Cosmology | Lecture 2

[@ericmelton4186]

I love this guy. I always told myself if I was rich I’d just go set in college classes just to learn stuff exactly like this. I love this man’s personality.

[@joabrosenberg2961]

Newtonian Cosmology 26:00; Escape velocity for the universe 41:00; FRW equations 1:09:00; FRW at critical velocity k=0 1:24:00; Big bang 1:34:00;

[@tylerw1774]

29y222y77w7747r74y838e6e8u3283837

[@hasanshirazi9535]

Such a delight to watch lectures of Prof. Susskind on cosmology. He makes difficult topics so easy to understand.

[@cochisewolf]

Thanks Stanford. These are outstanding lectures. Never thought I'd be able to sit in on these type of classes. Many thanks...

[@xxstephnayxx]

Leonardo and Stanford, much gratitude from me as well. I hope to see you in the classroom in the near future.

[@russellmakar579]

Thanks so much for posting these lectures, I could listen to Mr Susskind for hours on end. It's like going to college for free, Thanks again Stanford. I've only seen two of your videos,1,968 to go and looking forward to them.

[@lukemelvin9497]

Aaaasssazsss

[@lukemelvin9497]

Sess

[@lukemelvin9497]

AssasssrzsswsssD

[@lukemelvin9497]

Asssdaassd

[@lukemelvin9497]

aasa

[@ericmelton4186]

“You asked the question, nod your head when you understand “.

[@tabascoraremaster1]

I came to believe that this good man does not believe in the BS itself but is the right man to explain the calculations that proves the BS to be 'true'.

[@bishalbanjara2891]

At 1: 16:33 Susskind writes that units of 'a' in terms of length but quite before or in the usual sense he makes an assumption that 'a' be simple multiplicative factor corresponding to unit length. So, actually 'a' is just a number, not a length.

[@shiddy.]

1:48:26 I love the way he explained this aspect of relativity

[@NeedsEvidence]

It's amazing that the math necessary to derive basic insights about the expanding universe and its age is so simple, and that Newton could have gotten the same correct answer (corresponding to flat space) centuries before Friedmann.

[@phildurre9492]

+NeedsEvidence On the other hand Leonard Susskind is maby one of the 10 people on this planet who is able to explain these things in a way that they seem so simple

[@tishyamohan5673]

Newton could have derived the Friedmann equation if he had any reason to believe that the universe was expanding or even that galaxies were moving away from each other. Even Einstein believed that the universe was static until Hubble's discovery of expanding universe.

[@bendavis2234]

@@tishyamohan5673 keep in mind that Newton didn’t even know that other galaxies existed outside the Milky Way at that time. The concept of an expanding universe would have made no sense to him since his picture of the universe was so limited.

[@DragonSlave49]

@LeeJamison100 That part of the lecture can be misleading. He's actually saying the universe is like the earth, and from any observer's perspective they are in the middle of it. The hollow sphere example was used to illustrate how we can't identify a middle. So the observer will not feel himself being pulled in any direction, but will instead observe gravity by seeing everything moving toward himself.

[@will2see]

22:13 - That's what I needed to know from the first lecture! What does this t measure? Whose t it is? Ok, so it is the cosmic t made in the cosmic factory. ok :-) But has it to be a constant? What if it changes as a function of something? Like if there is a scale factor a(t), maybe there could be a time scale factor b(something)?

[@serbanmike]

Thanks for responding. You are saying that the speed of life in vacuum is constant on the other hand, as I indicated in my post above, Prof.Suskind says that c=c(t). I do know that at a constant speed (whatever that be) you need that time to travel from A to B

[@Onoma314]

Hahaha, I never noticed, I've watched these lectures about 100 times.

[@UdangMegane]

Jesus

[@conormacnessa7723]

And counting!!!

