and thus the war between the flat universers and the curved universers began

"the universe will expand forever, unless we were mistaken" Perfect description of physics

Since we have discovered that gravitational waves travel at the speed of light, this implies that the universe might be made up of all three geometries, depending on the mass of each section. As space expands, portions of it become causally disconnected as the rate of expansion from any given point becomes superluminal. Therefore, as matter passes beyond this event horizon, its mass effectively leaves the universre, and, that means the universe could, theoretically transition from positive curvature to negative curvature.

It's so cool how he always creatively manages to finish with the phrase spacetime. Good, deep episode.

“Is the universe infinite?” “No” “Could you check?” “No”

Can I just say I'm so grateful for PBS and this program and Matt? As a layman and as someone who never attended higher learning, I don't pretend to understand everything (or even most) of what is discussed, but physics in general fascinates me, and I'm so happy about the style of presentation. Matt talks like a calm and rationale adult, and treats us with the expectation that the audience doesn't need to be coddled or titillated with a pandering tone or memes bombarding the video. In a world where a lot of "educational" content almost seems to have contempt for its audience, Matt and Spacetime makes me feel respected, and for that I can only give the same respect right back. Thank you, please keep doing what you're doing. You are so appreciated!

I _want_ an infinite universe. A spatially finite universe could still be meaningfully infinite in time, for example under Penrose's eternally expanding Conformal Cyclic Cosmology model (in which expansion eventually causes scale to lose all meaning, and the expanded universe becomes indistinguishable from the big bang singularity, repeating the process). But really I want a universe that is infinite both in space _and_ in time. This is the reality I'd be most happy with. So, you know, make sure your scientific findings conform to my desires, please.

It's only when you consider how much we don't know that you get really can hold the universe in awe.

imagine if the universe being curved gets proven to be true and 100 years later Flat Versers will be a thing and worse yet is they would have little way of proving the other case

It is mind blowing to me that they can actually compensate for such much gravitational lensing at all. I mean, that has got to be a LOT of work.

put it on half speed and it's like you've just run into him at a bar and struck up a conversation

Questions I've had since I was just a boy: -What was the first thing created? What created it? What or who created the creator of the first thing? Is it possible at all to have a first thing created? -What is outside of the universe? What is outside of the things that are outside of our universe? What is outside that too?

Professionals in every other field: "Omg, our models are not correct, we are screwed!" Physicists: "Awesome, all we know might be wrong, more fun stuff to think about!!"

I LOVE PBS SPACETIME!!. I just wanted to say thanks for everything you guys do. I've been a follower for years now and I have seen every video multiple times. Keep it up PBS SPACETIME. Thanks for producing these videos and making them an essential part of my perception of... Spacetime :P

If the universe is curved/round. *That means there has to be a center of the universe.* what could be at the center of the universe?

A lot of people seem to be asking if the universe can change shape due to the ever faster expansion increases over-time. Like one that started off as a closed sphere, currently opening up to be flat, and then gets inverted to become whatever 4D-pringle-saddle-hypershape the negative curve was supposed to be

*starts talking about blobs* “I’m somewhat of a scientist myself.”

My problem with all this size/shape of the Universe stuff is that we can only see a tiny part of it - and what we can see right now is already billions of years out of date. For all we know, space civil engineers could have knocked down the entire Universe several light years away and built a car park around us and we won't even know about it for years.

Extra like 👍 for the “Hitchhiker’s Guide to the Galaxy” reference 😅 “Don’t Panic” and know where your towel is ✌️

It still boggles the mind that the universe is expanding at the speed of light *and its accelerating*. That makes about as much sense as magnets.

I love how every episode ends with ".…. of Spacetime."

Lord Kelvin : “There is nothing new to be discovered in physics now.” Physics : “hold my ......”

My high school math professor was occasionally saying something along these lines: "Parallel lines meet or diverge somewhere in the infinity of the universe." now this makes much more sense :D

11:30 If, if and buts were candy and nuts, we'd all have a Merry Christmas!

