Far out cosmology: expanding universe will not tear apart (Introduction)

by David Turell , Monday, January 15, 2024, 18:10 (102 days ago) @ David Turell

Hossenfelder adds her opinion:

https://www.patreon.com/posts/our-universe-not-96470792?utm_medium=post_notification_em...

"The Dark Energy Survey has surveyed dark energy and found that our universe is unlikely to rip into pieces. I think that’s good news. Let’s have a look.

"Dark Energy is the name that astrophysicists have given to a hypothetical ingredient of the universe that makes the expansion of the universe faster. So it’s not that dark energy causes the expansion itself, it makes it faster. Dark energy should not be confused with dark matter, which *does contribute to the expansion of the universe but doesn’t make it faster.

***

"In astrophysics the name “dark energy” isn’t used for just one thing. It’s a term that encompasses all kinds of different things which could cause this accelerated expansion. One of these types of dark energy is the cosmological constant, usually denoted with a capital lambda. So the cosmological constant relates to dark energy like a banana relates to fruits. It’s one of many.

"The cosmological constant is, would you have guessed it, constant, and it’s the simplest type of dark energy. But like there are other fruits besides bananas there are other types of dark energy besides the cosmological constant.

***

"And this brings me to the new data from the dark energy survey. They measured this w by looking at light from distant supernovae. The survey was conducted with a camera mounted on the Victor Blanco telescope in Chile that collects data in 5 different wavelength ranges. It closely monitored about one eights of the total sky for a total of more than 500 nights in the years from 2013 to 2019. In the past few years, the collaboration have analysed the data and are now publishing the results.

***

"In the new data analysis they now found that the best-fit value for w is minus 0 point 8. And the 95 percent confident band is within plus 0 point 14 and minus 0 point 16 of the best fit value.

"Now remember that the simplest type of dark energy is the cosmological constant which has w equals minus 1. So the best fit is barely just outside the 95 percent confidence band. It’s a very mild tension. Kinda interesting, but too insubstantial to think much about.

***

"More interesting is that it tilts the balance against w being smaller than even minus 1. This matters because the value of w determines the ultimate fate of the universe. With w at minus 1 or larger, the universe will expand faster and faster and get darker and darker and colder and colder in a rather boring way that’s sometimes called the Big Freeze. It might also recollapse, which is called the “Big Crunch”.

"But if w was smaller, so more negative than minus one it would be much more dramatic. It’s called the “big rip”. If that happened, eventually everything in the universe would move apart from each other at almost the speed of light, and this means that no forces could hold anything together, not even elementary particles.

"The new result from the dark energy survey therefore means that a big rip is unlikely to happen, luckily. I find it kind of hard to understand what a big rip would even mean, but I’m pretty sure I don’t want to be around when it happens."

Comment: so the universe will not tear up. We'll be all gone before it might happen, so all of it is theoretical.