If Our Part of the Universe is Less Dense, Would That Explain the Hubble Tension?

In the 1920s, Edwin Hubble and Georges Lemaitre made a startling discovery that forever changed our perception of the Universe. Upon observing galaxies beyond the Milky Way and measuring their spectra, they determined that the Universe was expanding. By the 1990s, with the help of the Hubble Space Telescope, scientists took the deepest images of the Universe to date and made another startling discovery: the rate of expansion is speeding up! This parameter, denoted by Lambda, is integral to the accepted model of cosmology, known as the Lambda Cold Dark Matter (LCDM) model.

Since then, attempts to measure distances have produced a discrepancy known as the “Hubble Tension.” While it was hoped that the James Webb Space Telescope (JWST) would resolve this “crisis in cosmology,” its observations have only deepened the mystery. This has led to several proposed resolutions, including the idea that there was an “Early Dark Energy” shortly after the Big Bang. In a recent paper, an international team of astrophysicists proposed a new solution based on an alternate theory of gravity that states that our galaxy is in the center of an “under-density.”

The study was led by Sergij Mazurenko, an undergraduate physics student at the University of Bonn. He was joined by Indranil Banik, a Research Fellow with the Scottish Universities Physics Alliance at the University of Saint Andrews; Pavel Kroupa, an astrophysicist professor with The Stellar Populations and Dynamics Research Group at the University of Bonn and the Astronomical Institute at Charles University, and Moritz Haslbauer, a Ph.D. student at the Max Planck Institute for Radioastronomy (MPIfR). The paper that describes their findings recently appeared in the Monthly Notices of the Royal Astronomical Society (MNRAS).

Simply put, the expansion of the Universe causes galaxies and large-scale structures in the Universe to move farther and farther apart. The speed at which they recede is proportional to the distance between them, where increases in distance lead to a twofold increase in velocity. Therefore, measuring the rate of expansion requires accurate distance measurements, which requires a constant by which these distances can be multiplied – Hubble-Lemaitre Constant. There are multiple ways in which this constant can be measured, which includes distance measurements of the Cosmic Microwave Background (CMB).

These measurements yield an estimate of about 244,000 km/h per megaparsec (Mpc), or about 269 km/s per light year. Other ways to gauge distances include using “standard candles” in the local Universe. However, when astronomers apply these measurements, they obtain a Constant value of about 264,000 km/hr per Mpc – hence the “Hubble Tension.” As Prof. Kroupa explained in a recent University of Bonn press release:

“But you can also look at celestial bodies that are much closer to us – so-called category 1a supernovae, which are a certain type of exploding star. The Universe therefore appears to be expanding faster in our vicinity – that is, up to a distance of around three billion light years – than in its entirety. And that shouldn’t really be the case.”

However, recent observations regarding local matter densities in our Universe could help resolve this problem. According to Dr. Kroupa and his colleagues, our galaxy may reside in a space cavity where matter density is lower than the surrounding matter. Gravitational forces emanating from this surrounding matter are responsible for pulling the galaxies inside the cavity toward the edges. “That’s why they are moving away from us faster than would actually be expected,” said co-author Dr. Indranil Banik from St. Andrews University. “The deviations could, therefore, simply be explained by a local “under-density.”

Similarly, they characterize these gravitational interactions using an alternate theory of gravity known as Modified Newtonian Dynamics (MOND). In the standard LCDM model, the distribution of matter throughout the Universe is homogenous and isotropic (evenly distributed), and under-densities should not exist. Said Kroupa:

“The standard model is based on a theory of the nature of gravity put forward by Albert Einstein. However, the gravitational forces may behave differently than Einstein expected. In our calculations, however, MOND does accurately predict the existence of such bubbles.”

This proposed resolution is supported by recent measurements by another research team of the average speed of a galaxy group located 600 million light-years away. According to the team’s results, these galaxies are receding from the Milky Way four times faster than the standard model of cosmology allows. This highlights one of the most appealing aspects of MOND: it does away with the Hubble Tension entirely. Rather than two constants, there would be only one for measuring the expansion of the Universe, and observed deviations are due to irregularities in the distribution of matter.

But of course, MOND also suffers from its share of issues that have prevented it from becoming the standard model of cosmology. Alas, all astronomers can do right now is continue to study the Universe in greater depth and detail in the hopes that future observations will help resolve the Hubble Tension and other cosmological mysteries.

Further Reading: University of Bonn