How Many Universes Are There?

How Many Universes Are There?

Space is big, but it’s peanuts compared to the eternally inflating multiverse. But just how many bubble universes does the eternally inflating multiverse contain? What are they like? And most importantly, what can they tell us
about aliens? Imagine it: the observable part of our universe
is 93 billion light years across, and that’s just a small fraction of the stuff created
in our Big Bang. But in the eternal inflation picture, ours
is just one among uncountable bubble universes. Bubbles that are continuously appearing and
growing within a vastly larger spacetime that itself expands at an exponentially accelerating
rate. A greater inflationary spacetime whose expansion
never ends. We looked at the bizarre idea of eternal inflation
in recent episodes – but we stopped short of exploring the full implications of this
proposition. Those implications are, frankly, completely
nuts. Some may also be true. Here’s the scenario: the default state of
the greater universe – or multiverse – is to expand exponentially due to the vacuum
of space itself having a large and constant energy density. Energy locked into something called the inflaton
field. Within that inflating space, tiny patches
stop inflating – the inflaton field in that patch loses its energy and so accelerating
expansion stops there. Within each bubble we get a new Big Bang that
kicks off a more slowly expanding universe. But that bubble has an edge, and the edge
spreads into the surrounding inflating spacetime at the speed of light, causing inflation to
stop within the growing bubble. If lots of these bubbles form, they could
collide with each other to produce a connected network of non-inflating universe. But the little patches where inflation persists
are expanding so fast that they quickly dominate the non-inflating network of bubbles, and
so the cycle of inflation continues. Some questions spring to mind: – I mean, besides
“What?!?” For example, how many bubble universes can
be made this way? How often do they collide? Are those universes similar to each other,
or wildly different? I’ll give you an idea of our best thinking
for all of these. Let’s start with the first two, because
these are basically our challenge question. To fully answer these we’d need to know
the true physics of the inflaton field. But we can figure some stuff out just based
on the exponential nature of the expansion. For example, in the challenge question I asked
you to calculate the RELATIVE number of new bubble universes that formed in one second
compared to the previous second. We assumed that there’s a fixed but unknown
probability that a bubble universe will form in any given volume of inflating space. That means the number of bubbles forming at
each point in time should be proportional to the volume of the inflating universe at
that time. So the rate at which that volume is increasing
is the same as the rate at which bubble universes multiply. We assumed the minimum inflation rate that
we think was needed to kick off our own universe. The scale factor – or radius of the universe
– increased by a factor of at least 10^26 in less than 10^-32 seconds. So how much did the volume increase over 1
full second at this rate? Well, a scale factor or radius increase of
10^26 means a volume increase of that cubed – so 10^78. Every 10^-32 seconds one cubic meter of volume
becomes 10^78 cubic meters, which is approximately the volume of our entire universe. And then in the next 10^-32 seconds each of
those 10^78 new 1-meter cubes spawns just as many new entire universe-sized volumes. So we have 10^78-squared, then 10^78-cubed,
etc. By the time we get to one full second we’ve
multiplied by 10^78 10^32 times, so our volume is 10^78 to the power of 10^32, or around
10^10^34. And our number of new bubble universes should
multiply by the same insane factor each second. This gives us a ridiculously large number
of universes even if the probability of making one per unit of volume is insanely low. The exponential nature of the expansion guarantees
that. As soon as the inflating spacetime is big
enough to make one universe, in the next second it should make 10^10^34 universes, and so
on. If you got this right then congrats. We’ll list the winners as well as answer
the extra credit question at the end of this video. As crazy as eternal inflation sounds, it’s
still fun to think about the consequences if all of this is true. Here’s another one: do all of these bubble
universes the same laws of physics, or could they be wildly different from each other? Based on standard ideas, they probably have
the same number of dimensions as ours – 3 space, 1 time – but their contents and physics
could be very different. For example the cosmological constant – the
strength of dark energy – could be different. Remember that it was the high energy density
of the inflaton field that drove inflation, and the loss of that energy density that ended
it. But what if the inflaton field retained just
a tiny bit of energy after its decay? That residual field might be what we observe
as dark energy. It’s an open and contentious question why
the current energy density of the vacuum should be so low without being exactly zero given
that it started out so large. Seems awfully lucky – if dark energy were
much stronger then our universe would have restarted its accelerating expansion too quickly
for galaxies and stars and life to ever form. Here’s a possible explanation: what different
bubble universes can end up with different vacuum energies? Very low vacuum energies like ours might be
extremely rare, but there are so many bubble universes that at least some will have a low
enough cosmological constant for life to form. Naturally, we would be in such a universe. This is an example of using the Anthropic
Principle – we must exist in a universe capable of producing us, so if there are many universes
it is natural that we find ourselves in one finely tuned for life. People have also invoked the anthropic principle
plus eternal inflation to explain a conundrum in string theory. As we’ve talked about before, modern string
theory is difficult to pin down because there are countless possible minimum-energy configurations
of its 6 curled up dimensions. More than 10^500 possible vacuum states – probably
a lot more. The vast space of possible configurations
of these compact dimensions is referred to as the string landscape. Each different configuration results in a
different family of particles and also a different cosmological constant. We don’t know why our universe has the particular
string vacuum state that it does. But it’s lucky it does – because the resulting
particles allow for things like complex chemistry. Enter the anthropic principle once again:
eternal inflation gives us enough universes to easily populate the entire string landscape. All different vacuum states exist, and our
universe necessarily has one that leads to life-friendly particles. As well as a decent cosmological constant. Some people aren’t comfortable with the
anthropic principle. Those people will be even less comfortable
with the next idea. This one is from Alan Guth, the guy who invented
