I find it very appropriate and appreciate that Prof. Sean can manipulate math models and equations to back up his topics and lectures. I love Neal Degrass Tyson as a speaker, representing the cosmos and being a great science communicator and advocate but he never busts out any math to back up his lectures which for me, is not as impressive as Prof. Sean's grasp of and display of the math that underpins and proves most or all of his videos topics and shows way more how and why we have the knowledge we have gained as a whole. And why the universe is the way it is. Bravo Sir.
Sean Carroll’s gently spoken manner plus his ability to explain this material is just the most wonderful way for us in the public to gain some insight into the beauty and wonder that is our universe. These videos are simply a gift to all.
Thanks Sean. I learn a little more every time I listen to your podcasts/lectures. Thanks for taking the time to educate and inform us. And thanks for referring to Schrödinger’s cat as awake or asleep. As a cat lover, that is so much easier to think about for me.
Sean Carroll is a superhero! Sir, you are the professor I wish I had and the professor I am so honored to have access to and could listen to you forever. What a mind and educator. Even when I don’t completely follow all the complexities, I find I learn something each time I listen. Your passion is infectious!
I am really impressed by your clear and professional presentation. I have the impression that I understand GR much more than e.g yesterday. Thank you very much indeed.
Denis Goddard Lol, I laughed at this. jsk, he explained in an earlier video in the Biggest Ideas in the Universe series. The last 2 Big Ideas were recorded before he got it formalized(cut).
Amazing episode! So satisfying when all the subjects from all the other videos come together and creates something new (knowledge), but still familiar (our universe!)
Many thanks again Professor Sean. I didn't get to see this until the Wed morning, but it was well worth waiting for, as always. And one of my personal favourite topics too. As an amateur astronomer, who happens to find himself living in a Universe, and wondering about that, I reckon that makes me an amateur Cosmologist as well !! Thank you.
Dropping a like at 1sec into the video, I was waiting for this topic. One of my favorite fields in all of science. And I know Dr Carroll will do it justice, having owned and read his fantastic book, " Spacetime and Geometry: An Introduction to General Relativity".
I fell asleep listening to this and dreamed that my former First Sergeant was actually my high school PE teacher, and that he happened to be very well-versed in cosmology for some reason
I had a major in the humanities, but took a Cosmology course in college. In retrospect, I value that one over any other and still have my notes, decades later. I know a lot has changed in the intervening decades, and have somewhat kept up with the field. I’m looking forward to this lecture with special interest.
21:00 I've heard you say a couple of times already that you don't like the balloon analogy, because space does not expand "into something". But, if you're living on the 2-dimesional surface of the balloon, it does not expand into another area either as the balloon grows. If you compare the radius of the balloon to our distance in time from the big bang (so you could call the entire thing "arearadius" or "areatime", just as we speak of "spacetime", the area is growing as the radius increases, just as our space grows as time progresses. I think that is a valid analogy, and by taking away one space dimension we get a three-dimensional "areatime" that we can at least comprehend with brains that are hardwired for imagining 3-dimensional constructs. I like that analogy especially for pointing out how little sense it makes to ask what was "before the big bang": In a balloon-like areatime this would be equivalent to asking, what's "inside the center"
Brilliant! The skill of the teacher has to be inversely proportional to the ability of the audience. I think I understood acoustic oscillations for the first time and where that CMB graph came from. What a privilege to have access to this great communicator. Making it simple is not easy. Many thanks
Inverse proportional would mean that teacher that has great knowledge of the subject and with a high pedagogical aptitude is unable to pass knowledge to a highly gifted student, but a teacher with low skills would be successful in bringing the same student to a high level of understanding the subject. But that is not quite right is it? The relation between teacher skill and student ability, in regards of successfully passing the knowledge can only be direct linear.
Ha. A Carl Sagan vid clip popped up on my youtube algorithm earlier today. Last night I watched a Freeman Dyson video & a Richard Feynman interview a couple days ago. I'm glad other people enjoying these as much as I am. Lol😹
Universe? Uni-verse? Hardly! It’s a novel at least; an epic, Opera, Trilogy, or Film series. And uni? Only one? Certainly multi is more likely, or at least a duet? So there is no universe, it’s a multopera! A Deutrilogy? A novel Multepic? C’mon, folks! Think outside the bun!
I appreciate Sean Carroll so much, I’m happy Joe Rogan had him on and introduced me to one of my favorite teachers. (Even though I’ve never taken an actual class, I learn so much from his videos)
Thank you Dr Carroll for that summary of the history of the universe based on temperature scale! Such a fantastic & understandable framework to connect all the major events! And the impact of dark matter on the temperature perturbations! Also fantastic!
A few questions. 1) Given the scale equations at 43:00, - For matter and radiation, H ~ 1/t - For vacuum, H ~ constant Can astronomical measurements see far enough in distance and time to observe H ~ 1/t? 2) Can you give a phenomenological explanation for the different scale equations (43:00) of the matter, radiation and vacuum domains of the expansion in terms of gravitation (the gravitational force being attractive), etc? 3) Vacuum energy is a quantum concept. Is it correct to say that quantum gravity is not needed for understanding the rate of expansion (except in the beginning of the universe), because the length scales of interest are much longer than the Planck distance? Thank you for these videos. They are interesting, informative and clear.
22:00 - "There is no center." Aren't we off-center from the CMB? Relative to the CMB, aren't we moving slowly toward Leo? (I've read that and heard that from a couple sources now.) That tells me there is a center identified with the big bang, and we're not there, not even from our own inertial frame.
I wonder if you can detect gravitational waves beyond/further back in time from the CMB. And will they be able to help you figure out what caused the perturbations?
I think Seans most impressive understanding to me is his stuff on the direction of time its a concept that i find very seductive yet ive never been able to wrap my mind around. Its in part though not wholly based on the fact that im not well versed at thermodynamics.
As someone here said Susskinds Lectures on relativity are really good. Ive watched all of em multiple times. The ones on cosmology too. In this channel theres some stuff ua-cam.com/video/saf-1OZrVh4/v-deo.html And Sean has a real good introductory to advanced book thats used in many universities to introduce the subject. Also recomend PBS spacetime. you need many sources to learn relativity. Its not easy subject.
