200 most important Astronomy topics - Sykalo Eugen 2023

The Dark Energy Survey

Close your eyes for a moment and picture this: you're standing on a remote Chilean mountaintop, the sky above you unpolluted and infinite. A wind cuts through the silence, but your focus is elsewhere—on the massive, mechanical eye beside you. It's the 570-megapixel Dark Energy Camera (DECam), one of the most sensitive instruments ever built for the purpose of unveiling the invisible. You’re not just looking at stars. You’re listening for a whisper from the very fabric of the cosmos. That whisper is dark energy.

What is it? We’re still not sure. But the Dark Energy Survey (DES), an ambitious five-year mission completed in 2019, was designed to help us grasp the contours of this mysterious force that is not only expanding the Universe—but accelerating its expansion.

The Elephant in the Cosmos: Dark Energy Isn’t Just 'Nothing'

Back in 1998, two independent teams of astronomers—one led by Saul Perlmutter, the other by Brian Schmidt and Adam Riess—announced a finding so bizarre that many initially dismissed it: distant supernovae were fainter than expected. The implication? The Universe’s expansion wasn’t slowing down, as gravity would suggest, but speeding up. This invisible hand pushing galaxies apart was dubbed "dark energy."

That term—"dark energy"—isn't just mysterious. It's a placeholder for our ignorance. We don’t know what it is. It might be a property of space itself, a new field, or even a modification to Einstein's theory of gravity. What we do know is that it comprises nearly 70% of the Universe. That’s right: most of the cosmos is made of something we can’t see, touch, or yet explain.

A Survey Like No Other: Mapping 300 Million Galaxies

How do you chase a ghost? With data—ridiculous, beautiful, galaxy-sized data.

The Dark Energy Survey operated from the Cerro Tololo Inter-American Observatory in Chile, scanning one-eighth of the sky. Over six years, the survey cataloged over 300 million galaxies, thousands of supernovae, and the subtle distortions in space-time caused by gravitational lensing. That’s when light from distant galaxies is bent by massive foreground objects, like cosmic fingerprints left on the fabric of space.

Imagine watching shadows dance on a cave wall and trying to reconstruct the shapes causing them. That’s how DES scientists inferred the shape, clustering, and movement of galaxies to understand the invisible energy propelling them apart.

The Four Cosmic Probes: How DES Peered into the Dark

The DES used four complementary techniques to triangulate dark energy’s effects:

Supernovae (Type Ia): Standard candles for measuring cosmic distances. Like cosmic lighthouses, their brightness tells us how fast space is stretching.
Baryon Acoustic Oscillations (BAO): Sound waves frozen into the distribution of galaxies—think of them as the ripples left by the Big Bang’s drumbeat.
Weak Gravitational Lensing: Subtle distortions of distant galaxies’ shapes by massive objects, revealing the distribution of dark matter.
Galaxy Clustering: The way galaxies group together helps us trace the underlying web of dark matter and the push of dark energy.

Individually, these probes are powerful. Together, they’re symphonic. Cross-checking them allowed scientists to reduce uncertainties and test the very foundations of modern cosmology.

What Did We Learn—and Why It Bothers Us

DES results confirmed the Lambda Cold Dark Matter (ΛCDM) model—the prevailing theory that includes dark energy (Lambda) and dark matter. But here’s where things get uncomfortable: the expansion rate of the Universe, known as the Hubble constant, doesn't seem to agree between measurements from the early Universe (Planck satellite) and those from the late Universe (DES and others).

It’s like trying to measure the length of a shadow at noon and again at dusk, only to find that the numbers refuse to match. This tension, called the "Hubble tension," may hint that our model is incomplete—or that something fundamentally new waits to be discovered.

Beyond Numbers: A Human Quest

You might ask, “So what? Why obsess over equations and galactic redshifts?” Because wrapped in these cold numbers is the fate of the Universe.

If dark energy stays constant, the Universe will continue expanding forever, galaxies drifting farther until the night sky grows empty and cold—a scenario called the "Big Freeze." But if dark energy grows stronger, we may face a "Big Rip," where even atoms are torn apart. Or perhaps dark energy will fade, and gravity will pull everything back in—a cosmic rebirth.

Each possibility is as terrifying as it is awe-inspiring. In this dance of matter and energy, we are participants, not spectators.

The Beauty of Uncertainty

Here's a confession: I want dark energy to surprise us.

We are at our most human when we reach into the unknown and name it—not with certainty, but with courage. The DES didn’t provide all the answers. It sharpened the questions. It built a cosmic map, yes, but more than that—it reminded us of the scale of our ignorance, and the thrill of trying to overcome it.

The philosopher Simone Weil once wrote, “The future is made of the same stuff as the present.” That might be true on Earth. But in the cosmos? The future is dark, expanding, unknowable—and we are building torches.

Looking Ahead: DESI, Euclid, and the Next Wave

DES may be over, but the journey continues. The Dark Energy Spectroscopic Instrument (DESI) is already measuring the light from over 30 million galaxies with even greater precision. The European Space Agency’s Euclid mission, launched in 2023, aims to chart the dark cosmos in 3D. And the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) will open its eyes soon, promising the most detailed dynamic map of the sky ever constructed.

Each of these missions stands on the shoulders of DES.

What Lies Beyond the Map

Have you ever stared at the night sky and felt both impossibly small and immeasurably lucky? That duality—that strangeness—is the essence of astronomy. The DES looked outward, but it brought us inward. In learning about dark energy, we learn about ourselves: our restlessness, our need to explore, our humility in the face of something larger.

The Universe is not just expanding. It’s inviting.

Will we accept the invitation?