Dark Energy Might Be Changing – This Is What It Means For Our Understanding Of The Cosmos

The standard model of cosmology – the best working theory we have to explain the universe – is known as ΛCDM (pronounced Lambda-CDM). Lambda is the symbol for the cosmological constant and CDM stands for cold dark matter. According to this, the universe is made of regular matter (5 percent), dark matter (25 percent), and dark energy (70 percent). Neither dark matter nor dark energy are understood and these new findings may upend our expectations of the latter. 

The Cracks In The Standard Model Of Cosmology

The Dark Energy Survey (DES) has collected data from millions of galaxies. This data provided a precise map of the universe that can be used to estimate the properties of dark energy by studying the expansion of the cosmos. It measures distances using specific supernovae (type Ia) which are used as “standard candles”: they always have the same luminosity so you can always work out of far they are. 

This would mean that dark energy is not simply vacuum energy, it would need to be something more exotic. [T]his could be a hint for a fifth force in nature, beyond the four well-known forces… or that there is some modification to Einsteins’s law of gravity, general relativity.

Dr Santiago Avilla

They also measured the Baryon Acoustic Oscillations (BAO), which can be envisioned as sound waves in the matter distribution of the universe – peaking about every 500 million light-years. DES data from 16 million galaxies suggested that the measured BAO is 4 percent smaller than predicted by ΛCDM.

“It is exciting to see the power of the Dark Energy Survey when combining its different probes. Whereas our supernova results and our BAO results from last year were already exciting. By putting them together, we can observe the cracks in ΛCDM, which is considered the standard model of cosmology,” Dr Santiago Avila from the Centre for Energy, Environmental and Technological Research (CIEMAT) in Spain, who was responsible for the BAO analysis in DES, told IFLScience.

An International Effort To Reach These Results

If understanding dark energy or dark matter was easy, we would have done it already. While it is believed that they dominate the universe, their effects are not immediately obvious in our labs. It is only by studying the large scales in the universe, that we can try to understand their properties.

“Getting to these cosmology results took years of collaborative effort by hundreds of people in the Dark Energy Survey. Their work spanned designing the camera on the telescope, conducting over 700 nights of observation, and developing a number of state-of-the-art analysis methods to characterize galaxies in images, finding and classify supernovae, and running simulations to develop a comprehensive understanding of uncertainties impacting the measurements, among other things,” Professor Jessie Muir of the University of Cincinnati told IFLScience.

“All of those developments represent challenges that had to be overcome to use DES data to test models of dark energy.”

What Even Is Dark Energy?

If the results are indeed correct and dark energy is not a cosmological constant, then scientists need to start rethinking what dark energy is. Its true nature continues to be wrapped in a mystery but the work by DES and other upcoming surveys will provide even more insights into the properties of this energy.

“We’re living in an exciting era for cosmology, and this work adds to the growing evidence in support of an evolving dark energy. The ΛCDM model, which has been the standard model in cosmology over the last decades, may not be sufficient to interpret the data of the latest cosmological experiments,” Dr Juan Mena-Fernández of the Subatomic Physics and Cosmology Laboratory in Grenoble, told IFLScience.

“This would mean that dark energy is not simply vacuum energy, it would need to be something more exotic. For example, this could be a hint for a fifth force in nature, beyond the four well-known forces. Another alternative could be that there is some modification to Einsteins’s law of gravity, general relativity. At this moment, these are just speculations, but with more data from DES or new experiments, we hope to find an explanation for the nature of dark energy,” Dr Avila told IFLScience.

The Standard Model Is Not Having A Good Time

One of the biggest points of discussion around the standard model in the last few years has been the Hubble Tension. Different methods of estimating the expansion rate of the universe today come up with different numbers.  

Using data from the Cosmic Microwave Background (CMB), scientists worked out that the universe is expanding at a rate of 67.4 kilometers per second per megaparsec, with 1 megaparsec being 3.26 million light-years. This means that if two galaxies are 1 megaparsec apart, the expansion of the universe would make them look like they are moving away from each other at a speed of 67.4 kilometers (42 miles) per second. A different method known as the distance-ladde,r which used standard candles known as Cepheid variable stars instead, found a value of 73 kilometers per second per megaparsec.

We were intrigued by the possibility that an evolving dark energy might solve this other conundrum, but the team doused our hopes.

“When interpreting the data from BAO and SN from DES (also including CMB from Planck) assuming an evolving dark energy, the values for the Hubble constant we obtain (~68+-1 km/s/Mpc) is consistent with the one we get when assuming ΛCDM (~67.0+-0.5 km/s/Mpc). This means that the tension with the measurements using distance-ladder methods (~73+-1 km/s/Mpc) is still there,” Dr Mena Fernandez explained.

Where Do We Go From Here?

The challenge to the standard model has been put down but it doesn’t mean that everything needs to be thrown out. A dark energy that is weakening with time might easily be fitted in the standard model following tweaks and improvements, without having to consider it completely obsolete. Much remains to be understood about it. The team is planning future observations as well as working to improve techniques, to make sure the analysis is as correct as they can possibly make it.  

“In the coming months we’ll be releasing more results from the completed DES, with studies of the clustering of matter probed with galaxies and gravitational lensing providing complementary information to the results that are out now, which are based on distance measurements probing the expansion history of the Universe,” Professor Muir told IFLScience.

“We also have an eye on exciting results coming out from other cosmology surveys, like BAO measurements from DESI, and looking ahead, to – among other things – the detection of huge numbers of additional supernovae with the Vera Rubin Observatory LSST, which will start taking science data later this year.”

As well as the Vera Rubin Observatory, the European Space Agency’s Euclid mission is also studying the dark universe. Dark energy’s intricacies might soon be revealed… and they might be weirder than we thought.