# Dark energy

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1. Jun 12, 2013

### Lsos

I'm no cosmologist and my understanding of dark energy is probably below that of a layperson's. But I'm misunderstanding not only what dark energy is (which apparently nobody understands), but even the concept of it.

The main thing I don't get is that they say it makes up ~70% of the mass/energy of the universe, because I guess that's how much energy is required to accelerate the galaxies away from each other.

However, my understanding it that the galaxies are NOT accelerating away from each other...just that the expansion of space between them is. Does this mean that if I expanded the space between two objects, would that require the same energy as simply moving the two object apart by the distance of the expansion? That doesn't seem right...from lay-reading of the big-bang, inflation, and proposed Alcubierre drives, I didn't think that expansion of space required any energy.

2. Jun 12, 2013

### Staff: Mentor

It is possible to associate the expansion of the universe to an energy density in the universe via General Relativity. It is not an energy density as regular matter has it.
With the expansion of the universe, the energy density of dark energy stays constant (probably), while the energy density of everything else goes down.

3. Jun 12, 2013

### phinds

"Local" things such as galactic clusters, galaxies, solar systems, you, atoms, etc do NOT move apart due to dark energy as these are all bound systems. Bound systems are getting farther away from each other at an accelerating rate and the reason for that acceleration of the expansion is called "dark energy" with the "dark" signifying that we don't know what it IS even though we know what it DOES.

There is some further discussion here:

www.phinds.com/balloonanalogy/

4. Jun 12, 2013

### Staff: Mentor

I think this is what you wanted to write (red part added by me).

5. Jun 12, 2013

### phinds

No, re-read what I said --- bound systems are getting farther away FROM EACH OTHER.

Of COURSE unbound systems are getting farther apart as well, but that's easier to understand.

EDIT: OK, I think I see how you interpreted what I said, but interpret it THIS way: Galactic cluster A is a bound system. Galactic cluster B is a bound system. A and B are moving apart.

6. Jun 12, 2013

### Chronos

The idea of dark energy arose in the late 90's with two supernova studies (re: Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant, http://arxiv.org/abs/astroph/9805201; [Broken] Measurements of Omega and Lambda from 42 High-Redshift Supernovae, http://arxiv.org/abs/astro-ph/9812133) that indicated the expansion of the universe was accelerating instead of slowing down [as expected]. Scientist quickly dusted off and restored credibility to Einstein's 'greatest blunder' - the cosmological constant. Since then, support for dark energy has continued to pour in. Scientists have long suspected there is not enough matter to explain why the universe appears to be flat [Euclidean]. WMAP confirmed the universe is flat to better than 1%, and also confirmed there is not nearly enough matter in the universe to achieve the density required to produce a flat universe. The contribution of matter, both baryonic and dark, only constitutes about 30% of the energy required for a flat universe (re: Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Data Processing, Sky Maps, and Basic Results, http://arxiv.org/abs/0803.0732). Measurements of the integrated Sachs-Wolfe effect also confirm the presence of a large amount of unidentified energy in the universe (re: Combined analysis of the integrated Sachs-Wolfe effect and cosmological implications, http://arxiv.org/abs/0801.4380). Baryonic Acoustic Oscillation distance measures also confirm an energy deficit, as well as confirm the distance assumptions used in the supernova studies (re: Measuring the Baryon Acoustic Oscillation scale using the SDSS and 2dFGRS, http://arxiv.org/abs/0705.3323; [Broken] The WiggleZ Dark Energy Survey: testing the cosmological model with baryon acoustic oscillations at z=0.6, http://arxiv.org/abs/1105.2862). The accelerated expansion of the universe is the best known, but, not the only evidence in support of dark energy.

Last edited by a moderator: May 6, 2017
7. Jun 12, 2013

### Staff: Mentor

Ah, okay. (A and B) are not a bound system then, just A and B individually.

