Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Rubinelli, Peter M on 02/02/2010 09:30:02
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Rubinelli, Peter M asked the Naked Scientists:
Hello,
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My name is Peter Rubinelli.
I would like to know why the big bang model is favoured over the steady state model for explaining the expansion of the universe.
What is the evidence that leads to the big bang as the favoured model?
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Thank you. I really enjoy your show (http://www.thenakedscientists.com/HTML/podcasts/).
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Peter M. Rubinelli, Ph.D.
Purdue University
USA
What do you think?
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It fits with a lot of observation - particularly Hubble expansion and the microwave background. The Bondi, Hoyle et al Steady State theory made valiant attempts to stave off defeat but became more or less untenable after the microwave background data was produced.
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I think there's also evidence in fundamental physics, epitomized by Einstein's stress-energy tensor. Stress is akin to pressure, and things under pressure tend to expand. We can see this in the Casimir effect (http://en.wikipedia.org/wiki/Casimir_effect), wherein space demonstrates an innate pressure.
The issue is that if the universe was in a steady state, there would have to be something to stop the galaxies moving together due to gravity. Einstein introduced his lambda (Λ) term for this, see http://en.wikipedia.org/wiki/Cosmological_constant. The problem isn't with Λ per se, it's making it exactly counteract gravity to maintain the steady state. I don't know how he missed the "pressure" expansion aspect, and I guess it's why he later said it was his greatest blunder. He could have predicted the expansion of the universe before Hubble spotted the redshift, see http://en.wikipedia.org/wiki/Edwin_Hubble.
As an aside, I read a really interesting article recently called Dark Energy: back to Newton? which suggests that Newton had a lambda term. See http://www.thenakedscientists.com/forum/index.php?topic=28014.msg294505
But having said all that, there are people who don't agree with me. See for example http://www.hiltonratcliffe.com/. Hilton describes himself as a "heretic" and advocates a steady-state universe. He's more cosmologist than fundamental physicist, but it's only fair that I mention the counter-argument.
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OK, there are a number of misconceptions here.
But before getting to that, it's worth noting that the Steady State theory has been out of favour for decades. Even those who promoted the Steady State theory switched to different theories by the 1990s.
Simply put, there is too much evidence for significant change in the nature of the universe over its observable history.
I think there's also evidence in fundamental physics, epitomized by Einstein's stress-energy tensor. Stress is akin to pressure, and things under pressure tend to expand. We can see this in the Casimir effect (http://en.wikipedia.org/wiki/Casimir_effect), wherein space demonstrates an innate pressure.
First, stress and pressure are two very different things in physics.
Second, while the Casimir effect might be a measure of the pressure of empty space, it is not necessarily one that influences the overall expansion of the universe. To date, no scientists has made a convincing argument linking the Casimir effect to cosmological expansion, primarily because there are not measurable comparisons between the two.
Third, the Steady State theory incorporated expansion, and it even incorporated a form of dark energy. As dark energy is what one often attempts to associate with the Casimir effect, the Casimir effect cannot be thought to be something that speaks against the Steady State theory.
The issue is that if the universe was in a steady state, there would have to be something to stop the galaxies moving together due to gravity. Einstein introduced his lambda (Λ) term for this, see http://en.wikipedia.org/wiki/Cosmological_constant. The problem isn't with Λ per se, it's making it exactly counteract gravity to maintain the steady state. I don't know how he missed the "pressure" expansion aspect, and I guess it's why he later said it was his greatest blunder. He could have predicted the expansion of the universe before Hubble spotted the redshift, see http://en.wikipedia.org/wiki/Edwin_Hubble.
Gamow claimed that Einstein called the cosmological constant a blunder, but there is no other source for this quote. Einstein did speak vehemently against the constant following 1929, though.
And, again, the steady state of the steady state theory does not mean that there is no expansion. Standard steady state theory had expansion, with the density of the universe being "filled in", so to speak, by the creation of matter and energy from a field that took the place of the cosmological constant.
As an aside, I read a really interesting article recently called Dark Energy: back to Newton? which suggests that Newton had a lambda term. See http://www.thenakedscientists.com/forum/index.php?topic=28014.msg294505
It is misleading to say that Newton had anything like a dark energy term. The authors of that article went too far in trying to sell their position to a popular science audience. Most of the article is good, and actually makes clear that what Newton examined (and correctly rejected) is only superficially like dark energy.
