Three mathematicians have a different explanation for the accelerating expansion of the universe that does without theories of "dark energy." Einstein's original equations for General Relativity actually predict cosmic acceleration due to an "instability," they argue in paper published recently in Proceedings of the Royal Society A.

About 20 years ago, astronomers made a startling discovery: Not only is the universe expanding—as had been known for decades—but the expansion is speeding up. To explain this, cosmologists have invoked a mysterious force called "dark energy" that serves to push space apart.

Shortly after Albert Einstein wrote his equations for General Relativity, which describe gravity, he included an "antigravity" factor called the "cosmological constant" to balance gravitational attraction and produce a static universe. But Einstein later called the cosmological constant his greatest mistake.

...

The original theory of General Relativity has given correct predictions in every other context, Temple said, and there is no direct evidence of dark energy. So why add a "fudge factor" (dark energy or the cosmological constant) to equations that already appear correct? Instead of faulty equations that need to be tweaked to get the right solution, the mathematicians argue that the equations are correct, but the assumption of a uniformly expanding universe of galaxies is wrong, with or without dark energy, because that configuration is unstable.

more ...