For nearly a century, the mystery of our insane dark universe has baffled theorists and astronomers alike. Let’s face it, even with the best telescopes, what’s visible on any clear night only reflects a tiny fraction of the universe as we understand it.
Dark matter — which makes up about 20 percent of the universe — neither emits nor absorbs light, but gravitationally attracts ordinary matter. Dark energy — the inexplicable acceleration of our expanding universe — accounts for 76 percent of our universe. Ordinary matter, flesh and bone and stars and planets, makes up less than four percent of the known universe.
Hopefully we will know more about this dark realm soon.
Next month the European Space Agency’s (ESA) Euclid mission, along with NASA-sponsored infrared detectors, should provide new clues about the physical nature of both dark matter and dark energy. No one expects the 1.2-meter telescope mission to answer everything, but it should give theorists a wealth of new data to chew on.
Why is this telescope so special?
ESA says Euclid will map the large-scale structure of cosmic time spanning the last 10 billion years. In the process, it will take spectroscopic redshift data on about 50 million galaxies while imaging 1.5 billion galaxies with Hubble Space Telescope-like resolution, the company says.
It has two science instruments, one that uses visible light and one that uses infrared light, US Euclid science lead Jason Rhodes, an observational cosmologist at NASA JPL, told me via email. An optical element called a 'dichroic’ separates the incoming light into visible and infrared components and sends those components to the appropriate instrument, Rhodes says. Both instruments have cameras with a high number of pixels, and both can simultaneously observe the sky and collect data, he says.
Assuming a normal launch in July, Euclid’s nominal six-year science mission will begin this December from the gravitationally stable Earth-Sun stable L2, about a million miles from Earth. Euclid covers 15,000 square degrees of the sky. This will produce a very large atlas with enormous resolution in both visible and near-infrared wavelengths.
What is the biggest technical challenge facing Euclid?
Dark energy studies are statistical in nature, says Rhodes. If there’s even the slightest systematic offset to those shapes or distances, that could affect our results, he says. That’s what prompted us to use a space telescope above Earth’s atmosphere for these measurements, says Rhodes.
The hope, Rhodes notes, is that Euclid narrows the error bars of current measurements and rules out a wide variety of possible expansions. He says that ruling out these possible explanations brings us closer to understanding the physical cause of dark energy.
A key feature of Euclid’s measurements involves a one-two punch. First the spacecraft will measure how galaxies are affected by dark matter, and then it will use those measurements to better characterize dark energy.
„From the shapes of the galaxy, you see it’s slightly distorted by dark matter,” René Lauriges, Euclid project scientist at ESA ESTEC in the Netherlands, told me recently in his office. And from the distribution of dark matter, you can get an accelerating expansion, says Larijs.
Our understanding of galaxy clustering is now so good that we can follow the dark energy content of the universe, University of Oslo theoretical physicist Hans Winter, a member of the Euclid Science Consortium group, told me recently. So, independent of the distance measurements for type 1a supernovae, we have massive evidence for the acceleration of the universe, says Winther.
How does dark energy affect the large-scale structure of the universe?
„We’re trying to understand what dark energy is by studying the large-scale structure of the universe,” says Winter. When you study structure formation, you are mainly concerned with two things — gravity, the attraction of objects; And says the expansion of the universe. It’s basically a battle between the two, says Winter.
In 1917, before the big bang theory and our observational understanding of the global expansion of the universe, Einstein temporarily added what he called his cosmological constant (a repulsive force) to his theory of general relativity. He did so to adhere to the theory of a constant universe in order to counteract the effects of gravity on ordinary matter. Einstein quickly retracted this aspect of his field equations when observational evidence of an expanding universe, known as the Hubble expansion, became available.
But in the late 1990s, theorists resurrected Einstein’s cosmological constant as a repulsive force in quantum mechanics called vacuum energy, a background energy embedded in spacetime itself. It was then, in 1997 and 1998, when two Nobel Prize-winning teams discovered that, some five to seven billion years after the Big Bang, our universe began to inexplicably accelerate its expansion.
Theorists in the current era posit the cosmological constant as a tentative solution to explain the force behind dark energy. This seems to be only a temporary solution until we better understand the physics of dark energy. Or perhaps, the universe has vacuum energy in the fabric of spacetime that somehow causes this acceleration. But why did the accelerating force of dark energy kick in only five to seven billion years after the Big Bang?
How is the value of the cosmological constant calculated?
„This is the value we measure to explain how much dark energy, or how much of the cosmological constant, must be in our universe today,” University of Oslo theoretical physicist David Mota, a member of the Euclid Science Consortium panel, told me recently in his office. There is a big difference between the value we calculate from particle physics and the value we observe astronomically, says Mota. When you do the calculations, he notes, this energy in a vacuum is 120 times larger than the value we measure from observations.
Which dark theory represents the biggest puzzle?
„My money is on dark energy because we know less about it than dark matter, but it’s an even larger component of the universe,” Rhodes said. „At a distance, dark energy is a tiny component of the universe.”
But in the future, dark energy will dominate, and dark matter will be a much smaller component, Rhodes says. He notes that the nature of dark energy will also determine the fate of the universe.
Mota goes even further and asserts that all theories used to define the universe as we currently understand it may be wrong.
From Newton, we went to Einstein and the curvature of space and time, says Mota. But a million years from now, I believe humanity will have evolved to embrace different mathematical and physical theories, he says. But at this point, we are taking baby steps, says Mota.
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