Expansion of the universe and quantum perturbations

[tim9000]

So I'm just learning about the higgs, electro-magnetic, strong and weak nuclear fields/forces.
Is dark energy one of those?
I understand there is as difference in the universe expanding from expansion and everything 'receding' from dark energy.

I was watching a UC video on expansion, the Hubble radius etc. and he said that expansion was like 'nothing' wasn't 'nothing', it was like static (quantum) noise on your telly, like full of particles popping in and out of existence. And it was this that when the universe got started there was this 'energy' in the quantum 'nothing'. More 'nothing' (space) - > more energy -> more space

(does this mean like a plank length is getting bigger, and if so or if not, what implications does this have for say the space inside an atomic nucleus?)

Which seems ridiculous to me as an explanation, though I'm sure it won a nobel prize. Also, it seems to me that there were still these fields / forces, 'around' before the universe / expansion / big bang / the quantum perturbation which caused particles and radiation to exist; according to the standard model. In which case the big bang isn't an explanation for existence, because the fields / forces have always existed, even when there was no 'space'...which is kind of weird because how can you have quantum fluctuations without space to have them in? (not to mention how can you not have space...that's like the universe doesn't exist yet, it's just as real as an imaginary theory in God's mind...which is like you need an observer which doesn't exist to conjure up something which is a meta concept of the properties of non-existance, that is: 'something' but without spatial dimensions. What I'm saying is I can see how 'time' and 'space' can break down (be compressed and stretched) asymptotically around a black hole, but for space to not exist at all is incomprehensible. I guess it's a bit like the tree falling when no ones around, if space exists with nothing happening, is time passing, or non-esistant? Or can time exist without space?

Can anyone elaborate on this, or clear anything up?

 

[Drakkith]

So I'm just learning about the higgs, electro-magnetic, strong and weak nuclear fields/forces.
Is dark energy one of those?

It is not. It might be part of General Relativity, aka gravity, but it doesn't have anything to do with any of those forces/fields you listed, as far as I know at least.

I understand there is as difference in the universe expanding from expansion and everything 'receding' from dark energy.

Expansion is the basic process by which unbound objects recede from each other over time. Dark energy isn't expansion itself, it is something that accelerates the expansion.

I was watching a UC video on expansion, the Hubble radius etc. and he said that expansion was like 'nothing' wasn't 'nothing', it was like static (quantum) noise on your telly, like full of particles popping in and out of existence. And it was this that when the universe got started there was this 'energy' in the quantum 'nothing'. More 'nothing' (space) - > more energy -> more space

That's not something I've heard of before. Sounds like a pop-sci explanation that has little real value.

(does this mean like a plank length is getting bigger, and if so or if not, what implications does this have for say the space inside an atomic nucleus?)

Nothing. Distances, as in units of measurements, are not getting larger and expansion doesn't affect objects like a nucleus which are very strongly bound together.

Also, it seems to me that there were still these fields / forces, 'around' before the universe / expansion / big bang / the quantum perturbation which caused particles and radiation to exist; according to the standard model. In which case the big bang isn't an explanation for existence, because the fields / forces have always existed, even when there was no 'space'...which is kind of weird because how can you have quantum fluctuations without space to have them in? (not to mention how can you not have space...that's like the universe doesn't exist yet, it's just as real as an imaginary theory in God's mind...which is like you need an observer which doesn't exist to conjure up something which is a meta concept of the properties of non-existance, that is: 'something' but without spatial dimensions. What I'm saying is I can see how 'time' and 'space' can break down (be compressed and stretched) asymptotically around a black hole, but for space to not exist at all is incomprehensible. I guess it's a bit like the tree falling when no ones around, if space exists with nothing happening, is time passing, or non-esistant? Or can time exist without space?

No one has any real idea of how the universe was created, or even if it was created at all. It's entirely possible that the universe has existed in some form forever, and the big bang is merely a cyclic phenomenon (Though it certainly doesn't appear that way right now). Questions about what things may have been like before the universe existed are essentially impossible to answer. As you say, if spacetime doesn't even exist, how can anything which occupies spacetime exist? I recommend accepting the fact that we will probably never have an answer to these questions and that we don't even know if these are even valid, answerable questions.