8. Jun 13, 2013

### Lsos

I understood (at least to a satisfactory level) the balloon analogy and the basics. I just don't understand why/ how it requires energy to expand space. Since earlier in the universe, space expanded faster than the speed of light (by the same mechanism as dark energy, I assume), does that imply there was infinite dark energy back then?

9. Jun 13, 2013

### phinds

No, why should it. Space is currently expanding MUCH faster than the speed of light (things at the edge of our observable universe are receding from us at about 3c) but there is no assumption of infinite dark energy. In fact, as I recall, the density of dark energy is TINY, it's just that over cosmological distances it really adds up.

On the other hand, if the universe is infinite, then both mass and dark energy are infinite, but I don't think that's what you are asking at all.

10. Jun 13, 2013

### Mordred

Inflation is considered due to a different energy field, though their is no agreement on that field. The inflaton field is one possibility.

As to why DE is needed. All energy and matter has a positive energy density. Energy density is the same as pressure. The ecxeption is DE which is negative pressure.
Gravity is the source of energy acting on all other energy mass densities. Without DE the universe would contract. This occured until the universe was 7.3 billion years old. The cosmological constant (negative pressure)
then started to dominate and the universe started expanding.
if it wasn't for DE then the universe would have continued to contract due to gravity.

11. Jun 13, 2013

### phinds

Uh ... no, not really. I think I see what you are meaning, but as written your comments are wrong. The universe EXPANDED where you say it was contracting. It's just that it was expanding at a slowing rate and WOULD have eventually started to contract had not dark energy taken over and caused the expansion to not only continue but to accelerate.

12. Jun 13, 2013

### Lsos

I guess Mordred's post is why I'm confused. Everybody talks about dark energy as if it's some kind of negative gravity: gravity should cause the universe to contract, but dark energy not only balances this out but exceeds it, causing the universe to expand.

But...should gravity really cause the universe to contract? I understand that it attracts the galaxies and stars together, but not the SPACE between galaxies and stars. And no matter how much gravity we have, it will never cause anything to move at the speed of light. But dark energy doesn't actually push the galaxies away, it pushes the SPACE between them, no? And apparently it does it faster than the speed of light. So how did they figure out that it takes such and such amount of mass/ energy to push space at 3x the speed of light?

13. Jun 13, 2013

### phinds

If, as you say (correctly by the way) gravity causes the universe to contract, why would you find it surprising that something that has the NEGATIVE of that effect causes it to expand? I don't really understand that as being confusing. It's like saying that if I call North the positive direction then if I go in the negative direction I'm going South.

It is, I agree, confusing in the following sense. Gravity pulls THINGS (neighboring galaxies for example) together but dark energy doesn't seem to act on things (neighboring galaxies for example) directly but rather just causes the distance between them to increase. Google "metric expansion" for more discussion.

14. Jun 13, 2013

### Mordred

yeah I see that what I had above was poorly worded, thanks for pointing that out.

15. Jun 13, 2013

### Mordred

DE doesn't necessarily push space, it creates or stretches space. the two terms are interchangeable. however it is also not creating space faster than the speed of light. When you hear that galaxies are moving away from us faster than the speed of light. They are referring to its recessive velocity. Recessive velocity is distance dependant. Ie observer dependant.

Hubbles law states that the greater the distance the greater the recessive velocity. In other words if you measure galaxy A from Earth. If you find its recessive velocity as greater than the speed of light and were able to teleport near that galaxy its recessive velocity would be less than the speed of light.

http://arxiv.org/abs/1304.4446 :"What we have leaned from Observational Cosmology." -A handy write up on observational cosmology in accordance with the LambdaCDM model.
http://arxiv.org/abs/astro-ph/0310808 :"Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe" Lineweaver and Davies
http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf: [Broken] "Misconceptions about the Big bang" also Lineweaver and Davies
http://arxiv.org/abs/1002.3966 "why the prejudice against a constant"
http://arxiv.org/abs/astro-ph/0203330 "On the Cosmological Constant Problems and the Astronomical Evidence for a Homogeneous Energy Density with Negative Pressure"

the papers above I posted are excellent reading articles I particularly recommend the prejudice against a constant

Last edited by a moderator: May 6, 2017
16. Jun 13, 2013

### Mordred

Using Jorries lightcone calculator I narrowed down the inflection point I described poorly earlier on.

