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I always ask such stupid questions when I encounter someone knowledgeable about astronomy
Astrophysics blog - Cosmology - Page 2
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mantequilla
Turkey775 Posts
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Teoita
Italy12246 Posts
On September 30 2016 22:04 mantequilla wrote: if space is really quantized something can't go "outside" of the universe right? + Show Spoiler + I always ask such stupid questions when I encounter someone knowledgeable about astronomy You can't go outside of the "known" Universe no matter whether spacetime is quantized or not. The Big Bang happened roughly 14 billion years ago - that means that light, the fastest thing in existence, could only travel a distance of 14 billion light years. Anything farther than that can't be reached, or communicated with, in any way by things in our Universe, which means that as far as science is concerned it might as well not exist, since we can't observe it or test it in any way. Also, when talking about relativity you should never think of space and time as two separate things. The basic structure of our Universe (at least in GR) is a 4 dimensional "volume" which we call spacetime. | ||
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eonrulz
United Kingdom225 Posts
 I'll admit that all I know about Cosmology is the occasional seminar about either AMS or Pierre-Auger results (particularly the AMS-02 positron results, which are interesting), so that's still coming from an experiment particle physics view point! I'm interested though - if you don't like the idea of QFT, or abstract hypotheticals like string theory, do you think that GR on its own can lead to a GUT-style concept? Or do you not think its possible to unify the different forces and scales in the universe, without the use of techniques like renormalisation at least? And do you think we'll ever see B-mode polarisation? | ||
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Teoita
Italy12246 Posts
It's not that i dislike purely theoretical things on their own, just that i feel like they tend to be overly flashy if that makes sense. You read all these popular science things like "scientists discover that black holes lead to parallel universes!" or whatever, and really to me it just looks like a pretty formula that none will ever verify for who knows how long. I prefer when models actually predict testable stuff. In QFT, that would be something like the Higgs boson which was actually observed in a relatively short time after its conception. | ||
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eonrulz
United Kingdom225 Posts
Yeah, I can understand that - though partly, I feel like that comes down to bad scientific journalism more than the science itself. I'm just as disappointed in that as you are! And string/M/F/GUT theories lose me too, you're not alone there. Also, I chuckled at "relatively short time" for the Higgs - think it was put forward in 1964, or something like that? I mean, I guess in the scale of things it's short, and we are taking about cosmology here xD So - we've talked about string theory and stuff. Are there any equivalently ludicrous but unprovable theories in Astro/Cosmology, that you know of? I'm interested in what counts as out-there in your field! | ||
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Teoita
Italy12246 Posts
1) wtf is dark energy 2) a new kind of transient sources discovered in 2007, called fast radio bursts 3) the mechanism for jet launching in accreting objects, particularly black holes None of these is particularly out there per se, but they are either absolutely mysterious (the first two) or incredibly hard to test with observations (the last one) | ||
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Cascade
Australia5405 Posts
On September 30 2016 19:46 Teoita wrote: Yeah im not debating that the Standard Model is crap, it isn't, it's just that from my astrohysics perspective between dark matter, dark energy (which, if associated with vacuum energy, QFT gets horribly wrong), matter/anti matter asymmetries, neutrino oscillations etc QFT as it is right now seems either lacking (the standard model) or a clusterfuck (supersymmetry and other extensions of the standard model). Plus you can't just add gravity to the standard model because (or so i hear from my theoretical phsyicist firiends) it can't be renormalized like the other three forces can, you need to go beyond into string theory, loop quantum gravity or whatever, and those sure aren't predictive at all yet. Yep, I agree with all that, and your friends are right. I just find it more likely that they come up with a standard model extension that solves gravity normalisation (and maybe even predicts some data..). Is there any opening for including standard model in a GR formalism? Edit: sorry didn't see your replies. Left a draft on the phone overnight. And just to be clear for everyone else, let's not confound the QFT of the standard model (super well tested, predicts essentially everything except gravity) with (super-)strings (purely mathematical, doesn't predict anything). | ||
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Mordanis
United States893 Posts
The second law of Thermodynamics essentially says that a system will evolve to maximum entropy. This drops pretty much out of statistical mechanics, which says that entropy will increase because, with overwhelming probability, the system will be very near the most probable (highest multiplicity) state, or if it isn't it will move towards the most probable state. Take the classic case of a container with two chambers, one a vacuum and the other filled with brown gas, say bromine. Open up the valve between the chambers, and the gas diffuses rapidly. Why does the gas not return to the original chamber, or move entirely to the chamber that previously was empty? Because the probability of getting all atoms on one side (without doing work on the system to get it just so) is less than infinitesimal. So far this supports the argument that the 2nd law causes dark energy. But, that example fails to account for potentials. If you ionize the gas, and apply a strong voltage across the two chambers, clearly the positive ions will mostly end up in the negative chamber, and the electrons in the positive chamber. Again, it is possible (if improbable) that all of the electrons and positive ions could trade sides for a bit, but the odds of getting that configuration are tiny compared to what we expect. If an electrical potential can cause this sort of effect, the same should be true for gravitational potentials. I think this argument is fairly subtle, so I want to make sure I'm clear. The idea that the 2nd law means that everything will be more uniform in the future than now is inaccurate. The 2nd law means that the universe will evolve to the most likely configuration, which may seem dis-ordered (diffuse brown gas in both chambers) or ordered (electrons on one side) On a side note: I'm publishing a paper in the American Journal of Physics (it's not as interesting as this though) and taking the GRE this month. Good luck with the PhD! Edit: Posted at 2 a.m. and missed the easier example of a more ordered, higher entropy system. A jar with water and oil. Shake it up, and all the little bubbles of water and oil combine to form larger bubbles and eventually distinct layers. | ||