Can the Universe transition between different geometries? Start off closed, then get more and more towards flatness and then 'invert' into negative curvature?

Your idea of a donut shaped universe is intriguing, Homer. I may have to steal it. - Stephen Hawking, 2016

"The universe is finite its resources finite if left uncheck life will cease to exist" *Purple Grape*

I’ve always said I think the Universe is pretty big. How big exactly, I don’t know, but definitely large. I’ve been saying that for a while and don’t see myself changing my mind anytime soon.

Just got out of school, literally learned more right now than in the seven hours past.

3:16 Isn't that technically a 2-D projection of a 3-D projection of a 4-D hypersphere.

I have been out of the university world for 40 years now and and I am amazed how much has changed in our understanding of cosmology and physics during these years. I think it is important to realize that it is not possible to gain a deeper understanding in stuff like this without studying the underlying math and science. This is probably the reason behind all the crackpot theories not worth wasting Your time on. On the other hand I would have been infinitely thankful for videos like this as motivation when studying abstract subjects like quantum physics etc. The attitude at the time was very much do the math first and refrain from applications until mathematical maturity was achieved. It is like reading a book, very valuable to get an overview first before diving into details and to have some clue why. Like all the discussions about the twin paradox easily debunked here when we know in detail how the Lorentz transforms work...

If expansion rates can effect the curviture of the universe then could it be possible to change from positive to negative curviture? Would this mean tearing apart an edge of the hypersphere?

It'd be nice if the universe was a hyper-torus. Then parallel lines would stay parallel, even while the universe is finite.

11:00 Dark energy is an evil super flower, I knew it!

This is literally performance art. I'm amazed he can keep from laughing for the entire video. What a pro.

I like how they took the time to deviate to explain everything you need to know to understand the conversation. While saying the universe curves in on itself is easy to visualize, it's hard to understand the universe mathematically without delving into some difficult geometry first! Amazing visuals that make it super simple to understand such a mind-shattering revelation of a finite universe!

When I was really young and learned about the expasion of the universe I thought of it like a Balloon. That hasn't changed

The visual presentation of this channel is breathtakingly well done.

2:19 When (La)TeX suddenly decided not to parse your symbols anymore

Thank you so much for uploading this video. It is helping me get through the pandemic!

so, will we have flatuniversers and globeuniversers now?

2019 flat earthers, 2020 flat universers

Dr. O'Dowd, Thank you for opening up the scientific research and related healthy debate to us.

1:23-1:39 What if everything we know as our "universe" is just a bubble in a solid rock on a much larger scale? And what if what we think is "freezing" is actually the wave's final impact residue?

"(Space/)Time is a flat circle." - Rustin Cohle, True Detective

hearing it from a science channel that finite might be a reality for our universe has somehow scared me much much more than infinity - what is this emotion.

Like Singapore Breaking News, I also listen to spacetime while I play games. I tell -myself- I'm getting brainy though. It might be working too, I recognize more words/diagrams than I did at the start. (obligatory "you may think it's a long walk down the road to the chemist but that's peanuts to space! Listen-!" and so on...)

Love this channel and the host. Watching these vids made me change my major to theoretical physicist.

I'll forever be thankful to this channel and the people behind it, i could honestly watch all these videos in a row and it's channels like this that keep my passion for space going and going!

Great video, Space Time team! ^_^ What about a closed but still flat geometry like a torus though (specifically a 3-torus embedded in 6D space so it stays flat)? Although, in the game Asteroids, the universe is a 2-torus (which would require 4D space to stay flat) but doesn't require an actual embedding space since it's a simulation. P.S. - Pretty please could y'all make a video about Sir Roger Penrose's objective collapse interpretation of quantum mechanics? ^_^

Thanks Matt of Earth, as of always, I appreciate your work always all ways CMB

was on sabines channel watching "if the universe is expanding, what is it expanding into"... and now im here!

2:18 small typo, your maths symbols briefly switched back to LaTeX commands before going off screen.

