Cosmology | Lecture 3
ฝัง
- เผยแพร่เมื่อ 11 มิ.ย. 2009
- Lecture 3 of Leonard Susskind's Modern Physics concentrating on Cosmology. Recorded January 26, 2009 at Stanford University.
This Stanford Continuing Studies course is the fifth of a six-quarter sequence of classes exploring the essential theoretical foundations of modern physics. The topics covered in this course focus on classical mechanics. Leonard Susskind is the Felix Bloch Professor of Physics at Stanford University.
Stanford University:
www.stanford.edu
Stanford Continuing Studies:
continuingstudies.stanford.edu/
About Leonard Susskind:
www.stanford.edu/dept/physics/...
Stanford University Channel on TH-cam:
/ stanford
With a History background, as a mere lecturer, I am often lost in calculus and general mathematics. But to 'see' Dr. Susskind explain these basic formulae and expressions of physics is a true joy.
I may have to watch this a few times to comfortably follow the progression, but one can not help but admire his humor and passion to instruct what is surely rudimentary to his vast knowledge.
He doesn't -have- to do this... but he does.
Wonderful!
Big thanks to Stanford for providing this.
I am fascinated. I woke up after falling asleep and this was playing. How? Idk. Am I grateful? Yes. If I had you as a math/calculus teacher, I'd have conquered the world of being a physicist.
Absolutely well done and definitely keep it up!!! 👍👍👍👍👍
When constructing the FRW equation he uses a sphere of radius 'a' and gets a 4/3*pi*a**3. However, when determining the energy density 'rho' he uses a unit cube to give M/a**3. Why a cube and not a sphere? A sphere would lead to rho being 3*M/4*pi*a**3. The FRW would then reduce to 2*M*g/a**3 - k/a**2
Cause spheres suck Eisenstein
bru are you really responding to a 13 year old comment
learning so much with these. thanks!!
Thank you, professor Leonard Susskind. I have enjoyed your video lectures. I am starting to understand them...
The matter-dominated universe 14:00; The scale factor increasing photons wavelength 27:30; Radiation-dominated universe 32:00; Energy gravitates 37:00; History of Radiation/Matter domination 43:00; Temperature of the universe 49:00; Why the Sun is opaque 51:00; Looking into the past (surface of the last scattering) 1:01:00; Dark (and light) Matter 1:13:00;
ID to see usages so is Isidro soon bye June yea Lou will see see see swhat is your grandma ok love mommy mommy mommy mommy mommy pffirst name Ikkk lol lol lol so sorry for you too too too bothering about the situation sorry for for for but not not really sad but we don’t don’t don’t have that big big big ibig toe Jaycee bothering her but I’m sure some are just us going over over reacting SS name she says so
Oh kinda udon’t forget sissy tell wells him issues like s and and and oooo him him too and I and you you are like Jaycee have are can arepa do you Lou sorry to be honest lol but you sü know sknow until is not not telling issue like have never told you I can’t was was crying hiiiii polo jü oh
Thanks, I just wondered since the sphere method gets rid of the pi elements which just seem to complicate the "look" of the equations not necessarily the result (as you said it only changes the constant portion). Keeping pi around for use in substituting elsewhere because it probably gets cancelled out when simplifying other equations.
I Thank for these lectures to the Stanford University!!!
it is unbelievable and beautiful regardless of the content that information of this mass let along magnitude is offered in my generation at the cost of internet access i Thank Whole heatedly stanford ucla any of the other universities which provide this intriguing content to those who thirst for knowledge
"Map out the dark matter by lensing" is one of my masters thesis options.
This was SO fascinating to listen to even if much of it went over my head. What I understood of it was so interesting that it makes me feel compelled to spend the time learning the basics of physics and working my way up to understanding the intricate concepts people like Leonard Susskind are able to comprehend.
Accidentally left my phone on while i slept and autoplay brought me here. Pretty cool
Like many others ,I have come back to Susskind's lectures on several occasions ,and got a little more out of them , each time I have done so .
Hence , I thought it might be worth noting that ,at around 1:11:00 ,when he was talking about the "surface of last scattering" , it would have been helpful for him to have emphasised that this "surface", is just an "observational artifact" (for us ).
That at both the surface of last scattering ,and beyond and into the farther universe ,we believe the universe to be just the same as the one we can see -- that is ,consisting of galaxies etc etc., that effectively go on forever .
If i were him, all i would seek is respect in the minds of those i taught to continue in this feild of knowledge. Brilliant.