 

[tim9000]

It is not. It might be part of General Relativity, aka gravity, but it doesn't have anything to do with any of those forces/fields you listed, as far as I know at least

Expansion is the basic process by which unbound objects recede from each other over time. Dark energy isn't expansion itself, it is something that accelerates the expansion.

Yeah, but as far as I know energy is just like potential in a field, like kinetic energy of a ball rolling down a hill, or of an electron, chemical energy, nuclear energy...
to say dark 'energy' is nebulous, not like we don't know what it is, hence 'dark' but 'energy' like energy isn't really a thing, its a man made concept useful for physics. So wouldn't dark energy hit at some connection to some other field?
I understand that it isn't causing the expansion, but presumably if there was no expansion, than it would cause the expansion? like why would dark energy be associated with the second deriative of expansion, but not the first?

That's not something I've heard of before. Sounds like a pop-sci explanation that has little real value.

Yeah it doesn't sit well with me either; between 15 and 16 minutes:

Nothing. Distances, as in units of measurements, are not getting larger and expansion doesn't affect objects like a nucleus which are very strongly bound together.

I understand that it doesn't effect bound systems, even just gravitationally bound systems like within galaxies. But the expanding space must still pass through the microscopic, and I'm just trying to picture what it would be like. Put aside the fact that we can see it in the sky: Otherwise it's just like an imaginary thing; what I'm saying is like we have a fixed real size of a nucleus and a theoretical (much smaller) size of a plank length based off light or something, relative to a nucleus due to expansion does a plank length grow? Because I know 'space' is getting bigger, but I'm trying to comprehend what that actually means, otherwise as I said, its just like it's an imaginary thing. I know it has no tangible effect to us as bound matter on a planet, but the space we occupy growing must mean something...is a meter longer today than it was 10 B yrs ago?

No one has any real idea of how the universe was created, or even if it was created at all. It's entirely possible that the universe has existed in some form forever, and the big bang is merely a cyclic phenomenon (Though it certainly doesn't appear that way right now). Questions about what things may have been like before the universe existed are essentially impossible to answer. As you say, if spacetime doesn't even exist, how can anything which occupies spacetime exist? I recommend accepting the fact that we will probably never have an answer to these questions and that we don't even know if these are even valid, answerable questions.

That is pretty much what I expected to hear. I am of an engineering background (recent Grad) so I have all the admiration possible for the dedication, genius, time, effort and sacrifice physicists have contributed to mankind, however I find it frustrating that people from all walks, from lay people to Stephen Hawking sort of say 'big bang, case closed'. I want to hear the most eminent mind say 'yeah, this makes my head hurt, this quantum business is bullshit'. (Not like it isn't real, or that Feinman said if you think you've got it, than you dont; but like it is fundamentally beyond rationalisation. Like science has given us philosophical avenues to pursue that philosophy couldn't dream of, and makes man more humble than any God ever could...not that I have any opinion either way about religion. I just don't really get that feeling of humble bewilderment from our society generally, like there is a lack of appreciation of how bizarre existence is. The fact that anything or nothing could or couldn't exist is so weird, at this point if there is an all powerful God I doubt it could give a satisfactory explanation.

 

[Drakkith]

to say dark 'energy' is nebulous, not like we don't know what it is, hence 'dark' but 'energy' like energy isn't really a thing, its a man made concept useful for physics. So wouldn't dark energy hit at some connection to some other field?

I'm afraid I don't know enough to help you with this question.

I understand that it isn't causing the expansion, but presumably if there was no expansion, than it would cause the expansion? like why would dark energy be associated with the second deriative of expansion, but not the first?

Given an equilibrium where the universe is neither expanding nor contracting, I believe it would lead to an expansion since its current effect is to accelerate the expansion.

Yeah it doesn't sit well with me either; between 15 and 16 minutes:

Okay, he's talking about inflation. That's a different concept from regular 'metric expansion' as the expansion of the universe is called.

