Is Dark Energy what we think it is?

Free energy will promulgate a forward leap in human progress akin to the discovery of fire. It will bring the dawn of an entirely new civilization -- one based on freedom and abundance. -Sterling Allan

Of course, when Sterling Allan talks about free energy, he's talking about natural energy from sources like wind and solar, not the violating-the-laws-of-thermodynamics type of energy.

There is, of course, no such thing as truly free energy, or energy that we can take out of nothing and use for something, which is why perpetual motion machines not only don't work, but are physically impossible. (Although it is amusing to try to get as close as possible.)

But as many of you have noted about dark energy, there is a non-zero amount of energy that seems to be inherent to space itself. As the Universe expands, it appears to create more space, and hence, more energy.

Now, the energy density is tiny. So tiny that we didn't even discover the existence of dark energy until 1998, and if you were to compare it to the energy stored in, say, the mass of a human body, you would have to spread a human being out over the entire inner solar system (to fill a sphere the size of the orbit of Mars) just to get the same density as dark energy, which is about two protons per cubic meter.

Now, while energy is ill-defined in general relativity, we understand energy and momentum well enough (as well as more complicated properties of metric spaces, which I will not go into) to know that dark energy should have the following properties based on our current observations:

  1. It should have a constant energy density everywhere in space.
  2. It should be impossible to add to or take away from that energy density.
  3. That energy density should also remain constant throughout time.

However, a recent paper has come out on the arXiv (and was discussed over on Cosmic Variance earlier today) that seeks to test that first assumption: is dark energy a constant everywhere in space?

Using a hypothetical improvement on a technique called atom interferometry (illustration above), they are proposing that changes in dark energy density could lead to changes in atomic motions, and could hypothetically exert a force on atoms.

Now, there are all sorts of reasons to believe that dark energy doesn't exert a force on atoms. Namely, the following big ones:

  1. Dark energy, as far as we can tell, affects the expansion of space and nothing else, meaning it shouldn't exert a force on atoms.
  2. You can only exert a force (whether you're dark energy or not) if your field changes from point-to-point. On the other hand, dark energy is observed to be a constant everywhere in space.

But, it isn't like we have a better proposal out there to try to perform some laboratory test on dark energy. Does it couple to matter? We don't think so, but we haven't tested it sufficiently to know for sure. Is it a constant everywhere in space? We think so, but we don't know if it clumps (even a little) around masses like the Earth or the Sun.

There is so much we don't understand about dark energy, and how the Universe's expansion on the largest scales relates back to what we can observe in a laboratory on Earth that this is possibly the most exciting prospect to come out concerning dark energy all year!

So do I think this is likely to produce anything new? No, probably not. There are many good reasons to believe that we know what we're talking about, but that doesn't really matter. What matters is that -- in order to know anything for sure -- we need to do the experiment. This idea deserves to get a little bit of a buzz, if for no other reason than we need to throw ideas around about dark energy, and be open to the notion that what we're seeing is so bizarre it could really turn our view of the Universe on its head.

But there still isn't any truly free energy out there, not even if dark energy does change from point-to-point. Which is too bad... because if there was some, I could then move to phase 3. I'll keep dreaming.

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Regarding dark energy, I think people would understand it much better if you said 'space-expanding awesome' rather than 'dark energy', which implies it acts on matter.

By Katharine (not verified) on 23 Feb 2010 #permalink

Dark energy reminds me of luminiferous aether. How long before the Michelson and Morley of dark energy appear on the stage?

I'm glad you put a picture of a turtle in this post.

Like Katherine, I get thrown off by the term dark energy. Vacuum energy or the cosmological constant or "what makes Edwin Hubble a really cool dood" are less confusing, but that's just me. The Casimir effect shows that vacuum energy can actually "do" something to real stuff on an observable scale, which is why I like it so much. I want a home science kit with the Casimir Effect included !!! And a cloud chamber.

You said to exert a force, a field needs to change from point to point. Then how does a uniform electric or magnetic field exert a force on a charged particle?