No nobel prize would fashion a wanting with this amount of illustrous brain activity! ; )
Peace
Question:
Previously for 2 lectures we applied conservation of energy to derive solutions. But now you say photons lose energy as the box expands, so where does their energy go. Why is it not conserved?
@37:50 Is there any empirical evidence that energy gravitates? Or is it what the Relativity Theory predicts?
Can it be possible that dark matter is gravity waves, distortions of space-time from mass in a galaxy interacting with one another? I know its calculated the mass and gravity of the stars traveling and it seems like more mass is needed to keep the outer stars at constant velocity, but since the galaxy is also moving via intergalactic space-time and there for besides the stars orbiting they are being shifted in someway, could this send their affects of gravity laterally through the system?
If you take the velocity of pluto and the radius of pluto from the sun and do the calculation does it show that dark matter exists in the solar system?
Is that the rate at which the milky way is rotating in relation to CMB?
@54:29 Temp. of surface of Sun is around 6000 K. Ionization temperature of Hydrogen is 10,000 K.
thanks a lot)
Is the velocities of the outer matter in a galaxy relativistic? If so, wouldn't this play an important role when determining the mass of a galaxy?
You got it 👌
"...In next ten years"...and now here we are with no sign from LHC of super symmetric particles
According to inflation, and our best observations, probably. WMAP has shown to incredible accuracy that the observable universe is flat. Since the Einstein tensor that determines spacetime curvature is zero over the observable universe, it implies the total energy is also zero.
Mc^2 is the rest energy in SR, a part of the stress-energy tensor. Therefore, as it gets larger, so does gravity. So, in a way, the large rest energy compensates for itself with negative gravitational energy.
What’s invisible light? Is there light or (Photons) we cannot see? How many photons has the universe emitted in its existence? Can photons and other particles exhibit pressure on Dark Matter or just the fabric of spacetime? Is the fabric of spacetime a thing? Is the conservation of energy causing space to expand? Take your time. Thanks
I don't know why I watch these, the maths is just too hard to even imagine.
I wonder all these years later; does JWST have the ability to "see" the early universal wall thingy they inquired about?
I was wondering (@1:47:37) if and how Dark Matter would be absorbed in the central super-massive black hole?
The expansion of the universe seems a lot like osmotic pressure and thermodynamics. If the ionization layers out in the very distance universe are semi-permeable, and/or the neutrino threshold is semi-permeable, then maybe the expansion is not from within. Think of a tiny sphere or "cell" in an ocean of "universe stuff", perhaps dark matter, and that tiny sphere hits a thermal source (an ocean vent) and rapidly expands, accelerating the osmosis of dark matter, but then cooling and slowing down.
So you think that the energy of the universe is zero, including the mc2 of all the masses?
Positive energy contributed from matter (or more specifically, Einstein's stress-energy tensor.) is negated by negative energy from gravity.
Is it possible that the supermassive Black hole and the scattering of black holes within a galaxy create a galaxy-radial disruption in the space-time fabric which causes dark matter to clump around galaxies since dark matter is weakly-interacting?
Crazy Susskind! Always making up words :D
In the future this will replace Miley Cyrus.
I am a bit confused. I understand that kinetic energy+ potential energy = 0, but is the energy of the universe equal to zero? The masses also have a positive energy mc2...
Wait a minute... it's the same thing! He's just blowing my mind: EVERY point in the universe is "the edge" of the universe-- so the universe's expansion causes the wavelength of a photon that connects any two points to increase. The red shift is just a boundary condition.
Dark Matter is Blackholes of a mass of 10 to the 25th power grams. Lots of them like 10 to the 40th power of them.
@joethepro3 oh i don't know, I think it would nice up there on the mantle between my childhood sport's trophies and my world's greatest dad mug even if purely decorative.
@TheLivirus I'm a layman but my understanding is that the energy is proportional to the frequency, not the wavelength. So when it refracts the wavelength increases or decreases but the frequency stays constant. The stretching that lengthens the wavelength reduces the frequency, making the photon less energetic and more red. I Am Not A Physicist (as you can tell) ;)
"the wavelength increases or decreases but the frequency stays constant"
i'm not an expert, but i am sure that frequency=1/wavelenght, so how the hell can that happen?
edit:
"lengthens the wavelength reduces the frequency"
exactly
To me it sounds weird that the energy of a photon would decrease over time since it gets stretched. I mean the number of wavelengths and their corresponding amplitudes are preserved right? If a photon's energy was proportional to the wavelength wouldn't it gain energy upon entering a medium with higher refractive index, thus violate the conservation of energy law?
