Is The Universe Finite?

**SPEAKER_1** (0:00)
This episode is brought to you by Accenture. When you're advertising operations fall out of sync, everything else follows. Spotify and Accenture are working together to reinvent the rhythm of ad sales. Using automation, analytics, and smarter workflows to simplify campaign delivery and access better data across the business. The result? Less time spent on operations, more time connecting brands with the moments and fandoms that matter most. Learn more at accenture.com/spotify.

**SPEAKER_2** (0:31)
This episode is brought to you by Google Chrome. You think you know a browser, but Gemini and Chrome, that's new. It can help you with practically anything on the web, like restoring a vintage motorcycle from a 50-page restoration block, or finally break down that long article you've had open for weeks. Gemini and Chrome is here for it. Ready to make anything online make sense? There's no place like Chrome. Check responses set up required compatibility and availability varies 18 plus.

**Matt O'Dowd** (0:56)
Thank you to brilliant.org for supporting PBS Digital Studios. Space is big. You just won't believe how vastly hugely mind-bogglingly big it is. Although according to a new paper, it may literally be infinitely smaller than we previously thought.
Every time you walk out the door, light from the big bang strikes your face, enters your eyes. This is the Cosmic Microwave Background Radiation, the leftover heat glow from the very early universe. We can't see this microwave light with our eyes. We can catch it with even a simple radio antenna. As soon as we became aware of its existence, we've been feverishly building better and better devices to collect it. Why? Because it encodes so many secrets. And within this light, a group of scientists have just found evidence of the limits of space.
A clue that our universe may actually be finite in size.
Today on Space Time Journal Club, we'll delve into the Nature Astronomy paper that just reported this. Planck evidence for a closed universe and a possible crisis for cosmology by Eleonora Di Valentino, Alessandro Melchiorri and Joe Silk. This is the map of the cosmic microwave background. The CMB made by the Planck satellite. We've explored in a previous episode how that speckled pattern is the frozen imprint of sound waves that reverberated through the first few hundred thousand years after the Big Bang, only to be frozen in place as the universe cooled. Analysis of the sizes of these speckles from previous satellites and the initial analysis from the Planck map pointed to a universe that is infinitely large and geometrically flat, and is dominated by the influences of dark matter and constant density of dark energy.
For the most part, this has agreed with our observations of the modern universe. But more detailed study of the Planck data has started to reveal tensions. We've talked about the so-called crisis in cosmology. The Planck team calculate an expansion rate for the universe that does not match the expansion rate observed today, particularly the modern expansion rate determined from supernova measurements. That's even accounting for the accelerating effect of a constant dark energy.
Despite this tension, the teams agree on lots of things, including the fact that the universe is, as close as we can tell, geometrically flat and infinite. But I'll come back to exactly what I mean by that. But even this agreement seems to be fading. In the new study, astrophysicists claim to have found clear evidence in the Planck data that the universe is not flat, but rather curved inward on itself. If they're right, the universe is not infinite in extent. Before we get to the new study, let's do a super quick review of geometry. Albert Einstein.

**SPEAKER_1** (3:56)
This episode is brought to you by Accenture. When your advertising operations fall out of sync, everything else follows. Spotify and Accenture are working together to reinvent the rhythm of ad sales, using automation, analytics and smarter workflows to simplify campaign delivery and access better data across the business. The result? Less time spent on operations, more time connecting brands with the moments and fandoms that matter most. Learn more at accenture.com/spotify.

**SPEAKER_2** (4:27)
This episode is brought to you by Google Chrome. You think you know a browser, but Gemini and Chrome, that's new. It can help you with practically anything on the web, like restoring a vintage motorcycle from a 50-page restoration block, or finally break down that long article you've had open for weeks. Gemini and Chrome is here for it. Ready to make anything online make sense? There's no place like Chrome. Check responses set up require compatibility and availability varies 18 plus.

**Matt O'Dowd** (4:52)
Einstein's general theory of relativity allows for three simple geometries for our universe. We have one, a universe with positive curvature. The 2D analogy for this is the surface of a sphere, so our 3D space would be like the surface of a four-dimensional sphere, also known as a hypersphere. Just like with the 2D spherical analog, lines that start parallel in such a universe eventually come together. Such a universe has a finite volume, just as a sphere has a finite surface area.