I understand that it doesn't effect bound systems, even just gravitationally bound systems like within galaxies. But the expanding space must still pass through the microscopic, and I'm just trying to picture what it would be like. Put aside the fact that we can see it in the sky: Otherwise it's just like an imaginary thing; what I'm saying is like we have a fixed real size of a nucleus and a theoretical (much smaller) size of a plank length based off light or something, relative to a nucleus due to expansion does a plank length grow? Because I know 'space' is getting bigger, but I'm trying to comprehend what that actually means, otherwise as I said, its just like it's an imaginary thing. I know it has no tangible effect to us as bound matter on a planet, but the space we occupy growing must mean something...is a meter longer today than it was 10 B yrs ago?

Plank length does not grow because units of measurement are not changed by the metric expansion of space. I prefer to think of expansion as an increase in the distance between objects, not as a literal expansion of the underlying space as if space were some kind of fabric being stretched. It doesn't matter how much space grows, you will only ever occupy the same volume of space that you always have. It's just that the distance between you and a galaxy 500 million light years away increases over time because you two aren't bound together.

That is pretty much what I expected to hear. I am of an engineering background (recent Grad) so I have all the admiration possible for the dedication, genius, time, effort and sacrifice physicists have contributed to mankind, however I find it frustrating that people from all walks, from lay people to Stephen Hawking sort of say 'big bang, case closed'.

Realize that the 'big bang', in modern cosmology, refers to a rapid expansion in the very early universe, not to a single, sudden creation event. The big bang theory does NOT state that the universe was created by the big bang, only that the universe was once in a very hot, very dense state and expanded from there, cooling as it did so.

 

[tim9000]

Okay, he's talking about inflation. That's a different concept from regular 'metric expansion' as the expansion of the universe is called.

I'll come across this in my research, but if you wanted to explain briefly what the difference was, I wouldn't stop you.

Plank length does not grow because units of measurement are not changed by the metric expansion of space.

Right, well why does light traveling from distant galaxies get red shifted, because wouldn't they need to be 'properly' moving for that to happen? That's why I thought the light was being stretched by the expansion of the space it was traveling though.

Realize that the 'big bang', in modern cosmology, refers to a rapid expansion in the very early universe, not to a single, sudden creation event. The big bang theory does NOT state that the universe was created by the big bang, only that the universe was once in a very hot, very dense state and expanded from there, cooling as it did so

You're right, however I was under the impression that the 'quantum fluctuation' was regarded as the creation event.

 

[PeterDonis]

relative to a nucleus due to expansion does a plank length grow? Because I know 'space' is getting bigger

No, you don't. This is one of those subjects where it's almost impossible to describe what is going on in ordinary language without inviting incorrect inferences. "Space is expanding" is one of those ordinary language descriptions that is pretty much guaranteed to invite incorrect inferences. "Space" is not a "thing" that can expand, and "space expansion" does not exert a force on anything. Dark energy can, but dark energy is not the same as "space expansion".

We don't know exactly what dark energy is, but whatever it is, it has one specific property that makes it different from all the other "stuff" we know of in the universe: it exerts gravitational repulsion instead of gravitational attraction. A big cloud of ordinary matter, or dark matter, or radiation, will tend to pull itself together--it has attractive gravity. A big cloud of dark energy will tend to push itself (and other things inside it) apart--it has repulsive gravity. Our universe, for the last few billion years has been like a big cloud of dark energy pushing itself apart; that's what we mean when we say the expansion of the universe is accelerating. Before that, when the density of matter was larger, it was like a big cloud of ordinary matter pulling itself together; the expansion of the universe was decelerating.

