Dark Energy: Too Big To Fail

[1/31/13 update: Many thanks to David Wiltshire for clarifying some points in the comments, and for addressing critics of his ideas. -EC]

According to today’s mainstream science, about 71% of the Cosmos is made of something that scientists have never seen, detected, directly observed, or even indirectly observed. It’s “dark energy,” first described in the late 1990s as a repulsive or anti-gravitational property of empty space. By measuring supernovae in distant galaxies, astronomers inferred that not only is the universe getting larger, the expansion is getting faster with time. Physicists started speculating about what might be responsible for the acceleration, and the newly minted concept of dark energy quickly became the accepted explanation. In fact, today it’s built into the standard cosmological model of a universe that expands with an ever-accelerating rate.

Dark energy has also gone mainstream: As viewers of popular-science TV programs know well, in the very distant future, the repulsive effects of dark energy may become so dominant that they tear apart galaxies, stars, planets — eventually even the atoms that currently make up your body — in a highly dramatic “big rip” that marks the end of the universe.

From the outset, the concept of dark energy seemed to me like a bad idea. We had a set of observations we didn’t fully understand, and in response, we externalized the problem onto some unseen but supposedly real agent “out there” that must be to blame. It didn’t sound like the best approach. It seemed very possible that we misinterpreted the supernova measurements, so why the rush to invent a brand new form of energy (which would need to comprise three-quarters of the universe!) to explain this interpretation? Surely, no one can say that all other alternative explanations had been exhausted. Inventing dark energy was a bit like losing your homework assignment, and then promptly deciding that your family must have an invisible and undetectable dog who ate it.

It turns out that in 2007, David Wiltshire of New Zealand’s University of Canterbury explained how the apparently accelerating expansion of the universe could be an illusion due to our perspective in the Milky Way. According to Einstein’s general theory of relativity, a clock can run at different rates depending on the local strength of gravity; near a massive star, for example, a clock will run more slowly than an identical clock positioned far away. From the perspective of an observer close to the star, everything (including the nearby clock) will appear to be running normally, while the distant clock seems to run fast; to an observer near the clock in deep space, meanwhile, the local time will appear to pass normally as usual, but from that perspective, the gravitationally bound clock seems to run slow. Gravity’s effect on time is universally accepted. If your GPS receiver didn’t account for general relativity, your navigation directions would lose accuracy in minutes.

Wiltshire’s insight was that light from the supernovae traverses huge stretches of space where there is little to no matter, so clocks there would run fast as judged by observers near galaxies, such as ourselves. When astronomers study supernovae on the other side of these voids, the measurements are affected by the fact that the voids are expanding faster than the space closer to us (as judged here on Earth). According to Wiltshire’s calculations, if you lived in one of these voids, you’d judge the universe to be about 18 billion years old — several billion years older than how things appear here, where matter and gravity slow clocks as well as the expansion of space. The end result is that the Milky Way resides in a “Hubble bubble,” a local region beyond which all of the rest of the universe appears to be expanding away, at an accelerating rate. Similarly, if you lived in a galaxy on the other side of a large void, the Milky Way would appear to be accelerating away from you. You would be living in a Hubble bubble there, too.

The first time I read about Wiltshire’s insight, my reaction was disbelief: “Surely they must have thought of that!” But apparently, they hadn’t. The equations that cosmologists use to study the universe’s large-scale structure are extremely difficult to work with, so some simplifying assumptions have been made in order to construct workable models. One of these is to describe space everywhere as a uniformly expanding fluid. This assumption has been a part of the mathematics of cosmology (in the so-called Friedmann equations) since 1922, before we even knew there were other galaxies beyond our own. Given the brutally difficult equations, which could not otherwise be solved, the Friedmann solutions have been a part of cosmology bedrock ever since. However, we now know that the large-scale structure of the universe is not at all uniform; there are walls of galaxy clusters separated by enormous voids, so if Einstein is correct, the universe is not a uniformly expanding fluid. Time flows, and therefore expansion happens, faster in some places than in others.

Wiltshire argues that a necessarily uneven expansion of the universe previously hadn’t been considered because (1) physicists are used to considering relativistic effects on time only in the extreme conditions of black holes and particle-accelerator experiments, and (2) they have assumed that such effects in intergalactic space would be extremely weak. They are indeed weak, but Wiltshire has done the math to show how the cumulative effects become huge over billions of years and across vast distances of space.

According to Google Scholar, Wiltshire’s original paper has been cited 123 times to date; it’s certainly not being ignored. There appear to be few or no papers that refute the concept of non-uniform expansion due to relativistic effects. In fact, most of the papers build upon the idea.

So why, in 2013, is “dark energy” still a thing? Beats me! In his excellent book The Trouble With Physics, Lee Smolin describes how modern physics can unwittingly construct opaque, monstrous theories that become intellectual dead-ends with no practical uses, but which nevertheless persist for sociological and financial reasons. First, a few innocent working assumptions are laid down, and then models are built on those assumptions, and then others build upon those models, and so on — until there’s a mini-industry of researchers working in the field and collecting funding, with the original assumptions rarely re-examined. This is what has happened with dark energy: Even though we now have a viable explanation for the appearance of cosmic acceleration, a bad alternative has become entrenched. Careers have been forged based on the cosmological model that includes dark energy. And in a potential embarrassment for the ages, the 2011 Nobel Prize in Physics was awarded for what the prize committee called “the discovery of the accelerating expansion of the universe.” Dark energy is too big to fail!

Beyond academia, the public never should have been asked to have faith in a mysterious, undetectable force that’s out there but at the same time all around us. Invoking such ideas makes it impossible to maintain the clear distinctions between religion and science, at least in the public eye. But even today, the History Channel tells us “we now know” that most of the universe is made of mysterious dark energy. NASA does the same, on a public-education page called “Universe 101.” Houston, we have a problem.

Perhaps there are physicists who secretly hope the dark-energy concept will gradually die out. But in popular science, which is usually well behind the curve, the myth of dark energy will undoubtedly linger for years on the websites and the TV programs. We deserve better.