Galaxies are at rest with respect to the early universe: study

Putting the theory of special relativity into practice, by counting galaxies.


Widely believed by astronomers, the principle of mediocrity states that the properties and evolution of the solar system are not unusual in any important way. New research from CU Boulder- led by CU Boulder astrophysics professor Jeremy Darling- adds to the case for mediocrity by demonstrating that galaxies are, on average, at rest in relation to the early cosmos.

According to Darling, we have a funny motion that is constant with everything known about the universe. He said, “There’s nothing special going on here. We’re not special as a galaxy or as observers.”

Cosmic Microwave Background (CMB) is electromagnetic radiation left over from the universe’s formation during the Big Bang. This CMB appears warmer in the direction of our motion and cooler away from the direction of our motion. Based on this glow, scientists hypothesized that the Sun and its planets are moving in a specific direction at a certain speed. Plus, our inferred velocity is a fraction of a percent of the speed of light—small but not zero.

This inference can be tested independently by counting the galaxies visible from earth. This is possible due to Albert Einstein’s 1905 theory of special relativity, which explains how speed affects time and space.

But, when experts tried to count galaxies in recent years, they’ve come up with numbers that suggest the Sun is moving much faster than previously thought, which is at odds with standard cosmology.

Undoubtedly, counting the galaxies over the whole sky is a challenging task. Plus, our galaxy gets in the way. It has dust that will cause you to find fewer galaxies and make them look dimmer as you get closer to our galaxy, said Darling.

Darling was intrigued and perplexed by this cosmological conundrum, so he decided to look into it further. He also knew about two recently released surveys: the Very Large Array Sky Survey (VLASS) in New Mexico and the Rapid Australian Square Kilometer Array Pathfinder Continuum Survey (RACS) in Australia, both of which could help improve the accuracy of a galaxy count—and shed light on the velocity mystery.

Data from these surveys help Darling study the entire sky by patching together views from the northern and southern hemispheres. Notably, the new surveys also used radio waves, which made it easier to “see” through the dust of the Milky Way, thus improving the view of the universe.

Analysis of surveys revealed that the number of galaxies and their brightness were in perfect agreement with the velocity scientists had previously inferred from the cosmic microwave background.

Darling said“We find a bright direction and a dim direction—we find a direction where there are more galaxies and a direction where there are fewer galaxies. The big difference is that it lines up with the early universe from the cosmic microwave background, and it has the right speed. Our cosmology is just fine.”

Because Darling’s findings differ from past results, his paper will likely prompt various follow-up studies to confirm or dispute his results.

Journal Reference:

  1. Jeremy Darling. The Universe is Brighter in the Direction of Our Motion: Galaxy Counts and Fluxes are Consistent with the CMB Dipole. DOI: 10.3847/2041-8213/ac6f08