22:20 ish, and forgive my ignorance here, but I understand that you are making the relativity point here, but if we were to be looking at the universe in its entirety, doesn't there by definition HAVE to be a center if everything is expanding at a constant rate as a whole. I may be missing something here, would love to be informed on this. Thanks for these videos, this series has been AMAZING!
Not every geometry needs to have a center, expansion or not. For example, there is no center point on the surface on a sphere, nor in a flat torus. Since the expansion is not an acceleration through space, there is no way to locally detect is as motion. If you were to define the universe in a more Newtonian way, then you might end up using a static background grid of space to plot out motion of galaxies and find an apparent center, but that grid is not physically real. You could just define a center into existence wherever you like just by choosing that grid. If you are taking into account that the space itself is expanding, and that the expansion looks the same no matter where you start looking, that apparent center goes away.
If you know the most basic cosmology 2d spacetime model youd not ask this question. You draw an x t coordinate axis. the x=constant lines are worldlines of galaxies (wlg )if you draw a t=0 vector it points between two of these (wlg) and the length of this vector is a(t) as t changes the distance changes equally between any two wlg that started at same distance and theres no spatial center to this. If you have an infinite forest of trees wheres the center. Nowhere. Wheres the center of an infinite plane Nowhere the is none or evrywhere every point can be center.
The fact that you can put those equations into words and vise versa off the top of your head speaks to your understanding of the subject matter. Pretty impressive. Thanks for the insights and not talking down to us. I find the mathamatics essential to understanding physics.
At 44:55, Sean writes: log(eˣ) = x From the context, the log's base is e (not 10) so it means: logₑ(eˣ) = x or ln(eˣ) = x Apparently, physicists assume *log* is a _natural_ logarithm (base e). However, engineers, calculators, and general convention all assume that *log* is a _common_ logarithm (base 10). That's a problem, isn't it? :-) Decades ago, I was taught that *log* has base 10 and *ln* has base e. The international standard *ISO 80000-2* (section 12 "Exponential and logarithmic functions") describes this notation: logₐ x : log to the base a of argument x. ln x = logₑ x (natural logarithm). lg x = log₁₀ x (decimal logarithm). lb x = log₂ x (binary logarithm). log x is used when the base does not need to be specified. log x shall not be used in place of ln x, lg x, lb x, or logₑ x, log₁₀ x, log₂ x. I recently adopted this standard a few weeks ago, which makes me the first person ever to do it! :-)
This one may be my favorite one yet!!! I actually feel like I understood everything he was saying. (As he said, cosmology is for simple minded people with short attention spans, hahaha!!!! :) ).
The exact math is a bit beyond me, but your accompanying narration does provide a nice overview and context to the underlying connections and principles
Fantastic! i like your calm way of of presenting. I hope you dont mind if i also use this particular video for meditation :-) (and i've also started studying maths and physics, it's a huge pleasure to be able to at least somewhat follow videos like this.)
The examples you spoke of about the Universe expanding are used to show how things further away are moving away faster, which was helpful at least for me.
Doc, great video and I loved the humor and irony…….. cosmology is easy and cosmologists only need a short attention span. However, the video is 1h 59 min long! Looking forward to finishing the series - thanks for all the good stuff.
A question: -- Is Einstein's cosmological constant == (same) as "dark energy" -- Why has over time the term "dark energy" replaced the original term "cosmological constant" -- Are the two terms identical or not -- and why Many thanks in advance
The "cosmological constant" is a factor in the equation, put in before there was any physics for it to correspond to. "Dark Energy" is the physics that was discovered in 1995, and it uses a non-zero value of the C.C. to describe its effects. It's like, "what's the difference between momentum in the lab and the symbol rho in the formula?" Another thing, Sean gave only the modern writing of Lambda. I believe Einstein's original C.C. was written on the other side of the equation. That is, do you interpret it as a fudge factor in the curvature of spacetime, or something _in_ spacetime that contributes to the total energy of a region? If you look at the Wikipedia page, you'll see that D.E. being the C.C. is just one possibility.
Physicists are lame anyway, they are practically mathematicians. No real urge to understand, and make sense of the fundamental stuff in this world. All they want is to calculate.
@@nafnist ? What exactly do You mean by "understand" ? And what group of people (if any) do You think are in pursuit of "understanding and making sense of the fundamental stuff in this world" ? Best regard.
nafnist There is no way to deeply understand the universe without the calculations. The quantification of the universe and its concepts is the only way to break through the biased lens of human perception and get a real glimpse at objective reality. I have a feeling you simply can’t handle the math and so you lash out against it altogether, like a child throwing a tantrum. Nice.
@@wavydaveyparker I mean, it will give you an average value, but it doesn't fit the curve perfectly. The actual curve looks more like an x^3 curve when you look at it - up, saddlepoint and then continues up. But I don't know how to tell Excel to calculate that. :o) Also: This is for ordinary episodes, not Q&A episodes.
@@wavydaveyparker No, you put in the episode number for _x_, and out pops the episode length. Actually, I managed to get a polynomial. t(s) = 1,81x^3-65,15x^2+808,31x+1124,4, where t (s) is episode length in seconds and x is episode number.
24:15 Could it be that the Universe is a finite, but very very very large sphere- that just _appears_ to be flat? Not unlike how we perceive the surface of the spherical Earth to be flat? But then, how might we be able to distinguish between this and a truly flat Universe?
At 43:25, do you use an initial condition when solving the Friedman equation to find that the universe starts expanding at all? So do you have to put that (the big bang) in, possibly even give the initial velocity a certain value? I mean I could imagine that otherwise, for a radiation and matter dominated universe, the expansion doesn't start at all because gravity keeps everything together. If you don't put that in as an initial condition, wouldn't the Friedman equation (and thus GR) predict the big bang, or make it inevitable, if expansion starts no matter what?
I left this on in the background while I was painting and now I have a strange urge to walk naked into my back yard and stare into the star strewn depths of an incomprehensibly vast and ancient universe, stare in breathless wonder and know with utter certainty that cosmology is so far beyond my ability to comprehend that I might as well be throwing twinkies into the sky to see if anything up there is close enough to poke with a stick. Ah well, plenty of content on UA-cam that will make me feel like a genius after watching it for ten minutes, plenty plenty. Actually I very much enjoyed the video and I will likely watch it again with my entire brain engaged.
