What Happens At The Edge Of The Universe? | Space Time | PBS Digital Studios

What Happens At The Edge Of The Universe? | Space Time | PBS Digital Studios


[MUSIC PLAYING] What’s at the edge
of the universe and what happens if
we try to get there? You might be thinking,
wait, how is there an edge to the universe
if it’s infinite? This gets talked about a lot. And people usually say
one of the following. The universe defines all of
space and time that exists. So that’s one
definition of universe. But is it even part of our
universe if we can never interact with it? And what if our universe
is very, very different beyond the edge? The universe is infinite because
general relativity tells us that the universe
is demonstrably flat and therefore the
galaxies go on forever. But how flat? Are you sure you measured
the universe’s curvature with infinite precision? And my favorite, we can
never know and never test it. So it’s not even a
scientific question. Oh, yeah. I’ll science any [BLEEP]
question I please. This is “SpaceTime.” OK. Sorry. Before we get carried away,
let’s talk about the edge or edges of the
universe and what it might take to get there. In a previous episode, we
talked about the size of what we call the observable universe. We even gave you a number,
93 billion light years in diameter, 46
billion in radius. Go ahead and watch it again. It’ll be useful. While you’re at it,
we’re also going to be talking about the CMB. So it wouldn’t kill you to
watch that episode also. OK. Let’s start with that 46
billion light year number. We defined that as the current
radius of the known universe. It’s the distance to that
blob of the CMB, the most distant thing we can
see in that direction. Now, it’s not 46 billion light
years to that actual blob. It is currently 46 billion
light years to whatever galaxy or galaxy clusters
that blob evolved into, racing away from us with the
expanding universe, as it did. We call this the particle
horizon of the universe. It’s the current
instantaneous distance to the most distant part of the
universe that could possibly have a causal connection to us. Anything inside the
particle horizon is referred to as
the known universe. Now when I say the “current
instantaneous distance,” I mean it’s the
distance that you would have to travel only if the
universe froze in its expansion and you were traveling
through static space. In cosmology, this sort
of instantaneous distance is basically what we call the
proper distance between two points. But nothing actually ever
travels the proper distance. That’s not how spacetime works. The shortest path
in spacetime is defined by the
geodesic, the path of light between two points. Even light takes time
to make any journey. So we have to factor in the time
interval, especially when space is changing. To travel to the
particle horizon, we need to move through
expanding space. And the closer we get
to our destination, the more space will have
expanded over the remaining distance. How far would you
have to travel? Bad news, you’d have to
travel infinitely far, even if you were in a
spaceship that could travel at the speed of light. Just as black holes have event
horizons, so too do universes. The event horizon of a black
hole is that point beyond which we can never receive information
because light from that point is redshifted into oblivion. It’s a boundary to the
observable universe. There’s a region of this
universe from which we can never receive any new signal. That is, any signal
that’s emitted today. This is because the
distance that signal has to travel to get to us
will be expanding faster than the speed of light
before the signal reaches us. The same thing applies to
our light speed spaceship. We can ever get to anything
beyond the cosmic event horizon because that space will be
moving away from us faster than light before we reach it. Now, here’s where it gets weird. The event horizon
of the universe is actually closer to us
than the particle horizon. Given our best measurements
of cosmological parameters, we think that the cosmic
event horizon is around 16 billion light years away. This means that
there are galaxies that we can see now that we
could never reach or even communicate with. We’re sort of
seeing ghost images from outside the
part of the universe that we could ever
interact with. As our universe expands,
more and more of it will cross the event horizon
and eventually almost all of that will be
lost to it forever. Kind of sad, really. And, of course, all
bets are off if we can break the cosmic speed limit. So let’s do just that. There’s no doubt
that Einstein was right in setting that limit for
objects moving through space. But two regions
of space can have superluminal relative speeds. That’s actually the motivation
behind the warp drive, which we might get to
in a later episode if you’re up for the challenge. But for now, let’s just assume
we have a nice Alcubierre-class warp-ship and we burn the
mass energy of entire stars to chase the particle horizon. What do we find? Almost certainly,
just more universe. Bummer. Remember, the particle