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There is no pressure that I know of to a 'free vacuum'?
If you are of the view of photons, or waves, traveling in space ala Feynman's 'many paths' you would have a radiative pressure, but then you would have to ask yourself how it introduces itself when there is no matter interacting?
outer Space (http://en.wikipedia.org/wiki/Vacuum#Outer_space)
Problems with the 'steady state' (http://en.wikipedia.org/wiki/Steady_State_theory#Problems)
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An expanding universe cannot, by definition, be a steady state universe.
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An expanding universe cannot, by definition, be a steady state universe.
Well, everyone who ever proposed a steady state universe disagrees with you.
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Umm... do you want to think that through again?
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No.
The steady state theory proposed that the universe always looked the same, including the expansion that we have seen since 1929. To account for this, the steady state theories proposed a field that created matter so that the average density of the universe remained the same. This means that the universe can always expand and always look about the same.
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There is no pressure that I know of to a 'free vacuum'?
If you are of the view of photons, or waves, traveling in space ala Feynman's 'many paths' you would have a radiative pressure, but then you would have to ask yourself how it introduces itself when there is no matter interacting?
outer Space (http://en.wikipedia.org/wiki/Vacuum#Outer_space)
Problems with the 'steady state' (http://en.wikipedia.org/wiki/Steady_State_theory#Problems)
There is in theory pressure in a vacuum. It comes from the idea of empty space as having a bunch of virtual photons popping into and out of existence. If you put two conducting plates close together, you can exclude some of those photons from the region between the plates. Since you have more possible photons outside than inside, they exert a net pressure on the plates, which is the Casimir effect. I think another way of putting the idea is that empty space still has a slight bit of energy associated with these quantum fluctuations, and this gives empty space an energy. However, quantum mechanics predicts an immense amount of energy in the universe from this--far more than we actually observe.
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Yor-on: re stress being akin to pressure, see http://en.wikipedia.org/wiki/Pascal_(unit). Also see http://en.wikipedia.org/wiki/Stress-energy_tensor which says "In particular Tii (not summed) represents normal stress which is called pressure when it is independent of direction." JP's reference to the immense amount of energy is known as the vacuum catastrophe.
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Yes I know about 'vacuum energy' JP :)
But if you read the experiments done by Ken Olum there is no 'pressure' to the vacuum. What you have when you introduce those Casimir-type systems is a new relation that produces a effect not seen before that. There's a crucial difference between that and an 'pressure' existent at all times to me. It's back to square one in our discussion about potential energy, sort of :)
And whatever else sort of 'energies' we attach to the vacuum it will still only be a 'potential' energy as long as we don't 'manipulate it', and even then it seems as if we only will create a 'debt' needed to be payed back. Can you see my point here?
“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.”
Rivers of time (http://www.fqxi.org/data/articles/Olum_Ken.pdf)
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The way I think of it (pressure) may be expressed like this. A pressure should introduce retardation in a free, or if you like, deep space for objects moving in it, but there is no such effect that I've heard of? Or at least be noticeable as being something more than just gravitational forces even if equally 'pressuring' objects from all sides. Is this making sense :)
Furthermore :) I don't really know what 'space' is more than it is a 'distance'
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Furthermore :) I don't really know what 'space' is more than it is a 'distance'
Yoron: I think the "vacuum" idea is highly misleading. (Substitute "Electrolux" [;D]) Seriously, "space" has very distinct, and quite measureable properties. If it was truly "nothing", why would it have any properties at all?
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I don't think Olum is ruling out the Casimir effect there. I think he's just ruling out using it to extract energy from the vacuum, which makes a lot of sense. That aside, I don't think anyone's questioning that there is some zero-point energy in the vacuum, the effects of which have been experimentally observed (the Lamb shift and spontaneous emission come to mind from quantum optics).
Thinking of it as pressure is problematic because you would only see it as pressure (i.e. a force per unit area) if you construct a Casimir-type system. Otherwise, you'd see it as energy fluctuations in the vacuum, whose (tiny!) forces would all average out to nothing when you looked at large objects.
