A vast, empty region in Eridanus may be giving us hints about the operation of dark energy in the distant universe. The region shows up on the Wilkinson Microwave Anisotropy Probe’s map of the cosmic microwave background (CMB) radiation. The remnant of the Big Bang, the faint radio waves of the CMB provide the earliest picture we have of the cosmos. What the WMAP displayed to us was a view of its structure at a time just a few hundred thousand years after the Big Bang.
The Eridanus region stands out on the WMAP data because it’s slightly colder, and I do mean ‘slightly’ — we’re talking about temperature differences in the area of millionths of a degree. Two possibilities thus arise: The cold spot could be intrinsic to the CMB itself, a structural anomaly in the early universe. Or it could indicate something through which the CMB radiation had to pass on its way to our detectors. Now a study using data from the National Radio Astronomy Observatory VLA Sky Survey offers a possible confirmation of the latter.
For the Eridanus region shows a marked drop in the number of galaxies that would be expected there. Says Lawrence Rudnick (University of Minnesota):
“Although our surprising results need independent confirmation, the slightly lower temperature of the CMB in this region appears to be caused by a huge hole devoid of nearly all matter roughly 6-10 billion light-years from Earth.”
And that’s abnormal indeed. Yes, the universe is known to feature voids largely empty of matter, but none on this scale, nor does the magnitude of this ‘hole’ gibe with computer simulations of the large-scale structure of the universe. And the observational effect being examined may involve dark energy. The lack of matter creates lower temperatures in the CMB, the team theorizes, because CMB photons that pass through the void before reaching Earth should have less energy than those that pass through space filled with a normal distribution of matter.
Here’s the gist of what the team is arguing. With the paper not yet available online, I’ll have to work solely off the news release:
In a simple expansion of the universe, without dark energy, photons approaching a large mass — such as a supercluster of galaxies — pick up energy from its gravity. As they pull away, the gravity saps their energy, and they wind up with the same energy as when they started.
But photons passing through matter-rich space when dark energy became dominant don’t fall back to their original energy level. Dark energy counteracts the influence of gravity and so the large masses don’t sap as much energy from the photons as they pull away. Thus, these photons arrive at Earth with a slightly higher energy, or temperature, than they would in a dark energy-free Universe.
Conversely, photons passing through a large void experience a loss of energy.
But this work, as Rudnick said above, needs confirmation. As team member Liliya Williams (also at the University of Minnesota) emphasizes, “What we’ve found is not normal, based on either observational studies or on computer simulations of the large-scale evolution of the Universe.” Accounting for the size of this void and its relationship to the rest of the WMAP data will doubtless yield new surprises. Let’s hope it also has more to tell us about dark energy itself. The paper is slated for publication in The Astrophysical Journal; full references when they become available.