Itisus but as I remember reading about it?
From Charting the river of time (http://www.fqxi.org/data/articles/Olum_Ken.pdf)
---Quote----from “Charting the river of time” by Ken Olum ---
The ANEC (Averaged Null Energy Condition) essentially says that, though you can borrow a little negative energy on your route through space-time, you wind up paying it back with the positive type. Exotic phenomena are unlikely because they require some form of energy with a density that violates the ANEC “they require that the total energy density be negative when we add up all the contributions over the complete path of the light ray,” as Olum describes it. Which is to say, a little trading in the energy margins is fine, but the end result will still be positive—or at least not negative enough, for long enough, over a large enough region to make any difference.. ..
Along with Noah Graham, a Junior Faculty Fellow at Middlebury College, Olum has already shown that the ANEC still obtains between Casimir plates, even if you put holes in the plates so a photon can pass through in the most negative energy-friendly direction. “What we found was the striking result that the region near the hole always contributed enough positive energy to overwhelm the ANEC violation,” said Graham. “This result could be a coincidence of this particular system, but it certainly suggests there is a deeper principle at work.” With Fewster and Pfenning, Olum showed that there is such a principle at work in flat space, a finding confirmed by his work with Graham. If it applies to more complex systems as well, that principle may be the barrier to time travel.
“No collection of Casimir-type systems in flat space can violate the ANEC,” Olum said. “This we succeeded in showing. So the next thing to do is to try this for interacting fields, and curved space.” Olum is skeptical of any exotic outcomes, however. There is no free negative energy lunch in the special case and, he suspects, there isn’t going to be one in the general case. “I have tried to construct these exotic things before, using what seemed to be promising ideas, and I have not been able to construct them. So I think that it’s impossible. And I have good reasons to think that it’s impossible,” he said. “Without constructing the proof, though,” he added, “one can’t be certain.”
----End of quote--------------
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Itisus, Sean Carroll says...
"There is no obstacle to imagining that all of the large and apparently unrelated contributions listed add together, with different signs, to produce a net cosmological constant consistent with the limit (20), other than the fact that it seems ridiculous. We know of no special symmetry which could enforce a vanishing vacuum energy while remaining consistent with the known laws of physics; this conundrum is the “cosmological constant problem”."
He is saying that there IS a problem in reconciling the 10^120 error. Although it is possible to imagine the QM forces somehow cancelling, it seems an unlikely stretch.
Personally, I am not that sure about the QM value. The Casimir Effect is often used as a demonstration of zero point energy but there are other potential explanations, as some experimental researchers freely admit.
Carroll has an interesting set of websites. I will take a more detailed look when I get chance. It is great that so many of the universities and individuals are openly publishing stuff on the web.
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Yeah, I'm reading him too.
There's an awful lot of math to his presentation
But he also have some articles.
Sean Carroll (http://preposterousuniverse.com/)
:)
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As for 'zero point energy' Graham, I see this as general experimental evidence that you can't play around with the energy existing inside space 'vacuum energy'. Which then question f.ex those expecting photons somehow to get its 'propulsion' through a vacuum through this 'energy', well, as I see it :)
The reasoning behind my view is that you have to 'pay it back' if you 'lend' from this 'energy'. Quite simple I think.