Could Vacuum Energy Be the Key to Understanding Dark Energy?

[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

 

[71STARS]

"Dark Energy" per se can be summed up as not existing simply because "Dark Matter" when it is in an overabundance state will naturally nudge galaxies apart. That does not imply energy. That is merely an accumulation of the unseen mass. When the cause of Dark Matter can be determined, the functions will show that an overabundance of this unseen mass is the reason for any distance fluctuation in galaxies (or suns and planets). The timeframe would be sooooo small, but simply a natural process. Yes, this is stictly my opinion. And yes, Einsteins cosmological constant can be equated to the term "Dark Matter" although he did not think in those terms.

 

[bapowell]

Einstein's cosmological constant in no way behaves like dark matter. Dark energy and dark matter are logically (and physically) independent concepts.

 

[mikejr82]

according to special relativity mass and energy are equivalent so because vacuum energy has mass so it should exert a gravitational force on matter . so , why can't dark energy be simply vacuum energy ?

This is something that has intrigued me for a while. When a matter and anti-matter colide,they destroy each other in a massive burst of energy.(cassimir effect).
Is it possible that these collisions create space between objects?
I believe this would better explain the expansion of the universe and how galaxies collide even tho everything in the universe is supposed to be moving away from each other.
No one has figured out how to calculate the true force generated by vacuum energy.

 

[twofish-quant]

This is something that has intrigued me for a while. When a matter and anti-matter colide,they destroy each other in a massive burst of energy.(cassimir effect).
Is it possible that these collisions create space between objects?

Casmir effect is something different, and no matter-antimatter collisions don't create unusual amounts of space. Anti-matter is something that gets produced in particle accelerators all of the time, and people use anti-matter routinely for brain and heart scans (google for positron emission tomography).

No one has figured out how to calculate the true force generated by vacuum energy.

They have actually, it's not a hard calculation

http://en.wikipedia.org/wiki/Casimir_effect

 

[twofish-quant]

I think General Relativity should be modified and the current theory is merely an approximation.

So do a lot of other people. The astrophysics database search for modified gravity has about 10000 hits

http://adsabs.harvard.edu/cgi-bin/basic_connect?qsearch=modified+gravity

The problem is not just saying "let's modify GR." The hard part is to say "if you modified GR in this way, then you will get observations that do or don't match what we see."

Also one way that physicists thinks instead of talking about *one* possible modification to GR, what you do is to try to classify all possible modifications to GR in several groups and then try to knock them over.

 

[bapowell]

No one has figured out how to calculate the true force generated by vacuum energy.

Why do you say this? Also, why do you think your suggestion better explains the expansion of space than the Friedmann solution? Have you worked out the relevant quantities in your theory: expansion rate, redshift relations, age of the universe, etc? Can you fit supernova, CMB, and large scale structure data with your idea?