Prof. Carroll, Thanks so much for making this series! I have a question and was wondering if folks in this field see any practical information to be gathered by looking at the CMB profile over time. If the maps we have represent a 2D projection of the inside of a sphere, then it would seem that every year we ought to be able to see 1 light year deeper into the boundary of recombination. However not sure what resolution span or depth is required to pick out any 3D variations of interest and if that is any function of d-rho (e.g., if ~10^11 light year circumference and 10^5 resolution that might suggest features to be noticed are 1M light years wide/deep). If it took ~1M years to notice any relevant T fluctuation in depth, then that doesn't seem practically useful. So is there any interest in plotting the T fluctuations visible over time? Thanks for any feedback or if you can touch on in the Q&A video.
Dr. Carroll- first of all, thank you again and again for this series. not only has it satisfied my curiosity on a level no science communicator has been able to, its helped keep me, and I'm sure so many others, sane during this ....interesting time. maybe I missed it, but aside from recombination happening at BB+380,000 years I was wondering if you could mention the times in relation to the distinct events in thermal history. how soon after the big bang did nucleosynthesis occur, for instance? thank you again.
AMAZING STUFF!!! I just found a multiverse in my bowl of quinoa. Will miracles never cease, including the miracle that miracles don't violate the law of cause and effect, and that people take seriously Sean Carroll's cosmological crappola.
Question for people who have read 'something deeply hidden', is it worth the read if i have already watched almost every one of Seans lectures on QM and spacetime emergence?
I taught this man everything he knows, and now I can't even get him to send me an autographed-copy of _The_ _Biggest_ _Ideas_ _in_ _the_ _Universe._ Back during his "Swiss Patent Clerk" days, I remember when he used to believe that Noether's Theorem had something to do with disproving the luminiferous aether and that "cosmology" was the study and application of beauty treatment. He was an okay student, and I'm very proud of him. I want that book now!
How exactly does expansion act on massive particles? If it stretches their wavelength, how does it know to stop at their rest-mass? Or do they keep themselves together by their own gravity at that point?
Maybe I'm missing something? but when we look at the most distant galaxies with large red shifts we are also looking back in time, so how do we know that space is expanding and causing the redshift rather than that things were moving apart faster in the early stages of the universe?
7:50 Sorry for the correction, the gaia mission and the billion stars is correct, but this is only about 1% of Milkyway. There is some evidence that the local group en.wikipedia.org/wiki/Local_Group may be a collapsing space... at least that the mass within the space is blue rather than red shifted.
Sean.. I promise to mention you when I receive the Nobel price for my work on the Inflationary theory. I plan to start working on it as soon as I retire from my current job in a few years. Thanks for another inspiring lecture.
Note #2: no center. How could there be no center? If the U started from a singularity wouldn’t all matter scatter uniformly from it. In other words I imagine it like some kind of an “explosion” which would leave some of the matter to be on the outer edge of the explosion and some going behind it (probably a little bit slower). I can imagine if that’s the case that the center could be tracked down by measuring the relative velocities between galaxies.
You can't do that, as the relative velocity bw galaxies depends on which galaxy you are in. Wherever you are the closer ones move away slower than the farther away ones.
I think imagining the start of universal expansion as a physical explosion is a limited analogy. That singularity is beyond/before the time where physics and intuition make sense. Something really different might happen at those really really high energy densities that we may be unable to predict when tracking backwards. It may be better to start imagining just after that moment, with an already centerless yet very dense U. Or, if you must take it back to a single "location", (even though there would have been nothing else to compare coordinates to define a location,) consider that all points of spacetime would have been at the same location in that singularity, with no one point being special. There is no outside rim if all the points are in the same place, or you could say every point is equally on the outside. In that light, you could say every point is equally the center of the universe, which I think is just another way to say there is no center.
@@michaelsommers2356 I like to think it as an infinite ruler. You can compress the notches infinitely close together (big bang) but you're still going to have an infinite ruler.
If you know the most basic cosmology 2d spacetime model youd not ask this question. You draw an x t coordinate axis. the x=constant lines are worldlines of galaxies (wlg )if you draw a t=0 vector it points between two of these (wlg) and the length of this vector is a(t) as t changes the distance changes equally between any two wlg that started at same distance and theres no spatial center to this. If you have an infinite forest of trees wheres the center. Nowhere Wheres the center of an infinite plane Nowhere the is none or everywhere every point can be center.
At the end of the video, you drew a graph of a(t) with its three phases. Could you quantify that graph? I'm asking because I was wondering whether the velocity of expansion was ever slower than c. I have problems envisioning looking at the sky at far distances when the expansion was slower than c such that photons can catch up with the expansion. For example, imagine a universe expanding with a constant velocity slower than c since the big bang. How would that picture about that backwards light cone would look like at 1:23:00, especially if the universe was not infinitely big?
There is no expansion of space "at some velocity", because relative velocities are proportional to distance. So expansion has units of velocity over distance, which is just time^(-1). Likewise it is meaningless to talk about "space that expanded slower than c".
I am very grateful that you've taken the time (significant amount) to do all these and answer questions is very commendable to say the least. I wish I had an opportunity to meet and learn from, no, exchange ideas with you. I sent you a invite on Linkedin.
Vacuum has energy, therefore our expanding universe is creating infinite amount of energy. Why? Because, if our universe is not embedded in a larger space and is expanding all by itself then it is creating infinite volume of vacuum for a long long time, perhaps a google years or longer. Does this sound right ?
Thanks professor. You talk about the observable universe and assessing a finite size on it. However you talk about infinite universe when you talk about a flat/negatively curved universe. You also mention that "we know how much stuff is in the universe". You also talk about "scaling infinite numbers - and the result being infinite". All these statements seems to be in conflict with each other (to a rookie - such as myself).
@ 49:20 (ua-cam.com/video/tZQadPmTd84/v-deo.html), Sean mentions that when two photons join to form an electron / positron pair, the pair is relativistic (moving close to the speed of light). I have a question: how is momentum conserved here, where photons (massless particles) create fast moving, massive anti/particles -> it seems momentum would increase. Is it that positrons have negative momentum? Thanks.
It's like your shirt is becoming one with the deep field 😂 I'm so glad I found this channel! The way you explain the Universe is very mind opening, and I really enjoy your lectures on time.
