A hunt for matched circles in the sky tests whether the universe loops back on itself

The reason the microwave background can be searched at all is that some topologies put the same pattern in two places. "The existence of such 'identifications' means that two areas of the sky that seem to be far apart might in fact be close together," Jaffe said ^[16]. In the simplest case, he said, "there is a circle on one side of the sky that has exactly the same pattern as a circle far away from it" ^[17].

That design fixes the reach of the test. The repeat has to fall inside the volume already observed, and a loop bigger than that volume leaves its copy outside the data, so a search that turns up nothing constrains the small cases and says little about the large ones ^[22]. The authors of a 2026 Nature Astronomy paper cited by phys.org characterise topology by the possible existence of nonshrinkable closed loops ^[7], and place its signatures in the microwave background and in the three-dimensional distribution of matter, where they would break homogeneity at the largest scales ^[8]. The account does not report a measurement excluding a closed topology, or a scale below which loops have been ruled out ^[21].

The microwave background is one surface. It is the light that last interacted with matter some 380,000 years after the Big Bang ^[9], and searching it means searching a sky map for anomalies rather than a volume for repeats ^[19]. "A proper three-dimensional map of that matter would give us even more information about the topology of the universe," Jaffe said, "possibly all the information that we could ever have" ^[18].

Cosmologists have been watching the microwave background since the 1960s, and the sensitivity needed to see topological patterns arrived only in the early 2000s ^[10]. "The modern era of cosmic topology started in the late 1990s when we realized we could use patterns in the CMB to look for topology," said Jaffe, a professor of cosmology and astrophysics at Imperial College London ^[11][5]. He credits WMAP in the mid-2000s and higher-quality data in the 2010s ^[12]. The phys.org report says the data has not improved much since ^[13].

COMPACT came after that. About 20 international scientists work on the problem under that name, in a group formed in the last few years ^[14], and Jaffe said the collaboration "has started to put these results into the full mathematical theory and details of the possible topologies that could describe our universe" ^[15]. The formalism postdates the last data improvement he names ^[24].

Jaffe drew the line himself between a loop and something stranger. "In some cases, there is an actual physical and straight path that can take you back to where you started, but not to when you started," he said ^[3]. A path returning to the original time and spatial point would be what he called a closed timeline curve, or a time machine ^[4].