Far out cosmology: space time is not curved (Introduction)

by David Turell , Saturday, June 03, 2023, 20:42 (537 days ago) @ David Turell

The paper says it is, Hossenfelder says no, no:

https://www.patreon.com/Sabine/posts?filters[search_query]=big%20bounce&filters[all...

"The paper is the report of an experiment. For this experiment, they trapped two clouds of about 20 thousand Rubidium atoms on a chip with a bunch of electric and magnetic fields. The clouds have the shape of a cigar and are a fraction of a millimetre long. They couple the two clouds together with more electromagnetic fields and then decouple them. This creates a kind of shock wave traveling through the clouds. Then they measure the correlations between the clouds after various time intervals.

"The rest is interpretation. The interpretation says that perturbations in the atom clouds behave as if they were perturbation in a curved space-time, if you identify the speed of sound in the gas clouds with the speed of light in space-time. This is because you can use the electric and magnetic fields to make the speed of sound in the gas dependent on the place. So, you can manipulate how the perturbations travel kind of like it would happen in a curved space-time. Then you can call that a “quantum simulation” of a curved space-time.

"Here’s what they don’t say in the press release. These atom clouds that they used for the experiment are cigar shaped which means they’re effectively one-dimensional. But there’s no gravity in only one dimension of space. You don’t have to take my word for it, you can look it up in Weinberg’s book on general relativity. So even the link to a curved space-time is somewhat of a stretch.

***

"A new paper that was just published in PRL casts doubt on the idea that the universe went through a big bounce, rather than starting with a big bang.

"Physicists have many ideas for the beginning of our universe. The simplest one is a Big Bang where the energy density in the universe was incredibly high. The Big Bang did not happen at a particular point but everywhere. If you find that confusing, you’re not the only one, and you’ll be happy to hear that I made a video about this in particular.

"The second most popular idea is that, yes, the universe had this phase of high energy density, but it wasn’t the beginning. Rather, it was itself the end of a previous phase in which the universe contracted. This is called a “Big Bounce”. There are several of these Big Bounce models. The best known one is probably Penrose’s Cosmic Cyclic Cosmology, which you can learn about in my previous video.

"However, it isn’t the best-studied one because it’s kind of difficult to figure out how to use Penrose’s model, I speak from my own experience. The most studied bounce model is probably that of Loop Quantum Cosmology . It’s based on the idea of Loop Quantum Gravity, the main competitor of string theory.

***

"In the new paper they now say that if you have a bounce model which does that, it would also have another property, which is to induce new correlations in the CMB. They’re known as the bi-spectrum or three-point correlations. I know this sounds somewhat intimidating but don’t worry, only thing you need to know is that these correlations don’t seem to be there.

"The authors now say you can’t have your cake and eat it too. Either you have a bounce model that explains the large scale CMB properties, but then it screws up the bi-spectrum. Or it agrees with the bi-spectrum, but then it doesn’t help with the large scales.

"This doesn’t rule out bounce models. To begin with it’s only a particular type of bounce models that they looked at. It does take away one of their appeals, but there’s still the appeal that you can make them more complicated and continue writing papers about them. (my bold)

Comment: My bold illustrates the problem, which is 'anything but God'. The big Bang can include God, but a bouncing universe is eternal, no God needed. 99% of the evidence favors the Big Bang.