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Cosmology has a problem: the two different methods of measuring the Hubble Constant, which is the value that determines the universe's rate of expansion, are in disagreement. The universe is expanding, and it is doing so at an accelerating rate. However, lingering questions about universal expansion do remain. Primary among these questions is the difference in measurements of the Hubble Constant — the value that describes how fast the universe is expanding — something that has to be resolved before we can truly understand the evolution of the universe.
However, a new paper published in Physics Letters B, authored by Assistant Professor of Theoretical Physics at the University of Geneva, Lucas Lombriser, puts forward an extraordinary suggestion to explain this disparity — could the Milky Way and surrounding galaxies be contained in a ‘bubble’ of lower-density than the rest of the universe?
To understand why this ‘Hubble Bubble’ of low-density can explain the disparity in the values of the Hubble Constant, it is necessary to examine the two different methods of measuring the Hubble Constant: one which can be described as ‘local’, and the other which astronomers describe as ‘global’ (though the title ‘universal’ may be more apt).
The local method of attaining a value for the Hubble Constant involves measuring the velocities of galaxies around us for their redshift : the change in the frequency of the light they emit as a result of their movement, and from observations of Type Ia supernovae. These massive star explosions are so uniform in emissions that they are referred to as ‘standard candles’ and are used by astronomers to plot highly precise distances in the vastness of space. This method delivers a value for the Hubble Constant of around 74 (no units are used for the Hubble Constant).
The ‘global’ method involves taking a measurement from the cosmic microwave background (CMB) radiation that permeates the entire universe. Discovered by accident in 1964 by astronomers Robert Wilson and Arno Penzias of Bell Laboratories, the CMB is the ‘fossilized remains’ of an event that took place in the very early universe, shortly after the rapid expansion period most commonly referred to as the Big Bang.
Unlike other forms of radiation that fill the universe, the CMB appears to come from ‘everywhere’ at once. It fills the universe isotropically and homogeneously, barring tiny variations called anisotropies, at a temperature of 2.7 K.
This uniformity helps confirm the idea that on a large-scale (a distinction that will become important soon) the universe is the same in all directions.
Data from the Planck space mission, with the application of Einstein’s theory of general relativity, put the Hubble Constant at around 67.4. This disagrees with the local technique by about 10%, not an insignificant difference.
To compound the problem, both values have continued to become more and more precise while remaining in disagreement with one another.
Hubble Bubble Solves the Trouble
In order to solve this disparity, researchers have put forward the idea that there may be some new physics at work. Lombriser’s idea of a low-density Hubble Bubble — named for Edwin Hubble, the man who first discovered that the universe was expanding — negates the need for new phenomena.
Lombriser’s idea of a low-density bubble relies on the fact that, while the universe is isotropic and homogeneous on a large scale, on a small scale this is anything but the case.
The densities of matter in the Earth and Moon system, for example, are much greater than if we move our lens outwards slightly. This inhomogeneous nature can be expanded to the Milky Way, which is much denser than the intergalactic space outside of it.
What Lombriser suggests is that the Milky Way and surrounding galaxies exist in a bubble that is 250 million light-years across, within which matter is 50% as dense as the matter in space around it.
This has a significant effect on measurements of the Hubble Constant taken with the local method. When factoring in this density, the local value of the Hubble Constant is brought into line with the global value given using the CMB.
Low-Density Bubbles in our Universe
The idea of a low-density Hubble Bubble is more than simply an ad hoc adjustment to obtain agreeable values. Astronomers have observed many low-density areas in the universe, and many have postulated that our region of the universe exists within one such pocket.
Additionally, this study is not the first to put forward the idea of a low-density bubble. The difference between previous studies and Lombriser’s is that in prior works researchers suggested that the Hubble Bubble would have to be as great as 4 billion light-years across to include all the supernovae used to make distance calculations. The problem with this is that the density of a bubble of such size is unlikely to vary from the density of the universe as a whole.
The need to be wide enough to include all supernovae in the data set can be neatly side-stepped because these stellar explosions are only used to calculate relative values. An absolute value is needed to make sense of these relative values, and that absolute value is given by the galaxy Messier 106, which exists 25 million light-years away.
If the low-density Hubble Bubble is wide enough to include Messier 106, which Lombriser’s is, then the theory is viable.
While the probability of existing in such a bubble is reasonably high, more evidence is certainly needed before the Hubble Bubble is widely accepted. This evidence could be delivered experimentally over the next few decades by increasingly detailed surveys of galaxy clusters and groups.
In addition, a new form of astronomy using gravitational waves in conjunction with observations in the electromagnetic spectrum could help clear up uncertainties around measurements. Performing this check is not currently possible, as the only gravitational wave event that has been observed in both that form of radiation and the electromagnetic spectrum (GW170817) lies inside our bubble.
References and Further Reading
Lombriser. L, ‘Consistency of the local Hubble Constant with the cosmic microwave background,’ Physics Letters B, (2020).