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A bit of a long post - still trying to get clear for myself what I really want to ask. You can perhaps skip the background info below and go straight to 'My question is'.

**Background**

On p300 of my Penguin edition of "The Fabric of the Cosmos", Brian Greene writes:

The recession speed of a supernova depends on the difference between the gravitational pull of ordinary matter and the gravitational push of the "dark energy" supplied by the cosmological constant. Taking the amount of matter, both visible and dark, to be about 30 percent of the critical density, the supernova researchers concluded that the accelerated expansion they had observed required an outward push of a cosmological constant whose dark energy contributes about 70 percent of the critical density.

He then stresses how, satisfyingly, results from two different empirical investigations - measuring/estimating the density of the universe (not discussed in detail

*how*it was done, cf p291), and measuring/estimating the expansion rate of the universe (via supernova study) - mutually confirm the conclusion that

[...] the outward push demonstrated by the supernova data can be explained by just the right amount of dark energy to

I don't get how one gets from the information about __account for__the unseen 70 percent of the universe['s density] that inflationary cosmologists had been scratching their heads over. The supernova experiments and inflationary cosmology are wonderfully complementary.__They confirm each other. Each provides a corroborating second opinion for the other.__[emphasis added]

*expansion rate*to the numbers about the proportions of stuff that make up the universe (or perhaps vice versa).

What do we have (forgive my no doubt very clumsy equations - just trying to get a rough idea across):

- There is a universe expansion rate
*e*(empirically found via supernovae). I may be dead wrong, but I'm guessing that, simplifying, something like this holds [eq1]:*e*=*e*where we know, or at least guess, the magnitudes of the impacts of_{unknown}- e_{dm}- e_{vm},**v**isual**m**atter/energy and**d**ark**m**atter/energy (negative terms since their gravitational impact is the 'regular' contractive one). There used to be an unknown*e*, the value of which has sprung from the equation after establishing_{unknown}*e*through the supernova measurements. Let's call whatever is causing the*e*term "dark energy", and we must assume it has negative pressure (for it causes repulsive gravity); but otherwise we have no clue what it is (is that so?)._{unknown} - There is a universe density
*d*(empirically found via means not quite detailed on p291), for which something like this holds [eq2]:*d*=*d*Expressed in proportions,_{unknown}+ d_{dm}+ d_{vm}.*d*corresponds to 100%, and our empirical guesses for*d*and_{dm}*d*are 25% and 5% respectively. So_{vm}*d*must be 70%._{unknown}

**My question is**: if we have no clue what the "dark energy" (from eq1, expansion rate related) is (i.e. what its properties are apart from what it does to

*e*), how can we say anything about its density

*from equation 1*? How is this a case of

__accounting__for the missing 70% in eq2, rather than just

__attributing__

*to this something ('dark energy') which we decided to use as a term to mean that what accounts for*

*d*_{unknown}*e*?

_{unknown}Here is an analogy which is imperfect on purpose - see next paragraph. Suppose that we find we have 100 apples in a basket (corresponding to

*e*). We know Don took out 10, and Val took out 10 too. We conclude that something or other, "dark energy" must have put in 120 apples. We also happen to know the total number of chihuahuas that all apple-handling entities have; let's say it's 100 too (corresponding to

*d*). And we know that Don has 5 chihuahuas and Val 25. Sure, it follows - trivially - that "dark energy" must have 70 chihuahuas. But we could have said that without knowing all the stuff about how many apples are in the basket. So it seems we are really just using the same term ("dark energy") for the things behind the up to that point unknown terms in our equations... but that doesn't seem to say much. You might as well call it "previously unknown remainder". And I don't see how one previously unknown remainder confirms another previously unknown remainder.

I figure the answer may be in the actual relationship between expansion rate and density. In my analogy this is missing: I implicitly assume there is no particular relationship between number of apples put in/taken out and number of chihuahuas owned. In contrast, density seems to have a lot to do with expansion rate, at least to my feeble intuition on this. Maybe for simplification, Greene has skipped the details? Or no less likely, I'm missing something. Enlightenment most welcome! Thanks.

Just for good measure:

- Post not meant as criticism of Greene's book. It's not faultless I think, but I'm grateful for all I'm learning from it.
- I don't mean to sound like "you (physicists) don't know what you're talking about". I bet you do. To the contrary, I don't understand what you're talking about. (I hope you understood what I'm asking :^)