[@ericmelton4186]

I’m on my fifth time. I’m starting to get it lol

[@geoffcunningham6823]

You are right - each pair of galaxies are attracted to each other. So no matter which two galaxies you choose and therefore which circle, you still get the right answer. All galaxies are attracted to all other ones, and the force is given by the mass in the sphere.

[@glutinousmaximus]

Simply, the speed of light in a vacuum is a constant. Obviously, It takes time to get from A to B; but is always a constant.

[@tempdeltavalue]

44:12 It's really strange , because outer part of mass in the sphere also should attract point , and actually if it doesn't universe matter should collapse into point

[@lsbrother]

I think (without being sure!) that the cosmologic principle is that homogeneity and isotropy are the case - the universe looks the same wherever you are in it. (And this can, to some extent, be justified). Consequently there is no meaning to a centre of the universe - all points are equal. Rather like asking which is the centre point on the surface of a sphere.

[@DrTruffaldino]

1:45 Friedmann wasn't killed during WW1: Friedmann died on September 16, 1925, from misdiagnosed typhoid fever. He had allegedly contracted the bacteria on his way back from his honeymoon in Crimea when he ate an unwashed pear he bought from a railway station.

[@serbanmike]

Professor Susskind says (13:35) that a= 1/c. When a becomes a function of time a (t )=1 /c, that means c becomes a function of time and therefore is not a constant as it was assumed initially. Do I miss something here?

[@tulliusagrippa5752]

Friedman did not die in the First World War. He produced his model of a non-static universe in 1922. The models produced before that were static, I.e., the universe was assumed eternal and unchanging. A serious problem with the "Newtonian" model is that masses at rest under the action of gravitational forces alone were shown by Earnshaw in the 1800's to be in unstable equilibrium. No stable static equilibrium is possible under the action of gravity alone.

[@TheLivirus]

One question. Since Newtonian physics forces us to only observe galaxies that are somewhat close to us, doesn't the galaxies particular deviations in velocity become a problem when determining the Hubble-constant? Doesn't these deviations become less important the farther away you look?

[@semperFi4ever100]

@scavengedfantasyname I think they aren't really physics students, this course is a general introduction and something Stanford offers to everyone who wants to learn about science.

[@jonrose665]

The shape doesnt matter, what does is the factors within that shape that makes an equation possible to be.

[@raykarush186]

Yes, indeed, this is the excellent lecture....

[@asimdahal3249]

We'll get there! We'll get there!

[@fertilizerspike]

I think I've just identified one of this professor's main problems. He said "equations that govern" the universe. Equations don't govern anything, equations describe reality, they don't govern reality.

[@xXZeroSynDromeXx]

Okay a bit of help...at around 53 min he begins his main derivation by imagining an arbitrary sphere in the universe and then claiming that the effects of gravity of objects outside that sphere cancel, and the gravity of objects inside acts as though they were at the center. I understand that for an actual sphere this holds. But for an arbitrary sphere in infinite space? It simply can't be valid. You could draw another sphere with another center and claim you are attracted to that center instead

[@aqwertgbvcxz]

We are at the center of the universe.

[@AlanAlan2001]

I like it when he says "We get there, we get there!"

[@aaronlyu133]

I think he should have described the expansion as a somewhat diffusion velocity (or divergence in a mathematical sense) that is uniform in space instead of a force. it accumulates in space and so the diffusion velocity would seem enormous when one considers a large distance (because there's more space in it) and would seem minor when one considers an ordinary object.

[@sherlockholmeslives.1605]

What is the Lagrangian in theoretical physics? I've looked it up on Wikipedia, but how is it related to research in theoretical physics?

[@samarthatilwankar8605]

Y

[@gustavobopsin5248]

We can find the equations of motion of everything using the action. And define the action using the lagrangian. Search for analytical mechanics

[@glutinousmaximus]

Be patient, you need to see all the lectures. Energy is conserved - It's that space itself increases the distance between objects (The 'acceleration of the universe') Don't ask me why - It's known currently as 'Dark Energy'.

