Exploring the Possibility of Vacuum Energy as Dark Energy: A Scientific Inquiry

[bapowell]

I'm not claiming to know what dark energy is. I'm simply saying that, if you put a stress energy into Einstein's Equations with $\rho \propto -p$, you get all the effects that I'm talking about -- including accelerated expansion and no effect on galaxy rotation curves. I'm not sure what two theses you think I'm simultaneously defending, nor am I aware of the strawman that you think I'm using. I'm saying that you can't argue that vacuum energy doesn't have gravitationally repulsive effects. This follows from two facts: that vacuum energy has $\rho \propto -p$ and that GR is correct. It isn't a matter of opinion.

 

[TrickyDicky]

I'm simply saying that, if you put a stress energy into Einstein's Equations with $\rho \propto -p$, you get all the effects that I'm talking about -- including accelerated expansion and no effect on galaxy rotation curves. This follows from two facts: that vacuum energy has $\rho \propto -p$ and that GR is correct.

I agree with those two facts and yet your conclusions are not the only ones possible from them, you are allowing yourself certain degree of interpretation from those facts so you must admit that there might be other interpretations of the same facts. We can agree to disagree, I'll leave it here.

 

[nismaratwork]

I agree with those two facts and yet your conclusions are not the only ones possible from them, you are allowing yourself certain degree of interpretation from those facts so you must admit that there might be other interpretations of the same facts. We can agree to disagree, I'll leave it here.

Um... I won't. I've been reading this, and I don't see this other possible interpretation THAT WORKS. You claim it exists, would you please show me, or link or otherwise support that claim?

 

[TrickyDicky]

Um... I won't. I've been reading this, and I don't see this other possible interpretation THAT WORKS. You claim it exists, would you please show me, or link or otherwise support that claim?

Well, bapowell made it clear that his interpretation is the one followed by most people in the community, I'm not completely sure about that but just in case that is the opinion of PF mentors too, I can't support it here any more than I've done, I don't want to risk an infraction, (still got many questions to ask), sorry.

 

[nismaratwork]

Well, bapowell made it clear that his interpretation is the one followed by most people in the community, I'm not completely sure about that but just in case that is the opinion of PF mentors too, I can't support it here any more than I've done, I don't want to risk an infraction, (still got many questions to ask), sorry.

Ahhhh... I see. Well, thanks for being up-front about that, I'm not trying to get you into any kind of trouble.

 

[Delta2]

No idea what this has to do with Automatic Teller Machines. I am not proposing a modified gravity -- just GR. In the weak field limit in the presence of a CC, $\Lambda$, one has for the Newtonian potential $\Phi$:

$$\nabla^2 \Phi = -\nabla g = 4\pi G \rho - \Lambda$$

where $g$ is the gravitational acceleration. This becomes

$$g = -\frac{GM}{r^2} + \frac{\Lambda r}{3}$$

and you can see the $\Lambda$ is a repulsive contribution.

Dont know much about GR and cosmological constant but can't CC be negative?

 

[bapowell]

Dont know much about GR and cosmological constant but can't CC be negative?

Yes. That geometry, called anti de Sitter space, is of particular interest to string theorists.

 

[Imax]

Dark energy is assumed to exist because of observations made on distant galaxies. The more distant a galaxy, the greater the red shift. So, for example, a galaxy at 10 billion light years has a greater red shift that a galaxy at 1 billion light years. This is interpreted to mean that the farther a galaxy is from our point of observation, the earth, the greater its’ speed, leading to the conclusion that the expansion of the Universe is accelerating.

What if instead of interpreting the observed red shift as a function of distance, it was interpreted as a function of time? So, for example, 10 billion years ago (i.e. a galaxy at 10 billion light years) a galaxy would have a greater red shift than a galaxy 1 billion years ago (i.e. a galaxy at 1 billion light years). That would mean that 1 billion years ago, the speed of a galaxy was less than that of a galaxy 10 billion years ago. This could be interpreted to mean that the expansion of the Universe is decelerating with time.

 

[nismaratwork]

Dark energy is assumed to exist because of observations made on distant galaxies. The more distant a galaxy, the greater the red shift. So, for example, a galaxy at 10 billion light years has a greater red shift that a galaxy at 1 billion light years. This is interpreted to mean that the farther a galaxy is from our point of observation, the earth, the greater its’ speed, leading to the conclusion that the expansion of the Universe is accelerating.