But then again, I'm altogeather uncomfortable with quantum mechanics.
I am not quite sure I understand the issue about the mc^2, but I am trying to learn GR now, maybe it will help me understand.
One last question if you don't mind: Let's say there is no dark energy. Would both the kinetic energy and gravitational energy tend to zero? Or would they tend to +mc^2 for the positive energy and -mc^2 for the negative energy? It seems to me that mc^2 cannot be lost like kinetic and potential energy.
Thank you very much for your time.
Theoretically makes sense, if one assumes the shape of the universe as a sphere, but to find the location physically is not possible today. Also I think the idea that the universe is flat is somehow misleading. Space is flat meaning there is no curvature in space but the universe itself must have a volume or else the radiation would not be equal from all directions.
@telephas1c Yeah that makes sense I guess.
I'm no physicist either, but wavelength/frequency=same thing. Get a picture in your mind or draw it. So, energy is greater with higher frequency, but then there's also amplitude of a signal which I guess would correspond to how many photons actually slap you in the eye. That will decrease with distance=attenuation=dimmer farther away. Redshift is lengthening wave, dimming is less photons. Redshift is less energetic wave. Dimming is less powerful signal. unless I'm missing something.
Figuring the average Christmas tree is about 22 kg and 5 strings of C9 lights at 1.2 kg, gives about 95% "dark matter" for the Christmas tree. This is in the very very rough ballpark of the "dark matter" in a galaxy at 90%.
17:00 hyperbolic noogie function lol what ?
i think your asking the wrong question because first we will say with a star UNLESS its in orbit of a blackhole or some gravitational anomaly it's at its own set coordinate in space, the star isn't moving, space itself is, space is expanding, now with that said outer stars at constant velocity, honestly thats added stuff, LIGHT WILL ALWAYS TRAVEL at the speed of light UNLESS it is in the deep freeze of space, so really, the speed of light has to be faster then expansion or we dont see stars :)
Relative to what?
sorry, what word does he say when he talks adout the sun - hot and opek(g)????
opaque: above 3000 to 4000 Kelvin atom nuclei and their electrons are no longer together. photons can no longer follow straight paths , because they are heavily disturbed by electrons "on the run". You can olnly see a blurred image. That is opaque
√3+2 divided by √3-2 how to solve that?,tell me,sir
Multiply by the conjugate ((√3)+2) on the numerator and denominator and you get a final answer of -4(√3)-7
I'm no cosmology major but I do know my fair share and i would personally say no just because of newtons law which is actually related to aerodynamics which is for every action there is an equal and opposite re-action and obviously there is gravity, now einstein never did his anti-gravity, but what he did do is the cosmological constant, which in essence is Dark Energy, and with your second question (Read next response)
when the wavelength get shorter, the frequency go up, and when the wavelength get longer, the frequency goes down. You can't change the wavelength without changing the frequency (unless I've missed something!)
i need a brain upgrade
As you are listening to this, you are moving at almost 2 million miles/hour in several directions.
Well he could have done that, but the basic law would still be the same, a const/a**3 ...so only constant would change, and that is not very important, u can have it both ways..it's a matter of personal preference.
relative to something stationary.
If the expansion of the universe causes photons' wavelengths to increase, can't this explain the redshift associated with photons that took longer to reach us? (Without having to invoke the idea that the further objects are receding from us with greater (or any) velocity))
Your question is contradictory. If the universe is expanding by definition objects far away from us would have to be receding.
Yes, I agree, and I think I realized this too, moments after I originally wrote it, in another comment:
“ LonelyQuark
13 years ago
Wait a minute... it's the same thing! He's just blowing my mind: EVERY point in the universe is "the edge" of the universe-- so the universe's expansion causes the wavelength of a photon that connects any two points to increase. The red shift is just a boundary condition.”
Age of the Universe is 13.81 billion years with an uncertainty of 0.05 billion. years. It contains less dark energy and more dark matter than previously thought, says astrophysicists after analyzing the first data from the Planck satellite.
The summ of Braun dwarfs is realy high
meso confused i like his voice and the small amount i can understand
In some universe
If the other knowd is the time natural
Save the laser point
Last?
No
If space is expanding should not time be expanding in some way as well. And if atoms are mostly made up of space should they be expanding as well. Perhaps if the expansion of space is slowing our perception of time may be also. What we see as billions of years may have appeared as less to someone living thousands of years ago
II'll respond to you via PM, so I can give a better answer.
