Do we really need dark energy to explain cosmic expansion?
The evidence for dark energy lies in our ability to relate the redshift of a galaxy with it’s distance. To prove dark energy is real we have to measure redshift and distance independently, and that takes a bit of doing.
New research questions the claim that the universe is accelerating. But this new work isn’t as strong as some claim.
One proposed model for dark energy known as the chameleon field has been put to the test, and failed.
We know the universe is expanding, and we know it is doing so at an ever increasing rate. This cosmic acceleration is part of the evidence for dark energy, which current observations put at about 68% of the observable universe. But beyond its existence as some kind of energy, we’re still trying to determine just what dark energy is.
Imagine a stadium filled with people. With everyone is in their seats, waiting for the game to begin, there is an undercurrent of noise. A few words between friends, the scuffle of shoes, the creak of a chair. All of these little sounds fill the stadium with a background of white noise. A similar “white noise” occurs with galaxies in our universe, and it helps us understand dark energy.
Dark energy is perhaps the least understood aspect of modern cosmology. We first obtained evidence of its existence via the 1998 discovery that the universe is not only expanding (which we’ve long known), but that the rate of expansion is accelerating. This was done by observing the redshifts of distant supernovae, and won the discovers a Nobel prize. Since then observations of the cosmic microwave background have found that dark energy makes up about 70% of the matter-energy in the universe. This is consistent with observations of acceleration.
The Soudan Iron Mine in Northern Minnesota is home to several experiments in particle physics and cosmology. I’ve written about one of the projects there, known as the Cryogenic Dark Matter Search (CDMS). Another experiment is Main Injector Neutrino Oscillation Search (MINOS), which detects muon neutrinos produced at Fermilab in Northern Illinois. MINOS is about 48 feet long, and contains 6000 tons of steel layered between scintillators. The entire detector had to be lowered down a narrow mine shaft piece by piece and then assembled on site.
From measurements of distant supernovae, we now know our universe is not only expanding, but that it is expanding at an ever increasing rate. This cosmic acceleration is driven by what we call dark energy. While we can see the effects of dark energy, and we know it makes up about 68% of our universe, we don’t really know what dark energy actually is. That means while the experimentalists scurry to get more data, the theorists work frantically to explain what’s going on.
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