Neil deGrasse Tyson


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Neil deGrasse Tyson

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juny 28, 2018, 1:53 pm

I'm listening to Astrophysics for People in a Hurry. At the very beginning (pun intended) NDT says that the universe went from 1/1,000,000,000,000th the size of a period to a few light-years across in a single second.

For those of you who know more about this, how is that possible? Is it that the laws of physics were different at the beginning of the universe, so the speed of light wasn't the speed limit of all things?

juny 28, 2018, 4:08 pm

I’m not qualified to give you a good answer, but I will share something I read in Brian Greene’s The Fabric of the Cosmos:
“Einstein showed that nothing can move through space faster than light…Einstein’s theory does not prohibit space from expanding in a way that drives two points—two galaxies—away from each other at greater than light speed. His results only constrain speeds for which motion from spatial expansion has been subtracted out, motion in excess of that arising from spatial expansion.”

So, I gather, the answer is that the laws of physics didn’t change. The idea that the speed of light is the maximum speed for everything in the universe is one that wouldn’t limit the initial expansion of the universe itself, which would be the initial expansion of “space.”

We should hope for comments from anyone who knows their stuff. It’s easy for folks like me to feel giddy when reading about these subjects.

juny 28, 2018, 4:12 pm

>2 dypaloh: Wow! Well that's a new idea for me.

juny 28, 2018, 4:43 pm

>1 neverstopreading: I was going to make a similar comment: not qualified, have read stuff including the Brian Greene that dypaloh has cited, that there is a difference between the expansion of space itself and moving through space. I too welcome someone with more expertise to chime in.

juny 29, 2018, 10:38 am

>4 stellarexplorer: dittoes, I was half way through writing a similar {but less erudite than >2 dypaloh:} response, feeling smug about sharing my expertise with everyone, then I realized I would sound like an idiot to someone who can actually explain the equations.

Editat: jul. 17, 2018, 4:44 pm

Sorry I haven't stopped by for the past month or so. It looks like y'all didn't need me in order to understand the key distinction between the expansion of space vs. propagation through space, the latter of which always occurs at or below the speed of light.

It sounds like Tyson was describing cosmic inflation, which you can read a bit about here. Perhaps I could contribute something non-negligible by elaborating a bit on that article's statement, "The basic inflationary paradigm is accepted by most physicists... however, a substantial minority of scientists dissent from this position."

Although cosmic inflation is an attractive way to explain several features of the observed universe (such as the "horizon problem" and "flatness problem" that have their own Wikipedia articles), this is somewhat 'indirect' evidence for it. In particular, it's not (yet?) feasible to rule out the possibility that some other process could be the actual cause of these features. Instead we have to judge how likely various possibilities seem, including both the "alternatives" listed by Wikipedia as well as the possibility that nobody has even thought of the correct explanation yet.

Perhaps another way to say it is that the fundamental dynamics that could cause the process of cosmic inflation remain unknown. (Wikipedia: "The detailed particle physics mechanism responsible for inflation is unknown... Various inflation theories have been proposed that make radically different predictions".) It would be nice to have some tests of such fundamental dynamics that could provide 'direct' evidence for or against cosmic inflation. However, as such dynamics necessarily combines quantum mechanics and general relativity, this is easier said than done. (Searches for low-multipole B-mode polarization in the cosmic microwave background seem the most promising path for the foreseeable future.) In this sense cosmic inflation is a bit like string theory---the best-developed proposal for a quantum theory of gravity, but one that has its critics, as I suspect y'all are already aware.

For my part, cosmic inflation is quite far from what I work on, so I'm not too invested in it. It certainly seems like the best story we've come up with so far (so you could say I 'accept the basic inflationary paradigm'), but I wouldn't be too shocked if someday it were superseded by something else.

jul. 17, 2018, 9:44 pm

>6 daschaich: I remember hearing Geoffrey Burbidge refer to cosmic inflation as “rubbish” back around 1989, presumably because of the indirectness of the evidence you mention as well as his own aversion to the Big Bang theory.

That Variable Speed of Light proposal in the “Horizon Problem” Wikipedia article caught my eye. I used to wonder whether such variations were possible. It was just an idle fancy on my part since I haven’t the competence to think about such a thing seriously. Interesting that some physicists have given it a whirl.

Thanks for the good info!

Editat: jul. 18, 2018, 5:08 am

Dear Group,

Are there any ideas on how this cosmic expansion is "done"? Does this mean there is a 5th force, with its own field and particles? I have never seen any suggestions sofar.

Are there any experiments - both gedunken and physical - suggested?

Is Dark energy the same as the Cosmic expansion. Where does Dark Matter fit into all this?

I realize there are no DIS-provable theory's about these topics, but are there any "novice friendly" explanations suggestions?


jul. 18, 2018, 6:07 am

>8 guido47: There is a particle called the inflaton and it's associated field but how these fit into the Standard Model I don't think anyone knows. People have criticized Alan Guth, who came up with inflation theory, for just sticking something in to make things work. However it works very well and nobody has come up with anything better. There are variations on the theory. Andrei Linde has a process called continuous inflation where our Universe is just an inflationary bud from another universe and will in turn bud off yet more universes.

Editat: jul. 18, 2018, 4:03 pm

Or there could be lots of different types of inflatons all interacting with each other in lots of different ways. Inflation could be driven by small inflaton field values, or by large inflaton field values (or, presumably, by intermediate inflaton field values, though that is speculation on my part). There's slow-roll inflation, hilltop inflation, double well inflation, power law inflation, Higgs inflation, Coleman--Weinberg inflation, radion gauge inflation, twisted inflation, logamediate inflation, and hundreds of other proposals at a minimum. I'm not well-versed in the details, and just copied some names of out this article, which analyzes a restricted set of 193 possibilities that have only a single inflaton field.

It seems to be a matter of some debate how many (and which) of these possibilities lead to eternal inflation, which has become popular in the past decade or two as a way to justify the hypothetical existence of a multiverse.

Anyway, there are lots of ideas "how this cosmic expansion is done", and not enough observational data to single out any particular one as the correct one---though plenty of possibilities have been ruled out and shown to be incorrect over the years, most recently through experiments such as BICEP and Planck. This is the situation I tried to summarize in the fourth paragraph of my previous post ("Various inflation theories have been proposed that make radically different predictions").

An obvious way forward is to get more and better observational data. This is easier said than done, in part because the universe was opaque for its first roughly 380,000 years, so we can't use light to directly 'see' back to times before that. The prospect that seems most promising to me is to look for (and disentangle) imprints of earlier epochs on the light dating back to that time period 380,000 years after the big bang, which is what I referred to in my previous post as "Searches for low-multipole B-mode polarization in the cosmic microwave background".

It seems sensible to speculate that cosmic inflation might have some connection to the qualitatively similar ongoing process of accelerating expansion of the universe attributed to "dark energy", but this is also quite hard to prove or disprove conclusively. The Wikipedia article on dark energy has some brief caveats.

Dark matter is most likely (but not necessarily) unrelated to inflation or dark energy.

>7 dypaloh: I should note that observational data has improved enormously over the past 30 years. Matt Strassler (an excellent resource for novice-friendly discussions) has emphasized that "These measurements could have ruled out the possibility of cosmic inflation, but instead they are (so far) quite consistent with that possibility." So inflation should no longer be dismissed out of hand.

jul. 19, 2018, 10:33 am

Interesting posts - thanks daschaich. Here is a recent article that is on topic more or less.

Universe Expansion 'Detective Story' Examined with New Tool

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