Not even Einstein should insult the work of Einstein. The great physicist called the inclusion of a cosmological constant in his general relativity equations the "biggest blunder" of his life. But perhaps Einstein shouldn't have been so hasty with his self-criticism. A new study, the most precise of its kind to date, shows that Einstein's cosmological constant may explain one of the greatest mysteries in modern physics: dark energy.
After analyzing only a tenth of their data, collaborating researchers from seven countries report that dark energy behaves like Einstein's cosmological constant to a precision of 10%.
"What we find is that the dark energy doesn't seem to be changing; it seems to be a constant," said Richard Ellis, an astronomy professor at Caltech. "And that, remarkably, is what Einstein predicted in 1917 would be the case for what he called his 'cosmological constant.'"
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"Maybe his mistake was that he didn't realize it wasn't a mistake."
The scientists aren't ruling out all other theories about dark energy quite yet. The results thus far still may fit models that show gradual changes in dark energy over time.
"At this stage we can only rule out fairly wacky ideas," said Ray Carlberg of the University of Toronto. "There are some viable ideas that are within about 10% of being a constant value, so with our current precision we can't say if that's okay or not."
Carlberg said that when the five year study is complete, they will be able to properly evaluate these slow-change models.
Researchers determine how dark energy varies over time by observing how quickly the universe expanded at different points in history. To do this, observers measure and analyze the light from Type Ia supernovae. These supernovae are "standard candles": They have a constant, known luminosity. Astronomers know how brightness varies with distance, so they can determine how far away each supernova is and, then, how long ago the observed light left the supernova.
They can also determine how much the universe has expanded by observing the light's redshift. When space expands, the wavelength of light travelling through it increases; light that was once blue might look red. The more space has expanded, the redder the light appears.
Using these two measures, cosmologists can deduce how quickly space expanded during different epochs.
When all the results are in—and the researchers determine whether dark energy is constant or whether it varies over time—science will be left with even more probing questions about the nature of the universe.
If it turns out that dark energy matches the cosmological constant, we will have to face the fact that we have no idea what dark energy actually is.
"Most ideas for how dark energy should work predict that it should vary, and if it turns out to be constant, it's extremely puzzling, because then it says that we have just an unbelievably large gap in our knowledge of physics," said Carlberg. "It means that we don't know what about 70% of all the energy of the universe is."
We're also damned if we find that dark energy isn't constant over time, according to the University of Victoria's Chris Pritchet.
"If we, in the future, can observe the dark energy and determine that it's different for [light emitted a long time ago] than it is locally, that would be a major discovery and it would be something that Einstein would not have predicted," Pritchet said. "If Einstein's cosmological constant turns out not to be the explanation, I think people would be very disturbed because it would mean that the universe is vastly more complex and mysterious than we ever suspected."
Saul Perlmutter, of the University of California Berkeley, said he hopes the full results of this study will allow some bright young scientist to re-conceptualize the acceleration of the universe.