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Dark energy is introduced as a constant inside Einstein's equations. Its primary purpose, from what I understand, is to make Einstein's equations compatible with the accelerating expansion of the universe. As a consequence, of the "predictions" of dark energy is the expansion of the universe according to Hubble's law.

I know there are numerous experiments that verify this expansion (and its acceleration), and thus indirectly support the dark energy theory. My question is:
Are there other factors that give credit to the existence of dark energy?
Are there any experiments that support this theory, but not only through the verification of Hubble expansion?

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Dark energy is NOT required to explain the expanding Universe. It is used to explain the accelerating expansion of the Universe. –  mtrencseni Dec 21 '10 at 20:39
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...and Einstein initially wanted that term to allow a static universe... –  dmckee Dec 21 '10 at 20:40
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@mtrencseni: Fixed that. Thanks. –  Malabarba Dec 21 '10 at 20:49
    
...and the $\Lambda$ term is not even part of the stress-energy tensor, so it's more like a geometrical feature of the universe... –  Sklivvz Dec 21 '10 at 20:54
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@Sklivvz: when you treat it as dark energy, it is part of the stress-energy tensor. –  David Z Dec 21 '10 at 20:57

5 Answers 5

up vote 7 down vote accepted

I think the strongest evidence comes from the CMB fluctuations, namely the location of the first acoustic peak. This gives the overall geometry of the Universe ($\Omega_{tot}=1$; the Universe is flat). Then with a multitude of observations of dark matter (e.g., galaxy cluster counts, large-scale structure, and weak lensing) to get $\Omega_{matter}=0.3$, we are left with

$$\Omega_{\Lambda}=\Omega_{tot}-\Omega_{matter}=0.7$$

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(ps: I am a newbie here; how does one include eqns and LaTeX fonts?) –  Jeremy Dec 21 '10 at 21:27
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use $'s around it –  Malabarba Dec 21 '10 at 21:34
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But to be clear: this evidence is, in the end, really just based on the expansion of the universe. In fact, we have different independent bits of evidence pointing to dark energy, of which the first is from type Ia supernovae, but they all amount to somehow measuring the behavior of the scale factor over time. (If it's just a cosmological constant, this is really the only way you ever could measure it.) –  Matt Reece Dec 21 '10 at 22:33
    
I see--you want to know about local (solar system or lab) tests? In principle, this might be measureable with something like a Pioneer satellite specifically designed to be very stable with "drag free" housing around a test mass, similar to the LISA satellite to detect gravitational waves. Then we could bounce a laser off of it from earth and measure its acceleration –  Jeremy Dec 22 '10 at 13:22
    
To essentially repeat what Matt Reece just said, the CMB's contribution to the evidence for dark energy is entirely based on the expansion rate as well. It has to do with the angle at which the sound horizon at recombination is seen, and the two sides of that triangle are the angular diameter distance - given by the expansion rate. I honestly can't think of evidence for DE that doesn't rely on the expansion rate in one way or the other, which is what I essentially said in my answer below. –  Dragan Huterer Jul 10 '11 at 1:07

No, there isn't much beyond the acceleration parameter of the universe to support DE. In fact, if you're willing to abandon homogenity and isotropy, you can even get away without DE by choosing a void model, where you replace a fine tuning of the matter distribution with a fine tuning of the dependence of the density on radius from the 'center of the universe'.

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Relating to dropping homogeneity and isotropy: I once heard that the cosmological constant might be purely a result of bad averaging. If you allow for microscopic fluctuations in the galaxy fluid, you can obtain interesting corrections to Friedman equations. Well, at least that's what I heard. Anyone cares to elaborate of whether this is just a rumor or it is possibly interesting thing to investigate? –  Marek Dec 21 '10 at 22:06
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You heard correctly. This is the relevant reference for cosmological averaging. –  user346 Dec 22 '10 at 0:49
    
@space_cadet: thanks. –  Marek Dec 22 '10 at 9:52

It is true that nearly all of the observational evidence for the accelerating universe - that is, the presence of dark energy - comes from the measurements of the expansion rate (the Hubble parameter H(z), for aficionados). These measurements are usually not direct measurements of the expansion rate, but rather measurements of distances (to type Ia supernovae, for example), preferred length scales in the distribution of galaxies or in the distribution of cold and hot spots of the cosmic microwave background, and other geometrical measurements. All of these things directly depend on the expansion rate, and they have been the fundamental means of how we found out about dark energy.

A qualitatively different signature of dark energy is its effect on the growth of density fluctuations in the universe - that is, the rate at which galaxies and other objects form in time. Recent measurements (of galaxy distribution at different cosmic epochs etc) indicate that measurements of this growth also indicates dark energy. [In fact, ALL of the measurements in cosmology are in perfect accord with the presence of dark energy.] However, in General Relativity, the growth rate can be shown to ALSO depend mainly on the expansion rate H(z).

Therefore, the answer to Bruce Connor's excellent question is unfortunately not clear-cut: while many distinct experiments indicate dark energy, essentially all of them are based off of measurements of the rate of expansion - or the Hubble parameter H(z).

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I may being hopelessly naive but if you substitute the claim "The farther out in space we look the faster the expansion" with the equivalent "The farther back in time we look the faster the expansion" then we might deduce Hubble's "constant" is reducing, which could be explained by gravitation, for example. It's just a question of overcoming the mental picture of the vast distance to high-Z galaxies by remembering that we are seeing a much smaller universe at that time. There wasn't room for "vast distances" then. which makes one wonder about inverse square law dimming for deducing distances as well as the effects on luminosity due to solid angle &c.

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Space expansion? yes. It is measured in relation to the atoms around us.
What if the atom can vary its dimensions thru time?
I'm pretty sure that no one presented evidence that the atom is invariant.
And yet, everybody is claiming that the universe expands.

Space expansion or matter shrinks ?

The search of a scaling model of the universe, a self-similar one or dilation, has been pursued by the scientific community since Dirac, Hoyle & Narlikar, and others without results.

A scaling model is born, derived from data, using standard physics and making no hypotheses, this model has only one parameter ($H_0$) :

A Self-Similar Model of the Universe Unveils the Nature of Dark Energy

So, from now on I'will ask for proper evidence (theoretical or experimental) that the atom is invariant every-time that I hear someone to say: the universe is expanding.

Dark Energy? NO, imo it is an artifact of the reference we use (invariant atomic units).

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