As someone whose loved physics my entire life yet never pursued an education or career in that direction it’s greatly appreciated you take the time to educate us. Shows how passionate you are for this. Thank you.
1:20:00 I don't like the term "make fun of", and I don't see why anybody whose ever been on the receiving end of a bully would. I always replace it with "ridicule", to promote a more accurate connotation. But, in the case of _recombination_ , I would have to go with "make light of". (Note: That's a pun, if you don't realize)
When Carroll speaks about "rapidly moving particles", in what rest frame is that measured? Relativity theory does away with the "fixed stars" of Newton.
When you state that the universe is expanding, do you mean that the scale and geometry (metric tensors) are expanding? - If any two points in space are moving apart, does it mean that objects are getting bigger and less dense? For example, are our bodies expanding, even by a very minuscule (unmeasurable) amount? Or is the physics at the local scale affected by the matter we see and not by the increasing scale of space? - Is "chemistry" changing? In other words, in principle, are the interactions (the model parameters) at the energy scale of 1 - 10eV changing or is the available energy to make chemical reactions just changing because the university is cooling?
Good point. The fine structure constant is independent of the scale of space (it does not reduce when the scale increases). Is that correct? H ~ 2 x 10^-18 m/s at 1 meter.
Is there a coloration between how much the universe expands and how much matter and spacetime black holes consumes? Could black holes be the reason why space is expanding? Since the biggest black holes are in the centers of galaxies and they are fundamentally the rulers of those galaxies. Meaning the galaxies follow the path of the black hole, like our solar system follows our sun.
I have a question: I believe that you said that energy is not conserved when we look at the entire universe, because of dark energy, and in fact energy is increasing as the universe expands. I have been watching Alan Guth's MIT Open Course lectures on cosmology, and he says that the increase due to expansion is offset by negative energy in the form of increased gravitation in the expanding space, and that the two balance out to preserve conservation of energy. Am I misunderstanding something here?
Leavitt wasn't using parallax to measure distances to Cepheid variables. She was cataloging stars in the Large Magellanic Cloud, and observed the relationship in the Cepheid variables there. Since all the stars in the Large Magellanic Cloud are all roughly the same distance away, she observed the relationship between period and APPARENT luminosity.
Is it accurate to say that the motion of the expansion is not due to any force or acceleration? It doesn't make sense to get a reading on a perfect accelerometer, yet there is clearly a d2/dx2 x.
22:02 I like to think that EVERY point is the "center" of the Universe. Under this perspective, the Big Bang happened everywhere. Also, under this perspective, each person can be said to be "The Center of the Universe."
overlaps---> gaps and symmetries, eqn to predict symmetry breaking outcome, even for spontaneous symm breaking, outcome can already be know, like kicking an acorn, or a [ine cone, and knowing which way each vertex will point before even kicking it. like called cards and dice, translated into book words, translated into heard convos in public domain while flipping pages in a boiok synched.
Question for Q&A: Regarding the Cosmic Microwave Background radiation, can you get cosmic background gravitational waves from the Big Bang that are even older than the CMB? As old as the Big Bang itself, and from this, get more insight of the early universe, dark matter and dark energy?
I have a question about time. The duration / time from the begining of the Universe, isn't that a universal constant which started with time zero and is the same everywhere in the universe. And then the relativistic time is like small "branches" of the universal time, depending on gravitational or motion local conditions. Is it reasonable to believe that we can use this "universal time" as a reference and then we can then measure all the relative times, relative to this "unique time, since the universe came to existence"?
I love how Sean is just a bust floating in space. Like a Boltzmann Bust
Two years later and I still love coming back onto these. ❤
We can come back 5 years or 10 years and still be amazed.
I find it very appropriate and appreciate that Prof. Sean can manipulate math models and equations to back up his topics and lectures. I love Neal Degrass Tyson as a speaker, representing the cosmos and being a great science communicator and advocate but he never busts out any math to back up his lectures which for me, is not as impressive as Prof. Sean's grasp of and display of the math that underpins and proves most or all of his videos topics and shows way more how and why we have the knowledge we have gained as a whole. And why the universe is the way it is. Bravo Sir.
Sean Carroll’s gently spoken manner plus his ability to explain this material is just the most wonderful way for us in the public to gain some insight into the beauty and wonder that is our universe. These videos are simply a gift to all.
It's just amazing how knowledgeable Sean Carroll is and how well he presents complex stuff in an understandable way.
One of the greatest communicators and ambassador of physics and cosmology since Carl Sagan.
I read Sean's books.
I listen to Sean's podcasts.
I watch Sean's videos.
I spend a lot of time with Sean... and I love it.
He’s awesome, his books are something special....
Isn't it weird to have a friend who knows nothing about you? lol
You’re not alone ;)
@@Psnym Wouldn't it be "he's not alone"?
Jeff Bass *we’re* not alone!
Thanks Sean. I learn a little more every time I listen to your podcasts/lectures. Thanks for taking the time to educate and inform us. And thanks for referring to Schrödinger’s cat as awake or asleep. As a cat lover, that is so much easier to think about for me.
Sean Carroll is a superhero! Sir, you are the professor I wish I had and the professor I am so honored to have access to and could listen to you forever. What a mind and educator. Even when I don’t completely follow all the complexities, I find I learn something each time I listen. Your passion is infectious!
I am really impressed by your clear and professional presentation. I have the impression that I understand GR much more than e.g yesterday. Thank you very much indeed.
Sean..Your content will be counted as a treasure as this playlist matures ❤❤
Sean: Cosmological Principle... homogeneous and isotopic...
Me: DUDE! Did you finally get a haircut?!
Denis Goddard Lol, I laughed at this. jsk, he explained in an earlier video in the Biggest Ideas in the Universe series. The last 2 Big Ideas were recorded before he got it formalized(cut).
@@adhdasian1896 renormalized
@@Psnym The Cosmological constant is a well known fixture of the Universe.
Fascinating,Sean keep it coming.
Amazing episode! So satisfying when all the subjects from all the other videos come together and creates something new (knowledge), but still familiar (our universe!)
Many thanks again Professor Sean. I didn't get to see this until the Wed morning, but it was well worth waiting for, as always. And one of my personal favourite topics too.
As an amateur astronomer, who happens to find himself living in a Universe, and wondering about that, I reckon that makes me an amateur Cosmologist as well !! Thank you.