5:00 Hahahahaha, that jab at flat-earthers! 😂😂😂

When we look out into space we are looking back in time at a universe that was denser, hotter, and smaller. Of course it is going to appear as though the expansion is accelerating when we are in a broader space here and now, broader than the space we are looking at in the past, in any direction

Very impressed with this video. I have always been interested in astronomy and physics. It was things like this that drove me to enter those professions. Thank you for feeding my insatiable curiosity about the universe and the wonders that we discove

Thank you for making this at least somewhat understandable by big dummies like me. I don't pretend to understand most of what is discussed, but feel a little proud of myself that I am at least follow along and not be totally lost....well most of the time. :p

Loved the Douglas Adams reference at the very beginning :)

For hundreds of years, theorists and experimentalists have done what I call the "Its your fault" dance. When theorists predict something and it doesnt quite fit the data from experiements, theres something wrong with their data. But when experimental scientists find something that does fit the mathematical model, its the models fault. Virtually every time there was a gap between data and theory, getting better data fixed it. So im on the theorists side on this one.

Thank you for making these videos for laypeople like me to (somewhat) understand, and marvel and ponder over.

Can you do a video on what it means to say the universe is ‘flat’? I find it confusing because if the universe is expanding in all directions, how can it be flat? Also, if you could explain what the gravity graphs mean in relation to to a flat but expanding universe that would be amazing!

Great question. The fact that space (perhaps not the universe) is infinite *_blows my mind_*

Please tell me how different our observations would be under the following set of assumptions, 1. The Universe has always been there 2. The Universe will continue forever 3. The Universe is infinite in size 4. There is no such thing as gravitational attraction. What we experience is the partial blocking of an all pervading force of repulsion

The Universe is therfore slightly positive curved. Using Complex geometry we can define multi and paralell universes.

MRW when he says "spacetime" at the end: "Hey, that's the name of the show!"

This changes everything. Literally.

You can believe me that the part of the universe we can’t see is much nicer than the part we can see.

From our frame of reference, light from the very edge the observable universe just reaching us was emitted right at the big bang, but the expansion of space has extended its journey. Does that mean we're looking at a magnified view of something tiny? And if so, as you go out and out (fixed in our frame of reference), the universe gets smaller, until it's a point. That would seem like closed to some degree, I'm not sure about in the wider sense.

This shows the importance of experimental thinking, where you create hypothetical scenarios in order to see how the physics works out.

When ʻOumuamua comes back around with Voyager 1 orbiting it, then I'll believe you, Matt....

Why can’t I for once hear: “ we are 100% sure that the universe is …..”

There is no possible way to know until we develop such advanced space travel that we can actually transcend the boundary or travel past the "edge" of the universe. OR die trying.

Love the occasional Hitchhikers Guide references. 👍

The visuals on this video are outstanding!

The title answers the question. Uni meaning single or one. Universe is less than the multiverse. Therefore, if there is an Uni, there must be multi; hence a multiverse: which can be different dimensions

“The team analysed all of the blobs”. Science !

According to the anthropic principle, it’s infinitely more likely we’re living in an infinite universe, because theoretically, one infinite universe contains infinite number of intelligent life forms, whereas one finite universe contains finite numbers of intelligent life forms, infinitely less than one infinite universe.

That Adam Douglas opening (from H2G2) got me at "mind-bogglingly"

The only channel i feel too dumb for, but yet find it incredibly interesting

Could the universe have had different geometries at different points in time as the universe evolved ?

remember at the end of Stargate Universe - they reveal that the actual mission for the ship was to reach the edge of the universe to study the object seen in the background radiation - is this why the show was cancelled - too accurate - MGM couldve funded it even during bankruptcy with partners. nah I rate this is something else here yo im loosing it.

An episode on better explanation for positive curvature finite universe and expansion so no big crunch type scenario. Is it like a sphere that keeps expanding in volume yet the surface will always loop?

In order for something to grow it has to grow into something even the incredible universe there is something else there there has to be and it blows my mind thinking about it, I love space

"Vegh's Theory of Infinity" by Theodore Alexander Vegh, B.S.

