so, what shape is the universe?
Early last month, a paper claiming that we’ve misunderstood the shape of our universe made the media rounds on popular science sites before being mocked by The Daily Show because the only nerds Trevor Noah is fine with are political ones. Apparently, after examining a map of the cosmic microwave background radiation, or CMBR, created by the Planck space observatory, a team of researchers proposed that some of the initially flagged anomalies are best explained if we assume that we live in a closed universe instead of an open one as we thought. Of course, this raises three important questions about their findings. What does that mean? Why did we think the universe was open? And finally, how sure are we about these results?
So, what does it mean to live in a closed universe? Imagine building a spaceship that could fly through space as long as you wanted. Point it in a certain direction and in an open universe, you’d be able to keep going forever because the universe stretches infinitely in every possible direction. But in a closed universe, you’d eventually return to where we started just like you would when navigating the surface of a planet because the cosmos will curl in on itself like a mobius strip. It also means that the universe wouldn’t just drift apart and slowly cool off into its lowest quantum state over the eons but eventually return to its starting point and cause a Big Crunch, ending with a bang, not an icy whimper.
taking a level to the universe
However, we’re pretty sure that the universe is open and flat, and we know that using two very well tested methods. The first is to look at features in the CMBR representing cosmic structures and form a triangle with them. By then seeing if those triangles in space and the CMBR have angles that add up to exactly 180 degrees, we could say that the universe must be flat and open since that’s the only way the numbers could possibly make sense. More than 180 degrees and the universe must be closed, and less than 180 degrees means that we’re looking at a curved universe in which everything on cosmic scales scatters.
The second method looks at the density of matter and how it affects the space around it based on predictions outlined by general relativity. As far as we can tell using both approaches, space as we know it is open, flat, and infinite, and its average density of just a bit under six hydrogen atoms per cubic meter confirms that arrangement. But there’s a slight problem. When you look closely enough at the CMBR data, tiny anomalies appear and that’s where the paper in question and others like it come into the picture. They’re the result of a disagreement currently playing out between cosmologists about why we see anomalies in CMBR maps.
reading the cmbr’s tea leaves
One camp says that they’re artifacts showing the limitations of our tools and data processing methods. The other insists that there may be something to that data and that it’s evidence that we don’t know as much about the universe as we think because the calculations they produce are different for small features of the cosmos compared to large features. So, the paper in question doesn’t really try to claim that we know for a fact that the universe is closed, but that if we just assume that it’s closed, we can resolve some of these scale-specific anomalies by adding one more metric into the dominant mathematical model of the universe known as ACDM to track the curvature of space.
But is that really necessary? If our only source of information about the shape of the cosmos was the CMBR map, maybe. But we also measure light and mass in ways showing us the basic shape of space between specially determined points and get confirmation of the universe’s flatness again and again, and larger CMBR features line up closely with those measurements. Instead of assuming that maybe space is closed and modifying numbers and theories that are useful in accurately describing 95% of literally everything, we just need a much better, higher resolution CMBR map to figure out if we’re just seeing the limits of our tools of if we’re really on to something profound when we examine the sky.
See: Valentino, E., et. al., (2019) Planck evidence for a closed Universe and a possible crisis for cosmology, Nature Astronomy, DOI: 10.1038/s41550-019-0906-9