First I kept the inputs as default, turned all my column selections on, increased the number of decimal places to 6 and set steps at 100. Click calculate.

then I looked for the period of time where A Ro was lowest. This showed around 7.4 to 7.8 G yr.

I looked over on the S column and picked two S values surrounding that period in time. In this case 1.68 which I set for S_upper, S_lower I set for 1.64. As I didn't need as many rows I set steps to 30.

Between S 1.653333 and 1.650667 A R0 is the smallest values. so the universe was almost balanced for a very short period of time cosmologically speaking. Roughly 200 million years.

to post on the forum I simply click PF format tab. then click calculate and then copy the results and post on the forum.

$${\scriptsize \begin{array}{|c|c|}\hline R_{0} (Gly) & R_{∞} (Gly) & S_{eq} & H_{0} & \Omega_\Lambda & \Omega_m\\ \hline 14.4&17.3&3400&67.92&0.693&0.307\\ \hline \end{array}}$$ $${\scriptsize \begin{array}{|r|r|} \hline S=z+1&a=1/S&T (Gy)&R (Gly)&D (Gly)&D_{then}(Gly)&D_{hor}(Gly)&D_{par}(Gly)&a'R_{0} \\ \hline 1.680000&0.595238&7.425739&9.821697&8.187321&4.873405&14.687083&22.664831&0.872703\\ \hline 1.678667&0.595711&7.433501&9.829625&8.174286&4.869511&14.690984&22.690599&0.872692\\ \hline 1.677333&0.596184&7.441368&9.837561&8.161085&4.865512&14.694792&22.716505&0.872681\\ \hline 1.676000&0.596659&7.449143&9.845502&8.148050&4.861605&14.698704&22.742355&0.872671\\ \hline 1.674667&0.597134&7.457022&9.853451&8.134849&4.857593&14.702524&22.768345&0.872661\\ \hline 1.673333&0.597610&7.464908&9.861406&8.121648&4.853575&14.706351&22.794376&0.872652\\ \hline 1.672000&0.598086&7.472702&9.869366&8.108612&4.849648&14.710282&22.820350&0.872644\\ \hline 1.670667&0.598563&7.480600&9.877334&8.095411&4.845617&14.714120&22.846464&0.872636\\ \hline 1.669333&0.599042&7.488505&9.885309&8.082210&4.841579&14.717964&22.872620&0.872628\\ \hline 1.668000&0.599520&7.496417&9. 893290&8.069008&4.837535&14.721815&22.898818&0.872621\\ \hline 1.666667&0.600000&7.504334&9.901278&8.055807&4.833484&14.725671&22.925058&0.872615\\ \hline 1.665333&0.600480&7.512259&9.909272&8.042605&4.829427&14.729534&22.951340&0.872609\\ \hline 1.664000&0.600962&7.520189&9.917273&8.029404&4.825363&14.733403&22.977664&0.872603\\ \hline 1.662667&0.601443&7.528126&9.925280&8.016202&4.821292&14.737278&23.004031&0.872599\\ \hline 1.661333&0.601926&7.536070&9.933293&8.003000&4.817215&14.741159&23.030440&0.872594\\ \hline 1.660000&0.602410&7.544119&9.941314&7.989633&4.813032&14.744947&23.056990&0.872591\\ \hline 1.658667&0.602894&7.552075&9.949341&7.976432&4.808942&14.748841&23.083484&0.872588\\ \hline 1.657333&0.603379&7.560038&9.957374&7.963230&4.804845&14.752740&23.110021&0.872585\\ \hline 1.656000&0.603865&7.568106&9.965414&7.949863&4.800642&14.756547&23.136700&0.872583\\ \hline 1.654667&0.604351&7.576082&9.973461&7.936661&4.796531&14.760459&23.163322&0.872582\\ \hline 1.653333&0.604839&7.584164&9.981514&7.923294&4.792315&14.764278&23.190087&0.872581\\ \hline 1.652000&0.605327&7.592252&9.989573&7.909927&4.788091&14.768102&23.216895&0.872581\\ \hline 1.650667&0.605816&7.600247&9.997639&7.896725&4.783961&14.772033&23.243647&0.872581\\ \hline 1.649333&0.606306&7.608348&10.005712&7.883358&4.779724&14.775871&23.270541&0.872582\\ \hline 1.648000&0.606796&7.616456&10.013791&7.869991&4.775480&14.779714&23.297480&0.872583\\ \hline 1.646667&0.607287&7.624570&10.021876&7.856624&4.771229&14.783564&23.324462&0.872585\\ \hline 1.645333&0.607780&7.632691&10.029968&7.843257&4.766971&14.787420&23.351487&0.872588\\ \hline 1.644000&0.608273&7.640819&10.038066&7.829890&4.762707&14.791282&23.378557&0.872591\\ \hline 1.642667&0.608766&7.648953&10.046171&7.816523&4.758435&14.795151&23.405670&0.872594\\ \hline 1.641333&0.609261&7.657093&10.054283&7.803156&4.754157&14.799026&23.432828&0.872599\\ \hline 1.640000&0.609756&7.665341&1 0.062400&7.789624&4.749771&1 4.802807&23.460130&0.872604\\ \hline \end{array}}$$

