Year in Review: Dark energy gets more confusing

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New measurements of light from distant exploding stars were supposed to illuminate the dark energy that is pushing the cosmos apart. Instead they have further shrouded the universe’s fate.

Dark energy first made headlines in 1998, when researchers found that light from faraway supernovas was dimmer than expected, suggesting that the universe is expanding at a faster and faster pace. To explain this acceleration, scientists surmised the existence of dark energy, which pushes space outward (SN: 4/7/01, p. 218). Most physicists suspect that dark energy is a form of vacuum energy known as the “cosmological constant” because its strength never varies. If so, a number called w, which relates the pressure pushing space apart to the density of dark energy, must equal –1.

But this year, scientists using a powerful new telescope in Hawaii, part of a project called Pan-STARRS, arrived at a different value for w. By combining the supernova data with previous results from other studies, the researchers calculated w to be –1.186.

This value, if confirmed, would force cosmologists to pursue more complicated theories in which dark energy’s strength increases over time. If it does, the universe might ultimately be torn apart in what scientists call the Big Rip (SN: 11/30/13, p. 8).

So far, though, nobody is writing off the cosmological constant. The researchers say they and other groups must first look for sources of measurement error, starting with the telescopes they use to study far-off parts of the universe. Even slight discrepancies in how telescopes gather starlight that has traveled billions of light-years can introduce sizable differences in the data the instruments collect.

In two years, when cosmologists have more precisely calibrated their instruments and analyzed more data, they may know whether the cosmological constant needs to be scrapped. For now, says study coleader Armin Rest of the Space Telescope Science Institute in Baltimore, “My hunch is that w is –1.”

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