Boy do I love knowledge and theories of infinite possibilities. The mind is filled with madness and is full of vanity.

You may think it's a long way down the road to the chemist's, but that's just peanuts to space.

"If if and buts were candy and nuts, we all would have a merry Christmas...say it"

this is just something I've dreamed but: the universe is a manifold. we are in the vicinity of all other galaxies to begin with, then we break apart into separate "tubes" and at the end we come back together. i hate to bring deity into this equation but there is such a thing bringing us all back together in the end.

In a recent episode, you responded to a viewer's invocation of the law of energy conservation in objection to something you discussed by stating that the conservation of energy applies on a local scale or to a closed system but not to the universe as a whole. So, here is my question: If some process at play in the broad universe creates or destroys energy, could you not draw an arbitrary boundary around the event and show that energy was created or destroyed locally or within a closed system?

The thought of a positively curved Universe is what makes me feel safe at night

Maybe it can help : Logical problems 1- infinity ∞+ , ∞- ( becoming bigger or smaller) ( infinitely bigger/smaller ) 2- beginning ( moment zero) ( without time) 3-before beginning 4- beyond light ( light is the limit,what is beyond the limit?) 5- light nature ( particle or wave ?) Or both ? How it is possible ?! Is it a junction among 6- creation of space ( what was beginning of space ,what was before space ) is the space older than big bang ? Or have the same time 7- creation of time ( what was moment zero , what 8- before space ( other dimension) 9- before time . ( other dimension) 10 - end of space ( impossible to imagine ) 11- end of time . ( impossible to imagine ) 12- end of matter ( impossible to imagine) 13- energy( definition concrete, source of energy ) 14 - finish of energy ( where did go the energy, the light ,the photons , the heat, etc.... Questions about big bang When happened the big bang ?( moment zero ,( the beginning mean : when the time was not existing ) did the time existed before the big bang ? Did it was : - ∞ ? Did the space exist during( with) the big bang or the space is older than big bang ? If the space is older than big bang ,it mean : the big bang happened in the middle of space ( barycenter) , then what caused the space ? What/ who created the space ? How it is created ?! What was before the space ? Nothing ? ( absolute nothing ? Can you imagine the nothing ?! When will end the space ?( or is it infinite ? I think no ) Concept ( of infinite) is a problem which need to be solved. The space is not infinite becaIn the Min 1.25 : singularity...density have been infinitely large ,this is the problem ,because it means : the density of matter is not a thing ( static thing ,) but is a thing which is increasing infinitely ,so it is not a thing ,but a process .use the infinity concept has to do with motion,or transforming, which is a kind of motion too. So if the space is infinite it means : it is increasing (quickly), it is moving ( becoming bigger ) ,what/which is increasing ? The vacuum ? Doesn't have sense So it is not infinite. Where it will end ? When it will ends ?! In the Min : singularity...density of matter have been infinitely large ...,this is the problem ,because it means : the density of matter is not a thing ( static thing ,) but is a thing which is increasing infinitely ,so it is not a thing ,but a process which has not end . so I'm telling : the phrase : 'infinitely large' should not be involved to the equation , because : the equation can't calculate the process which is changing infinitely , does not have sense .

With all the various cosmological options theoretically, it sounds like the elephant and the five blind men metaphor. Depending on exactly what we’re observing, that’s which aspect we see. What if we put them all together?

So the game Asteroid was even more physics-accurate than we thought -- keep going in one direction and you *will* eventually wind up back where you started!

Yohoo!! I pretty much understood this whole episode! Half-way to the video, I was getting so excited, getting so far... and as the ending started to near, it became a nail-biter.

swear down, i'm not even a physicist and this is best show on YT

Einstein was not sure about the universe being infinite, but he had no doubts about the abilities of humanity to be silly. 😁

could it be possible that there is an unusual amount of mass in our local visible area that could cause a local curvature deviation while the rest of the universe maintains a different state?

Colossus: “ together we will solve all the mysteries of the universe “

Man.. you're a clever boy... nice work again...

Wanted him to say "according to a paper, it may be as big as yo' mama"