[@terrafirmaterrafirma]

@FrenchExpat1 Ummm, e=mc^2 is small in the whole scheme of things.... so is y = mx + b, or the fundamental theorem of calculus.

[@bollyfan1330]

Do all the effects of relativity apply based on the peculiar velocity of an object?

[@bunkmasterflex]

i would say intelligence lies in the understanding of the consequences of an action through inference our prediction, wisdom lies in the ability to understand how insignificant an inference can be rendered at any point in time..knowing but knowing that what you know is a fragment of what could be known or what is. now we can all stop talking about how right we are, intake the amazing amount of information that is available to us, have the most educated opinion we can.. our social responsibility

[@anmolraina3509]

Can somebody please tell me where can we ask him questions currently (probably some email address or other stuff) as his blog was last edited 7 years ago. Please reply as soon as possible

[@xinzeng-iq7zv]

experimenting to see if this game is viable for multiplayer or single player campaign

[@will2see]

Not knowing what the units of the Hubble "constant" are and at the same time teaching modern cosmology at Stanford University is a bit strange...

[@will2see]

Could gtt in the metric tensor be a function of something?

[@190ecky]

I really want to ask him if the universe is expanding and contracting like a harmonic osilater than would the trough and crest being the most expanded part of the wave move slower in time compared to the other parts jw

[@Gismho]

Superb presentations. Thank you Prof. Susskind.

[@jamesdowns72]

Only part I didn't follow was where he adds an x^2 out of nowhere to the const on right hand side and is then able to divide them all out. He says it's the only way to make the equation make sense... could you also just say x is a const because of how he's defined x as just a label that moves with the galaxies and a(t) is actually the variable?

[@tishyamohan5673]

Actually, X can not be a constant. It is different for each galaxy. Therefore the constant on the RHS is not a true constant either - it will have a different value for each galaxy. However, the different values of the constant will all be proportional to X^2. Hence both sides can be divided by X^2, yielding an equation independent of the position of galaxy.

[@justingreen1887]

Thank you, Dr. Zaius

[@glutinousmaximus]

I think you missed that he is talking here in Newtonian terms.

[@daujok4375]

Newton's theorem is my favorite!!!

[@adityadas.mr.cosmos357]

Very nice lectures👍👍👍👍👍👍👍

[@naimulhaq9626]

I am not very comfortable with Newton's idea that matter outside the shell does not count.You say that he did not use his usual calculus to prove it, and you also said you discussed the mathematics somewhere else.Could you kindly tell me in which video this proof is discussed? Considering it true,and that g gets diminished as I approach the center of the earth,, where g=0, does it not follow that the pressure P=mg should be zero?How is it that great pressure at the center heats it up to melt it?

[@hjembrentkent6181]

The statement is that matter outside the circle doesn't contibute to the gravitational force. Newton used both calculus and geometry to prove it, but none of the other scientist knew calculus, so he had to prove it to them by geometry. P=mgh is not pressure, its the potential gravitational energy, which of course is 0. You were thinking about hydrostatic pressure P=ρgh (ρ is not the letter p its the greek letter rho) and the pressure is of course not 0

[@dorwinblack5934]

+Naimul Haq: It's not that the mass outside the shell doesn't count. It's that, under the assumption of homogeneity, the gravitational effects of all the mass outside the shell cancels out. If the assumption of homogeneity were not valid, mass outside the shell might have to be accounted for. For example, on earth, when computing the gravitational effect on the tides, the mass of the sun and moon must be accounted for even though they are outside the "shell' of the earth. Without the mass of the sun and moon, there would be no tides. The assumption of homogeneity is not valid for the earth-moon sun system when computing gravitational effect on the tides

[@naimulhaq9626]

Dorwin Black There are other aspects of gravitation that is puzzling to me. For instance galaxies are moving away from each other. The further they are the faster they move, yet most galaxies seem to have tumbled into other galaxies, and still are. Are we having celestial mechanics all wrong?

