This really is a very understandable article from the NY Times that covers the effort in pinning down the value of the Hubble constant, and how it relates to dark energy. The nature of this field of study is such that any parameters used is highly dependent on a number of important factors. This makes any kind of precise determination subject to be challenged.
“I think Adam’s work is nice,” said Dr. Freedman, who has led a large space telescope effort to measure the constant. But she and others added that some parts of Dr. Riess’s scheme could be vulnerable to the sorts of so-called systematic errors that have embroiled previous generations of astronomers in controversy — the effects of dust and galactic chemistry, for example, on the brightness of distant stars.
In an e-mail message, John Huchra of the Harvard-Smithsonian Center for Astrophysics wrote, “we know of several big bugaboos.”
So stay tune....