What if instead of interpreting the observed red shift as a function of distance, it was interpreted as a function of time? So, for example, 10 billion years ago (i.e. a galaxy at 10 billion light years) a galaxy would have a greater red shift than a galaxy 1 billion years ago (i.e. a galaxy at 1 billion light years). That would mean that 1 billion years ago, the speed of a galaxy was less than that of a galaxy 10 billion years ago. This could be interpreted to mean that the expansion of the Universe is decelerating with time.

AFAIK: Because this would be inconsistent with observations, and experiment since Hubble first chewed on a pipe.

 

[bapowell]

What if instead of interpreting the observed red shift as a function of distance, it was interpreted as a function of time? So, for example, 10 billion years ago (i.e. a galaxy at 10 billion light years) a galaxy would have a greater red shift than a galaxy 1 billion years ago (i.e. a galaxy at 1 billion light years). That would mean that 1 billion years ago, the speed of a galaxy was less than that of a galaxy 10 billion years ago. This could be interpreted to mean that the expansion of the Universe is decelerating with time.

10 billion years ago, a galaxy 10 billion light-years away would have had a redshift of around 0. The key is that today that galaxy has a greater redshift than a galaxy 1 billion light-years away. The galaxy at 10 billion light-years has a greater recession velocity today than the galaxy 1 billion light-years away simply because it is further away from us. This is a general trend that follows from Hubble's Law, and doesn't by itself tell us anything about the expansion history of the universe. What matters here is the detailed dependence of redshift on distance -- this is provided by observations of type 1a supernovae.

 

[Imax]

AFAIK: Because this would be inconsistent with observations, and experiment since Hubble first chewed on a pipe.

Not necessarily. The observation is the same (i.e. red shift), but the interpretation is different.

 

[nismaratwork]

Not necessarily. The observation is the same (i.e. red shift), but the interpretation is different.

That still conflicts with observations (see bapowell's post!).

 

[71STARS]

Gravity has nothing to do with so-called dark energy, in my opinion. Gravity is the attraction between two core entities. Its function in Empty Space diminishes greatly. When dark matter is found to be a steam/mist/quantum foam, the energy associated with it may be minimal as to be nil. However, the quantity of dark matter (aka pressure ether in Dwyer's), the sheer volume of dark matter, this would be the factor of force, but one that is completely non-hindering. Einstein's CC was strangely on point, even though it was not created for such.

 

[nismaratwork]

Gravity has nothing to do with so-called dark energy, in my opinion. Gravity is the attraction between two core entities. Its function in Empty Space diminishes greatly. When dark matter is found to be a steam/mist/quantum foam, the energy associated with it may be minimal as to be nil. However, the quantity of dark matter (aka pressure ether in Dwyer's), the sheer volume of dark matter, this would be the factor of force, but one that is completely non-hindering. Einstein's CC was strangely on point, even though it was not created for such.

What the HELL are you talking about? Gravity is, currently, believed to be the geometry of spacetime; you're describing Newton's view of gravity. Then you start rambling...

 

[TrickyDicky]

Not necessarily. The observation is the same (i.e. red shift), but the interpretation is different.

Simplyfying a lot. What is known as accelerated expansion is just an observation made from 1997 and well confirmed since, that a type of Supernovae, the type Ia, that due to its features is considered a good "standard candle" for large distances doesn't look as bright from here as the models predicted. This is done by comparing observations in the two main sources of information in astrophysics:spectrometry (redshift) and photometry (brightness).
Redshift can be used as an independent measure of the distant an object such a galaxy is, and if we have a good standard candle, that is, a reliable way to guess the intrinsic brightness of a distant object we can relate that supposed intrinsic brightnes with the brightnes we measure with our photometers.
And what was observed was that certain Supernovae redshifts when converted to distance didn't fit with the expected brightness, it appeared less bright than models usually predicted, and that could logically be interpreted as the Supernovae being farther away for a given redshift than the distance the model predicted for that redshift.
Considering redshift as velocity it leads to think that the Supernovae is accelerating from us, thus accelerated expansion. As simple as that, although the implications for cosmology are not as simple.

 

[PhilKravitz]

We should be a little careful here. Dark energy isn't repulsive anymore than ordinary matter is attractive. Rather, a homogeneous distribution of dark energy will cause spacetime to accelerate -- this is a gravitational phenomenon. So it's more correct to say that dark energy is gravitationally repulsive, and ordinary matter is gravitationally attractive.

So as the universe gets bigger there is more dark energy and the rate of expansion increases. What does this feedback lead to? Does it create a BANG!? If so, what happens next? If not, how fast will the universe be expanding in the future? Is there a limit to the expansion rate?

 

[nismaratwork]

So as the universe gets bigger there is more dark energy and the rate of expansion increases. What does this feedback lead to? Does it create a BANG!? If so, what happens next? If not, how fast will the universe be expanding in the future? Is there a limit to the expansion rate?

