Surely one of the biggest ideas in the universe has to be the universe itself, no? Or, as I claim, the very fact that the universe is comprehensible — as an abstract philosophical point, but also as the empirical observation that the universe we see is a pretty simple place, at least on the largest scales. We focus here mostly on the thermal history — how the constituents of the universe evolve as space expands and the temperature goes down.
The Biggest Ideas in the Universe | 22. Cosmology
And here is the associated Q&A video:
The Biggest Ideas in the Universe | Q&A 22 - Cosmology
A very recent talk on Everettian QM by professor Carroll followed by David Albert and then a lively discussion with Carroll, Albert, Lev Vaidman and Q&A. Good stuff.
https://www.youtube.com/channel/UCPRe-yID_EaQwvCZM7hU9Hw/
At the time of its birth, the universe was very small . Since then it has been expanding at a finite rate for a finite period of time so how can we speculate that it may be infinite?
Assuming that it is finite, could we imagine cosmologists on a planet at the edge trying to find out why half the sky is an empty void?
Thank you so much for this amazing series. Are you going to make videos about “String Theory” and/or “Loop Quantum Gravity”?
So as a photon travels through space, over ordinary distances, energy is conserved. But over cosmological distances, where dark energy appreciably expands space, the wavelength of the photon is stretched, the photon loses energy, and energy is NOT conserved. Is that right?
Hi Sean,
In one of the entropy videos, I believe you said that absent a future crunch, the universe will empty out to a “flat, empty” universe (or at least you say this in your 2004 paper with Chen). So “flat” is the ultimate fate of the universe. But suppose we had an FRW universe with no cosmological constant and negative spatial curvature (saddle shape). How does the dilution of energy density over time make this universe more flat? The spatial shape/curvature is a function of energy density, and the flat FRW universe is denser than the negatively curved one. So how does decreasing the energy density in a negatively curved universe make it more flat in the future? I assume I am just equivocating two separate meanings of the word “flat”, but I don’t know exactly how to distinguish these uses.
Are there any constraints on combinations of spatial curvature and cosmological constant values? For example, can there be a spatially closed universe with negative cosmological constant (aka a spatially closed anti de Sitter universe)? This seems unacceptable because AdS/CFT has the boundary CFT at spacelike infinity, which wouldn’t exist if the space is closed like a sphere.
Regarding dark matter’s effect on the CMB perturbations: I thought that dark matter (according to WIMP models, etc.) doesn’t ‘collapse’ under gravity, since there’s no way for it to lose the kinetic energy it gains. Baryonic matter will accelerate, then lose some of that kinetic energy in collisions and radiation, and end up “stuck” in the lower-energy state; e.g. accretion discs, planetary nebulae, stellar nurseries, galaxies, etc. Since (as far as we know) dark matter doesn’t collide or radiate, it doesn’t lose this kinetic energy, so it will go straight through these over-dense regions and climb back up to the same ‘height’ it started with, essentially orbiting forever; which is why a galaxy’s dark matter is thought to form a spherical ‘halo’ much larger than the galactic disc. I would have thought this ‘orbiting’ behaviour would have a similar effect to the ‘bouncing’ of the baryonic matter; except even more pronounced since it’s not damped?
As I understand expanding universe creates event horizon so light from some distant objects can never reach us. How many properties this event horizon share with black hole event horizon? Should it radiate? Could it be responsible for some initial energy considering inflation happened? Could radiation at inflation and potential “big rip” be that huge?
Should the event horizon shrink?
For any followers of this series who, like me, are struggling with the concepts of symmetries and group theory, I’d like to recommend the latest upload from the Three Blue and One Brown channel on You Tube. The subject is handled with the usual clarity and high standard of helpful graphics.
https://youtu.be/mH0oCDa74tE
Hi,
I wonder how the cosmological models with a universal time parameter t and a distinguished rest frame (relative to the CMB) fits in with what we have learned so far: There is no universal time, local coordinate systems cannot generally be continued globally, and the time direction within spacetime depends on the observer’s state of motion. How, for example, is the cosmological time parameter t related to the proper time of a celestial body that has just narrowly bypassed a black hole on its journey? How can we talk about “the age of the universe” when even my feet and my head differ in age?
I understand the explanation that the cosmological models average over large distances and are a good approximation for largely homogeneous situations. However, the temporal development is then determined by nonlinear equations that mix large and small scales. Therefore, it remains unclear whether the initially good approximation is maintained during the course of the dynamic evolution. With what justification are the cosmological models therefore extrapolated into the distant future?
Thanks as always for a wonderful talk.
1. I especially enjoyed the CMB explanation. Do you have any recommended reading on that subject?
2. Has there been any progress on why there is more matter than antimatter? Are the Sakharov conditions still the only ones being explored?
