Comprehending cosmic conflicts

In some ways, the motions of galaxies in the Universe can be compared to the flow of traffic through a city. Imagine galaxies are vehicles. Just like traffic, they can experience congestion in areas and traffic is low in cosmic voids. Just has having more cars on the road can affect the severity of traffic having more matter in the Universe can affect how galaxies move and clusters. But while cars follow a path set by roads galaxies follow a path set by gravity. Traffic in a city can be affected by the speed limits we set on the roads and likewise the strength of gravity can affect how fast galaxies become clustered together and move. So, when cosmologists measure the motions of galaxies across a patch of the sky, it tells us something about the amount of matter in the Universe and strength of gravity.

However, cosmologists can’t seem to agree on what they think of their measurements of galaxy motions; for over a decade conflicting measurement of the bulk flow, which can be thought of as a net flow of galaxies in a volume, has resulted in conflict over what the best cosmological model is for our Universe. However bulk flow measurements are hard – imagine that you tried to measure how fast cars are moving past you, but you can only measure their speed if they move directly away from you or towards you. This is the problem cosmologists have with measuring galaxy motions. To get around this you need to use clever statistical tricks and measure galaxies across many angles in the sky. But evidence in the literature is beginning to suggest that the assumptions included in these ‘tricks’ might be responsible for conflicting bulk flow measurements.

For my research I have been using simulations to test how well our methods of measuring the bulk flow work. You can imagine this like me using these methods to measure how fast cars are going, then checking that result against the actual speed from the cars speedometer for comparison. My results found that while we do get the right bulk flow on average, we underestimate our uncertainty on the measurement. This causes us to overestimate the precision and thus could be responsible for the disagreement we see in the literature.

I then applied our methods, considering how sure we can be based on the results from simulations to make a measurement with the latest real data. I found we are moving at a speed of over 400 km/s, and my analysis still indicates this is larger than what we expect in the Lambda-CDM model (the standard cosmological model). Thus, possibly our model does need some revision – which could have big implications our understanding of gravity and fundamental physics. More research needs to be done to understand the gravity of the situation.

by Abbé Whitford