DESI Map Suggests Dark Energy May Not Be Constant Over Time

For decades, the standard model of cosmology has relied on a specific assumption: dark energy is a constant, unchanging force pushing the universe apart. It was thought to be the “cosmological constant,” represented by the Greek letter Lambda. However, new data released from the Dark Energy Spectroscopic Instrument (DESI) has sent shockwaves through the physics community. The largest 3D map of the universe ever created suggests that this mysterious force might actually be evolving over time. If validated, this finding would require scientists to rewrite the history—and future—of our cosmos.

The Instrument Behind the Discovery

To understand the magnitude of this finding, you first need to look at the machine that made it possible. DESI is not just a standard telescope. It is a complex instrument mounted on the Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory in Arizona.

Managed by the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), DESI is unique because of its robotic eyes. The instrument contains 5,000 tiny robotic positioners. These robots automatically swivel to capture light from distinct galaxies and quasars. By using fiber-optic cables to feed this light into spectrographs, DESI can measure the distance to millions of galaxies with incredible precision.

The data released in early 2024 stems from just the first year of DESI’s observations. In that short time, the collaboration mapped over 6 million galaxies and quasars, covering 11 billion years of cosmic history. This volume of data is already three times larger than the previous leading map created by the Sloan Digital Sky Survey over two decades.

Breaking the Standard Model

The current leading theory of the universe is known as the Lambda-CDM model.

• Lambda: Represents dark energy as a constant force.
• CDM: Stands for Cold Dark Matter.

Under this model, the density of dark energy never changes, even as the universe expands. It pushes outward at a steady rate. However, when the DESI team analyzed the expansion history of the universe using “standard rulers” known as Baryon Acoustic Oscillations (BAO), the data did not perfectly line up with the Lambda-CDM prediction.

The results hint that dark energy might be dynamic rather than static. Specifically, the data suggests that the influence of dark energy may have weakened in the more recent epochs of the universe’s history. Instead of a steady foot on the accelerator, the universe might be easing off the gas pedal slightly.

What Are Baryon Acoustic Oscillations?

To measure expansion, scientists look for patterns called Baryon Acoustic Oscillations (BAO). These are essentially frozen sound waves from the early universe.

380,000 years after the Big Bang, the universe cooled enough for light to travel freely. The pressure waves (sound) traveling through the plasma froze in place. These waves created a distinct pattern in the distribution of matter. By looking at how far apart galaxies are today, scientists can see the “echo” of these original waves. Because they know how big the waves were originally, they can use them as a cosmic ruler to measure how much the universe has expanded at different points in time.

Implications for the Fate of the Universe

If dark energy is truly weakening, the implications for the future of the universe are profound. The standard theory (constant dark energy) predicts a “Big Freeze.” In this scenario, the universe expands forever, galaxies move out of sight, stars burn out, and the cosmos becomes a cold, dark, and lonely place.

However, if dark energy is evolving and weakening, other scenarios become possible:

1. The Big Crunch: If dark energy weakens enough, gravity could eventually become the dominant force again. The expansion of the universe could slow down, stop, and reverse. Billions of years from now, the universe could collapse back in on itself.
2. Cyclic Models: A collapse could lead to another Big Bang, suggesting the universe goes through infinite cycles of expansion and contraction.

While these are theoretical extremes, the DESI data opens the door to models that were previously discarded in favor of the cosmological constant.

The Statistical Significance

It is important to note that this is a “hint” and not yet a confirmed discovery. In physics, researchers look for a statistical threshold known as “5 sigma” to claim a definitive discovery. This means there is only a 1 in 3.5 million chance the result is a fluke.

The current DESI results, when combined with data from other projects like the Planck satellite and supernovae studies, reach a significance of roughly 2.5 to 3.9 sigma. This is considered “tantalizing evidence” but requires more data to confirm.

The variance appears specifically when researchers combine the DESI map with supernova data. The discrepancy suggests that the equation of state for dark energy varies with time. While this could still turn out to be a statistical anomaly, the fact that such a precise instrument found the deviation has captured the attention of cosmologists worldwide.

What Comes Next?

The findings released recently represent only the first year of a five-year survey. DESI will continue to collect data until it has mapped roughly 40 million galaxies and quasars.

As the dataset grows, the statistical noise will decrease. By the end of the five-year run, scientists will likely know if this discrepancy is a real crack in the standard model or just a statistical fluctuation.

Furthermore, upcoming space missions will provide backup. The European Space Agency’s Euclid mission and NASA’s Nancy Grace Roman Space Telescope (set to launch by May 2027) will also map the dark universe. If these instruments observe the same weakening of dark energy, the Lambda-CDM model will officially be obsolete, marking the biggest shift in cosmology since the discovery of cosmic acceleration in the late 1990s.

Frequently Asked Questions

What is dark energy? Dark energy is an unknown form of energy that permeates all of space and accelerates the expansion of the universe. It makes up roughly 68% of the total energy-mass of the cosmos.

How is the DESI map different from previous maps? DESI creates a 3D map by measuring the “redshift” of galaxies. It is significantly larger and more detailed than previous surveys, such as the Sloan Digital Sky Survey, allowing it to look much further back in time with greater precision.

Does this prove the Big Bang theory is wrong? No. The Big Bang theory describes the early expansion of the universe from a hot, dense state. The DESI findings support the general framework of the Big Bang but challenge our understanding of how the expansion has evolved specifically over the last few billion years due to dark energy.

Where can I see the DESI data? The data is open to the public and hosted by NERSC (National Energy Research Scientific Computing Center) at the Lawrence Berkeley National Laboratory.

When will we have a final answer? DESI is currently in its operational phase. The full five-year dataset is expected to be analyzed and released in stages over the next few years, likely providing a definitive answer before 2030.