This NASA/ESA Hubble Space Telescope image shows a massive galaxy cluster, PLCK_G308.3–20.2, glowing brightly in the darkness. This is what huge swaths of the distant Universe looks like. But how far does the Universe as-we-know-it, including the unobservable part, go on for? (ESA/HUBBLE & NASA, RELICS; ACKNOWLEDGEMENT: D. COE ET AL.)

Ask Ethan: How Large Is The Entire, Unobservable Universe?

If we know how big the observable Universe is, why can’t we figure out how big the unobservable part is?

13.8 billion years ago, the Big Bang occurred. The Universe was filled with matter, antimatter, radiation, and existed in an ultra-hot, ultra-dense, but expanding-and-cooling state. By today, the volume containing our observable Universe has expanded to be 46 billion light years in radius, with the light that’s first arriving at our eyes today corresponding to the limit of what we can measure. But what lies beyond? What about the unobservable Universe? That’s what Gray Bryan wants to know, as he asks:

We know the size of the Observable Universe since we know the age of the Universe (at least since the phase change) and we know that light radiates. […] My question is, I guess, why doesn’t the math involved in making the CMB and other predictions, in effect, tell us the size of the Universe? We know how hot it was and how cool it is now. Does scale not affect these calculations?

Oh, if only it were so easy.

The history of the Universe, as far back as we can see using a variety of tools and telescopes, has been well-determined. But our observations can only, tautologically, provide us with evidence about the observable parts. Everything else must be inferred, and those inferences are only as good as the assumptions which underlie them.(SLOAN DIGITAL SKY SURVEY)