And this is the reason why we got along so well for so long by saying the vacuum had zero energy and was completely empty. In most cases, and for large objects, it averages to zero, so that classical theories work quite well by just assuming that the vacuum is completely empty.
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No.
The steady state theory proposed that the universe always looked the same, including the expansion that we have seen since 1929. To account for this, the steady state theories proposed a field that created matter so that the average density of the universe remained the same. This means that the universe can always expand and always look about the same.
First of all, if the universe is expanding it is not in a steady state, but even if new material were to be created (by some sort of undefined field) in such a way that the universe 'looked' the same it would have to be in the form of 'new' galaxies, which although 'new' would have to appear to be 'old'.
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First of all, if the universe is expanding it is not in a steady state, but even if new material were to be created (by some sort of undefined field) in such a way that the universe 'looked' the same it would have to be in the form of 'new' galaxies, which although 'new' would have to appear to be 'old'.
Well, you could take up this issue with everyone who worked on the steady state theory, but they've all abandoned it, so the point is pretty much moot.
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Why would I want to take up such a pointless and flawed issue, which I was, in any case, disagreeing with ?
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JP, I work in an industry that actually makes practical use of the Casimir effect. It seems to be widely assumed that this effect demonstrates zero-point energy, and it may do, but it is far from certain. There are other, more mundane, possible causes that can't be ruled out.
But, that aside, it does seem likely that there should be ZP energy. Can you explain why you think such spontaneous pair creation and annihilation would not create a pressure? Unless you assume that there is a sea of particles and virtual particles (those on the "other side" waiting to spring into existence) such that the virtual particles create a negative pressure balancing the positive pressure from those in existence at any one time. However, isn't a feature that dark energy has to have negative pressure to give the right sign for the cosmological constant to explain accelerated expansion? That is even harder to explain, although I admit I am looking this rather crudely.
Then there is the factor of 10^120 (or maybe only 10^55 with some other theories) of error! I think there is one thing everyone agrees on: nobody understands the reasons for the cosmological observations and the QM theory does not tie in with any of the current hypotheses in any consistant way.
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I should have set "net force." I think you should get a pressure, but usually from all directions equally. You wouldn't be able to see it except for in special cases, such as the Casimir effect, where you set up a system so that the pressures on both sides are unequal.
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I don't think you get no pressure, JP. A vacuum is not a vacuum if it has particles in it with a temperature that is not at zero Kelvin. That means there is a pressure. There is not net force on the walls of a box of air at the same pressure as the air outside the box, but it would be wrong to say that there is no atmospheric pressure, just, as you say, it is not exerting any net force.
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Could you explain a little more how you think here Graham?
"A vacuum is not a vacuum if it has particles in it with a temperature that is not at zero Kelvin. That means there is a pressure."
Do you mean that any vacuum with a particle in it isn't a real 'perfect (free) vacuum'
Or that it just isn't a vacuum?
Or that only when the temperature (heat?)of those particles is at zero K will it be a vacuum.
And how you see that proving a pressure?
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If I would guess I think you mean that as long as there are 'real particles' with a nonzero energy there will be a 'pressure'? Is that it?
And then you see this 'energy' as undefined in space perhaps?
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I'm wrong heh :)
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The atmoshere, as a gas, has a particular measurable pressure. If you surround a cubic metre of it by a box there will be no net force on the box walls, but that does not mean you cannot speak of the pressure of the gas. Even without the box, the volume of gas has a pressure which it exerts on the surrounding gas (and vice versa). If you take the box of gas up a mountain and into a very large sealed room (say), the pressure will exert a net force on the walls of the container. If released it will disperse into the room and the room pressure will go up and the box pressure go down until they are equalised. The same would be true if you took the box into space although the pressure change in space would be rather small :-) In the milky way galaxy there is a measurable density of hydrogen ions and electrons (and some other particles) buzzing about and they have a pressure. In the galaxy the distances of pulsars are estimated by how much this gas density spreads the emitted sharp pulses by virtue of the finite refractive index that the gas has.
Yes, real particles with energy produce positive pressure. The mysterious thing about the idea of dark energy is that it must produce negative pressure to give the observed effect. I don't know enough about the QM side of this to see how this happens. When the particles appear and then annihilate they are, for their period of existence, real particles and should have positive pressure and kinetic energy (I think). Maybe there is something that can be said about the virtual particle (the ones that have potential to exist) that more than balance this by having negative energy and pressure, but I don't understand this. Maybe someone can explain.