The reason "space expansion" gets brought into it is that all of the above--dark energy pushing things apart, or ordinary matter pulling things together--does not take place in flat spacetime. It takes place in curved spacetime. So it isn't the same as our ordinary concept, based on Newtonian physics, of things pushing or pulling each other in an underlying absolute flat Euclidean space. That's why, for example, we here in the Milky Way, and an observer in some galaxy a billion light years away that is moving away from us at high speed, can both see the universe as homogeneous and isotropic; that wouldn't be possible in flat spacetime. But this sort of thing is not, IMO, best thought of as "space expanding", since that invites all those incorrect inferences. It's best thought of as "curved spacetime", without saying anything about "space". (Note that "space" in any case is coordinate-dependent in relativity, and trying to give absolute physical meaning to coordinate-dependent things is one of the first errors one needs to learn to avoid when learning relativity.)

 

[Drakkith]

Right, well why does light traveling from distant galaxies get red shifted, because wouldn't they need to be 'properly' moving for that to happen? That's why I thought the light was being stretched by the expansion of the space it was traveling though.

I don't know the detailed, technical explanation, but I think about it like this:

Imagine an EM wave (light) emitted from a star in a distant galaxy. The wavefront of that EM wave isn't bound together with any forces like matter is. So, as it travels through intergalactic space, expansion stretches out that wavefront and redshifts it. Note that in GR curvature of space is a result of geometry. Gravity is the result of the geometry of spacetime being such that the shortest distance between two points is a curved line. Expansion, on the other hand, is a result of the changing geometry of spacetime. The front of the wavefront occupies a different portion of space than the back of the wavefront. As the EM wave travels, this change in the geometry of spacetime causes the front of the wavefront to get further away from the back, stretching and redshifting the wave. This really isn't that special. If an EM wave passes close to a massive object part of the wavefront would be 'pulled' away from the rest under gravity (by pulled I mean that part of the wavefront takes a different path through spacetime than the rest, curving it away from the rest). Notable examples are the gravitational lensing of distant galaxies or the testing of GR by Eddington in the 1919 solar eclipse.

I'm sure that's a mangled, half-accurate explanation, so someone correct me if I've made any grievous mistakes.

You're right, however I was under the impression that the 'quantum fluctuation' was regarded as the creation event.

There is no creation event in the mainstream big bang theory. All creation events are simply hypothetical possibilities, not part of the BBT.

 

[tim9000]

I don't know the detailed, technical explanation, but I think about it like this:

Imagine an EM wave (light) emitted from a star in a distant galaxy. The wavefront of that EM wave isn't bound together with any forces like matter is. So, as it travels through intergalactic space, expansion stretches out that wavefront and redshifts it. Note that in GR curvature of space is a result of geometry. Gravity is the result of the geometry of spacetime being such that the shortest distance between two points is a curved line. Expansion, on the other hand, is a result of the changing geometry of spacetime. The front of the wavefront occupies a different portion of space than the back of the wavefront. As the EM wave travels, this change in the geometry of spacetime causes the front of the wavefront to get further away from the back, stretching and redshifting the wave. This really isn't that special. If an EM wave passes close to a massive object part of the wavefront would be 'pulled' away from the rest under gravity (by pulled I mean that part of the wavefront takes a different path through spacetime than the rest, curving it away from the rest). Notable examples are the gravitational lensing of distant galaxies or the testing of GR by Eddington in the 1919 solar eclipse.

I'm sure that's a mangled, half-accurate explanation, so someone correct me if I've made any grievous mistakes.
There is no creation event in the mainstream big bang theory. All creation events are simply hypothetical possibilities, not part of the BBT.

I think I understand what you're saying about the geometry of space-time, but what I was saying was that wouldn't expansion mean that the light reference frame that is traveling from alpha centory to us see the space it is traveling through...actually I don't know, stay the same I suppose. If I'm light and I leave at a certain frequency, than presumably I arrive at the same frequency I left. But from our reference frame the light left at one frequency and arrived with a longer wavelength...

Ok so the quantum fluctuation wasn't the beginning of this infinitely dense point, the universe was conceivably always just sitting there as an infinitely dense / small, point.

Thanks

 

[Drakkith]

I think I understand what you're saying about the geometry of space-time, but what I was saying was that wouldn't expansion mean that the light reference frame that is traveling from alpha centory to us see the space it is traveling through...actually I don't know, stay the same I suppose. If I'm light and I leave at a certain frequency, than presumably I arrive at the same frequency I left. But from our reference frame the light left at one frequency and arrived with a longer wavelength...

