Far out cosmology: huge universe (Introduction)

by David Turell , Sunday, April 09, 2023, 16:00 (375 days ago) @ David Turell

Hubble expansion rate faster than the speed of light at far out edges:

https://www.discovermagazine.com/the-sciences/how-big-is-the-observable-universe?utm_so...

"Our universe is about 13.8 billion years old, and the observable bubble of that cosmos has a diameter of about 93 billion light-years across. And we all know the famous maxim from Albert Einstein’s special theory of relativity: nothing can travel faster than light.

***

"Locally, no galaxy is moving. But the space between them is. So there are no restrictions here based on the speed of light because they’re literally not moving. There’s no limit to how quickly space can expand (because “expanding” isn’t a motion as far as relativity is concerned) and so the universe can grow as quickly as it pleases.

"Essentially, the universe is so big because it can expand faster than light. In fact, it’s doing so today. We measure the present-day expansion rate of the universe with something called the Hubble constant, which is around 68 kilometers per second per megaparsec.

"That means for every megaparsec in distance you get away from the Milky Way, the universe’s expansion speed will increase by 68 km/s. A galaxy two megaparsecs away appears to recede at 136 km/s, a galaxy ten megaparsecs away will recede at 680 km/s, and so on.

"The Hubble constant guarantees that once you reach a certain distance — about 13 billion light-years (a distance known as the Hubble radius) — galaxies will appear to move away from us faster than light.

***

"The light from galaxies beyond the Hubble radius was released billions of years ago and is only just now reaching the Earth. We calculate where these galaxies are right now based on our understanding of cosmology, and that’s how we’re able to estimate the size of the universe. The fact that they appeared to move faster than light means that any light that they send now will never reach us — because that light will not be able to overcome the expansion of the universe.

"Since most of the universe is beyond the Hubble radius, all those galaxies are forever out of reach. As time goes on, those galaxies will, one by one, disappear entirely from view. Not through any cheating of the laws of physics, but through simple (and inevitable) stretching."

Comment: dhw wonders why God made it so big. Because that is what God wanted.