By Crux Australis (not verified) on 23 Feb 2010 #permalink

Electric fields aren't uniform, they still have a gradient depending e. .g on the distance of the particle to the plates generating it.

@Crux Australis: Ethan didn't mean a "force field" like the electric or magnetic field, but a "potential field" (notice that he talks about energy!). As you perhaps know, both the electric and the magnetic fields can be expressed as derivatives of potentials. If the potential is constant, the electric and/or magnetic field will be zero, and there won't be a force.

Crux Australis,

I am sorry for being so vague. You are correct: a uniform electric field and a uniform magnetic field are both perfectly capable of exerting forces on charged particles.

But dark energy is "the other" kind of field: a scalar field, with a magnitude only, as opposed to electric and magnetic vector fields, with a magnitude and a direction.

If you take the gradient of a scalar field, you get a vector field, which is necessary to exert a force. (Forces require a direction.)

Glad we got to clarify (and thanks to Bjoern and Mu for the help)!


You'll be pleased to know that I have found dark matter, however it was only after I completed step 1 of your 3 step plan.

I apologize if I missed the post, crd2 and Dr. Siegel, but was the plan

1. Magically transport yourself to the edge of the universe and become an all-knowing being who can detect such things as 'dark energy' and 'dark matter'
2. ???


By Katharine (not verified) on 25 Feb 2010 #permalink

So does dark energy as a non-directional, scalar field require an infinitely large Universe in which it operates? Or is the Universe so large, but not infinitely large, that the directionality is for all intents and purposes, undetectable? Or does Hubble red-shit expansion make it impossible for any two sufficiently distant parts of the Universe to be unable to communicate with each other, thereby destroying any possibility of directionality?

Even you have detected dark matter in your underpants at some point katharine. Dont play coy with me.

When you suggest that the hypothesized dark energy does not interact with atoms; I assume that you mean baryonic matter. But what about quarks, neutrinos and the other fermi-dirac particles of the stantard particle theory; and what about photons and gluons and the other bose-einstein particles of the standard model. And if none of those particles (and/or their fields) then does the hypothesized dark energy interact with supersymmetry particles (BE or FD)? Any thoughts; I just don't know what is currently hypothesized?

As well, I really do not have a clear idea what exactly the "dark energy" observations and correlations are? (e.g. are they redshift data or something else?) As well I don't understand how dark energy is is somehow analogous to the Casamir effect?

Nathan, I was rather disappointed that the dark energy density is so low,2 protons per M3.
I thought that quantum entanglement could be the separated particles interacting into and out of a dark energy field when separated by a distance eg some km apart.
The analogy being an electric current in a conductor where an electron "in" is balanced by an electron "out" at the far end.The actual rate of individual electron flow in the conductor is slow MM per minute?
This idea would require dark energy (or dark matter?)density to be relatively high.
How do you estimate DE density? I guess on a galactic scale?
If we were living in a dense soup of darkness, some of the more puzzling observations like identical twins knowing at a distance when the other is distressed might have a rational but dark explanation?

By Ron Horgan (not verified) on 05 Mar 2010 #permalink

Nathan , sorry for the idiot question.
I see the density of the large scale universe estimated from WMAP as 5.9 protons M3 with dark energy component 72% as about 4.2.
I still like the dark dense soup as a possible medium for very odd events.
Does your cafe have anything like this on the menu?
Quantum entanglement is so cute!

By Ron Horgan (not verified) on 05 Mar 2010 #permalink

Ethan, Sorry about getting your name wrong.
I have some more innocent questions
If the model of the vacuum energy of space is a sea of transient particles and antiparticles coming into "our "universe , and going back into into nothingness,
might the notional protons of dark energy represent leakage and escape from this equilibrium.?
Development of this idea might start to look like "steady state" with the source of the expansion being an inherent property of space rather than Fred Hoyles sartirical Big Bang?
Coming back to the dense dark soup notion,might the vacuum energy of space represent a low proportion of the total dark matter which is present "through the looking glass"
How much "material " would be required to act as a "conduction medium" for quantum entanglement?
A lot easier to ask than answer I expect!