Absolutely fantastic summary! Congratulations. Never stop giving these lectures.
I love this series! Half of what he says goes way over my head. The other half goes way, way over my head.
Sean you have been very very productive lately. I think that about 60% of my online listening is content of yours.
Exciting!!!
Prove it.
Same for me.
sundial is only possible on a flat earth. They have lied to us.
And 40% is pornhub
"The Best Content"
Amazing series going on
Thank you so much Prof.Carroll
Dropping a like at 1sec into the video, I was waiting for this topic. One of my favorite fields in all of science. And I know Dr Carroll will do it justice, having owned and read his fantastic book, "
Spacetime and Geometry: An Introduction to General Relativity".
I fell asleep listening to this and dreamed that my former First Sergeant was actually my high school PE teacher, and that he happened to be very well-versed in cosmology for some reason
Made up. It didn't even sound cool either...
@@josephhall5681 hello there little troll, aren't you cute...
Lol.
Well Sean Carrol does have quite the soothing voice.
I fall asleep listening to these every night for a month Atleast lok
I had a major in the humanities, but took a Cosmology course in college. In retrospect, I value that one over any other and still have my notes, decades later. I know a lot has changed in the intervening decades, and have somewhat kept up with the field. I’m looking forward to this lecture with special interest.
Thanks for taking the time to do this. I've really learned a lot.
nice hippo
yeah. it looks like a really really good hippo
How have I not found this channel until now?! Brilliant content, sir! 😊👍
21:00 I've heard you say a couple of times already that you don't like the balloon analogy, because space does not expand "into something". But, if you're living on the 2-dimesional surface of the balloon, it does not expand into another area either as the balloon grows. If you compare the radius of the balloon to our distance in time from the big bang (so you could call the entire thing "arearadius" or "areatime", just as we speak of "spacetime", the area is growing as the radius increases, just as our space grows as time progresses. I think that is a valid analogy, and by taking away one space dimension we get a three-dimensional "areatime" that we can at least comprehend with brains that are hardwired for imagining 3-dimensional constructs. I like that analogy especially for pointing out how little sense it makes to ask what was "before the big bang": In a balloon-like areatime this would be equivalent to asking, what's "inside the center"
good point
i like the balloon analogy also. flatland but not flat
Brilliant! The skill of the teacher has to be inversely proportional to the ability of the audience.
I think I understood acoustic oscillations for the first time and where that CMB graph came from.
What a privilege to have access to this great communicator. Making it simple is not easy.
Many thanks
Excellent comment
Making it simple is not easy......
Copyright that
Or as Steve Jobs puts it: Simplicity is the ultimate sophistication 😉
Inverse proportional would mean that teacher that has great knowledge of the subject and with a high pedagogical aptitude is unable to pass knowledge to a highly gifted student, but a teacher with low skills would be successful in bringing the same student to a high level of understanding the subject.
But that is not quite right is it?
The relation between teacher skill and student ability, in regards of successfully passing the knowledge can only be direct linear.
Thank you so much for this amazing series. Are you going to make videos about "String Theory" and/or "Loop Quantum Gravity"?
"We're made of star stuff. We are a way for the cosmos to know itself"
- Carl Sagan
"Our universe is the ultimate spherical cow"
- Sean Carroll
LOL!
Ha. A Carl Sagan vid clip popped up on my youtube algorithm earlier today. Last night I watched a Freeman Dyson video & a Richard Feynman interview a couple days ago. I'm glad other people enjoying these as much as I am. Lol😹
Universe? Uni-verse? Hardly! It’s a novel at least; an epic, Opera, Trilogy, or Film series. And uni? Only one? Certainly multi is more likely, or at least a duet? So there is no universe, it’s a multopera! A Deutrilogy? A novel Multepic?
C’mon, folks! Think outside the bun!
I appreciate Sean Carroll so much, I’m happy Joe Rogan had him on and introduced me to one of my favorite teachers. (Even though I’ve never taken an actual class, I learn so much from his videos)
Thank you Dr Carroll for that summary of the history of the universe based on temperature scale! Such a fantastic & understandable framework to connect all the major events! And the impact of dark matter on the temperature perturbations! Also fantastic!
Brilliant, very lucid and balanced exposition.
The universe must be energy, because it doesn’t matter
This is a truly great series because of so many interesting topics. Are Axions a possible source of dark matter in the early universe?
Thank you very much for taking the time to produce these outstanding videos.
A few questions.
1) Given the scale equations at 43:00,
- For matter and radiation, H ~ 1/t
- For vacuum, H ~ constant
Can astronomical measurements see far enough in distance and time to observe H ~ 1/t?
2) Can you give a phenomenological explanation for the different scale equations (43:00) of the matter, radiation and vacuum domains of the expansion in terms of gravitation (the gravitational force being attractive), etc?
3) Vacuum energy is a quantum concept. Is it correct to say that quantum gravity is not needed for understanding the rate of expansion (except in the beginning of the universe), because the length scales of interest are much longer than the Planck distance?
Thank you for these videos. They are interesting, informative and clear.
0:49 My new favourite phrase is "the universe is the ultimate spherical cow."
22:00 - "There is no center." Aren't we off-center from the CMB? Relative to the CMB, aren't we moving slowly toward Leo? (I've read that and heard that from a couple sources now.) That tells me there is a center identified with the big bang, and we're not there, not even from our own inertial frame.
not a center so much as a rest frame
I wonder if you can detect gravitational waves beyond/further back in time from the CMB. And will they be able to help you figure out what caused the perturbations?
yes for sure we will be able to do this
An excellent speaker and so so sharp. Your intelligence fascinates me sir.
I think Seans most impressive understanding to me is his stuff on the direction of time
its a concept that i find very seductive yet ive never been able to wrap my mind around.
Its in part though not wholly based on the fact that im not well versed at thermodynamics.
Sean Carroll, the Bob Ross of science
Prof. Sean Carroll isn't there going to be any video on Relativity? By the way, love your videos
Look up the Gravity episode. Lots of stuff about general relativity there.
As someone here said Susskinds Lectures on relativity are really good. Ive watched all of em multiple times. The ones on cosmology too. In this channel theres some stuff ua-cam.com/video/saf-1OZrVh4/v-deo.html
And Sean has a real good introductory to advanced book thats used in many universities to introduce the subject. Also recomend PBS spacetime. you need many sources to learn relativity. Its not easy subject.