horizon is just defined by the limit
of our current view. Move to my left, and
my observable universe moves with me. Wait a minute, and my
particle horizon expands. Travel to the particle horizon
instantaneously and you’ll see the Milky Way as a cute baby
CMB blob on your new particle horizon. And presumably, a pretty
similar distribution of galaxies and clusters all around you. But what if we keep going? What’s far beyond that edge? Well, that all depends on
the geometry of the universe. On the largest scales,
the geometry of spacetime is very flat. It’s lumpy on small scales
due to stars and galaxies, but smooth on large, sort of
like ripples on the ocean. Measurements of the distribution
of galaxies and the CMB confirm this flatness with
very high, but not infinite, precision. If spacetime really is
perfectly flat, then, with the most
simplistic application of Einstein’s equations, we get
that the universe is infinite. Now, people claim this a lot. So if it’s true, what happens if
you cross the particle horizon? The universe just goes on, and
on, and on, and on, and on, and on, and on, and
on, and on, and on. And infinity is its
own amazing beast. And there are many
types of infinity, including some that involve
infinitely repeating versions of this bit of the universe. But is our universe
really perfectly flat? Think about it this way. The surface of the Earth
looks pretty flat to us because we really can’t
see the curvature locally. But get some
perspective by taking a ride on the
International Space Station and it’s clearly curved. What if the curvature
of the universe is so small that we’re
just not seeing far enough or measuring precisely
enough to detect it? It’s very possible that the
universe has curvature just inside the uncertainty range of
the best measurements to date. If that curvature
is positive, then it may be that the
universe is really the surface of a hypersphere,
the 3D surface of a 4D sphere. In that case, our
warp-ship would eventually travel all the way around
this curved hypersphere and get back to
where it started. OK, so how far would
it have to travel? Based on a recent estimate
of the minimum radius of the curvature
of the universe, you’d need to travel
an absolute minimum of 18 times the distance
to the particle horizon to get back to where you
started, assuming expansion froze for the whole journey. We also have to keep in mind
that these geometries assume that we can just extrapolate
Einstein’s equations in the most simplistic way. In addition, although general
relativity is pretty cool, it’s not a theory of everything. Non-crazy ideas for the
origin of cosmic inflation suggest that our universe may
just be a slowly expanding bubble in an exponentially
infinitely-inflating multiverse. Now, bubble universes may
be finite in size regardless of internal geometry. And so they may
have a true edge. But what’s on the other side? Are the laws of physics, or even
the number of dimensions, the same? Tell us what you
think in the comments. We’ll cross that edge into the
multiverse in another episode of “SpaceTime.” Squishina and others ask whether
it’s contradictory or circular to use Einstein’s theory
of general relativity to prove itself? Well, the only way to test
a theory is to use it. However, you’re right in
thinking that just one prediction is not enough. A model has to make multiple
independent and accurate predictions to be
accepted as a theory. And there are few
theories with as many independent and
accurate predictions as general relativity. For example, the predictions
GR makes for planetary orbits can give us a mass for the Sun. And that mass predicts
the deflection angle for light passing
the Sun perfectly, that is its gravitational
lensing effect. The same with galaxy clusters. The galaxy orbits give us
a mass for the dark matter in the clusters and the lensing
gives us a mass consistent with this. Shadowmax889 asks
why stars and planets aren’t filled with dark matter? Now, this is a great point. Dark matter is cold
and clumpy, which means it can bunch
together to form galaxies. But it doesn’t
interact with itself in any other way
besides gravitationally. This means there’s a
limit to its clumpiness. To collapse completely
into a star-sized object, it would have to lose
a lot more energy. This is really
hard unless you can interact electromagnetically. By comparison, the cold hydrogen
gas that fills our galaxy clumps together in giant clouds. But then these clouds radiate
light in different ways, allowing the gas to cool even
more and collapse into stars. Dark matter doesn’t do that. So it stays sort of puffed up. MrLewooz asks if we can
please stop throwing monkeys into black holes? Don’t worry. No monkeys were harmed in
the making of “SpaceTime” and any events that can
be consistently assigned to our clocks at PBS. Izvarzone informs us
that upon winning a Nobel Prize for discovering
dark matter particles, he or she would spend all of the
prize money on Phoenix cases. And, yeah, I guess that’s cool. But I only play 1.6. [MUSIC PLAYING]

