New Distance Measurements by Dan Scolnic Add to the Puzzle of Universe’s Expansion Rate

The universe is expanding, but not at the rate scientists once anticipated. Recent measurements conducted by researchers at Duke University reveal that the expansion is occurring more rapidly than what theoretical models have predicted. This discrepancy is challenging the foundation of modern cosmology and raising the possibility that current models may be flawed. 

This issue, known as the “Hubble tension,” has emerged as a pressing mystery for physicists. According to findings recently published in Astrophysical Journal Letters, there is now even stronger evidence supporting this accelerated expansion. 

“The tension now turns into a crisis,” stated Dan Scolnic, the leader of the research team.

The Expansion of Universe and Hubble Tension 

The universe is continuously expanding—a concept that has been firmly established and recognized by scientists for almost a hundred years. This idea was initially introduced by Russian physicist Alexander Friedmann in 1922 and independently revisited by Belgian astronomer Georges Lemaître in 1927. Observational proof supporting this theory was later provided in 1929 by American astronomer Edwin Hubble.

The scientific community largely agrees that the Universe is expanding, but there is a noticeable disagreement between two highly precise calculations of its rate of expansion. This inconsistency, known as “Hubble tension,” could hint at a significant gap in our current understanding of the Universe’s origin and development.

While it is possible that one or both measurements contain errors, recent data supports the idea that the discrepancy is genuine. As a result, researchers are compelled to reexamine the entire framework of their cosmological theories.

Scolnic, a physics associate professor at Duke University, describes the process as creating the Universe’s growth chart. While we know the Universe’s size at the time of the Big Bang, the question lies in understanding how it expanded to its current dimensions. In his analogy, the Universe’s early image symbolizes the distant Universe, which includes the initial seeds of galaxies. Conversely, the Universe’s current representation signifies the local Universe, encompassing the Milky Way and its surrounding galaxies.

The standard model of cosmology serves as the proposed growth curve linking these two states. However, the issue arises because the connection doesn’t seem to align as expected.

Solnic said, “This is saying, to some respect, that our model of cosmology might be broken”.

Understanding the Hubble Constant

The rate at which the Universe is expanding can be a perplexing idea, and it may be helpful to explain it through an analogy. Imagine a rubber band that measures two units in length, with a mark indicating the midpoint. If you secure one end of the band to a fixed hook and pull the other end straight, the end you are holding will be positioned two units away from the hook, while the mark will be located one unit away.

Now, envision that you take the loose end of the rubber band and pull it to double its length, completing this action in one second. As a result, the end is now four units away from the hook, while the midpoint is two units away. Therefore, the midpoint has shifted one unit in one second, whereas the loose end has shifted two units in that same time frame. 

The crucial observation is that the point farther from the hook moved at a faster rate than the point closer to it. In cosmological terms, the speed of a point on the rubber band is one unit per second for each unit of distance from the hook.

The expansion of the universe operates in a similar manner: objects that are farther away from Earth are receding at a faster rate than those that are nearer. 

For instance, a galaxy located one megaparsec from Earth is moving away at a speed of 70 km/s, while a galaxy two megaparsecs away is receding at 140 km/s. (A parsec, an older unit of astronomical distance, is equivalent to 3.26 light years.) This speed is referred to as the Hubble constant (symbolized as H₀), and the fundamental concept is well-established in the field.

New Measurements by Scolnic’s Team

To measure the universe, a cosmic ladder is employed, consisting of a series of techniques for determining the distances to celestial bodies. Each technique, or “rung,” depends on the preceding one for accurate calibration. 

The ladder utilized by Scolnic was developed by another research team that gathered data from the Dark Energy Spectroscopic Instrument (DESI), which observes over 100,000 galaxies each night from the Kitt Peak National Observatory.

Bridging Gaps in the Coma Cluster

Scolnic recognized that this ladder could be anchored nearer to Earth by obtaining a more accurate measurement of the distance to the Coma Cluster, which is one of the closest galaxy clusters to us.

“The DESI collaboration did the really hard part, their ladder was missing the first rung,” said Scolnic. “I knew how to get it, and I knew that that would give us one of the most precise measurements of the Hubble constant we could get, so when their paper came out, I dropped absolutely everything and worked on this non-stop.”

The Role of Type Ia Supernovae

In order to determine an accurate distance to the Coma cluster, Scolnic and his team, supported by the Templeton Foundation, analyzed the light curves of 12 Type Ia supernovae located within the cluster. 

Similar to candles illuminating a dark pathway, Type Ia supernovae possess a consistent luminosity that is directly related to their distance, rendering them dependable for distance measurements. 

The researchers concluded that the distance to the cluster is approximately 320 million light-years, which aligns closely with the range of distances reported in studies conducted over the past 40 years, thereby providing a reassuring indication of its precision.

Scolnic said, “This measurement isn’t biased by how we think the Hubble tension story will end. This cluster is in our backyard, it has been measured long before anyone knew how important it was going to be.”

Resultant Value of Hubble Constant

By utilizing this precise measurement as a foundational step, the research team adjusted the remaining steps of the cosmic distance ladder. They determined the Hubble constant to be 76.5 kilometers per second per megaparsec. This indicates that the local Universe is expanding at a rate of 76.5 kilometers per second for every 3.26 million light-years.

This outcome is notably distinct from the forecasts derived from early Universe observations, which indicate an expansion rate of approximately 67.4 kilometers per second per megaparsec. Although the difference between these figures appears minor, it presents significant challenges for our comprehension of physics.

The value currently observed aligns with previous measurements of the local Universe’s expansion rate. However, similar to those earlier measurements, it contradicts the Hubble constant values derived from observations of the distant Universe. 

This means that while it corresponds to the recent findings regarding the Universe’s expansion rate, it does not align with what our current physics models suggest. The ongoing debate is whether the discrepancy lies in the measurements themselves or in the theoretical models used. Scolnic’s team’s new results adds tremendous support to the emerging picture that the root of the Hubble tension lies in the models.

“Over the last decade or so, there’s been a lot of re-analysis from the community to see if my team’s original results were correct,” said Scolnic, whose research has consistently challenged the Hubble constant predicted using the standard model of physics. 

“Ultimately, even though we’re swapping out so many of the pieces, we all still get a very similar number. So, for me, this is as good of a confirmation as it’s ever gotten.”

“We’re at a point where we’re pressing really hard against the models we’ve been using for two and a half decades, and we’re seeing that things aren’t matching up,” said Scolnic. “This may be reshaping how we think about the Universe, and it’s exciting! There are still surprises left in cosmology, and who knows what discoveries will come next?”

Conclusion

“We’re at a point where we’re pressing really hard against the models we’ve been using for two and a half decades, and we’re seeing that things aren’t matching up,” said Scolnic. 

“This may be reshaping how we think about the Universe, and it’s exciting! There are still surprises left in cosmology, and who knows what discoveries will come next?”

Questions regarding dark energy, dark matter, and other unknown phenomena persist. Many researchers speculate whether there might be an overlooked factor that could clarify the unexpectedly rapid rate of cosmic expansion. Some are exploring possibilities for new physics, while others are enhancing measurement techniques to achieve greater accuracy. 

Scientists are continuously analyzing data from observatories around the globe, collecting additional evidence to comprehend the behavior of the expansion rate over time. Whether through minor adjustments to existing models or the development of entirely new frameworks, the narrative of the cosmos is still evolving, driven by measurements that consistently exceed predictions.

This research was supported by funding from several organizations, including the Templeton Foundation, the Department of Energy, the David and Lucile Packard Foundation, the Sloan Foundation, the National Science Foundation, and NASA.