22:20 ish, and forgive my ignorance here, but I understand that you are making the relativity point here, but if we were to be looking at the universe in its entirety, doesn't there by definition HAVE to be a center if everything is expanding at a constant rate as a whole. I may be missing something here, would love to be informed on this. Thanks for these videos, this series has been AMAZING!
Not every geometry needs to have a center, expansion or not. For example, there is no center point on the surface on a sphere, nor in a flat torus. Since the expansion is not an acceleration through space, there is no way to locally detect is as motion. If you were to define the universe in a more Newtonian way, then you might end up using a static background grid of space to plot out motion of galaxies and find an apparent center, but that grid is not physically real. You could just define a center into existence wherever you like just by choosing that grid. If you are taking into account that the space itself is expanding, and that the expansion looks the same no matter where you start looking, that apparent center goes away.
Ben Marolt thank you!
If you know the most basic cosmology 2d spacetime model youd not ask this question. You draw an x t coordinate axis. the x=constant lines are worldlines of galaxies (wlg )if you draw a t=0 vector it points between two of these (wlg) and the length of this vector is a(t) as t changes the distance changes equally between any two wlg that started at same distance and theres no spatial center to this. If you have an infinite forest of trees wheres the center. Nowhere. Wheres the center of an infinite plane Nowhere the is none or evrywhere every point can be center.
1:56:40 Should not the yellow inflation curve be steeper than the regular curve (since the scale factor grows much faster during inflation)?
I’m just waiting for a new series of lectures, Seannnnnnn
What an absolutely wonderful lecture. er, I mean Video.
"Lecture in disguise"
The fact that you can put those equations into words and vise versa off the top of your head speaks to your understanding of the subject matter. Pretty impressive. Thanks for the insights and not talking down to us. I find the mathamatics essential to understanding physics.
At 44:55, Sean writes:
log(eˣ) = x
From the context, the log's base is e (not 10) so it means:
logₑ(eˣ) = x
or
ln(eˣ) = x
Apparently, physicists assume *log* is a _natural_ logarithm (base e). However, engineers, calculators, and general convention all assume that *log* is a _common_ logarithm (base 10). That's a problem, isn't it? :-) Decades ago, I was taught that *log* has base 10 and *ln* has base e.
The international standard *ISO 80000-2* (section 12 "Exponential and logarithmic functions") describes this notation:
logₐ x : log to the base a of argument x.
ln x = logₑ x (natural logarithm).
lg x = log₁₀ x (decimal logarithm).
lb x = log₂ x (binary logarithm).
log x is used when the base does not need to be specified.
log x shall not be used in place of ln x, lg x, lb x, or logₑ x, log₁₀ x, log₂ x.
I recently adopted this standard a few weeks ago, which makes me the first person ever to do it! :-)
This one may be my favorite one yet!!! I actually feel like I understood everything he was saying. (As he said, cosmology is for simple minded people with short attention spans, hahaha!!!! :) ).
The exact math is a bit beyond me, but your accompanying narration does provide a nice overview and context to the underlying connections and principles
Q: Does the mass of particles also vary with the temperature of the universe? 1:09:30
Fantastic! i like your calm way of of presenting. I hope you dont mind if i also use this particular video for meditation :-)
(and i've also started studying maths and physics, it's a huge pleasure to be able to at least somewhat follow videos like this.)
The examples you spoke of about the Universe expanding are used to show how things further away are moving away faster, which was helpful at least for me.
Doc, great video and I loved the humor and irony…….. cosmology is easy and cosmologists only need a short attention span. However, the video is 1h 59 min long! Looking forward to finishing the series - thanks for all the good stuff.
A question:
-- Is Einstein's cosmological constant == (same) as "dark energy"
-- Why has over time the term "dark energy" replaced the original term "cosmological constant"
-- Are the two terms identical or not -- and why
Many thanks in advance
The "cosmological constant" is a factor in the equation, put in before there was any physics for it to correspond to. "Dark Energy" is the physics that was discovered in 1995, and it uses a non-zero value of the C.C. to describe its effects.
It's like, "what's the difference between momentum in the lab and the symbol rho in the formula?"
Another thing, Sean gave only the modern writing of Lambda. I believe Einstein's original C.C. was written on the other side of the equation. That is, do you interpret it as a fudge factor in the curvature of spacetime, or something _in_ spacetime that contributes to the total energy of a region?
If you look at the Wikipedia page, you'll see that D.E. being the C.C. is just one possibility.
Carroll's Theorem: All parallelograms tilt to the right.
That way they look like Tennessee.
@@beenaplumber8379 uuhb bb ubbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbububybbbbbbbbbybubbbbbbbbnbnbbbbbnbnrnnnnnnnnnn
@@beenaplumber8379 uuhb bb ubbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbububybbbbbbbbbybubbbbbbbbnbnbbbbbnbnrnnnnnnnnnn
Both you and they are right
I hope we all get theoretical physics degrees at the end of this series.
Only if you do the associated mathematics.
Physicists are lame anyway, they are practically mathematicians.
No real urge to understand, and make sense of the fundamental stuff in this world.
All they want is to calculate.
@@nafnist ?
What exactly do You mean by "understand" ?
And what group of people (if any) do You think are in pursuit of "understanding and making sense of the fundamental stuff in this world" ?
Best regard.
@@mrdr9534 Shut up and calculate
nafnist There is no way to deeply understand the universe without the calculations. The quantification of the universe and its concepts is the only way to break through the biased lens of human perception and get a real glimpse at objective reality. I have a feeling you simply can’t handle the math and so you lash out against it altogether, like a child throwing a tantrum. Nice.
The length of an episode (in seconds) is given by kx+m, where is 161, x is the episode number and m is 2579 (least squares method).
@@wavydaveyparker I mean, it will give you an average value, but it doesn't fit the curve perfectly. The actual curve looks more like an x^3 curve when you look at it - up, saddlepoint and then continues up. But I don't know how to tell Excel to calculate that. :o)
Also: This is for ordinary episodes, not Q&A episodes.
@@wavydaveyparker No, you put in the episode number for _x_, and out pops the episode length.
Actually, I managed to get a polynomial. t(s) = 1,81x^3-65,15x^2+808,31x+1124,4, where t (s) is episode length in seconds and x is episode number.