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SpecKROELLchen
Germany150 Posts
On October 01 2016 04:45 Teoita wrote: Depends, astrophysics is a lot more based on data than quantum gravity for obvious reasons. I'd say the least understood things in astrophysics right now are, in no particular order: 1) wtf is dark energy 2) a new kind of transient sources discovered in 2007, called fast radio bursts 3) the mechanism for jet launching in accreting objects, particularly black holes None of these is particularly out there per se, but they are either absolutely mysterious (the first two) or incredibly hard to test with observations (the last one) I am also an astrophysicist (finishing my PhD in about 2 month, less time for sc2 -_-). You should add dark matter (DM) here. About 20% of the scientists in astrophysics don't even believe in DM. Some of them argue that the standard model of cosmology has to be updated and that modified newton dynamics (MOND) might be the answer to the rotation curve problem (e.g. former professor, where i wrote my master thesis) So this is still highly debated and very complex. Because both sides argue on many different fronts. I am a little bit more neutral about this and try to understand both sides but i wanted to stress out how hot of a topic this still is. | ||
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Teoita
Italy12246 Posts
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SpecKROELLchen
Germany150 Posts
On October 03 2016 19:04 Teoita wrote: Oh ops i cant believe i forgot DM. You're 100% correct, except i thought the MOND community was actually much smaller than that. I think you are right. It also depends which university you are from. In Bonn it is highly discussed, in Garching or Munich not at all ;D. But i think it is also a difference of MOND believers and Standard model of cosmology disbeliever. But you are right the number might be significantly smaller. Btw. i have nothing to do with this and don't know much about either theories (just from basic courses). I am just a radioastronomer studying the interstellar medium and supernova driven shells. So i focus on the smaller scales :D | ||
Whitewing
United States7483 Posts
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mantequilla
Turkey775 Posts
https://www.reddit.com/r/askscience/comments/5dgwjl/does_the_universe_have_an_event_horizon/da5dbfe/ the big bang being a singularity always bothered me. can you say something about the correctness/validity of the above comment? | ||
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mantequilla
Turkey775 Posts
On September 30 2016 23:31 Teoita wrote: You can't go outside of the "known" Universe no matter whether spacetime is quantized or not. The Big Bang happened roughly 14 billion years ago - that means that light, the fastest thing in existence, could only travel a distance of 14 billion light years. Anything farther than that can't be reached, or communicated with, in any way by things in our Universe, which means that as far as science is concerned it might as well not exist, since we can't observe it or test it in any way. Also, when talking about relativity you should never think of space and time as two separate things. The basic structure of our Universe (at least in GR) is a 4 dimensional "volume" which we call spacetime. Is "dark flow" an in-universe effect or something "outside" the universe is related with it? | ||
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Teoita
Italy12246 Posts
The reddit comment is correct, minus a tiny detail about "dark energy" causing inflation. Unless dark energy has the incredibly strange property of being very very important at the start of the Universe, then being negligible, then becoming important again just around the time when we humans can observe it, then inflation was caused by something similar to dark energy, but unrelated to what we observe today. Then again, pretty much anything goes when it comes to dark energy because we really are completely clueless about it. The "singularity" at the start of the Universe comes from General Relativity breaking down at small scales (ie, very very very small times and spaces), so he is correct that it's a product of the (incorrect) math. Currently we do not know anything about any law of the Universe for scales smaller than the Planck scale (about 10^-35 meters) because we would need to tie relativity and quantum field theory, which hasn't been done fully yet. This means that the singularity at the start of the big bang is very likely to be a product of our flawed description for it, exactly like the singularity inside black holes. By dark flow i guess you mean the non-random peculiar velocity of matter that some people found a few years ago. That is still very poorly understood and it might not even exist in the first place; it is possible that it's caused by something tied to inflation (i wouldn't know how though), or something even more strange, but at this time we really don't know | ||
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mantequilla
Turkey775 Posts
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mantequilla
Turkey775 Posts
I found a book lying around the house, the Elegant Universe by Brian Greene. Since it is published on 1999, most of the knowledge in it is probably obsolete now. Do you think it might be worth sinking time into or should I grab a more recent book? | ||
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Teoita
Italy12246 Posts
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ninazerg
United States7291 Posts
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Cascade
Australia5405 Posts
On December 01 2016 13:36 ninazerg wrote: So, here's a weird question: if the universe is expanding faster than the speed of light, would we be able to observe distant objects such as quasars vanishing as they pass the light horizon? No. The last light that can arrive would be stretched out over the rest of time. So the light from the last (say) year of quasar that can reach us would spread out from now until infinity. The light would be increasingly red-shifted and faded as we travel away, but it'd never completely stop. The ants-on-balloon may or may not be useful here, but I'm sure you can stretch the analogue to cover this situation with some imagination. | ||
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