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Finding Dark Energy in the Data

We always thought that the real impetus to the theory of ‘dark energy’ came from the discovery that the expansion of the universe seems to be accelerating. But an article in New Scientist points out that Allan Sandage (Carnegie Observatories, Pasadena) had studied evidence that might have led to the theory of dark energy way back in 1972.

Sandage was working with ‘peculiar velocities,’ deviations in the normal rate of cosmic expansion caused by the gravitational pull between groups and clusters of galaxies. And he had seen that galaxies just outside the Local Group showed velocities that were below what was expected. Fabio Governato of the University of Washington has now plugged dark energy into a computer model of galaxy formation and finds that this force matches nicely with the peculiar velocities for galaxies in regions like the Local Group. The Sandage data plus the new computer model, it can be argued, point to dark energy.

You can find an abstract of Governato’s study, “The signature of dark energy on the local Hubble flow,” here. From the preprint:

The environment of our Local Group provides new, independent evidence for the existence of dark energy on scales of just a few Mpcs [a megaparsec equals one million parsecs], corroborating the evidence gathered from observations of distant objects and the early Universe.

But as the New Scientist article indicates, some astronomers argue that factors other than dark energy can explain the galactic slowdown. Clearly we’re a long way from understanding dark energy, or even knowing if we are observing its effects in this case, but it’s always intriguing to find the seeds of potential discovery in observations that are decades old, like Sandage’s. It makes us wonder what data might currently be staring us in the face that could change the way we view other aspects of the cosmos.