Hubble Trouble: How Dark Energy may solve a cosmic conundrum

The fact that the universe is expanding is in little doubt — the question troubling cosmologists is just how rapidly. But, a new model of dark energy may unite conflicting observations.

The rate at which the universe is expanding has troubled cosmologists for decades. Thus far only approximations have been gathered by instruments such as NASA’s Hubble Space Telescope. Even more concerning, recent data from the Hubble telescope have revealed a discrepancy between the two primary techniques used for estimating the universe’s expansion rate.

Data collected via the Hubble telescope suggests that the rate of expansion of the universe — known as the Hubble flow — is more rapid than the rate inferred from cosmic microwave background (CMB) observations.

The CMB is light left over from an event known to cosmologists as the last scattering — the moment at which electrons and photons ceased to be in thermal equilibrium and the universe became ‘transparent’ to photons. Part of the reason the CMB is so remarkable is because of how uniform it is across the entire sky.

2018 Planck map of the temperature anisotropies of the CMB, extracted using the SMICA method. The grey outline shows the extent of the confidence mask. Credit: ESA.

In fact, mapping small deviations in the overall temperature of 2.73k— known as anisotropies — has allowed cosmologies to make predictions about the Universe’s structure — including its rate of expansion.

Hubble tension and dark energy models

The discrepancy between these two methodologies— referred to as the Hubble tension — has sparked a growing body of research within physics and cosmology — but various attempts to resolve it have, thus far, proved fruitless.

Now, researchers at Johns Hopkins University and Swarthmore College have put forward an alternative model of dark energy — which accounts for approximately 70% of the Universe’s energy density, despite remaining largely a mystery — that could potentially solve this cosmic conundrum.

Their study, published in the journal Physical Review Letters, applies a model of dark energy — generally modelled as a fluid with negative pressure driving the expansion of the Universe— that proposes it as evolving but non-interactive with respect to the Hubble tension.

A portrait of our universe’s history is called the Hubble Ultra Deep Field (or HUDF). It is a minuscule patch of the sky first targeted by the Hubble Space Telescope in 2002 (ESA)

As Vivian Poulin, one of the researchers who carried out the study told Phys.org: “Despite the lack of success, previous attempts at solving the Hubble tension allowed us to understand roughly what characteristics a solution should have.

“At the same time, we were working on testing consequences of string theory with cosmological observables, which predicts the existence of an “axiverse,” i.e., a huge number of extremely light particles with very peculiar physical properties.”

The team realised that a simple modification of the physical properties of these particles gave them the characteristics needed in the context of the Hubble tension.

He continues: “Thus, we decided to push forward in this direction and test this alternative model.”

Testing EDE models against the CMB

Using the data collected during research collaborations such as Planck’s CMB observations and the SH0ES H0 measurements, the theoretical physicists applied a model of early dark energy (EDE) to the Hubble tension.

Poulin explains further: “An EDE just means that these particles, in the cosmological context, acts like a dark energy component at much earlier time than the current dark energy does.

“In practice, these particles modify the expansion rate of the universe around the time at which CMB photons were emitted — roughly 380,000 years after the Big Bang — increasing it slightly compared to the standard prediction.”

Part of the aim of the study was to calculate what the CMB might resemble if an EDE component is present. These predictions were incredibly detailed thanks, in most part, to the sheer precision of the data gathered by Planck’s CMB mission.

A visual timeline of the Big Bang and the expansion of the Universe. (NASA)

As Tanvi Karwal, another researcher involved in the study also told Phys.org: “We needed to figure out exactly how our model would behave, evolve and fluctuate, and how it would affect the cosmic microwave background — the oldest light in the universe.

“The CMB is complex and its shape must be calculated numerically, so we added code describing the EDE to a pre-existing code to extract cosmological information from the CMB.”

Poulin, Karwal and their colleagues used a supercomputer to sample hundreds of thousands of different cosmological models — thus allowing them to identify which best fits existing observations of the universe. They found that the cosmological model including an EDE component could solve the Hubble tension.

The results imply that a slight modification of the universe’s expansion rate in the remote past produced by an EDE relieves the Hubble tension. Of course, this is only a model, thus it is possible that it is not realised in nature.

Poulin suggests that this isn’t as much of a problem as it might sound. “In cosmology, what really matters are the dynamical properties of an ensemble of these particles (more accurately, it is their total energy density and pressure), and not so much their individual micro-physical properties.

“In fact, there already are alternative realizations of the EDE proposed after our work was published, whose collective properties are similar to the one we proposed.”

A springboard for future research

This new research not only bolsters the current understanding of how influential EDE must at various times in the universe’s history, but it also promises to inform the development of more effective future cosmological models.

Karwal expands on this: “My main takeaway from this project is that anomalous cosmological observations can help us to explore new physics.

“This research has inspired other groups to investigate similar models of EDE as a solution to the Hubble tension. We have some more work to do in refining and understanding our EDE model, but are also interested in different solutions to the Hubble tension altogether.”

The researchers plan to test their model further in several ways — gathering important details about the properties of EDE. In fact, one thing that concerns Poulin, Karwal and their team is why their EDE model produces better predictions than others. They suspect that their findings highlight the sensitivity of data to the characteristics of EDE.

Poulin continues: “We also want to see whether there are additional signatures of these particles in cosmological observables. For instance, we already realized that next-generation CMB experiments could test this model independently of the supernovae observation.

“It means that one could tell unambiguously that this fluid exists in nature without needing to invoke the Hubble tension. But we also showed that these models can affect the statistical properties of ensembles of galaxies, for which we have numerous observations.”

The team believe that future data collected with improved accuracy using space-based instruments such as the EUCLID satellite and LSST telescope could also contain the fingerprint of EDE.

They also acknowledge, that achieving an accurate prediction of this fingerprint will require additional work that goes way beyond the numerical computations they performed.

Original research: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.122.221301