A NEW SPACE probe set to launch Saturday morning has the power to shed light on the universe’s biggest questions. If all goes as planned, the Euclid telescope will scan billions of galaxies, poring through the past 10 billion years of cosmic time. It will give astrophysicists the data they need to better understand two persistent mysteries: dark matter and dark energy.
Euclid is more than a space telescope. It’s really a dark energy detector,” said René Laureijs, the mission’s project scientist, at a press briefing last week.
After more than a decade of hard work, the European Space Agency, or ESA, is planning for liftoff at 11:11 Eastern time on July 1 from Cape Canaveral, Florida. A SpaceX Falcon 9 rocket will provide the ride to space. (The agency will broadcast the launch live here, and they’re reserving Sunday as a backup launch date.)
Euclid will survey more than one-third of the sky—just about everything that can be charted without pointing the telescope through the disk of our Milky Way. Such coverage will allow scientists to study in exquisite detail how the expansion of our universe has accelerated, likely driven by an unseen phenomenon called dark energy.
Astrophysicists only really understand about 5 percent of the universe, the atoms that make up normal matter—everything from stars to planets, and from people to toasters. But according to research done using Planck, another ESA space telescope, about 25 percent of the universe is dark matter, the hidden scaffolding of the cosmos that determines where and how galaxies form. The rest is all dark energy, an elusive—and hypothetical—repulsive force that shapes the universe’s evolution by driving it apart. Several billion years ago, dark energy became the dominant component of the universe, ensuring not only that it keeps ballooning, but that its expansion rate accelerates.
A crucial quantity Laureijs and his colleagues want to investigate is called w, or the ratio of the pressure of the universe’s dark energy to its density. Einstein hypothesized a “cosmological constant,” or the notion that the universe is filled with empty space, which nonetheless has its own energy and couples to gravity. If that theory is true, then the pressure of dark energy should be equal to the negative of the energy density. In other words, if dark energy is the cosmological constant, then w should equal -1.
So far, that appears to be the case, but studies with previous telescopes have large uncertainties in their measurements. Data from Euclid will show whether or not a cosmological constant is the right explanation for the universe’s acceleration by creating more accurate measurements for w and seeing if it turns out to be anything other than -1. It will also show whether w has changed throughout cosmic history.
We’re looking at some of the most fundamental questions of cosmology,” says Carole Mundell, the ESA’s director of science. “What this mission will do for us with incredible precision is let us map out the cosmic structure and the expansion history of the universe.
Another explanation for dark energy is that it is a new kind of dynamical energy fluid or field, something that fills all of space but something whose effect on the expansion of the universe is the opposite of that of matter and normal energy. Some theorists have named this "quintessence," after the fifth element of the Greek philosophers. But, if quintessence is the answer, we still don't know what it is like, what it interacts with, or why it exists. So the mystery continues.
A last possibility is that Einstein's theory of gravity is not correct. That would not only affect the expansion of the universe, but it would also affect the way that normal matter in galaxies and clusters of galaxies behaved. This fact would provide a way to decide if the solution to the dark energy problem is a new gravity theory or not: we could observe how galaxies come together in clusters. But if it does turn out that a new theory of gravity is needed, what kind of theory would it be? How could it correctly describe the motion of the bodies in the Solar System, as Einstein's theory is known to do, and still give us the different prediction for the universe that we need? There are candidate theories, but none are compelling. So the mystery continues.
The thing that is needed to decide between dark energy possibilities - a property of space, a new dynamic fluid, or a new theory of gravity - is more data, better data.