Less than 'c' in their own frame...

 

[PhilKravitz]

Less than 'c' in their own frame...

If I understand correctly during inflation points in the universe had the space between them increase faster than the speed of light. Can this happen at some point in the future due to dark energy repulsive force?

 

[bapowell]

So as the universe gets bigger there is more dark energy and the rate of expansion increases. What does this feedback lead to? Does it create a BANG!? If so, what happens next? If not, how fast will the universe be expanding in the future? Is there a limit to the expansion rate?

Good question Phil. The destiny of the universe depends on the nature of the dark energy. In its simplest incarnation, dark energy can be chosen to have a constant energy density (a cosmological constant.) In this case, as the universe expands, the dark energy in a comoving volume increases. The (logarithmic) rate of expansion given by the Hubble parameter,

$$H = \frac{\dot{a}(t)}{a(t)}$$.

where $a(t)$ is the scale factor (governing in the growth of length scales in the universe. ) In this case, when the dark energy is constant, the Hubble parameter is constant as well. The universe goes right on expanding.

However, consider the case in which the dark energy density grows, $\dot{\rho} > 0$. Then, it can be shown that a future singularity is hit (see: http://prl.aps.org/abstract/PRL/v91/i7/e071301) because the scale factor goes to infinity in finite time. This has been termed the Big Rip, and occurs at a time determined by

$$t_{\rm rip} - t_0 \propto |1+w|^{-1}H_0^{-1}$$

where the subscript '0' denotes present values, and $w=p/\rho$, with $w<-1$ for $\dot{\rho} > 0$. Dark energy that behaves in this way is called phantom energy, and has met with serious theoretical difficulties. In any case, as an effective equation of state, it leads to a cataclysmic dooms day.

 

[nismaratwork]

If I understand correctly during inflation points in the universe had the space between them increase faster than the speed of light. Can this happen at some point in the future due to dark energy repulsive force?

bapowell gave the good answer... I was just going to say, "nobody knows... we just know some options."

It is a very good question indeed.

 

[bapowell]

If I understand correctly during inflation points in the universe had the space between them increase faster than the speed of light. Can this happen at some point in the future due to dark energy repulsive force?

This is a popular misconception. Even during ordinary, decelerated expansion, there exist points in the universe that separate at speeds surpassing that of light. This follows simply from Hubble's law: $v=Hr$, where v is the relative velocity of the two points and r their separation. When their separation reaches a value of $r=c/H$ then you can see that the relative velocity surpasses that of light. Nothing funny going on -- this is what defines an important distance known as the Hubble radius.

The main distinction to be made with inflation is that during decelerated expansion, the Hubble distance grows faster than the expanding spacetime, gradually illuminating (literally) distant regions of the universe. During inflation, the background spacetime is expanding more rapidly than the Hubble distance is increasing with the result that the boundary of the observable universe becomes an event horizon. During inflation, the expansion can (and did) pull things outside of the observable universe.

 

[nismaratwork]

This is a popular misconception. Even during ordinary, decelerated expansion, there exist points in the universe that separate at speeds surpassing that of light. This follows simply from Hubble's law: $v=Hr$, where v is the relative velocity of the two points and r their separation. When their separation reaches a value of $r=c/H$ then you can see that the relative velocity surpasses that of light. Nothing funny going on -- this is what defines an important distance known as the Hubble radius.

The main distinction to be made with inflation is that during decelerated expansion, the Hubble distance grows faster than the expanding spacetime, gradually illuminating (literally) distant regions of the universe. During inflation, the background spacetime is expanding more rapidly than the Hubble distance is increasing with the result that the boundary of the observable universe becomes an event horizon. During inflation, the expansion can (and did) pull things outside of the observable universe.

That was... elegantly put!

 

[Imax]

Redshift can be used as an independent measure of the distant an object such a galaxy is, and if we have a good standard candle, that is, a reliable way to guess the intrinsic brightness of a distant object we can relate that supposed intrinsic brightnes with the brightnes we measure with our photometers.
And what was observed was that certain Supernovae redshifts when converted to distance didn't fit with the expected brightness, it appeared less bright than models usually predicted, and that could logically be interpreted as the Supernovae being farther away for a given redshift than the distance the model predicted for that redshift.

OK, maybe I’m a fuzz brain (i.e full of dark matter), but I don’t understand why differences in photometric (i.e. brightness) and spectroscopic (i.e. red shift) measurements of type 1a supernovae imply an acceleration in the expansion of the Universe.