24:15 Could it be that the Universe is a finite, but very very very large sphere- that just _appears_ to be flat? Not unlike how we perceive the surface of the spherical Earth to be flat?
But then, how might we be able to distinguish between this and a truly flat Universe?
It's possible but we havent found any direct evidence of a curved universe.
At 43:25, do you use an initial condition when solving the Friedman equation to find that the universe starts expanding at all? So do you have to put that (the big bang) in, possibly even give the initial velocity a certain value? I mean I could imagine that otherwise, for a radiation and matter dominated universe, the expansion doesn't start at all because gravity keeps everything together. If you don't put that in as an initial condition, wouldn't the Friedman equation (and thus GR) predict the big bang, or make it inevitable, if expansion starts no matter what?
wonderful ideas are sprinkling at us from such a great mind of our time........
Thanks Sean for the making of this videos!
Fantastic stuff Sean...thank you!
I left this on in the background while I was painting and now I have a strange urge to walk naked into my back yard and stare into the star strewn depths of an incomprehensibly vast and ancient universe, stare in breathless wonder and know with utter certainty that cosmology is so far beyond my ability to comprehend that I might as well be throwing twinkies into the sky to see if anything up there is close enough to poke with a stick. Ah well, plenty of content on UA-cam that will make me feel like a genius after watching it for ten minutes, plenty plenty. Actually I very much enjoyed the video and I will likely watch it again with my entire brain engaged.
Prof. Carroll,
Thanks so much for making this series! I have a question and was wondering if folks in this field see any practical information to be gathered by looking at the CMB profile over time. If the maps we have represent a 2D projection of the inside of a sphere, then it would seem that every year we ought to be able to see 1 light year deeper into the boundary of recombination. However not sure what resolution span or depth is required to pick out any 3D variations of interest and if that is any function of d-rho (e.g., if ~10^11 light year circumference and 10^5 resolution that might suggest features to be noticed are 1M light years wide/deep). If it took ~1M years to notice any relevant T fluctuation in depth, then that doesn't seem practically useful. So is there any interest in plotting the T fluctuations visible over time? Thanks for any feedback or if you can touch on in the Q&A video.
Dr. Carroll- first of all, thank you again and again for this series. not only has it satisfied my curiosity on a level no science communicator has been able to, its helped keep me, and I'm sure so many others, sane during this ....interesting time.
maybe I missed it, but aside from recombination happening at BB+380,000 years I was wondering if you could mention the times in relation to the distinct events in thermal history. how soon after the big bang did nucleosynthesis occur, for instance?
thank you again.
AMAZING STUFF!!! I just found a multiverse in my bowl of quinoa. Will miracles never cease, including the miracle that miracles don't violate the law of cause and effect, and that people take seriously Sean Carroll's cosmological crappola.
Question for people who have read 'something deeply hidden', is it worth the read if i have already watched almost every one of Seans lectures on QM and spacetime emergence?
I taught this man everything he knows, and now I can't even get him to send me an autographed-copy of _The_ _Biggest_ _Ideas_ _in_ _the_ _Universe._ Back during his "Swiss Patent Clerk" days, I remember when he used to believe that Noether's Theorem had something to do with disproving the luminiferous aether and that "cosmology" was the study and application of beauty treatment. He was an okay student, and I'm very proud of him. I want that book now!
Hmm, perhaps the limit as ego goes to infinity sets the boundary condition for book transmission?
How exactly does expansion act on massive particles? If it stretches their wavelength, how does it know to stop at their rest-mass? Or do they keep themselves together by their own gravity at that point?
Maybe I'm missing something? but when we look at the most distant galaxies with large red shifts we are also looking back in time, so how do we know that space is expanding and causing the redshift rather than that things were moving apart faster in the early stages of the universe?
i think... space HAS to be expanding if distant galaxies are moving away from each other. Otherwise there's nowhere for them to go
7:50 Sorry for the correction, the gaia mission and the billion stars is correct, but this is only about 1% of Milkyway. There is some evidence that the local group en.wikipedia.org/wiki/Local_Group may be a collapsing space... at least that the mass within the space is blue rather than red shifted.
@@michaelsommers2356 Give it time
"We're gonna predict it, we're gonna get it wrong, and we're gonna fix it". 1:31:05 Classic!
Sean.. I promise to mention you when I receive the Nobel price for my work on the Inflationary theory. I plan to start working on it as soon as I retire from my current job in a few years. Thanks for another inspiring lecture.
At 43:25, does dark matter just behave like matter or radiation, depending on its velocity? Or is there a new relation between a and t for it?
Note #2: no center. How could there be no center? If the U started from a singularity wouldn’t all matter scatter uniformly from it. In other words I imagine it like some kind of an “explosion” which would leave some of the matter to be on the outer edge of the explosion and some going behind it (probably a little bit slower). I can imagine if that’s the case that the center could be tracked down by measuring the relative velocities between galaxies.
You can't do that, as the relative velocity bw galaxies depends on which galaxy you are in. Wherever you are the closer ones move away slower than the farther away ones.
I think imagining the start of universal expansion as a physical explosion is a limited analogy. That singularity is beyond/before the time where physics and intuition make sense. Something really different might happen at those really really high energy densities that we may be unable to predict when tracking backwards. It may be better to start imagining just after that moment, with an already centerless yet very dense U. Or, if you must take it back to a single "location", (even though there would have been nothing else to compare coordinates to define a location,) consider that all points of spacetime would have been at the same location in that singularity, with no one point being special. There is no outside rim if all the points are in the same place, or you could say every point is equally on the outside. In that light, you could say every point is equally the center of the universe, which I think is just another way to say there is no center.
@@michaelsommers2356 I like to think it as an infinite ruler. You can compress the notches infinitely close together (big bang) but you're still going to have an infinite ruler.
If you know the most basic cosmology 2d spacetime model youd not ask this question. You draw an x t coordinate axis. the x=constant lines are worldlines of galaxies (wlg )if you draw a t=0 vector it points between two of these (wlg) and the length of this vector is a(t) as t changes the distance changes equally between any two wlg that started at same distance and theres no spatial center to this. If you have an infinite forest of trees wheres the center. Nowhere
Wheres the center of an infinite plane Nowhere the is none or everywhere every point can be center.
