So, how’s it going with the Hubble tension?

Title: Interacting Dark Energy after DESI Baryon Acoustic Oscillation Measurements

Authors: William Giarè, Miguel A. Sabogal, Rafael C. Nunes, Eleonora Di Valentino

First Author’s Institution: University of Sheffield

Status: Published open access in Physical Review Letters

If you follow cosmology, you’ll probably be familiar with the oft-covered Hubble tension. When we measure the rate at which the Universe is expanding (called the Hubble constant, H0) using the Cosmic Microwave Background (CMB), we get a value of about 67.4 kilometers per second per megaparsec, meaning that objects 1 Mpc away from us should be, on average, moving away from us at a speed of 67.4 km/s. Objects 2 Mpc away should be moving away from us at 2 x 67.4 = 134.8 km/s, and so on. Our Milky Way is only about 30,000 parsecs across, so on the scale of one megaparsec this isn’t too large of an effect–but on the scale of the entire Universe, it adds up!

However, when we actually measure the velocities of objects at these distances directly, using standard candles like Cepheids and type Ia supernovae, we get a very different value of the Hubble constant: about 73 km/s/Mpc. These measurements are so precise (with uncertainties of just 0.5 for the CMB and 1 for supernovae), and so separated from each other, that they’ve given rise to a “crisis in cosmology”. Both methods have been tested and verified again and again, but the discrepancy between their results has, if anything, increased! It’s still possible that one or both of them is giving an incorrect result, but it’s also possible that we’re looking at new physics beyond the standard model of cosmology (called ΛCDM).

That’s the possibility that today’s authors choose to explore. They test an alternate model of dark energy (the not-yet-understood force driving the expansion of the Universe) in which it interacts with dark matter, and use this model to test the Hubble tension!

The law of equivalent exchange

It’s impossible to create something from nothing. In a closed system, energy and mass are always conserved. In normal ΛCDM cosmology, we think that the energies of dark energy and dark matter are conserved separately–in other words, the total amount of energy (or momentum) contained in dark matter hasn’t changed since the Big Bang, and neither has the amount of dark energy in the Universe. Today’s authors, though, take a different tack. They consider a model called Interacting Dark Energy (IDE), in which dark energy and dark matter are allowed to interact with each other, transferring energy and momentum from one to the other. Rather than dark matter and dark energy being conserved separately as in ΛCDM, then, the total amount of energy contained in both is conserved.

This new model adds a seventh parameter to the six that are standard in the ΛCDM model, a parameter which describes the amount of energy that’s transferred between dark matter and dark energy. The authors can then use Bayesian statistics to find the values of the seven ΛCDM parameters that are most likely to have produced the cosmology we observe, using CMB data from the Planck mission, baryon acoustic oscillation (BAO) data from the Dark Energy Spectroscopic Instrument (DESI), and supernova data from the SH0ES collaboration. 

New physics?

A plot depicting the correlation between the coupling parameter and the Hubble constant, with dashed contours representing the values derived from Planck and DESI and solid contours representing values derived using supernovae as well as Planck and DESI. A green shaded region represents the Hubble constant measured from supernovae by SH0ES, 73 +/- 1. A blue shaded region represents the Hubble constant measured from the CMB by Planck, 67.4 +/- 0.5. The contours without supernovae overlap quite a bit with the SH0ES region and a little bit with the Planck region, while the contours with supernovae overlap only very slightly with SH0ES.
Figure 1: A contour plot of possible values using IDE for both the Hubble constant and the coupling parameter ξ between dark matter and dark energy, given only the Planck and DESI datasets (dashed contours) and including data from type 1a supernovae (solid contours). Green vertical bounds show H0 as measured by the SH0ES collaboration using only supernovae, and blue vertical bounds show H0 measured by Planck using only the CMB. Modified from Figure 1 in today’s paper.

Although the authors find that their IDE model explains the data equally well to the ΛCDM model, they also find that by including coupling between dark energy and dark matter, they push the Hubble constant towards higher values, reducing the tension. Using a combination of CMB and BAO data from Planck and DESI, they find a significant overlap between their H0 estimate and the supernova estimate from SH0ES. This overlap is shown above in Figure 1–the purple contours are the H0 estimate using the IDE model, and the green shaded region shows the SH0ES estimate. For reference, the blue shaded region in Figure 1 shows the Planck estimate.

Ironically, when they fit to the SH0ES data as well as DESI and Planck (the filled contours in Figure 1), the discrepancy with the supernova-derived H0 increases. This is because in order to compensate for the transfer of energy away from dark matter and still produce a model that fits the data, we have to decrease the density of normal (“baryonic”) matter in the Universe. The lower the coupling parameter, the more energy is transferred, the lower the baryonic matter density. Supernova data prefers a high matter density, and thus a higher coupling parameter and a lower Hubble constant. However, the IDE model, even when considering supernovae, still reduces the tension significantly compared to ΛCDM!

Despite this exciting result, there’s still hurdles to overcome if we want to conclude definitively that IDE resolves the Hubble tension fully. Still, the Hubble tension has been plaguing scientists for years. Perhaps it’s time to consider a new model. The Universe, after all, is complex–and so much more interesting than we know.

This bite was written and published as part of Astrobites’s new partnership with the American Physical Society (APS). As part of this partnership, we cover selected articles from the Physical Review Journals, APS’s premier publications covering all aspects of physics. For more coverage as part of this partnership, see our other PRJ posts.

Astrobite edited by Dee Dunne

Featured image credit: Katherine Lee/imgflip meme generator

Author

  • Katherine Lee

    Katherine Lee is a software developer working on stellar spectroscopic analysis for PLATO and 4MOST at the Max Planck Institute for Astronomy in Heidelberg, Germany. In 2023 they received a master’s degree from the University of Oslo, where they worked on cosmological parameter estimation using CMB anisotropies and FIRAS data. In their spare time, they play the cello, run D&D, and practice an ever-increasing list of fiber crafts.

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