The Case Against the Cosmological Principle (and/or FLRW) - Jenny Wagner

[@GravityGrinch]

Hello wonderful people also from my side! If you have questions or comments, feel free to ask! I said I wanted a dialogue and I mean it!

[@madeleinebirchfield7658]

There is the KBC void in our universe. Whether the KBC void is consistent with the FLRW metric is also up for debate; if not, there is the LTB metric to model such local voids. In that vein, I wonder if there is a simple metric which combines the Bianchi metrics and the LTB metrics in a way to model a universe which has both a local void and a dipole.

[@GravityGrinch]

@@madeleinebirchfield7658 good question! Depends on what you consider "simple", because, LTB is inhomogeneous and the dipole universe is only isotropic in 2 spatial dimensions and prefers the 3rd one. Thus, you end up with an inhomgeneous and anisotropic universe in the simplest case. Depending on how much symmetry you want to have in terms of homogeneity and isotropy, you can classify cosmologies like it's done on p36 of this nice review: arXiv:gr-qc/9812046 (homogeneity is decreased from s=4 to s=0, isotropy decreases from q=3 to q=0). Thus all the universes that fit your requirements can be found in the table with s

[@gerry2718]

What?! She should get a really good position soon and I hope that the right people see this video and make some offers! @GravityGrinch: Yet, as you write, creative persons like me and my colleagues, we even struggle hard to obtain funding in these times because most funding still goes into mainstream. I haven't been paid for almost two years and hope to get a permanent position soon because I cannot continue pursuing high-risk-high-gain-projects without having financial security anymore.

[@GravityGrinch]

🥰 thanks!!

[@lindsayforbes7370]

Many thanks Jenny Wagner for that excellent presentation on the challenges to the Cosmological Principal. The data is right. The evidence for dipoles has passed the 5 sigma threshold as has the Ho data. The FLRW model is well tested. If the data and the model are correct perhaps we should question our assumptions about the playing field. The assumptions we make about the universe. How big is the cosmos? Could the cosmos be much bigger than we assume? Could gravitational fields from beyond our universe be responsible for the dipoles we observe and the higher value of late time Ho? Why do we assume that ours is the only universe in the cosmos? It's a wishy washy speculation but it could open up a whole new cosmology.

[@GravityGrinch]

Thanks for the kind words and the inspiring questions. Indeed, I have not talked much about the very-early-universe modifications because I am more of a late-universe explorer. Nevertheless, you are right that there is a lot of potential still sleeping in the early era. Some ideas have already been tested (like arXiv:2207.01569). My personal opinion is that we should first see whether we can still reconcile the probes by synchronising them over scales and time, as I suggested here: arXiv:2203.11219. This endeavour is already under way, for instance, when setting up the inverse-distance-ladder approach to reconstruct the cosmic expansion function starting at the CMB on large scales and then synchronising all other probes from that. It seems to do well so far and the idea is supported by other calculations that the non-linearities in the power spectrum need to be taken into account to resolve tensions (like arXiv:2206.11794). Understanding the limitations of our mathematical modelling and finding resolutions there before predicting new physical effects is important for me to be sure that new physics is _really_ on the horizon. It's important to maintain the trust in science that people out there have by carefully taking all options into account.

[@rockfordlow571]

Sounds like a very interesting talk- good thing there is a guest speaker because the moderator is talking so fast he is literally incomprehensible-perhaps listen to yourself ?

[@CosmologyTalks]

It is quite fast, isn't it. Oops, ah well. Some of the more recent talks might be at a better pace?

[@GravityGrinch]

If it's too fast or slow, you can adjust the speed yourself, as workaround -- that's the best thing when watching video recordings, at least in my opinion.

[@isonlynameleft]

Has this been sped up?

[@CosmologyTalks]

It might have been, I don't remember. It certainly seems like it when watching it back now, 12 months later. I'm surprised nobody mentioned it for 12 months though and now two people have within a few weeks of each other. It could also just be that I had too much caffeine before recording and spoke too fast?

[@Thomas-gk42]

new perspectives should be good for science. what Jenny said about camp thinking should be taken seriously. She´s a really good communicater

[@GravityGrinch]

Thanks a lot, @Thomas-ws6lk Hopefully, more people will join the camp "whatever the data tell us about the universe, is also fine for me" and we can have an inspiring dialogue about the differences in the different model predictions. I have already initiated a similar pathway for my own field, strong gravitational lensing, with quite interesting outcomes (arXiv:1709.03531, arXiv:2207.01630, arXiv:2306.11779) that have helped us a lot to accelerate the data evaluations immensely and to make our interpretations more robust. So it seems a promising path to go for cosmology as well!

[@Thomas-gk42]

@@GravityGrinch I'm honored by your attention Dr. Jenny. I'm just a layperson who's interested in this astrophysics stuff since early youth. But I understood about half of your lecture and of course, the results and the debate with your interviewer. Really a pleasure, to see you so kind, cool and balanced. I think there are a lot of difficulties and problems in physics, cosmology and QM, but could be a chance too? In times of low budgets, the distribution of funds can lead to more conflicts, I guess. I read the book 'Lost in Math' by Sabine Hossenfelder last year, and what she says there seems quite obvious for me, in particular since the search for DM is so unsuccessful. How do you think about it?

