We’ve developed scientific laws and theories that help us understand the cosmos. While our theories are powerful, they are still rooted in some fundamental assumptions. One of these is that the laws of physics are the same everywhere. This is known as cosmic isotropy, and it allows us to compare what we see in the lab with what we see light-years away. Isotropy is a good working assumption. But we could still be wrong, as a recent study of distant galaxy clusters suggests.
The study looks at distant galaxy clusters. When we look at these clusters, we find they are speeding away from us. The more distant the galaxy, the faster it appears to be moving. This is true for galaxy clusters in every direction we look. This is consistent with a cosmic expansion driven by dark energy.
This new study looked at cosmic expansion by observing hot gas within galaxy clusters. This gas emits powerful x-rays, and by observing the spectrum of these x-rays the team could calculate the temperature of the gas. The higher the temperature, the brighter the gas. By measuring the gas temperature the team could determine how much x-ray light it emits. This is true regardless of how the universe is expanding.
But then the team compared the actual brightness of the gas in these galaxy clusters with their apparent brightness. From this, they could measure the Hubble constant in the direction of the galaxy cluster. They did this with hundreds of clusters all over the sky, and the result they got depending on the direction they looked. Based on this research, the universe is not isotropic. If this is true, we need to take a serious look at the basic assumptions of cosmology.
There are several reasons to be cautious about this result. To begin with, if this non-isotropic behavior held across the entire universe, its effect should be seen in the cosmic microwave background (CMB). Since the CMB doesn’t show this effect, this could be a local property. Local in this case is about 5 billion light-years. There could also be issues with the calibration of the data that needs to be resolved.
Source: “New observations show that the Universe might not be expanding at the same rate in all directions” Universe Today, 9 April 2020.<https://www.universetoday.com/145605/new-observations-show-that-the-universe-might-not-be-expanding-at-the-same-rate-in-all-directions/>