Expansion of the Universe could End Soon!

[The Admiral]

Well, this is what I have just read, and I don't know enough to critically examine it. In as little as 100M years the expansion could come to an end and the Universe could start to contract, ending in a big crunch. All very conjectured of course. However, I must say that I have never felt easy with the concept of eternal expansion, somehow it just doesn't seem neat enough for physics.

See for example, https://www.livescience.com/end-cosmic-expansion

Ian

 

Edited May 5, 2022 by The Admiral

[vlaiv]

It is definitively plausible, but far from conclusive.

We won't be able to tell for certain until we figure out what the dark energy really is.

Our currently widely accepted LCDM model assumes dark energy to be constant - and that fits the data. There is "competing" theory of dark energy - that it is some sort of scalar field / a force that depends on distribution of kinetic and potential energy thorough the universe.

Problem with either of those two is that we just don't yet know. We have different types of measurements and we fit our models to those measurements and when we have a good fit - then we have higher confidence that our model might be correct.

It is like having a curve and fitting different mathematical equations to that curve. Some curves can be fitted with variety of functions with small error - for example:

Apart from constant function (green) - other seem to match all the way up to x=1.5.

If we have data only for 0 - 1.5 range - and there is some noise - we could say that any of those lines "fits" our data (although they are different functions - like sine or polynomials of order 3, 5, 7. ...)

Similarly - both LCDM and Quintessence fit observational data well. There is no currently way of telling which one is correct model. Most people prefer LCDM model as it is simpler. Both have dark energy as repulsive force (actually with Quintessence it depends on configuration of universe, but is repulsive now and has been for most of history) so it is not completely different theory - it is just that one component that is modeled slightly differently.

Thing is - for past data - they both agree, but for future they disagree completely - one ends up in big rip and other in big crunch

Until we have more understanding on what dark energy really is - we are free to choose our favorite model

 

[George Jones]

I think this work illustrates how, more than we like to admit, part of science is a social construct. The senior author of the work, Paul Steinhardt, is a maverick  who previously developed a cyclic model of a universe that cyclically expands and contracts. Steinhardt realized that for his personal model to be viable, he needed a way to connect the expanding past and present universe to a universe that contracts in the future. Steinhardt and his collaborators, being clever people, found a mathematical function which does just that.

In spite of the way I have phrased the previous paragraph, this is not a bad thing. Suppose scientists X, Y, and Z all come up with personal theories that are consistent with present data. It is then up to science (not just X, Y, and Z) to find experiments that will produce data that that will differentiate between the models Until then scientists, being human, will different subjective opinions on the models. One scientist might say "I think X's model will turn out to be true" while another scientist says "X's model is crap; Y's model is just the ticket".

In this way, personal opinion helps to drive the progress of science. For this to work well, it is important to have enough scientists that a wide variety of opinions are represented.

My personal opinion on the work of Steinhardt et al.:

Is it plausible? No.

Is it possible? Yes.

Is it interesting? Very.

Is it worth funding as research? Absolutely.

I hope that some folks have the same answers to these questions; I hope that some folks have different answers to these questions.

[Gfamily]

Steinhardt should be looking for where to find the datapoints that would provide the disproof of his hypothesis.

If they're there, he's done good science, if they're not there, it makes his hypothesis stronger.

But I know l people aren't like that in real life, not after they've got their doctorates  at any rate ! (I am joking of course)