Well, we can't assign a reference frame to light, but we can imagine a co-moving observer traveling at near-c between the emission point and Earth who left right when the light was emitted. As far as I know, if both the light and the observer leave a galaxy and travel for a billion years or so (our time, not the co-moving observer's time which is highly, highly dilated from our frame), the light would arrive redshifted and the observer's velocity relative to Earth would be lower than the observer's velocity relative to the emission point when they left. That is, the observer's velocity relative to the emission point, at the time of emission, was X. The observer's velocity relative to Earth when they arrive is less than X.

Ok so the quantum fluctuation wasn't the beginning of this infinitely dense point, the universe was conceivably always just sitting there as an infinitely dense / small, point.

That is my understanding.

 

[tim9000]

That is interesting, so the light sees itself traveling slower, the longer it travels.
Fascinating, thanks

 

[Drakkith]

That is interesting, so the light sees itself traveling slower, the longer it travels.

No, there is no reference frame for light. That's why I had to create an observer traveling 'close' to c. What I mean by that is that a physically realizable observer can never travel at c, and if we explore the consequences of an imaginary observer traveling at c we would find that most/all of our physical laws that we can ascribe to an observer would cease to work. For example, it is commonly said that light experiences zero time because it travels at c. But that makes no sense. How can light travel across the universe and interact with things if it experiences zero time (and by extension, infinite length contraction)? That's just a single example of how things no longer make sense if you treat light as having a frame of reference.

 

[tim9000]

No, there is no reference frame for light. That's why I had to create an observer traveling 'close' to c. What I mean by that is that a physically realizable observer can never travel at c, and if we explore the consequences of an imaginary observer traveling at c we would find that most/all of our physical laws that we can ascribe to an observer would cease to work. For example, it is commonly said that light experiences zero time because it travels at c. But that makes no sense. How can light travel across the universe and interact with things if it experiences zero time (and by extension, infinite length contraction)? That's just a single example of how things no longer make sense if you treat light as having a frame of reference.

I understood what you said in your previous reply, but that explanation really gives me an appreciation of/for it.
If we can't understand how time has no frame of reference (beyond the hyperbolic dilation/contraction), it is a fools errand to try and understand a frame of reference outside/before the creation/expansion of space.
Thanks!

 

[phinds]

Ok so the quantum fluctuation wasn't the beginning of this infinitely dense point, the universe was conceivably always just sitting there as an infinitely dense / small, point.Thanks

No, there was never an "infinitely dense" point. There was an incredibly dense plasma of undetermined size. It might have been infinite, in which case the universe is and always has been infinite, or it might have been finite but unbounded (no center, no edge) in which case that topology still applies to the universe today.

 

[tim9000]

No, there was never an "infinitely dense" point. There was an incredibly dense plasma of undetermined size. It might have been infinite, in which case the universe is and always has been infinite, or it might have been finite but unbounded (no center, no edge) in which case that topology still applies to the universe today.

But how do you know the gravity of a concentrated infinite or even huge but finite universe wouldn't be enough to crush the elementary particles? And you couldn't exactly call that a plasma. Yeah undetermined size, sure, I would have thought before the expansion of space would be infinitesimal, but whatever, but I mean how do you know it wasn't infinitely dense exotic mass?

 

[phinds]

But how do you know the gravity of a concentrated infinite or even huge but finite universe wouldn't be enough to crush the elementary particles? And you couldn't exactly call that a plasma. Yeah undetermined size, sure, I would have thought before the expansion of space would be infinitesimal, but whatever, but I mean how do you know it wasn't infinitely dense exotic mass?

Good question, and I'm sure one of our members who understands this stuff better than I do can/will explain it.

