Our universe is a mysterious place. Only 4 percent or so is made up of the ordinary matter we see around us, including all those galaxies filled with stars. The rest, physicists believe, is made of up weakly interacting dark matter, and a mysterious substance called dark energy that is causing the cosmos to expand at an accelerating rate.
At least, that’s the working hypothesis, and it seems to fit the data, although there are scientists who question its existence and tout alternatives to explain that accelerating expansion.
A new, two-year study by scientists at the University of Portsmouth and Ludwig-Maximilians-Universitat Munchen concludes that dark energy does, indeed, exist. Those results just appeared in the journal Monthly Notices of the Royal Astronomical Society.
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In the early 20th century, scientists believed the universe was in a steady state. But when Albert Einstein was working on his theory of general relativity, the math just didn’t add up: the universe should have been expanding. So he invented something called the cosmological constant — a mathematical trick to balance everything out so that the equations described a static universe, rather than an expanding one.
But then astronomer Edwin Hubble discovered the universe actually was expanding — Einstein’s original equations were correct. He dubbed the cosmological constant (lambda) his “greatest blunder.”
The universe wasn’t done surprising us, however. In 1998, astronomers studying distant exploding stars called a Type 1A supernovae discovered that not only was the universe expanding, but that the rate of expansion was accelerating due to some type of unknown force or dark energy. And one of the explanations for this effect is — you guessed it — Einstein’s cosmological constant.
While that discovery snagged its team leaders the 2011 Nobel Prize in Physics, it’s not the only evidence in favor of dark energy. For instance, last May, a team of scientists from Melbourne’s Swinburne University announced their independent confirmation of both the existence of dark energy and its rate of expansion, based on four years of data collected by a powerful spectrograph at the Australian Astronomical Observatory.
That study included more than 240,000 galaxies going back over seven billion years and showed that the growth of galaxy clusters and super clusters has slowed down. This means that in the most distant parts of universe — those further back in spacetime — gravity dominates. It’s only in our current part of the cosmos where dark energy dominates, and hence we are seeing accelerated expansion.
The Swinburne researchers also looked at the distances between pairs of galaxies, and the ripples in the cosmic microwave background radiation (CMB). They found that the average distance between galaxy pairs (about 500,000,000 light years) has been growing because of the expansion of space-time, providing further confirmation of dark energy.
One of the strongest pieces of evidence can be found in a unique feature of the CMB, known as the Integrated Sachs-Wolfe Effect, based on a 1967 prediction that light from the CMB would show a redshift — i.e., it would become slightly “bluer” — as it passed through gravitational fields of clumped matter.
Scientists didn’t detect the Sachs-Wolfe Effect until 2003 — Science magazine deemed it the “discovery of the year.” It showed up as tiny gains in energy among photons in the CMB, based on comparing the temperature of the CMB with maps of galaxies in our local part of the universe.
As exciting as that discovery was, it was pretty weak signal, and might have been caused by something else — space dust, for instance. So Tommaso Giannantonio and Robert Crittenden took the lead on a two-year study to re-examine that data and improve the galactic maps used in the original work.
Their conclusion: there is a 99.996 percent chance that dark energy is responsible for those observed variations in the CMB where the photons are just a wee bit hotter than the rest. That’s equivalent to the level of certainly of the recent announcement of the discover of a Higgs-like particle.
We still don’t know what exactly the dark energy is, of course, and what all this will ultimately mean for modifications of general relativity. But its existence is looking like much more of a sure thing, although we can expect to see some pretty close scrutiny of these new results from those who remain skeptical.
This isn’t the final word, is the point — science marches on. As Giannantonio said via press release, “The next generation of cosmic microwave background and galaxy surveys should provide the definitive measurement, either confirming general relativity, including dark energy, or even more intriguingly, demanding a completely new understanding of how gravity works.”