[@GravityGrinch]

​@@Thomas-gk42 It's great that you understood about half of the talk here, so your interest as a layperson brought you to learn a lot about cosmology already. Well done! For me personally, the unresolved scientific tensions are a great chance to explore new paths instead of just following mainstream and obtain a solution. Yet, as you write, creative persons like me and my colleagues, we even struggle hard to obtain funding in these times because most funding still goes into mainstream. I haven't been paid for almost two years and hope to get a permanent position soon because I cannot continue pursuing high-risk-high-gain-projects without having financial security anymore. I haven't read that book (as I want to be more positive and constructive about scientific knowledge gain), but being at the crossroads in cosmology now as an interdisciplinary researcher, it's not a big deal for me to keep or abandon LCDM. Nature decides the cosmology, not us, so we have to adapt our models if it becomes necessary... However, for people having worked on our standard cosmology for decades, it's clear that they want to keep their favourite model, as most of them also have other obligations in administration and teaching. Sometimes they are too tired of re-thinking their research and I can fully understand such a viewpoint. When conditions become more encouraging again to have more creative time and competition ceases to be so fierce as it is now, I am sure that more of them would change their minds and become curious again to enjoy the pleasures of finding things out.

[@Thomas-gk42]

@@GravityGrinch the book, I mentioned, is not destructive against science, but critical of theory building in physics in the last decades. It confirms what you said about mainstream. Science should be openminded. Think there are many people like me, who are happy and thankful, that young people like you and your colleges try to find new ways for a better understanding of nature. Thanks a lot for your great and engaged work. I wish you all the best. PS: If the concordance model would has to be rearranged or ruled out, would that also have consequences for the idea of cosmic inflation?

[@GravityGrinch]

@@Thomas-gk42 thanks a lot for the kind words! And, of course, consequences would also affect inflation. Depending on which cosmology we want to explore, it may not even needed and alternatives to that are also being investigated, for instance, the model that Paul Steinhardt from Princeton put forward some years ago. Planck recently announced signatures that could hint at a closed universe, i.e. our cosmos could be curved, so inflation also has to be in agreement with the non-flat universe, if the Planck results are further supported with more evidence.

[@redshiftdrift]

[1:04:57] "LCDM is sufficiently close to correct" Not convinced you can affirm this...

[@GravityGrinch]

Let's put it like this: if we want to reconstruct probes with a 10-20% precision, we can fit LCDM without any issues. It depends now which probe you consider and which precision you want to achieve for it to say that LCDM is not good enough anymore. This is why I also showed the transition from the spherical to the realistic cow and why I mentioned the "Fitting Problem in Cosmology" by Ellis & Stoeger (1987). We need to remind ourselves that the cosmo-principle came from a sparse-data era (almost a no-data era) and without it, we wouldn't have found anything. But now, we can use the vast amount of data we have gained until now to look for the next-to-leading order approximation/ fit to an obviously complex universe.

[@CosmologyTalks]

Adding to Jenny's response, what I was saying in that specific part of the video is that LCDM as an empirical model is a reasonable fit to the data (at worst, at a 20% level - and when the numbers involved vary by many orders of magnitude from early to late universe, getting something right to a 20% level is highly non-trivial). This doesn't necessarily mean that a more complete model is similar to LCDM at a fundamental level (my statement was agnostic about that). It is similar to Newtonian gravity and General Relativity in the solar system. Newtonian gravity and General Relativity, at the fundamental level are very different theories. However, within the solar system Newtonian gravity is sufficiently accurate that one can indeed parameterise General Relativity as Newtonian gravity + correction terms and model observations with high precision. So, my claim was that even though observational data has been pre-processed assuming LCDM, whatever the correct theory is, it must be sufficiently close *empirically* to LCDM that one could (for the first calculation) re-process that data by parameterising the more fundamental model as LCDM + corrections. The alternative is to become an expert in supernovae, galaxy clusters, cmb, bao, cosmic shear, quasars, etc by oneself and re-analyse all the data that has been pre-processed with LCDM, which just isn't feasible - no group working on beyond LCDM has the time for all of this. But, if a group did do this LCDM + corrections analysis and found the pieces started to fit, then the next step would be to convince the observers to re-analyse their data in this new framework. The more precise one wants to get the more important this would become, but one shouldn't refuse to take a first step because it doesn't get you to the final destination (and other groups shouldn't criticise a group that tries that first step either).

[@GravityGrinch]

@mtheory85 Yes, right! For cosmology, this idea that small deviations from an exact Friedmann-Lemaitre-Robertson-Walker Universe can give us an immense grow in possibilities how structures can grow and look like today has already been put forward by Ehlers, Geren, and Sachs ("Ehlers-Geren-Sachs-Theorem" is even in Wikipedia!) in 1968. Still today, it's something we have to explore further and deeper. Work in progress!

[@andersjohansson1889]

A takehome message should be possible to formulate in a sentence with 20-30 words or so. 😄😄😄

[@GravityGrinch]

@andersjohansson1889 That's indeed possible and would read: we analysed multiple complementary probes and found excess anisotropies beyond LCDM that reach larger than 5-sigma significance but we do not know where they come from. Happy with that? 😇 (Shaun told me that we should speak a few minutes about that, so I went a bit deeper into details...)