 

[Farmar John]

I have been reading all this and I get confused by formula and terms that I cannot follow. Here is one thing I have thought for over 30 years.
Say, for argument sake that the universe was not expanding and time itself was speeding up for the whole universe, then I believe that light emitted from a distant object would arrive at Earth at a lower frequency. Let's take segments of distance over the total journey. The light travels over the first segment at the speed it left the object. Let's say there were 1,000 oscillations in that segment, Now it travels over the second segment at the same speed but time itself has sped up slightly so the second segment will, in order to contain the same number of oscillations, be shorter. The observer on Earth will receive shorter segments containing 1.000 oscillations. The speed of the light is the same but unknown to the observer, the measurement of time when the journey started to the present time, when the journey ended will be greater. The observer wonders why there is a red shift and, after more observations, discovers that the frequency is even lower the for more distant objects. Then the observer wrongly concludes that the only possible explanation for this a a doppler effect caused by an expanding universe. Now I don't say I am right, heavens no, but I can't see why I am wrong. I am not educated enough to know all about this sort of thing and this came to me long ago. Can someone tell me where I have gone wrong with my theory in simple terms please?

 

[PeterDonis]

Say, for argument sake that the universe was not expanding and time itself was speeding up for the whole universe

"Time speeding up" doesn't really have any meaning. "Time" is not an absolute.

Can someone tell me where I have gone wrong with my theory in simple terms please?

What you have come up with isn't really a "theory". To make it one, you would have to do two things: come up with a consistent framework in which you could make quantitative predictions; and compare those predictions with actual observations. And the observations in question would be much more than just the simple fact that light from distant galaxies is redshifted. You would have to account for the detailed way in which the redshifts are related to other observations, like the observed brightness and angular size of distant galaxies. You would have to account for the cosmic microwave background radiation, its observed temperature and spectrum and how it varies from one part of the sky to another. And so on.

In other words, when cosmologists say the universe is expanding, it isn't just a simple model based on a simple observation. It's a detailed model based on a lot of detailed observations. So it isn't necessarily going to be possible to take an alternative model like the one you are proposing and give one simple reason why it's wrong. I did give one reason above (that time is not an absolute); but that's really only part of the answer. The other part is what I said above, which isn't really simple.

 

[Drakkith]

But how do you know the gravity of a concentrated infinite or even huge but finite universe wouldn't be enough to crush the elementary particles?

My question would be whether or not matter and energy would create gravity (as in a net force) if they are spread homogeneously/near-homogeneously throughout space.

Yeah undetermined size, sure, I would have thought before the expansion of space would be infinitesimal, but whatever, but I mean how do you know it wasn't infinitely dense exotic mass?

We don't. We are looking back into the past from here in the present and we can't even see back beyond the CMB, let alone back to the near-beginning of the universe.

 

[tim9000]

My question would be whether or not matter and energy would create gravity (as in a net force) if they are spread homogeneously/near-homogeneously throughout space.

I'd have thought so, but I don't know enough about the Higgs field...my thoughts just trailed off then: its funny that energy and mass are linked, but only some matter has mass (gravity associated), because wouldn't a high energy photon when it splits into an electron and a positron (which do have mass)...

 

[PeterDonis]

only some matter has mass (gravity associated)

Gravity is produced by the stress-energy tensor; even things that have no rest mass, like light, have a nonzero stress-energy tensor.

 

[tim9000]

Gravity is produced by the stress-energy tensor; even things that have no rest mass, like light, have a nonzero stress-energy tensor.

I understand that light has momentum due to non-rest mass, relative mass. But what is a stress-energy tensor for a layman? I'm not familiar with that term, I assume it has nothing to do with the Higgs field.
Thanks

 

[PeterDonis]

what is a stress-energy tensor for a layman? I'm not familiar with that term, I assume it has nothing to do with the Higgs field.

The Higgs field is a quantum concept. If you're thinking it has something to do with gravity because it "gives mass" to particles, that's not correct; it doesn't.

The stress-energy tensor is a classical concept; it is described fairly well here:

https://en.wikipedia.org/wiki/Stress–energy_tensor

The section on the electromagnetic stress-energy tensor describes the stress-energy tensor for light.

