Dark energy, chunky or smooth?

One of the primary questions about dark energy is whether it is the same everywhere ("smooth") or varies in density from place to place ("chunky").

One of the primary questions about dark energy is whether it is the same everywhere (“smooth”) or varies in density from place to place (“chunky”).

The discovery of dark energy was the most surprising scientific breakthrough in my lifetime. Many physicists consider it the most baffling of nature’s mysteries, and still little is known about it.

To say that it caused the textbooks to be rewritten is literally true. When I first studied cosmology, every textbook had a section on how the fate of the universe depends on the amount of matter it contains. As the universe expands, the gravitational force of all matter pulls against this expansion, slowing it down. If there is enough matter, the expansion can be reversed, pulling everything together into a big crunch. If there is too little, the universe will merely slow down as it flies apart. Dark energy is the discovery that the universe is not slowing down at all, but speeding up.

That’s pretty much all we know: There’s a force that more than counteracts gravity, blowing the universe apart at an ever-faster rate. Even the word “dark energy” is an empty label, since we don’t really know whether it’s a form of energy or not. It might not even be a thing. For instance, it may be that gravity repels, rather than attracts, on cosmic scales. There’s even a term in Einstein’s theory of gravity, called the cosmological constant, that might represent this large-scale repulsion.

But then again, dark energy might be a substance. This substance, often called quintessence, would have unusual properties like negative pressure. Substances can be distributed from place to place, so an observation of clumpy dark energy—more in some places than others—would teach us that it is a thing, rather than a law.

One way to search for clumps in dark energy is to map the sky with extraordinary precision. The gravitational effects of dark energy (and dark matter) leave imprints in the distributions of galaxies and the rate at which they form. These influences can be measured statistically in a sufficiently detailed map. The next generation of sky-surveying telescopes, the Dark Energy Survey, just began its five-year mission last week.

The nature of dark energy is also relevant for predicting the fate of the universe. If dark energy is a cosmological constant, a property of space, then the force it applies will get stronger as space expands—a runaway effect known as the big rip. If it is a substance like quintessence, then there are many possibilities, depending on the exact nature of the substance. Apart from just wanting to understand the world around us, learning about dark energy could have consequences in the long, long, very long term (eons).

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