[@kartikkaranth2683]

Naimul Haq nope we are missing something once we find it things are gonna be pretty clear similar to classical quantum divide in 20th century

[@naimulhaq9626]

Kartik Karanth Maybe you are right, do you know what are we missing?

[@kkonstantinosss2]

@kkonstantinosss2 And of course the Greek mathematicians then fought about the issue of the Heliocentric and Geocentric models, and guess what, The equations worked anyway you wanted them to work, so for some it was heliocentric and for some it was geocentric, My question is (Does it really matter?) What if you took the earth to be still and the sun to be moving, practicaly it is the same thing. Speed relative to what? Even if the sun is so much more massive still our measurements are subjective

[@caladvwlch10]

Gracias a la StanforUniversiti por estas clases.

[@glutinousmaximus]

Well because we know the net force on any particular galaxy is non-zero. Nothing would move if the value was zero. Think of this as a plum pudding rising in the oven. It remains homogeneous and isotropic but keeps expanding. It's more complicated than that, but a reasonable general picture.

[@aqwertgbvcxz]

If you think of the universe as the surface of the sphere then you can only look around but not up or down.

[@ellenetdesign]

Actually, Friedmann died on September 16, 1925, at the age of 37, from typhoid fever that he contracted while returning from a vacation in Crimea.

[@Fritzybedeek]

I thought he might be confusing Friedman with Schwarzschild who died in 1916 from a disease he contacted on the Russian front.

[@xinzeng-iq7zv]

is cosmology a relevant subject matter when rocket fuel cost proportional to distance

[@frankieli98]

1:05:00 Why is further away particle has more energy? I thought energy is conserved.

[@27merk]

Potential energy becomes less negative the further away it is. It has more potential energy and less kinetic energy. You're right. Energy in this instance is conserved.

[@rlhoward47]

This is great. I wish more universities would record great lecturers. I wonder if there are any class notes to go with the lectures ?

[@xXZeroSynDromeXx]

I know that the conclusion must be valid, but this method of derivation is not convincing to me since it requires an assumption which is clearly absurd...that the NET force on an arbitrary m is REALLY in the direction of the center of an arbitrary center of a sphere. You could make the same argument to conclude that the net force is literally in any direction you wish. It seems rather obvious the NET force in a homogeneous and isotropic universe on a galaxy is zero. What am I missing?

[@frankieli98]

shouldn't a(t) have no unit? since it's a scale factor

[@senselesseldritch]

I fell asleep and woke up with this on… I don’t know how I got here

[@arminhansarian2268]

Eating cookies during the lecture like a baws!

[@jonrose665]

Thats exactly y the general relativity law exist, to make sure every equation comes to a representation of a relative constant.

[@frogolfish]

my brain doesnnt want to let me understand how velocity increases with distance if k=0 or if K is any # for that matter. if its a hyperbole as it gets closer and closer to the tangent it slows down???????

[@stevenrichardsonsr932]

Is he lecturing or being lectured?

[@adiacheng4417]

what kind of cookie was that? 1:11:56

[@aqwertgbvcxz]

If this was the case, then we could've only look into around us but not below or obove(atlease not as deep as around us). I don't think that is the case according to the current model.

[@lsbrother]

Sorry I didn't explain very well. We definitely live in a 3-d universe. But, like the 2-d surface of a sphere where all points are the same - there is no special center, our 3-d universe could be a surface within 4-dimensions and also have no special centre. No good trying to envisage a 3-dimensional surface - you won't be able to. But mathematically they are solutions to Einstein's equations and a perfectly valid model.

[@smangano64]

@EMOjamesy23 Well then at no point did I criticize "wondering" in the way you attribute to Hawking and others. Of course we must imagine and wonder as well as theorize and compute. (cont..)

[@sylviomello1979]

One correction: Friedmann died in 1925. Who died during the World War I was Schwarzschild.