By Ron Horgan (not verified) on 07 Mar 2010 #permalink

I was trying to find out more about expansion and google calculated an equivalent in hertz.

72 ((kilometers / second) / Mpc) = 2.33335907 Ã 10 to the minus 18 hertz.

Does that mean the expansion sort of blinks or vibrates?

I can't believe it's Pluto's 80th birthday. Having said this, the planet has yet to be visited by a spacecraft. We do know that the planet is mostly made up of ice which is one of the reasons it might be very difficult for any mission to take off.

The world's largest steerable telescope - Robert C. Byrd Green Bank Telescope of the National Science Foundation - will provide scientists with valuable clues about the nature of the mysterious 'dark energy' believed to constitute nearly three-fourths of the universe's mass and energy.
Cellex-C Serum

By Cellex-C Serum (not verified) on 21 Jul 2010 #permalink

i love these

Yet for all their disdain for the former Alaska governor, liberals still can't quite put their finger on her

By vegan protein powder (not verified) on 20 Oct 2010 #permalink

Great list, I really enjoy reading these posts and I've already downloaded (and subsequently gotten hooked on) Drop7 based on your video game list.

Hi Ethan,
Love your "Macho Man Randy Savage" halloween costume. I saw a recent picture of Randy and he has completely white hair now, can you believe it!?

You have a good money if you make a lot of tourist want to visit your town. To make them want to visit your town, you need to make sure that you have the clean town area. There are no people that want to go toi a dirty place like this.

Their government know if they can manage that potential change by their own. That is why they need people that have some experience to help them and we can stay in their country for free.

it shouldnt be called "DARK" its actually more "Light" than dark in fact its so clear its transparant. The first cosmologists missed the opportunity to coin it properly and put the wrong spin on it. Imagine if it was first coined white matter or even golden matter or god forbid ... holy matter

By Roy Mendez (not verified) on 03 Dec 2010 #permalink

This is the application project. They need to make a good one to make sure that tou have alot of people that will use this application. More people mean we sucess in this

By Helosboing (not verified) on 21 Dec 2010 #permalink

Development of this idea might start to look like "steady state" with STD Symptoms the source of the expansion being an inherent property of space rather than Fred Hoyles sartirical Big Bang?

hm.. funny thing... i've heard that in physics, light is synonym with energy, but this doesn't sound ok: dark light... light is bright, not dark :)

The term "dark" refers to how much we know about it. Just as to illuminate something can also mean to become more aware of, or learn more about something. (Example: To shed light on a topic). So dark just means that we know very little, not necessarily that it doesn't give off any photons. I agree, the name is far from optimal. If we can down grade Pluto out of planetary status and have the collective world accept it, I don't see any reason why changing the name of dark energy should be so difficult.

Wouldn't it be possible for a particle to travel _faster_ then light if it some how had negative mass? (yes i know, it may be silly physically but come on the math works doesn't it?), I'm pretty sure this would also mean it traveled in the reverse direction in time but i haven't played with the relativity equations enough yet to be sure.

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I agree, the name is far from optimal. If we can down grade Pluto out of planetary status and have the collective world accept it, I don't see any reason why changing the name of dark energy should be so difficult.
This is the application project. They need to make a good one to make sure that tou have alot of people that will use this application.

I think this is likely to produce anything new? No, probably not. There are many good reasons to believe that we know what we're talking about, but that doesn't really matter. What matters is that Cherished Teddies

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Why would dark energy be a constant? I don't think any form of energy is constant in space, in terms of how it is diffused or spread.

Why would dark energy be a constant? I don’t think any form of energy is constant in space, in terms of how it is diffused or spread.

Did you drop by just to spam lysine, or do you want an explanation of density?

^ Oh, wait, that was necromancing of a five-year-old post. I'm going with spam.

Odd thing though, this sort of scumming really only puts people off. Like cold calling or hard selling, if people have a choice, they avoid the products that "need" that form of selling like the plague.

If they're going to be scummy just trying to advertise, what the hell must the product be like?!?!