4. The Kinematics of the Homogeneous Expanding Universe

MIT 8.286 The Early Universe, Fall 2013
View the complete course: ocw.mit.edu/8-2...
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
More information at ocw.mit.edu/terms
More courses at ocw.mit.edu

[@robpatterson2861]

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

[@rakshitverma5016]

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

[@parsleypalace3272]

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!

[@Paxientas]

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

[@macieyid]

It's not that bad, considering originally it's been in parsecs per week

[@davidwilkie9551]

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.

[@davidhand9721]

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?

[@timjohnson3913]

Here’s how I think of it. When we observe the CMB, we are viewing a shell of light from the CMB that left ~13.8b years ago. When we view the CMB the next second we are seeing light from ~13.8b years + 1 second (i.e. we see different light each second from a slightly further shell vs the previous second). Now imagine there are slight heat differences in that CMB light. When we piece these spherical shells of radiation together over time, we would be able to tell if we are rotating. It’s not a huge step from here to imagine there are ways with precise enough measurements to tell if we are moving as well as rotating. We actually know we are doing both because we are on a planet that rotates about its axis and revolves (moves) around the sun. And our solar system is in a galaxy that is revolving around the center of the galaxy. If you want to be relativistic about it, you could validly argue that it is equivalent to say the CMB is rotating around us and thus, there is no contradiction with relativity.

[@wagsman9999]

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

[@vitaminprotien1644]

They have got velocity and distance and take avg both of them ....then divide avg velocity and avg distance ......and do this calculation for many years then from these data , we'll get value for hubbles parameter 😀

[@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?

[@maxtabmann6701]

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?

[@tonybarrera2897]

We proove Allan Guth's theory!

[@davidhand9721]

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.

[@maxtabmann6701]

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.

[@skhalid360]

@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! :)

[@felipeblin8616]

None of theories of the universe consider an spherical universe. The best analogy is a balloon surface where the universe is (the surface) and is inflated. So this universe analogy (in 2 dimensions ) grows without having any edge or border.

[@skhalid360]

Yes, that's what I meant. Consider a balloon surface and now consider two points on diametrically opposite sides of the balloon (in other words, as far away from each other on the manifold as possible). If the universe were isotropic with respect to two such points, homogeneity would not necessarily follow. For example, if you call the two points the poles, then if all of the latitude circles (or curves of fixed distance on the manifold from each pole) had different uniform distributions of matter that varied symmetrically around the equator (or the subset of the manifold containing points equidistant from each pole), then any observer on either pole would look in any direction she desired and see a uniform picture. However, such a universe is clearly not homogenous. To be sure, a sphere in mathematics is a surface like that of a balloon. The entire solid balloon would instead be called a ball in mathematics and not a sphere.

[@antoniolewis1016]

No, a sphere would be homogeneous. Any two diametrically opposite points cannot be distinguished from the original pair. However, if you squeezed the sphere and made it more cigar shaped, then it would be isotropic but not homogeneous

[@Tom-vu1wr]

@@antoniolewis1016 very very late but when they said sphere they will have meant with non uniform concentric layers

[@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.

[@AmxCsifier]

+mechtheist The CMB is really just waves coming toward us so it's not a fixed background; Even the source of this CMB was expanding so it's not fixed. To talk about absolute velocity, you need a fixed point in space. But due to the homogeneous expansion of the universe, there is no such point. The variation of red shift we observe on the CMB means that we have a relative motion to the CMB but that's not an absolute motion for the lack of a fixed reference

[@brainstormingsharing1309]

👍👍👍👍👍👍👍👍👍👍

[@randytheram1]

12 parsecs is how many light years?

[@meetghelani5222]

1 parsec is about 3.26 light years

[@tonybarrera2897]

Yes!!

[@mattreilly4013]

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.

[@timjohnson3913]

It accelerates the Hubble constant, which is a big part of the reason many now call it a rate and not a constant

[@while_coyote]

Ouch, please fix the sound balance.

[@adamkubiak7132]

You can adjust sound ratio in your PC settings.

[@UnforsakenXII]

My ears = )

[@titusnicholson3734]

🐈

[@lufiadaos6560]

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

[@mihirnatani4479]

Ya everyone ever coughed in human history had covid but they didnt knew.

[@edwardgalliano9247]

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