Question concerning density of dark energy

  • #1

Main Question or Discussion Point

Ok this might be quite a stupid question but I can't seem to find a clear answer. The Wikipedia page on dark energy says that work done by WMAP and the Planck spacecraft indicate that the universe is made up of about 68% dark energy. But looking at the Wikipedia page titled List of unsolved problems in physics it says the following:

Why is the energy density of the dark energy component of the same magnitude as the density of matter at present when the two evolve quite differently over time; could it be simply that we are observing at exactly the right time?
Also, in one of the Lawrence Krauss lectures he presents the following graph:
Screen_Hunter_06_Jun_05_07_50.jpg


For those unaware, cosmological constant is another term for dark energy, assuming the density of dark energy never changes (as new space is created new dark energy is created with it they say).

So I think my question at this point should be fairly predictable: doesn't the cosmological coincidence problem tell us that there is essentially 50% dark energy if the densities are the same? Has Krauss placed the "NOW" label slightly after where the two lines intersect in order to match the 68% dark energy estimation? If so doesn't that mean the densities don't match all that well at this point in time and there is no real coincidence?
 

Answers and Replies

  • #2
Simon Bridge
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Welcome to PF;
By "magnitude" I suspect the cosmic coincidence article is referring to "order of magnitude".
There is not enough information to make such statements about absolute magnitudes.
It is fair to say that 50% and 68% have "about the same order of magnitude".
Cosmology is like that.

Krauss has, indeed, placed the "now" label to illustrate the empirical measurements.
This is the data that any model of cosmology has to conform to after all. Reading off the graph, the red and blue lines both have order 10-30 and continue to do so until well after "our sun dies".

Note: Wikipedia articles will disagree with each other when they are not updated, to reflect new discoveries, at the same rate. In general, Wikipedia is not a reliable source for information - though it can be useful as a starting point.
 
  • #3
It is fair to say that 50% and 68% have "about the same order of magnitude".
Ah that makes sense, especially considering the y axis is using powers of 10 as the units.
 

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