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[dupe] Dark matter may be older than the Big Bang (sciencedaily.com)
55 points by howard941 72 days ago | hide | past | web | favorite | 22 comments

Saying that it happened before the Big Bang is super misleading. Most readers would assume that the inflationary period is part of the Big Bang.

The paper's authors are surely writing for an audience of physicists and not a general audience. As mentioned here[1], their audience will expect this usage. ScienceDaily may not have, though if they did, it would have been nice for them to highlight the distinction.

[1] https://news.ycombinator.com/item?id=20666140

Agreed. I wonder what the authors think “Big Bang” refers to if it occurred after inflation.

That's what I was going to ask.

But from https://news.ycombinator.com/item?id=20666140, cited by sls:

> They're using "Big Bang" to mean the hot, dense, rapidly expanding state that is the earliest state of the universe for which we have good evidence. In models with inflation, this state occurs at the end of inflation, when "reheating" transfers all the energy stored in the inflaton field to the Standard Model fields (quarks, leptons, and radiation).'

The scientific discussions of Dark Matter seem to be the scientific version of political doublespeak which boil down to- "We aren't really sure what is going on but since you have questions, we should have answers..." I'm about to check the previous article and comments and kudos for krastonov for the suggested reading.

Still trying to make sense of this dark matter subject besides the usual, no one is really sure...

This article says that 'mathematically' it could have happened before but leaves it at that. Does anyone know what kind of math? And the prerequisite knowledge I might need to know (subjects, bonus points for good textbooks). I myself have a pretty firm knowledge of undergrad and some grad level physics. But I find myself recently interested in (big time/spacial scale) cosmology and also quantum phys, but I don't really know where to begin.

This pretty much all falls under the umbrella of mathematical physics- any mathematician will tell you it's not "real" math. Don't let that stop you from studying it if you're interested, but it's not for the faint of heart and most people require a mentor (aka an advisor and PhD program)- many self proclaimed "autodidacts" stall out once there are no longer online tutorials and they have to slog through textbooks written by physicists and mathematicians who seem to enjoy making a subject hard to understand.

Starting with analysis, measure theory, and I suppose the obligatory linear algebra should first if you don't already have those. Spectral theory and maybe group theory are probably next steps to having enough background to do the quantum and relativistic stuff. I glanced at the abstract for the source material for the HN post (https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.12...), and it almost certainly requires spectral theory.

I'm not an expert in this area, so a mathematical (or theoretical) astrophysicist might chime in and fill in the gaps.

A typical suggestion is to check "The Theoretical Minimum" series (books and lectures). That would also inform you on what to read if you want to delve deeper in a particular subfield.

Could someone explain to a layman what is our best theory as to the limits of the universe as it pertains to the start of the big bang? How do we know the big bang is not just a tiny spec in the middle of a giant ocean (similar to a supernova, but on a larger scale) that's part of something much much bigger? I am guessing it's just beyond our knowledge to figure out what lies beyond the big bang, but if that were true, wouldn't dark matter truly exist way before any big bang(s) took place?

> How do we know the big bang is not just a tiny spec in the middle of a giant ocean

This is referred to as the Cosmic Landscape, or, Eternal Inflation. In eternal inflation, the cosmological constant becomes a field in its own right, the Inflaton field, and our universe is a pocket of spacetime where that field has randomly adopted a value allowing nontrivial structure to form.


The big problem is that science is based on repeatable experiments, and it's very difficult to make a theory of what happens outside the observable universe that can be tested even in principle. If a theory doesn't make falsifiable predictions, then it isn't science. Until someone comes up with a prediction that can be tested, one might as well ask, "How do we know the world wasn't created by God?"

There are infinitely many things we don't know, the challenge is coming up with something we can know.

no debates from me here.

i will add though that from “it’s not science” it does not necessarily follow that “and therefore it’s not useful.”

the obvious example is that the idea of falsifiability as a criterion for what is and is not science is itself unfalsifiable — but very useful!

Thanks for the explanation and the link!

The answer is that we don't know if big bang is a tiny spec in the entire universe. What we do know is that big bang is a huge deal in observable universe.

Definitely. Before the advent of telescopes and modern astronomy, we naturally assumed that the sun revolved around us because that's as far as we could physically (and cognitively) see. We were being egotistical to assume that the universe was here just for us.

In a contemporary context, it would be similarly egotistical to assume that the whole of the universe began with the furthest event we puny humans are able to infer from our vantage point on this little speck of a planet. Context should teach us that we have no reason to assume that our local "big bang" is special or unique in any way.

Just because we can't see or infer any further out (and we might never be able to) gives us no right to declare the big bang to be any sort of beginning.

I still feel like MOND would make alot more sense than all this weird dark matter stuff https://en.m.wikipedia.org/wiki/Modified_Newtonian_dynamics

Dark matter doesn't necessarily need to be viewed as this super weird thing. It seems to be simply some unknown substance that exerts gravitational force while not interacting or only very weakly interacting with most other forms of matter / energy.

We have seen somewhat similar things before. Neutrinos interact very weakly with matter. A single neutrino needs to pass through a stack of lead that is an entire _light year_ in length just to have a 50% chance of hitting a single atom (yes, 1 actual light year).

And we have certainly have encountered unknown substances and particles before.

My layman's understanding is that no variation of MOND has managed to get that close to the comprehensive and robust scope of explanation provided by general relativity.

If your friend talked to you about a topic and spoke about it in the same manner that scientists talk about dark matter you would call them an idiot and change the subject.

'Dark Matter' is physicist code for 'gravitating stuff that we cannot see'.

'Dark Energy' is physicist code for 'anti-gravitating non-stuff vacuum that we cannot see'.

I feel like this is all insane and doesn't make any sense. At least to the general public. What difference does it make? It doesn't even improve our understanding of reality in any meaningful way, it doesn't impact us in any way. We might have as well been where we are now technology wise without knowing about Bing Bang at all. What is it even? Because when you actually ask that question - where did it come from - it's a question you can't really answer within the framework of our consciousness. And if you can't answer that question, yet another layer above Big Bang doesn't really matter.

Dark matter seems to comprise about a quarter of the mass-energy of the Universe, and we don't know what it is.

It seems worth investigating.

Basic research has an incredible ROI over the long run. It's one of the most wisest economic investments a society can make.

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