A r0 is the proper distance history of recessive velocity (hubble radius)

Last edited: Jun 13, 2013
17. Jun 14, 2013

### Lsos

That's the thing, I'm not so sure that it's as simple as "gravity causes the universe to contract" and "dark energy causes the universe to expand", because they are pulling on two very different things. Which one is the actual "universe"? I imagine the space-time, and not the objects in it. If I smash two eggs together, I didn't cause the universe to contract locally. If an asteroid hits a planet, the universe didn't contract locally. But If I remove the actual space between the two eggs or the planet and the asteroid, causing them to smash, then I can say the universe DID contract.

That's exactly what confuses me. Gravity works on objects, so it's easy for me to say "it requires x amount of mass/energy to provide y amount of attractive force". However, it's not so easy to say "it requires x amount of mass/energy to provide y amount of repulsive force”, when talking about dark energy, because it’s not even a repulsive force we’re talking about. A force pushes and pulls on objects with mass, it doesn’t create space-time. It seems to me that they’re such fundamentally different concepts that even trying to talk about mass/energy in the context of dark energy is non-sensical. And yet scientists have seemed to pin down an exact value for the mass/energy of dark energy. And hence why I started this thread.

Unfortunately it looks like the answer is not as simple or intuitive as I had hoped :)

Thank you all for the replies. Looks like I have a lot of reading to do. I started on some of the linked papers, but so far too much is going over my head.

18. Jun 14, 2013

### phinds

Yes, I think the problem/confusion is that dark energy is sometimes described as "negative gravity" because it has that EFFECT, NOT because it is an exact opposite force to gravity.

19. Jun 14, 2013

### Mordred

Your right they do act differently in several aspects. What helped me understand the behavior of DE may or may not work for you. Their is a useful analogy that I like to describe DE. Unfortunately that analogy doesn't work very well with gravity. Also keep in mind its an analogy only, all analogies can mislead.

I mentioned before that energy mass density is identical to pressure in units/volume.
take a piston with pressure x, when you pull on the piston shaft the pressure within the cylinder remains the same, also the pressure is uniform throughout the volume of the piston. This describes the behavior characteristics of DE. The energy density of DE inside the piston has the following properties.

1) negative pressure
2) uniform throughout the volume
3) constant
4) has positive energy mass, but negative pressure as you increase the volume the pressure stays the same however the total energy mass increases. (where that energy came from we don't know, in a piston its from the action of pulling on the shaft. In DE we don't know, so we'll just say some QM field).