Dark energy might be fading - so what does that mean?

Over the past two years, scientists using the 10-meter South Pole Telescope (SPT) have mapped about 1/25 of the sky using 16,000 ultra-sensitive millimeter-wave detectors. The data offer new details about the early universe and help scientists probe the nature of dark energy, the mysterious force driving cosmic acceleration.

Now, the international team behind the effort has released their data, revealing the most detailed maps yet of the cosmic microwave background’s (CMB) temperature and polarization in that region.

The snapshot captures conditions just 400,000 years after the Big Bang. At that point, the hot primordial plasma had cooled enough for hydrogen and helium atoms to form.

Complementing the SPT images are three-dimensional galaxy maps from the Dark Energy Spectroscopic Instrument (DESI). By cross-matching the two data sets, the researchers have refined key cosmic yardsticks such as the sound-horizon scale – the imprint of primordial sound waves frozen into the CMB.

What the universe is made of?

Cosmologists describe the Universe’s history with the ΛCDM (lambda-cold-dark-matter) framework. It begins with a burst of cosmic inflation 13.8 billion years ago, followed by expansion and gradual cooling of a fireball of photons, electrons, and ionized nuclei.

Tiny density ripples – one part in 100,000 – visible in the CMB grew into today’s web of dark-matter haloes, galaxies, and clusters.

According to ΛCDM, about 5% of the Universe is ordinary matter, 25% is cold dark matter, and 70% is dark energy represented by Einstein’s cosmological constant Λ.

Dark energy was proposed in 1998 after observations of distant supernovae revealed that the universe’s expansion has been accelerating for roughly the last five billion years.

This acceleration defied expectations based on gravity alone, which should have slowed the expansion over time.

Dark energy may evolve

The new SPT data, when combined with DESI and earlier CMB surveys such as Planck, lean away from the simplest cosmological constant.

The experts argue that the combined data set reduces the likelihood of a cosmological constant. It also increases support for dark energy models that evolve over time. Adding supernova measurements further strengthens this new understanding.

The implication is that dark energy, the force driving cosmic acceleration, might be weakening with time. If so, expansion could eventually slow, stop, or even reverse – an outcome not predicted by a constant Λ.

The result does not yet reach the stringent “5-sigma” threshold (which means less than one chance in 3.5 million for the standard model be false) beloved by particle physicists, but the statistical weight is building.

What if dark energy fades?

Should the hints solidify, ΛCDM would need modification. Einstein originally introduced his cosmological constant in 1917 to balance gravity and keep the Universe static, but he abandoned the idea after Edwin Hubble revealed the Universe was expanding in 1929.

The constant was revived in the late 1990s and has since explained virtually all precision cosmological data.

A weakening acceleration could point to a dynamic field – sometimes dubbed “quintessence” – rather than a simple constant.

Alternatively, general relativity itself might need adjustment on cosmological scales. Either route would transform fundamental physics.

Clarity from the South Pole

The South Pole Telescope sits at the Amundsen-Scott station, where bone-dry, ultra-cold air offers ideal transparency for CMB wavelengths.

The new release covers only a fraction of the final survey. Future observations with an upgraded receiver in 2028, along with forthcoming results from the Simons Observatory and the 2030s CMB-S4 project, promise an order-of-magnitude increase in sensitivity.

On the galaxy side, the DESI collaboration plans a DESI-2 instrument and, further ahead, the ambitious Spec-S5. These surveys will map tens of millions of galaxies. They will track how large-scale cosmic structure has grown under the tug-of-war between gravity and dark energy.

Cosmic verdict still pending

It is not yet clear how soon decisive evidence might arrive. Current indications fall short of the gold-standard probability of less than one in 3.5 million of being a fluke.

Yet each new dataset shortens the odds. By the early 2030s, combined CMB and galaxy surveys are expected to either confirm a dynamic dark-energy component or vindicate the cosmological constant once more.

The physics of dark energy

One of the SPT’s key achievements is a sharper measurement of the sound horizon – the maximum distance sonic waves could travel in the primordial plasma before recombination.

That scale left a pattern in the CMB and galaxy distribution, acting as a “ruler” for measuring cosmic expansion. Pinning it down more precisely tightens every subsequent inference about dark-energy physics.

Whether the expansion is truly “losing steam” remains an open question. But the latest SPT–DESI results highlight the power of combining high-resolution microwave maps with vast galaxy surveys.

Each new observation slices deeper into the cosmic enigma of dark energy and helps scientists build a fuller picture of how the Universe began, how it grew, and where it is heading next.

The study can be found here.

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