Understanding Negative Pressure: Physical Description Explained

[helenwang413]

Hello, everybody!

I find it hard in understanding negative pressure. What is the physical description of a negative pressure?

Thank you!

 

[pervect]

You have some idea of what pressure is, right? For instance, if you take a hollow sphere, if you put it deep in the ocean, the pressure of the water will try to crush the sphere.

Negative pressure would make the sphere try to expand rather than to try and crush it.

Negative pressure is weird, but is hypothesized in some cosmological theories (dark energy, the cosmological constant) based on observations that the expansion of the universe is not slowing down, but speeding up.

 

[Garth]

Hello Helen (?),

Welcome to these Forums, you have chosen the right website to ask your question, keep them coming!

Think of negative pressure as tension.

Now an expanding universe with matter in it decelerates because the internal gravity between the galaxies and stars pulls everything together.

The really weird thing in General Relativity (GR) is that pressure is a form of energy, and energy is equivalent to mass so that it increases the gravitational field. So in GR a universe with pressure decelerates more quickly than one without it, other things being equal.

When cosmologists explained the observation that Type Ia supernovae in really distant galaxies were fainter than expected, by saying they were further away than expected, because in the mean time the universe had been accelerating and not decelerating in its expansion, they needed a reason to cause this acceleration. As pressure would cause the universe to decelerate faster then negative pressure would cause it to accelerate! Hence the need for the standard model to have Dark Energy to provide this negative pressure. If you do a search on PF you will find many posts about this subject.

I hope this helps.

Garth

 

[Chronos]

What Garth said . . . . I can't add much to what he said . . . . he is very good at this stuff. Welcome to PF, Helen[?]!

 

[robousy]

Nice explanation Garth.

Say, do you find the conclusion that the universe is accelerating a little extreme (i.e there is some other explantion) from the astronomical evidence or pretty solid?

 

[Garth]

Nice explanation Garth.
Say, do you find the conclusion that the universe is accelerating a little extreme (i.e there is some other explantion) from the astronomical evidence or pretty solid?

Hi robousy, and thanks for the compliment Chronos!
It is one explanation for the observation and a necessary one for the standard model.

However all remote sensing observations, and you cannot get much more remote than cosmological observations, are theory dependent; change the theory and the interpretation of the observation changes too. For example the most accurate and nearest astronomical distances are determined from parallax as the Earth orbits the Sun, now there is no way I wish to question it, but it is instructive to realize that these observations are dependent on Euclidean geometry and trigonometry. If, for example on the contrary, we should be in the middle of a powerful gravitational lens of some kind then these distances would have to be severely modified. Now I emphasise I do not for one moment think that we are in such a powerful lens, I have only introduced this example for instruction.

On the other hand, when we approach the limits of observation at z~1 -> z~6 or so, then we do have to be much more circumspect about our conclusions. I do not criticize the standard LCDM theory as a theory but I do question the confidence that is placed in it, especially as it has demanded the invocation of Inflation, non-baryonic Dark Matter and Dark Energy, none of which has been discovered in laboratory physics, even after about thirty years of intense looking! These all could be mere 'epicycles' added to the standard GR Friedmann cosmological models to 'save the appearances'.

The observation itself on which so much is placed is that these Type Ia SN are fainter than expected. There could be other explanations for this. Perhaps the geometry of space-time is correctly predicted by GR but the super novae are intrinsically fainter in the past as they evolve over cosmological history. Perhaps there is an unaccounted absorption of their light, after all there is much baryonic matter that has not been observed even in the standard model, (BB nucleosynthesis and WMAP suggest \Omega_b = 0.04 whereas observed \Omega_o = 0.003), so this dark baryonic matter could be in some unknown form that absorbs light.

Perhaps the geometry is wrong and GR is not the last word in gravitational physics at cosmological distances. There are several alternative theories that claim to explain the distant supernovae without acceleration, such as my own published SCC (you can search for that on PF and the physics ArXiv if you wish). The important thing is to be able to independently test these conclusions, thankfully that is being done at the moment with the Gravity Probe B satellite data analysis and we should know soon!

Garth

 

[Astronuc]

Please excuse the intrusion, but would this be useful to the discussion or as background?
http://map.gsfc.nasa.gov/m_uni.html
It also has a page concerning the Wilkinson Microwave Anisotropy Probe (WMAP) - http://map.gsfc.nasa.gov/m_mm.html

I was going to respond, but then realized that the question involved cosmology, and I thought that 'negative pressure' would have a different connotation than that with which I am familiar as an engineer.