3. I have often heard that Einstein’s field equations “would have predicted either an expanding or contracting universe before he added lambda”. Can you show us how that works?
4. Similarly, you showed us how the Schwazschild metric was derived from Einstein’s equations. Could you do that for the Friedman equation?
I very much enjoyed the spirited discussion triggered by your (Mad Dog) Everettian talk at the Harvard Foundations of Physics. Bravo!
Thanks for the great series!
I understand space is expanding in all directions and is expanding into nothing. Since our galaxy/solar system and we aren’t being ‘ripped’ apart and the Andromeda galaxy is heading towards us do we theorize that space is expanding between the galaxies that aren’t gravitationally bound i.e where gravity is weak?
Is there anything that gives us a way to directly see how fast a clock is going — relative to our local clock — when observing something from the cosmologically distance past — like 12bn years ago? How do we test any assumptions we have on the rate of time across cosmological scales? Could it be true that time changes (rather than space? or addition to space?) at cosmological scales?
Could you please say more about energy not being conserved in radiation and vacuum dominated expanding Universes?
My question is about the curvature of spacetime in the early universe (Plank era).
“Based on data from Wilkinson Microwave Anisotropy Probe, in order to achieve the current flatness value for Ω, the density of the early universe can’t have departed from its current density by more than one part in 10^62.” https://en.wikipedia.org/wiki/Flatness_problem
This blows my mind, since the mass of the universe is only 10^56 grams. So if there was one gram more, or less, in the universe, it would be significantly curved?
Quantum Entanglement Joke:
https://www.youtube.com/watch?v=kTcRRaXV-fg
Amazing content yet again. Thanks a lot, professor.
If I got it right, GR requires that there can’t be any global inertial frames of reference. And yet we always hear that the universe is 13.8 B years old. My question is: with respect to which clock? To my puny little physics enthusiast brain, it sounds very newtonian to put a global time reference to the universe. It’d be awesome if you could address this in the Q&A.
The universe expands into nothing. This ‘nothing’ is not equal to vacuum. Vacuum can only exist within the universe, as outside of the universe there is ‘nothing’. Do the laws of physics stop at the edge/frontier of the univers? Are there no quantum wave functions and forces beyond the edge?
Even with a cosmological constant, would a steady state universe be stable?
Would it require exact fine tuning?
(I suppose instead of a constant, it could be a variable that adjusts itself to the density.
But them with enough free parameters a theory could predict anything,
— which is the same as predicting nothing)
Wouldn’t dark matter also bounce under gravitation?
Potential energy would convert to kinetic energy, then back to potential energy.
does the CMB imply a special reference frame for the universe? if you see uniform temperatures in all directions, wouldnt that be different for an observer who was moving relative to us? how does this related to special relativity
Thanks for the wonderful video. A question: is “the age of the Universe” a well defined concept?
I mean: by GR we know that near massive objects (like black holes) the time “flows slower” than in the interstellar vacuum. I guess that with “the age of the Universe” one means an “average age”, in the same spirit/approximation as one says that the universe has constant curvature at cosmological scales. However could you please clarify this point? Many thanks!
Thanks for a great set of videos. I’m still working my way through Cosmology, so apologies if this is covered towards the end. It would be interesting if you could say some more about the empirical evidence and assumptions (if any) that lead to the conclusion that the universe is (very nearly) flat. Does the evidence support a universe that is flat at all times, or just now? Also any comment on whether a flat universe is expected or is it a surprising result?
Why is it that a positively curved universe must be finite and close in on itself like a sphere? It seems like that’s embedding the space in some higher dimension. Why couldn’t it be positively curved but go on forever? Or does it only have to be of finite duration in time?
Einstein, with his thought experiment, showed that the proper way to think of spacetime is that its structure is curved. But the LIGO experiment showed that gravity is a field propagated by gravitons emitted when neutron stars or black holes coalesce. Isn’t the LIGO phenomenon evidence that gravity propagates like a field, that gravity does not act uniformly throughout space, and hence that gravity is a field?
With random perturbations, wouldn’t + and – density fluctuations be equally likely? So how does a positive (i.e. excess energy / temperature) 1st peak arise?
Also, does the discrepancy in amplitudes for even and odd numbered peaks allow fitting the relative amounts of ordinary and dark matter in the univers?
Hi Sean,
Great series… One comment on Cepheid Variables though : Henrietta Leavitt discovered the period – luminosity relationship by looking at Cepheids in the Small (?) Magellanic Cloud . The point was that they were all at the more or less the same distance so it was clear that ghe bright one pulsated slower than the faint ones. I don’t think parallax measurements of the day were up to the job.