 

[PhilKravitz]

During inflation, the background spacetime is expanding more rapidly than the Hubble distance is increasing with the result that the boundary of the observable universe becomes an event horizon. During inflation, the expansion can (and did) pull things outside of the observable universe.

This is a new idea to me. Very interesting. Thanks bapowell.

 

[PhilKravitz]

However, consider the case in which the dark energy density grows, $\dot{\rho} > 0$. Then, it can be shown that a future singularity is hit (see: http://prl.aps.org/abstract/PRL/v91/i7/e071301) because the scale factor goes to infinity in finite time. This has been termed the Big Rip, and occurs at a time determined by

$$t_{\rm rip} - t_0 \propto |1+w|^{-1}H_0^{-1}$$

where the subscript '0' denotes present values, and $w=p/\rho$, with $w<-1$ for $\dot{\rho} > 0$. Dark energy that behaves in this way is called phantom energy, and has met with serious theoretical difficulties. In any case, as an effective equation of state, it leads to a cataclysmic dooms day.

bapowell thanks for the reference I will go and read it. And I quite enjoy the terminology "Big Rip" and "phantom energy".

 

[Janus]

OK, maybe I’m a fuzz brain (i.e full of dark matter), but I don’t understand why differences in photometric (i.e. brightness) and spectroscopic (i.e. red shift) measurements of type 1a supernovae imply an acceleration in the expansion of the Universe.

1a supernovae all put out the same amount of light. By measuring their brightness we can tell how far away they are. Red-shift tells us how fast they are receding. So if we plot brightness against red-shift we are plotting distance against recession. Also, since light travels at a set finite speed, we are looking at them as they were and not as they are. The further the supernova, the further in the past we are looking. It's like taking snapshots of the universe at different points of time.

If the universe were expanding at a constant speed, we would expect to see a one to one match of distance and recession. Double the distance and double the recession speed.

But we don't see this, instead, we see a pattern that indicates that, in the past, the universe did not expand as fast as it does now.

The initial study expected to find the opposite. They expected that the universe would slow its expansion over time due to gravitational attraction. What they were trying to determine if it was slowing fast enough to ever stop the expansion and cause the Universe to collapse back on itself. The results they got surprised them.

 

[Imax]

Hi Janus:

Thanks for your reply. It’s given me a greater appreciation for the relationship between distance and recession. But (and I like buts) I can see it only for nearby galaxies. The problem with galaxies at a distance of 10 billion light years is that those photons are 10 billion years old.

 

[nismaratwork]

Hi Janus:

Thanks for your reply. It’s given me a greater appreciation for the relationship between distance and recession. But (and I like buts) I can see it only for nearby galaxies. The problem with galaxies at a distance of 10 billion light years is that those photons are 10 billion years old.

OK... I'll bite: Why would that matter, assuming a photon that was never absorbed and re-emitted (read: new photon!) to begin with?

 

[Imax]

When trying to gauge the current expansion of the Universe, points in the data set from millions to billions of years ago need to be treated carefully.

 

[nismaratwork]

When trying to gauge the current expansion of the Universe, points in the data set from millions to billions of years ago need to be treated carefully.

When making a statement, you have to actually say something, not intimate an unnamed caution like an old man wagging finger. This PF, not Dagobah.

To put it in better terms: what do you mean by, "careful", and what are you cautioning against?

 

[Imax]

When making a statement, you have to actually say something, not intimate an unnamed caution like an old man wagging finger.

Sorry nismaratwork if I sounded like an old man wagging my finger (naughty, naughty, naughty). That wasn’t the intent of my post. I used the word “carefully” because what’s happening now may not be what happened a long time ago. As a poor analogy, what’s the average speed of vehicles on an interstate? If you include old data from Model-T Fords, then it could bias results.

 

[Janus]

Sorry nismaratwork if I sounded like an old man wagging my finger (naughty, naughty, naughty). That wasn’t the intent of my post. I used the word “carefully” because what’s happening now may not be what happened a long time ago. As a poor analogy, what’s the average speed of vehicles on an interstate? If you include old data from Model-T Fords, then it could bias results.

But that's the whole point. By looking at more distant galaxies we are looking into the past, and this is how we know that the universe is expanding faster now than it was then.

 

[Imax]

So, vacuum energy < dark energy ?

 

[nismaratwork]

So, vacuum energy < dark energy ?

Bingo!

 

[scottbekerham]

I think General Relativity should be modified and the current theory is merely an approximation . If we arrive at the correct theory for gravitational physics that can be incorporated with quantum physics in a unified manner then the theory should be able to predict accelerated cosmic expansions , dark energy . and Inflation .May be Supergravity theories should be able to predict it