59 thoughts on “What Happens At The Edge Of The Universe? | Space Time | PBS Digital Studios”

  • Karen Shahgeldyan says:

    Imagine the expansion acceleration rate of timespace being related to the amount of mass of all the black holes being in that timespace ^^, hence the inflation

  • If possible at all,if one would travel beyond the edge of the universe they would cease to exist since the laws of physics acting In anything natural do not exist beyond our uinverse,so forget about seeing what's beyond the universe

  • If there is another universe other than ours, Then it might also be expanding constantly. So, there might be a time when both of these universes run into each other.
    Also, what is between the two universes?? There must be something!! But what???

    My mind is totally fucked up and I feel so helpless and puny. Fuckkkkk!!! I cant even imagine a genuine theory!!!

  • i think on the other side of the universe there will be nothing im no genius but if you think about the double slite experiment they found that when you don't look at the photon of light it is a wave.on the other hand, if you look at the photon it turns into a particle. (crazy i know) so if humans can not travel to the edge or beyond then there will be nothing. fun answer you will out of the laws of physics space and time so you could live forever because time would not exist.

  • I have so many questions… If we were able to achieve super luminal speeds would we even notice IF we were in a 4D hypersphere? Can we observe a dimension we might not be apart of? What if we are apart of it? Is that consequential to relativity when traveling that fast? So many more questions…

  • A way to summarize this video in 30 seconds would have been to say we dont know if there is a edge to the Galaxy or not. Its easy to assume its infinite but who really knows lol we wont know until we can travel light years away 😥 i hope im still alive to see that day.

  • If the universe is expanding then it has to have a center right?
    Earth is A and some random star beyond cosmic horizon is B
    So if the space is "running" away from A then is it also running away from B in the same direction or in every direction? So if A is in the middle and space moves in every direction so it does from B. So are we moving away from B and getting closer at the same time? Surely Earth isn't in the middle but where is the middle? If we could find it we would be able to travel towards it and it would make us go faster because that central thing would be pushing thing towards us.

  • Outside the edge of the universe there is space. Someone or something legged that cosmic egg of our universe once upon a time.

  • Dayyan Zahid Khan says:

    Thanks for the video.
    hey i just have a good proposal for you, try also uploading your videos on steem blockchain ( a social media platform), a platform called dtube is built on it and you can Earn some good amount of crypto currency. Or if you want i can run an affiliate account for you? Want to know more just reply and i will contact you in detail. 🙂

  • Man, you are so cool and smart. If I had one unselfish wish that a genie could grant me, I would wish that you could meet Einstein and palaver with him.
    But then… a genie couldn't exist in the same reality as Einstein's theories, so… sorry, man, I tried…

  • The universe is all there is so must be infinite in space and time.
    The Big Bang is nonsense. many scientist say so. It requires a miracle!
    The pope says its true, so it must be false. He was wrong about Galileo!
    Mathematicians have ruined science.

  • Peter Alton Mertz says:

    It almost has to be a four dimensional hypersphere. That is the only way that one can have all parts of the universe expanding at the same rate. My estimation would be the observable universe is less than one percent, meaning our universe is very big. How I get that estimation is another story all together, but it also has to be very old, no older then that, no way older then that. Anyway it is all a crazy thought, but it can be calculated if we look at how much energy is in each field.

  • If universe is expanding at speed of light, earth observer is moving away from the light source at speed of light, thus how will the emitted light catch up or reach earth observer?

  • Chris Champion says:

    We think we know the dimensions of the entire universe but we arrogantly think that our " observable" universe ( with our technology) is infact all there is which is stupid. We think what we see is the outer limits but we havent got a clue

  • Hope someone can answer this. Why is the cosmic event horizon closer than the particle horizon? The rest makes sense except that. He just moves on.

  • How can quantum uncertainty result in quantum tunneling? If a particle "passes through" a solid object doesn't this mean that particles could travel vast distances in an instant under specific conditions? If you were to define momentum so precisely (like in the creating a blackhole video) couldn't you just make so many possible positions that the likelihood of traveling this far would become *likely*?

    How can particles travel like this without traveling faster than light? Is this bypassing that law because the momentum of the particle is never tehnically faster than the speed of light?

  • In practicale .
    If you look at a human cell system closley in microscop you will realize they looks quite the same has the universe in structure and that all things in universe has 1. Cycle 2. They dieing or actually transforming.
    Means in persoective if you look from a distance even our entire universe has its own cycle & it could be in a round buble shaoe bext to other billon of other glaxies bext to it that also in a giand buble system and even this one next to other mega galaxy bubkes etc .. thats mean its has no end to it .. ir thst all is a dream
    Nit even exist . Metrix dreamers

  • If the universe is expanding due to dark energy, I would imagine that it could be expanding into an infinite void that needs to be filled acting like a vacuum on the universe, thus causing it to expand.

    I would say the odds are infinite that there is an infinite number of universes expanding in this void.

    Due to the void being infinite the number of universes in the void would determine whether or not there are universes near us.

    Wrap your head around that.

    For me, the term infinity is impossible for me to grasp.

  • Maybe outside of the uiverse there is something like virtual particles? Quantum states that have no real impact, pure potential without reality?

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