[@blogdanzehavi]

Friedmann died on September 16, 1925, at the age of 37, from typhoid fever that he contracted while returning from a vacation in Crimea.[5]

[@LeeJamison100]

I come late to this discussion but, since we are clearly in a universe that is the same in every direction, farther than the eye can see, how is that not like being inside the hollow sphere example Susskind uses? Why, then, is there any assumption gravity EVER slows the expansion of the universe? Secondly, the galaxies should have been Bob Carol Ted and Alice,

[@audreymciver3087]

I may be a key to the cure for all of the problems.

[@terrafirmaterrafirma]

Ummm, it would be nice if Leo would just give a few words of how Newton's Theorem follows from calculus....

[@claudebo]

Ty!

[@lsbrother]

"only look into around us but not below or obove" - sorry but I have not a clue what you mean

[@fapalisok97]

Mind bending

[@Kg277]

Mind straightening..

[@fapalisok97]

Ear twisting...

[@bithynian3700]

Mac eye stretching

[@Kg277]

Thanks ol'man!

[@Onoma314]

lol at 4:15 when he realizes the guy in his class has missed some things in physics class.

[@upsdwatrzy2252]

that was a valid question and clarification of the model

[@Biednymaniek]

Thats why All move around this point

[@Biednymaniek]

And everybody who want Tuch it play with death

[@Biednymaniek]

Live the cat?

[@Biednymaniek]

Iam so unmotivated 🤣

[@190ecky]

Crap let me make myself clear the universe by no means has to be a wave but if it contracts in doing so moves faster in time then when it expands and reaches maximum frequency for which it can expand at that point would time begin to alter

[@realcygnus]

super duper coolness

[@aqwertgbvcxz]

The equations are right but the assumptions are wrong.

[@smangano64]

@EMOjamesy23 no, i visit your profile to see who it is I am talking to. And I think your profile speaks for itself. good luck at CAMBRIDGE.

[@Onoma314]

1:47:23 the classic implied Susskind Facepalm

[@lsbrother]

presumably that shirt is his cosmology lecturing shirt

[@danielnofal]

Amazing classes that anyone can watch. Too bad most young people are watching TikTok instead.

[@Biednymaniek]

Team plane cut save point

[@aqwertgbvcxz]

I don't know if that make sense but I guess a donut shape could work.

[@apoxonxyourxexes]

boot fits s t if too b

[@Biednymaniek]

I think now we can talk

[@johntitor9096]

badass at 59:47

[@adamsblanchard836]

And oddly better

[@r3d0c]

am i the only one who cones to these videos to get some sleep?

[@fertilizerspike]

@mrbubtube Ah, now I see why you're confused, you think I'm arguing. I'm not. I'm trying to educate people and disabuse them of utter nonsense disguised as science.

[@smangano64]

@EMOjamesy23 My original post was in response to a bunch of commentary about God and the bible (which I don't seem to see here anymore so maybe it got deleted). The point is that if someone goes to a lecture about cosmology and raises their hand and starts asking questions (or making assertions) about how the bible fits in then they are just being plain rude.

[@pathfinder2557]

70 yr ol Lenny easily solves highly abstract maths while a 30 yr ol me barely solves elementary fractions. wish I am as smart as him. i feel so dumb and worthless and it's hurting my self esteem so bad. please anyone help me.

[@Biednymaniek]

Log 🤔

[@philosophicalleo405]

Susskind teaching cosmology, testing conformity? Delivering hypothetical constants in order to make mathematical abstractions appear tangible? Make data agree with points of scientific dogma by factoring equations designed specifically to hide facts anomalous to a cannon of thought. Comparing sensory inputs under meditation all principles will become apparent. The Nomenclature to describe these principles is best described using or condensed or vocal imagery, possibly dance, not mathematics.

[@apoxonxyourxexes]

Empty vessel less ev y t p m e Fill ab ball if Shell ab Is this jumping ahead I jumped ahead I will be jumping ahead ^ one of these statements are

[@The_House_Velaryon]

The heckler is not getting it.