4. The Kinematics of the Homogeneous Expanding Universe

MIT 8.286 The Early Universe, Fall 2013
Instructor: Alan Guth

In this lecture, the professor first talked about the properties of the universe, then discussed Hubble's Law, gave an example of isotropy without homogeneity, etc.

License: Creative Commons BY-NC-SA

[rakshitverma]

50:30 that's exactly what I had been wondering about. beautifully answered

[robpatterson]

I love watching these videos! I'm very poor and how else can a person like me get an education that's so awesome!

[parsleypalace]

I so appreciate having access to these kectures. Thank you MIT, and thank you Professor Guth. I struggle to understand a lot of this subject, but these lectures in their clarity help .

[sfitzsi]

Clear as bell. I feel like I learned a lot of fundamentals. Thank you Professor Guth for making this lecture available to the public!

[davidhand]

I'm very uncomfortable with the idea that we are "moving through the CMB". The CMB didn't happen "at" a particular place. It occurred everywhere. If we can be moving relative to that radiation background, we can be moving relative to every point in space. That would strongly suggest that there is one frame of reference that is "at rest". This is more or less forbidden by relativity, is it not? I almost have to protest and demand an alternative explanation. Perhaps the universe is not as isotropic as we think, but rather it, or at least the portion of it that is accessible to us, has a polarity. Or a gradient of some kind.

Thoughts?

[Paxientas]

"They measure velocity as a normal person would... in kilometers per second"

[davidwilkie]

On topic, "homogeneous" => Zero Kelvin sync-duration Singularity positioning, by default. Any referencing to universal phenomena is projected infinitely via Singularity positioning ONE-INFINITY Eternity-now Interval Conception, because it's AM-FM Modulation Mechanism pure-math relative-timing ratio-rates Perspective Principle in logarithmic time-timing motion.

[wagsman]

Great lectures. Wondering how WMAP data is used to determine H? Not obvious to me at all.

[robheusd]

Isn;t the Hubble constant - besides not constant - a rate for the average historic expansion rate and not the momentary rate of expansion? Woudn't this need to be expressed in the equations?

[davidhand]

Is the expansion manifested as a measurable force or acceleration on every galaxy? I expect not because if it did, what direction would it be? But if it's not an acceleration, how can it result in a relative velocity? The kinetic energies do not add up, it would seem.

[maxtabmann]

If the Hubble valie H0 were constant, the scale factor a(t) would have to grow exponentially, simply from the differential equation. Isn't that a problem?

[skhalid]

@18:04 Two diametrically opposite points on a spherical universe would be an example of two points relative to which isotropy wouldn't necessarily imply homogeneity! :)

[mechtheist]

Around min 49.5 he mentions that there is no absolute motion in space, I hear that a lot, and it's simply not true.  There is a fixed background that can be used and it's the cosmic background radiation.  Using doppler shift measurements, you can measure an absolute velocity.  If you look into how they have to massage the data for those images of it, they will discuss how one half of the sky looks blue in the raw data, and the other half red, and that is due to the overall motion of the sun in the milky way and the milky way in its local cluster etc. wrt the background, bluest in the direction of motion, and reddest behind. This has to be removed by processing because it swamps the 10 e-5 variation found in the blackbody radiation. If someone who sees this and actually knows their stuff, unlike me, I'd appreciate a comment as to why you hear what you hear considering what I just laid out.

[maxtabmann]

Can anyone explain to me, when the universe changed from a plasma sphere to a transparent universe, why the photons did not travel outward, toward the edge of the universe, but back to us, so that we can see them? Now we know that the universe is flat and not the surface of a 4D sphere. Thus nothing should come back to us.

[mattreilly]

This might seem like a silly question but How does the observed acceleration of the expanding universe affect the Hubble constant? Since it is framed as a constant velocity.

[edwardgalliano]

The answer is obvious. We are in Euclidean space inside an elliptic plane. It's just light is red-shifted by elliptic gravity.

[lufiadaos]

7:01 A person coughed in this classroom. She/he probably has CoVid.