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Thanks Graham, I can see how you thought now, and it makes perfect sense to me. Well as much as anything can do those days :) But it makes me wonder if there can be any 'free (perfect) space' considering the pressure you describe here?
That is assuming that the pressures behavior resembles waves?
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And, can we measure that pressures influence on invariant mass if so?
Like heavenly objects?
(Nope, not Marilyn Monroe, but it was a near guess:)
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It's a positive pressure, graham, despite what the equations say. A negative pressure would make the universe contract, not expand. Gravity is a pressure gradient, and if the pressure is uniform there is no gravity, but the universe still expands like a ball of compressed gas. Let's face it, the early universe was very dense, but it didn't collapse. Look up stress-energy along with stress and pressure, and think stress ball. Squeeze it down in your fist, and let it go. It expands.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Fthumb%2F6%2F66%2FStress_ball_manipulation.jpg%2F180px-Stress_ball_manipulation.jpg&hash=1de94169eafd6b6b053665e35fc58fba)
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It's a positive pressure, graham, despite what the equations say. A negative pressure would make the universe contract, not expand. Gravity is a pressure gradient, and if the pressure is uniform there is no gravity, but the universe still expands like a ball of compressed gas. Let's face it, the early universe was very dense, but it didn't collapse. Look up stress-energy along with stress and pressure, and think stress ball. Squeeze it down in your fist, and let it go. It expands.
This may be good pseudoscience, but it has nothing to do with the relevant science.
To properly assign an energy density in General Relativity, one uses a 4X4 tensor, Tμν, the 00 component of which is the energy density proper. Under certain general conditions allowable in cosmology, we can remove the off-diagonal elements of the tensor, but we cannot get rid of the diagonal elements. These are all equivalent, T11 = T22 = T33 = p and they are a function of density and the behaviour of the particles or field in phase space. This function agrees with the usual definition of pressure in its use with ideal gases, but its role is as a source of gravity, not a pressure as classically used. This is especially important because the expansion of the universe is a gravitational effect, not one of force imparted through some other mechanism.
If we assign an energy density ρΛ to the vacuum, then we must assign it in such a way that the energy density is associated with Lorentz invariant tensor. This is so because the properties of the vacuum itself don't transform along with a change in coordinates. The only Lorentz invariant tensor is ημν, that tensor with 1 in the 00 position, -1 on the other diagonal positions, and 0 on all off-diagonal positions. So we assign the vacuum energy tensor as TΛμν = ρΛ ημν.
Combine these two requirements, and we see that the "pressure", p, associated with the energy density of the vacuum must be -ρΛ. Thus if the energy density of the vacuum is positive, its pressure term, such as it is, is negative.
(The above adapted from pp. 130-132 of James Rich, Fundamentals of Cosmology, Springer 2001.)
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As I have said before, it is very hard to explain some of the less intuitive results of GR without some knowledge of differential geometry.
To address your point and ball analogy, farsight, it is not that the universe is expanding that is suggestive of dark energy (or any sort of pressure), but the acceleration of the expansion.
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Many people who wish to propose alternatives to expansion to explain the red shift forget some of the more subtle aspects of observations of this in the form or indirect observations of the many other gas clouds that light has passed through at different distances. This caused what is known as the "Lyman alpha forest" on the Lyman alpha emissions from very remote active galaxies.
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It's a positive pressure, graham, despite what the equations say. A negative pressure would make the universe contract, not expand. Gravity is a pressure gradient, and if the pressure is uniform there is no gravity, but the universe still expands like a ball of compressed gas. Let's face it, the early universe was very dense, but it didn't collapse. Look up stress-energy along with stress and pressure, and think stress ball. Squeeze it down in your fist, and let it go. It expands.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Fthumb%2F6%2F66%2FStress_ball_manipulation.jpg%2F180px-Stress_ball_manipulation.jpg&hash=1de94169eafd6b6b053665e35fc58fba)
Farsight, look here for an explanation of how it seems to be thought to work.
Negative energy (http://www.thenakedscientists.com/forum/index.php?topic=28906.msg301609#msg301609)