In my field, negative pressure is relative, much the same as a negative temperature is relative to some positive reference temperature in °C or °F, whereas it is not possible in the absolute scales K or R.

Also in mechanics of materials or structural engineering, a positive pressure on the outside would put a vessel in compression, in which the principal stresses in the vessel wall would be negative.

Thanks Garth for the interesting and informative explanation!

 

[Garth]

Hi Astronuc,
Yes those are helpful and informative links.

If I could make just two comments:

First, the standard model has been described as standing on many pillars (as in that first link), knock over one and it still stands on the others. However, as I said above three of the pillars, inflation, exotic DM & DE are invisible as they have not been found in the laboratory. Inflation requires a force carrying particle such as the undiscovered Higgs boson or Inflaton, non-baryonic DM has not been discovered and the same could be said for DE, unless you identify it with the Casimir force somehow.

First find these necessary force-mediating particles in the lab, then measure their properties and then compare these properties with those required to explain the cosmological observations and check that they are concordant, then and only then will we know what we are talking about.

Secondly, the power spectrum of the CMB fluctuations has a series of peaks that fit a spatially flat, or nearly spatially flat, universe as predicted by inflation. However as the data is angular in nature and conformal transformations are angle preserving the data also fits conformally flat space models. The largest angle fluctuations appear missing to several sigma confidence and those present seem to be aligned to local geometry - possibly to the galaxy's motion through the CMB - and so may well be local contamination that has not been properly masked out. Therefore the large scale modes are really missing. The simplest resolution to this enigma in my opinion is the universe is finite (so the largest fluctuations could not have existed) and conformally flat, such as Einstein's original 'cylindrical' model (time being the axis).

Garth

 

[Astronuc]

However, as I said above three of the pillars, inflation, exotic DM & DE are invisible as they have not been found in the laboratory.

I guess one problem is that we are a small part of that laboratory, and until we can venture far, far away from our location, we are limited to what we observe from our Petri dish (Earth).

Unfortunately, the laboratory (our Universe) is much bigger than we can get our hands around, and perhaps our minds sometimes, but therein lies the challenge.

I had asked a question on another science forum, but I don't know if it has been asked here, and if not I'll start another thread, so as not to derail this one. The question is "where exactly (or with some reasonable certainty) is the earth/Milky Way with respect to the center of the universe?"

 

[Garth]

I guess one problem is that we are a small part of that laboratory, and until we can venture far, far away from our location, we are limited to what we observe from our Petri dish (Earth).
Unfortunately, the laboratory (our Universe) is much bigger than we can get our hands around, and perhaps our minds sometimes, but therein lies the challenge.

Quite

I had asked a question on another science forum, but I don't know if it has been asked here, and if not I'll start another thread, so as not to derail this one. The question is "where exactly (or with some reasonable certainty) is the earth/Milky Way with respect to the center of the universe?"

The universe does not have a centre any more than the surface of the Earth has a centre. We can at least locate the poles and the equatior on the Earth's surface, but there is no equivalent in the universe.

The Big Bang happened everywhere, or rather everywhere was once located in the event of the BB. Now everywhere is bathing in the (nearly) isotropic radiation (the CMB) that came from that BB.

Garth

 

[Chronos]

Astronuc, your engineering insights are still valid at cosmic scales, but require GR corrections and a background metric. The GR part is pretty firm. Unfortunately, selecting the proper background metric is very tough. FRW is the most popular choice, but the error bars do not rule out a variety of alternatives [e.g., Garth's conformally flat model]. GPB is very important for this reason. It currently appears possible GPB results may be delayed due to some unexplained 'artifacts' in the data.

 

[Garth]

GPB is very important for this reason. It currently appears possible GPB results may be delayed due to some unexplained 'artifacts' in the data.

Hi Chronos! Do you have a reference/link for that?

The GP-B data analysis is expected to last until the summer when the satellite tracking data and the guide star tracking data will be merged - what the gyros have been doing will not be known until then.

Garth

 

[SpaceTiger]

Say, do you find the conclusion that the universe is accelerating a little extreme (i.e there is some other explantion) from the astronomical evidence or pretty solid?

There was skepticism about this result when it first came out because there are a lot of potential problems with the supernova observations. However, the fact that we've independently measured the same cosmological parameters with multiple experimental methods has given the community a lot of confidence in LCDM. It is, of course, possible that the concordant measurements are a coincidence, but that seems to most folks a bit of a stretch.

All in all, I would say that yes, the astronomical evidence is solid, but it never hurts to test alternative theories. Although I seriously doubt his theory's veracity, I applaud Garth for putting up specific predictions for the Gravity Probe B results in the coming year.