MadSci Network: Astronomy
Query:

Re: Why is negative energy portrayed as fact in current literature

Date: Sat Mar 29 13:58:37 2003
Posted By: Joseph Lazio, Radio Astronomer
Area of science: Astronomy
ID: 1048866101.As
Message:

Leaving aside the vitriol in the question, Why is there continuing talk about "dark energy"? (I've addressed elsewhere on MadSci why astronomers think that the Big Bang model is the best explanation for the evolution of the Universe.)

The reason that there is continuing talk is because we have a problem. The problem is summarized in this diagram, stolen from Tony Smith's Cosmology document:

On the horizontal axis the matter density of the Universe, relative to what it would take to cause the Universe to recollapse. So if Omegam is larger than 1, the Universe recollapses; if it is smaller than 1 the Universe expands forever. On the vertical axis is OmegaLambda or the equivalent energy density contributed by a cosmological constant, "anti-gravity," or dark energy. If this value is greater than 0, dark energy exists.

The shaded regions show what values of Omegam and OmegaLambda are allowed by a subset of the currently available data. There are actually two shaded regions shown. One shows the allowed values as indicated by measurements of the cosmic microwave background. The other shows the allowed values as indicated by measurements of the distances to Type Ia supernovae. The essential region of the plot is where the two shaded regions overlap. It is quite obvious that the overlap region is around Omegam ~ 0.3 and OmegaLambda ~ 0.7.

That's the problem. If we look at only one or the other of the measurements, then the shaded region for that particular measurement encompasses OmegaLambda = 0, and we have no need of dark energy. We're not allowed to pick and choose the data in science, though. We have to consider all of the measurements.

Moreover, as I indicated above, this graph doesn't show all of the data. If one includes measurements from things like the masses of clusters of galaxies, the constraints on Omegam and OmegaLambda become even tighter, and it is even more difficult to avoid dark energy.

(Heuristically, why is dark energy needed? One simple explanation is that distant Type Ia supernovae appear too dim. One way to explain why these supernovae appear too dim is to say that they are more distant than we thought, which means there must be some kind of repulsive force or dark energy in the Universe, accelerating its expansion.)

So what are the possible solutions? There are three:

In the absence of any other, better explanations, astronomers generally have settled on the third option. The result has been a combination of elation and unease. On the one hand, we're learning things about the Universe! That's one of the neatest feelings one can experience, and it's why many astronomers do astronomy. On the other hand, we don't understand dark energy and there's always the nagging worry at the back of one's mind that maybe we've missed something.

In conclusion, I think that dark energy is far from being accepted as "fact," though there are a number of strong indicators that the Universe might really work this way. Until somebody comes up with a better explanation, most astronomers will continue to think that the Universe contains dark energy.


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