By Vivian Ford, Physics BS ’27

Have you ever wondered if the Universe we inhabit is finite? Does light traveling through space eventually reach a boundary and bounce off, like a ball rolling on a billiard table? Does it come back to where it started, like the space station orbiting the Earth? Or does light travel through boundless infinite space forever? Researchers at the Case Western Reserve University Department of Physics are leading members of a study to explore this very question – Is our Universe spatially finite and small enough for us to tell?

The research of CWRU scientists and collaborators Glenn Starkman, Amirhossein Samandar, Anna Negro, Deyan Mihaylov, Craig Copi, Yashar Akrami, Fernando Cornet-Gomez, Andrius Tamosiunas, Catherine Petretti, Mikel Martin Barandiaran and more is in the September issue of Nature Astronomy. ‘The topology of the Universe‘ is an invited review showing that existing data from space missions has the power to let us explore – and look farther – than we have realized.  

But first, how do cosmologists go about even measuring these properties of the Universe?

The Cosmic Microwave Background (CMB) provides the earliest observable snapshot of the Universe. The early Universe was extremely hot, causing particles of light called photons to have very high energies. The Universe was also very dense, so photon-electron collisions occurred constantly, quickly knocking off any electron that had bound to a nucleus. As photons were repeatedly scattered by electrons, the photons couldn’t travel very far in a straight line. This made the early Universe appear opaque, almost as if a thick fog were obscuring everything. 

Eventually, about 380,000 years after the Big Bang, the Universe was cool enough that photons were no longer energetic enough to knock the electrons out of the atoms. In a process called recombination, the electrons therefore bound themselves to ions – mostly just free protons – to make long-lasting neutral atoms, mostly hydrogen. With far fewer free electrons around, light was able to travel freely through space. These photons reach Earth today as the CMB – a snapshot of the early Universe that we can see with our microwave telescopes.

This snapshot may contain clues about the overall shape and connectedness of the Universe itself! Topology describes how space is “connected” – whether there are directions in which you can draw a straight line that returns to its starting place.  On the surface of a sphere, great circles are such curves. A loop around the surface of a cylinder is another example.  Researchers have long wondered about the possibility of our Universe having such loops, or what they refer to as a non-trivial topology. Think of playing Pac-Man, when your character runs off the screen it pops back out on the other side. A non-trivial topology is just that, it would describe a Universe that is ‘multiply’ connected.  

For the past six decades the CMB has been observed with increasing precision and resolution from the ground, from high altitude balloons, and from the space missions COBE, WMAP and Planck. These space missions can observe the whole sky, and evidence of non-trivial topology is most likely to be found in the maps of CMB intensity and polarization that they can make. In the past, a team including CWRU scientists showed that the topology scale – the length of the shortest closed loop around the Universe – exceeds 98.5% of the diameter of the observable Universe. An international collaboration called COMPACT, including researchers at CWRU, has been exploring the potential for detecting signals of non-trivial topology even beyond that. As reported in an invited review for Nature Astronomy, they’ve shown that existing data from the Planck satellite has the power to let us look farther than was realized, while future CMB experiments such as the Japanese/European LiteBIRD satellite and the CWRU-led NASA-supported Taurus balloon experiment will provide new more accurate maps of the polarization of the CMB sky. The Universe may be infinite or just too large for us to tell that it is not, but we have not yet exhausted the possibility that we will someday soon map the contours of the entire Universe.

SEE NATURE ASTRONOMY INVITED REVIEW ‘THE TOPOLOGY OF THE UNIVERSE’ HERE