@bunkmasterflex
Hahaha yeah, maybe my friend. Though, im very sure that worlds greatest dad is much more important in your eyes ; )
Peace
1:22:57 there i can stop losing my damned spot
The earth is moving. The point is an imaginary stationary point!
What makes the shape of the universe very hard to imagine is the time and constant movement. I don't think we have grasped the exact shape, age and size of the universe yet. There are several contradicting theories about the specifications of the universe today. I guess at some point we might know but there is a possibility that we might not at all because of loss of information due to the location of the earth.
I detect any more
It isnt shine why? 😂
@colin0630 and the hair of a monk.
Think t
Think secand t
RED 🤣
There is no real stationary thing in the universe. But you mean something like the sun or so
Hahahaha at "hyperbolic noogie function"
Why would a photon expand with the universe and the electron not? both can be described as particle or wave. yes photons behave more often like a wave and electrons more often like a particle. But i see no clear boundary or reason why one would expand with the universe and not the other. Or is the real reason why photons from microwave background get colder just because they travelled further since the big bang as time increases, and thus are redshifted more?
I'll try to explain what you may be asking by trying to give you a sense of intuition for it. Well, if you think of a lightbeam traveling from the center of a star to, say, the rim or boundary of the universe (the outer edge that's observable to an observer in a reference frame relative to it). This edge crosses or expands 67km per second from the reference frame of an observer that's roughly a million parsecs away from it (one parsec is roughly equal to 31 trillion km). Say that this is possible, you'd need a beam of light to track that, for sure, a redshifted one (i.e. one moving away from the observer's eyes). You'd also know that the universe expands at a rate on the lightyear scale (literally the parsec scale, or that 67km/s). I say all this just to show you the relevance of light in answering your question. Now here's the answer: A beam of light consists of a stream of quanta that we call "photons". There's also a difference between an electron and a photon (this is particle physics territory). On the Standard Model the photon is a massless boson and an electron is not. Under the principle of superposition two waves will also have physical implications, where one beam can be both a wave and a particle (to answer that uncertainty). All things considered, the electron and photon may both be electromagnetic radiation, but they're not matter of the same kind. An electron has mass, so it's subjected to gravitation. Photons will travel more freely as they are massless (it's a boson like the Z and W bosons of the weak nuclear force). Photons will reach the rim as light (that is formed by photons) is the universal speed limit. In this line of thought you'll see that the photon will be more "likely" to be able to reach the nearest point to the edge of the expanding universe. Here's why and also what's discussed in the lecture: The wavelength of a photon expands and this shows that it will be subjected to the expansion of space (as a matter of fact, a photon's wavelength is proportional to the expansion of space), unlike the electron in the more localised sense. If you think of the universe as a box by itself, the wavelength of all the photons in the universe will be proportional to the size of the universe (a wider wavelength gives you a "redshift", as the observer near the rim or boundary observes). This may also be why an electron acts more like a particle, because it is less subjected to the "stretching" of spacetime. Some high energy photons will act more freely, as they would be on the gamma section of the energy EM spectrum (due to their shorter wavelengths, i.e. gamma rays. These ones will resemble material points rather than waves, i.e. in a more "photonic" particle form). The ones being spoken about in the lecture are less energized, near to the microwave marker, i.e. the lower end of the EM spectrum. So they'll have wider wavelengths and resemble waves more. I think the same case applies to an electron as gravity would more or less reduce it to a material point, due to the fact that it has mass. Thus, resembling a particle instead of a wave). I may be wrong, but this seems to be a solid answer (I doubt I'm wrong). I think this is what you wanted to know.
CRITICAL COMMENTS FOR THE PAPER"COSMIC REPULSION IN PRESENCE OF MATTER ":gr-qc/0106029 in xxx.lanl.gov; invited
Relative to an imaginary stationary point.
You mean gravity.
Treba jači signal
enjoying these lectures immensely... but why is Susskind insistent on killing off Friedmann in WWI -- he died in 1925
He might mean Karl Schwarzschild who died at WW 1
btw did you not know that that the earth is being held by the turtles? XD
Its dominated by dark Energie 🙄
I laugh at last
lol ok
or... the imaginary stationary point is moving relative to the earth lol
bro wtf is this i just woke up on this video
And if you create dark matter 🤣
Higher Intelligence thinking 🙄
😂🤦♂️
Thay want produce dark Energie but if i right it must be inpossible
😂
If you create log 🤣😂🤣