# Dark Energy

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1. Jun 28, 2014

Dark Energy is concept of total nuisance to physicist, deriving no crystal-clear concept to stand on.
So why are wasting a loads of time and effort on it?

2. Jun 28, 2014

### Staff: Mentor

It's the best hypothesis so far for explaining observations that suggest that the expansion of the universe is accelerating. We have these observations, and we can't just ignore them.

3. Jun 28, 2014

### Chronos

A analogous case existed for the orbit of mercury at the beginning of the 20th century. Scientists knew something was amiss, but, try as they might, they could not reconcile its insolent behavior in the face of Newtonian gravity. They tried everything - even undiscovered planets. Einstein finally figured it out. Observation is the lifeblood of science. If observation does not fit the model, the model is on trial. True to form, modern scientists have tried everything to make dark energy go away - even MOND. Dark energy is still the consensus choice among scientists.

4. Jun 29, 2014

### Chalnoth

Why do you think the cosmological constant isn't a crystal-clear concept?

5. Jul 1, 2014

But friends, are their any alternative to the Dark Energy concept?

6. Jul 1, 2014

### Chalnoth

Certainly. But they've all failed.

One idea was that because the accelerated expansion is a result inferred from far-away supernovae being dimmer than we would otherwise expect them to be without the accelerated expansion, perhaps light is losing energy ("tired light"), for instance via a slow oscillation between photons and an axion akin to neutrino oscillations. This turned out not to work very well to begin with, then we had some observations of really far-away supernovae that were brighter than they would otherwise appear (as predicted if dark energy/cosmological constant were the cause, as the early universe decelerated), and we also have estimates of distance that are not dependent upon brightness (such as the CMB observations and baryon acoustic oscillations), and those don't match a tired light hypothesis.

Another idea was that perhaps the FRW equations weren't quite right because they assume a homogeneous, isotropic universe, and some subtleties of the fact that our universe isn't quite homogeneous or isotropic show up in the estimated expansion rate. This is the alternative possibility that has stuck around the longest, but it just doesn't hold up under detailed observations. First of all, it was demonstrated that this kind of thing could only be explained if we were situated in the very center of a gigantic, spherical underdense region (a void). This seemed, on its face, to be highly unlikely to many cosmologists. But it was also shown that it just doesn't match observation.

There have been other, more exotic ideas, such as one proposing that maybe dark matter behaves such that it doesn't actually scale in density as $1/a^3$, but something slower like $1/a^{2.5}$. Then the cosmological constant appears just because we've made an error in understanding dark matter. But this doesn't really fit with observation either (it can't cause the expansion to accelerate, for one).

So yeah, lots and lots of ideas have been tried. But so far only dark energy remains, with the cosmological constant being the simplest explanation that fits the observed data.

7. Jul 1, 2014

### phinds

Try yourself to create an alternate hypothesis that holds water. THEN we'll talk about why physicists "waste loads of time" on it

8. Jul 1, 2014

### bapowell

In addition to Chalnoth's list, there have also been attempts to tweak the geometry side of the Einstein equations. Such approaches are known as "modified gravity", and they appeal to higher-order curvature terms. Perhaps someone else can comment on their overall success, but my impression is that they have generally met with difficulty.

9. Jul 2, 2014

So dark energy is said to be a sort of energy. So how can energy accelerate the expansion of our universe? Plz do not underestimate the question.

10. Jul 2, 2014

### phinds

"Dark energy" is a NAME. I does not actually mean energy, it means "something ... we don't know WHAT it is, but the guy that gave it the name decided to call it energy. He may be wrong"

11. Jul 2, 2014

### bapowell

Why can't energy accelerate the expansion of the universe? What physical misunderstanding are you struggling with that compels you to challenge this?

What are you getting at with "Plz do not underestimate the question." Either you have a problem with Einstein's general relativity or you don't have all your ducks in a row, or both. Which is it?

12. Jul 2, 2014

### Chalnoth

Consider a type of energy density that is constant, and does not change with expansion, and imagine that said energy density is the only stuff around. The Friedmann equation for this situation is:

$$H^2 = {8\pi G \over 3} \rho$$

...where $\rho$ here is a constant. Since this is a constant, we can rewrite the right hand side of the equation in terms of the current expansion rate:

$$H^2 = H_0^2$$

Now, we add to this the definition of of the Hubble parameter:

$$H = {1 \over a}{da \over dt}$$

Take the square root of both sides and shuffle things around a bit and we have:

$${da \over dt} = H_0 a$$

The above is one of the simplest differential equations around. It is the equation for exponential growth, and the solution is:

$$a(t) = a(t=0)e^{H_0 t}$$

And there you have it, an accelerating universe. Bear in mind that the Friedmann equation is derived directly from the assumptions of homogeneity and isotropy combined with General Relativity. There's nothing particularly special here except for the seemingly counterintuitive notion of an energy density that does not change as the universe expands.

13. Jul 3, 2014

So dark energy is some sort of energy pushing the boundaries of space-time. But I have heard that energy never gets destroyed or gets created, so does it mean that the Dark Energy is eternal?

14. Jul 3, 2014

And if so, then the energy must be finite and must have a limit to power the expansion of our universe. Then what's the time period of its effect over the expansion? Will there be a change over the acceleration of the expansion of our universe in future?

15. Jul 3, 2014

And whats the source of Dark Energy?

16. Jul 3, 2014

### Chalnoth

Conservation of energy isn't a law that applies in a curved space-time.

For one thing, for conservation of energy to apply, certain aspects of the system have to be independent of time (If you're interested in the details of how this works, look up Noether's Theorem). In an expanding space-time, the expansion of the universe breaks that symmetry, and thus breaks energy conservation.

For some more information on energy conservation in General Relativity, see here:
http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

One additional thing: that energy conservation doesn't strictly apply in a curved space-time doesn't mean that energy can increase or decrease arbitrarily: rather you have a different conservation law, the conservation of the stress-energy tensor. Under certain conditions, this conservation law reduces to conservation of energy. Under other conditions, it forces energy to not be conserved.