At the end of the video, you drew a graph of a(t) with its three phases. Could you quantify that graph? I'm asking because I was wondering whether the velocity of expansion was ever slower than c. I have problems envisioning looking at the sky at far distances when the expansion was slower than c such that photons can catch up with the expansion. For example, imagine a universe expanding with a constant velocity slower than c since the big bang. How would that picture about that backwards light cone would look like at 1:23:00, especially if the universe was not infinitely big?
There is no expansion of space "at some velocity", because relative velocities are proportional to distance. So expansion has units of velocity over distance, which is just time^(-1). Likewise it is meaningless to talk about "space that expanded slower than c".
This was really a good video. Thank you so much.
Is there a time dilation or Lorentz contraction for objects at great distances, as there is for objects traveling at great difference of speed?
This connection between dark matter and the scale deviations of the CMB.... a dynamite explanation. 1:46:38
I am very grateful that you've taken the time (significant amount) to do all these and answer questions is very commendable to say the least. I wish I had an opportunity to meet and learn from, no, exchange ideas with you. I sent you a invite on Linkedin.
Vacuum has energy, therefore our expanding universe is creating infinite amount of energy. Why? Because, if our universe is not embedded in a larger space and is expanding all by itself then it is creating infinite volume of vacuum for a long long time, perhaps a google years or longer. Does this sound right ?
Dont know what to tell you cuz as I understand it from something I read energy conservation can be violated in general relativity
Why Rotation & Revolution are universal processes across scales (fractal property) in the universe? Can we extrapolate that to the Universe itself?
Thanks professor. You talk about the observable universe and assessing a finite size on it. However you talk about infinite universe when you talk about a flat/negatively curved universe. You also mention that "we know how much stuff is in the universe". You also talk about "scaling infinite numbers - and the result being infinite". All these statements seems to be in conflict with each other (to a rookie - such as myself).
@ 49:20 (ua-cam.com/video/tZQadPmTd84/v-deo.html), Sean mentions that when two photons join to form an electron / positron pair, the pair is relativistic (moving close to the speed of light). I have a question: how is momentum conserved here, where photons (massless particles) create fast moving, massive anti/particles -> it seems momentum would increase. Is it that positrons have negative momentum? Thanks.
It's like your shirt is becoming one with the deep field 😂 I'm so glad I found this channel! The way you explain the Universe is very mind opening, and I really enjoy your lectures on time.
As someone whose loved physics my entire life yet never pursued an education or career in that direction it’s greatly appreciated you take the time to educate us. Shows how passionate you are for this. Thank you.
Related to cosmology, do you have any thoughts on redshift quantization, AKA redshift periodicity? Is it real, and what might explain it?
I love your stuff ~ pity about the multiverser..Everett Song
1:20:00 I don't like the term "make fun of", and I don't see why anybody whose ever been on the receiving end of a bully would. I always replace it with "ridicule", to promote a more accurate connotation. But, in the case of _recombination_ , I would have to go with "make light of". (Note: That's a pun, if you don't realize)
When Carroll speaks about "rapidly moving particles", in what rest frame is that measured? Relativity theory does away with the "fixed stars" of Newton.
The observer.
Thanks for a wonderful presenta. Why is the model of background rather is shaped like a rugby ba okll. It surrounds use doesn't it?
Background radiation
When you state that the universe is expanding, do you mean that the scale and geometry (metric tensors) are expanding?
- If any two points in space are moving apart, does it mean that objects are getting bigger and less dense? For example, are our bodies expanding, even by a very minuscule (unmeasurable) amount? Or is the physics at the local scale affected by the matter we see and not by the increasing scale of space?
- Is "chemistry" changing? In other words, in principle, are the interactions (the model parameters) at the energy scale of 1 - 10eV changing or is the available energy to make chemical reactions just changing because the university is cooling?
Good point. The fine structure constant is independent of the scale of space (it does not reduce when the scale increases). Is that correct?
H ~ 2 x 10^-18 m/s at 1 meter.
Amazing content. Much appreciated.
Yeah man i love cosmology and math
Is there a coloration between how much the universe expands and how much matter and spacetime black holes consumes? Could black holes be the reason why space is expanding?
Since the biggest black holes are in the centers of galaxies and they are fundamentally the rulers of those galaxies. Meaning the galaxies follow the path of the black hole, like our solar system follows our sun.
I have a question: I believe that you said that energy is not conserved when we look at the entire universe, because of dark energy, and in fact energy is increasing as the universe expands. I have been watching Alan Guth's MIT Open Course lectures on cosmology, and he says that the increase due to expansion is offset by negative energy in the form of increased gravitation in the expanding space, and that the two balance out to preserve conservation of energy. Am I misunderstanding something here?
Leavitt wasn't using parallax to measure distances to Cepheid variables. She was cataloging stars in the Large Magellanic Cloud, and observed the relationship in the Cepheid variables there. Since all the stars in the Large Magellanic Cloud are all roughly the same distance away, she observed the relationship between period and APPARENT luminosity.
Is it accurate to say that the motion of the expansion is not due to any force or acceleration? It doesn't make sense to get a reading on a perfect accelerometer, yet there is clearly a d2/dx2 x.
22:02 I like to think that EVERY point is the "center" of the Universe. Under this perspective, the Big Bang happened everywhere. Also, under this perspective, each person can be said to be "The Center of the Universe."
overlaps---> gaps and symmetries, eqn to predict symmetry breaking outcome, even for spontaneous symm breaking, outcome can already be know, like kicking an acorn, or a [ine cone, and knowing which way each vertex will point before even kicking it. like called cards and dice, translated into book words, translated into heard convos in public domain while flipping pages in a boiok synched.
Question for Q&A: Regarding the Cosmic Microwave Background radiation, can you get cosmic background gravitational waves from the Big Bang that are even older than the CMB? As old as the Big Bang itself, and from this, get more insight of the early universe, dark matter and dark energy?
I have a question about time. The duration / time from the begining of the Universe, isn't that a universal constant which started with time zero and is the same everywhere in the universe. And then the relativistic time is like small "branches" of the universal time, depending on gravitational or motion local conditions. Is it reasonable to believe that we can use this "universal time" as a reference and then we can then measure all the relative times, relative to this "unique time, since the universe came to existence"?
Your vlogs are fascinating.