 

[tim9000]

I didn't think it was to do with giving mass, but making matter with mass attract. So gravity isn't rectified classically and with quantum theory, they both in ways I don't understand, explain the same thing?

The section on the electromagnetic stress-energy tensor describes the stress-energy tensor for light

I checked that out before I posted my last response, I really didn't understand that (I wish I could makes sense of it), all I really recognised was the permeability of free space, my higher level maths is really in disorder.

 

[PeterDonis]

I didn't think it was to do with giving mass, but making matter with mass attract.

No; it doesn't have anything to do with that either. Things with no rest mass, like light, or like all fundamental particles before the electroweak phase transition, still attract.

So gravity isn't rectified classically

Huh? Where did I say that?

and with quantum theory, they both in ways I don't understand, explain the same thing?

I don't understand what you mean by this. I think perhaps you need to step back and rethink your questions in the light of what I've said about the Higgs field--basically, that it has nothing whatever to do with "gravity" in the sense you are using the term.

I checked that out before I posted my last response, I really didn't understand that (I wish I could makes sense of it)

It's worth taking some time to understand it, because the stress-energy tensor is how GR models anything that can produce "gravity" in the sense you are using the term. Note that "gravity" in this sense does not have to be attractive--it can also be repulsive. In fact, that's what "dark energy" is: it's a substance that produces repulsive instead of attractive gravity.

A somewhat oversimplified view of how this works can be seen by looking at an idealized "perfect fluid" type of stress-energy tensor (this is also described briefly on the Wikipedia page I linked to). Basically, in an appropriately chosen coordinate system, this stress-energy tensor is purely diagonal, and has diagonal elements $[\rho, p, p, p]$, where $\rho$ is the energy density and $p$ is the pressure. (We're using units in which $c = 1$.) However, note that in this simplified model, "pressure" can also be negative--in this case it is more appropriately called "tension".

Now, the key thing about this simplified model is that "gravity" is produced by the quantity $\rho + 3 p$, i.e., the energy density plus 3 times the pressure. For the details of why this is the case, I recommend reading John Baez's article on the meaning of Einstein's Equation:

http://math.ucr.edu/home/baez/einstein/

In theory, the relationship between energy density and pressure could be anything; but cosmological observations indicate that there are basically three kinds of "stuff" that are significant for the dynamics of the universe as a whole:

(1) "Matter". This includes all the ordinary matter that we are made of, plus "dark matter", which acts basically the same as far as the dynamics of the universe as a whole goes but we don't know exactly what it's made of. This kind of stuff has (in our simplified model) a pressure of zero; so its gravity is entirely due to energy density $\rho$, which is always positive, so $\rho + 3p$ for matter is just $\rho$, and is positive, meaning attractive gravity.

(2) "Radiation". This is stuff which is either massless, like light, or at such a high temperature that it behaves similarly to a massless substance, like the matter in the very early universe. This kind of stuff has pressure equal to 1/3 of its energy density (for example, if you work out the details of the electromagnetic stress-energy tensor in the Wikipedia article, you will see that it comes out this way), so $\rho + 3p = \rho + 3 \left( \frac{1}{3} \rho \right) = 2 \rho$. This is positive, once again, so radiation, like matter, has attractive gravity.

(3) "Dark energy". This is stuff which acts like the cosmological constant in the Einstein Field Equation. It turns out that this means it has pressure equal to minus its energy density, i.e., $p = - \rho$. (The pressure, as I noted above, is better thought of as tension in this case.) This means $\rho + 3p = - 2 \rho$, which is negative, so this kind of stuff, unlike matter and radiation, has repulsive gravity. This is why dark energy causes the expansion of the universe to accelerate (ordinary matter and radiation cause it to decelerate).

Note that nowhere in the above did I have to say anything about the actual underlying quantum fields that make up matter or radiation or dark energy. (Which is a good thing since for dark matter and dark energy, which are most of the universe, we don't know what those fields are.) This is all a simplified model at the classical level and works for any quantum field that gives rise to the appropriate relationships between $\rho$ and $p$ at the classical level.