inflation in the first place. Here it is: eternal inflation may solve the
Fermi Paradox. It may explain why we don’t see aliens. The argument goes like this: if the number
of new universes increases by a factor of some impossibly large number every second,
then the vast, vast majority universes in existence were born recently. There are vastly more universes born one second
after ours than were born at the same time as ours. Now, imagine that there’s a set amount of
time for the first intelligent life to form in any one of these universes. Let’s say 10 billion years. Pretend that it’s the same in all universes
down to the second. The real timescale is going to be much more
fuzzy than this, but it also doesn’t really matter for this argument. So, the first intelligence appears in our
universe after exactly 10 billion years – we’ll call that moment “second one”. The same happens in the exact same second
for universes that formed at the same time as ours. One second later – “second two” – you
might get some more intelligent lifeforms forming in those same universes. Maybe just as many as in second one. But consider the wave of universes that formed
one second after our own. These are only now reaching the 10 billion
year mark and producing their first intelligent lifeforms. And of course there are vastly more of those
slightly younger universes – more than all the atoms in all the universes that are one
second older. So first civilizations in those younger universes
will always outnumber all the civilizations in all older universes. The result is that at any one moment, almost
all of the intelligent beings in the eternally inflating multiverse are in the youngest universes
that have had time to form intelligent life. So if we imagine that we are a typical intelligent
lifeform, then we’re most likely the most common type across the multiverse, which means
we’re the first to appear in our universe. Ergo we shouldn’t be surprised that we don’t
see aliens. Guth calls this argument the Youngness Paradox. Let’s be clear – he doesn’t necessarily
believe it to be right. He admits that, while it seems logically sound,
it does seem weird enough to force us to question the premises. He proposes that there may be something off
in the logic of weighing up probabilities over bubble universes. He also doesn’t rule it out. OK, the last thing I want to talk about is
colliding universes. How often does it happen? Let’s start by answering the extra credit
question: how close to bubbles need to be in order to collide? I won’t go through the detailed calculation
because it takes a wee bit of calculus. Instead I’ll provide a link to the answer
in the description. But the too-long-didn’t-calculate is this:
remember that bubble edges expand at the speed of light. If they form too far apart then the intervening
inflating spacetime will throw them apart at faster than light speed before they can
merge. Assuming the crazy rate of inflation in the
question, you get that two bubble edges need to be closer than 6×10^-50 m in order to reach
each other. This is something like 15 orders of magnitude
smaller than the Planck length. The implication is that universes don’t
collide very often unless their rate of production is extremely high. And it might be very high – especially if
the inflaton field is highly correlated from one point to the next, as in slow-roll inflation. But it still should NOT be surprising that
we don’t see evidence of bubble collisions in our observable universe. If such collisions happened, they’re probably
too far away for us to see. OK, so we’re come up with a lot of ridiculously
large AND small numbers, and convincingly shown that none of this is easily testable. Where does that leave eternal inflation? Well we probably shouldn’t just accept it
– but at the same time, current lack of proof doesn’t mean something is wrong. Just that the universe – or multiverse is
playing hard to get. But here’s something that’s not hard to
get – Space Time swag. If your name is listed below, you’re a winner
of the challenge question and you get your pick of a swag item from our merch store. Email us your choice, any relevant size info,
and your mailing address and we’ll get it out to you. Of course you can reward yourself for just
listening to the answer by buying your own swag. Link in the description. It’s the best humorous science apparel in
all of spacetime. Did you get enough spacetime today? No? Need more of a fix of that existenial awe
at the wonder and weirdness of the universe? Got burning question about the nature of reality? I may have what you need. We just launched a spacetime discord for 24-7
conversations on all of the above. It’s for our patreon members – but don’t worry
– it’s the lowest of the lowest tiers – 2 bucks a month to help support the show and
to indulge that voracious curiosity of yours. Hope to see you over there. And one the subject of Patreon, I wanted to
give a big, big thanks to Alexander Tamas, one o our big bang supporters. Alexander has been traveling the bubble universes
for many years, supporting local youtube space shows where he finds them. Alexander, we’re glad you found us, and safe
travels, wherever they may lead. Last time we visited that old argument – Is
Pluto a planet? The question gets to the heart of what a scientific
definition really is. And a number of you found the International
Astronomy Union’s new defition of planet – the one that excluded Pluto – not very scientific
at all. I thought it worth focusing on that for this
comment response. A number of people in the comment section
echoed these thoughts – but special props to Regolith on the spacetime discord, who’s
a bona fide planetary scientist. I’m also echoing some of the thoughts of Alan
Stern, lead scientist on the new horizons mission to pluto. There are two main arguments – firstly that
the IAU definition is arbitrary and ambiguous, second that it should have been planetary
scientists that made this decision, not astronomers. I acknowledged the arbitrariness in the episode
– but ambiguity is a more serious concern. that’s something a scientific definition should
never be. Here the IAU definition again: “A planet is
a sun-orbiting body massive enough to be round and to have cleared it’s orbit of debris”. It’s that last bit – cleared its orbit – that
excludes Pluto. This might be considered arbitrary because
“clearing the orbit” doesn’t necessarily reflect a fundamental difference in the physical nature
and formation process of the body. And that’s an issue that planetary scientists
take. In fact, whether or not a body clears its
orbit depends strongly on where in the solar system it forms – the further out, the more
massive a body needs to be to clear its orbit. If Earth formed at 100 times its current distance
from the sun it wouldn’t have cleared its orbit either, and so wouldn’t be considered
a planet. But Jupiter in the same location would have. So we end up with this somewhat complicated
relationship between mass and distance from the Sun that determines whether you get called
a planet – and that relationship may not relate to any scientifically interesting properties
of the body. So there’s your arbitrariness. But what about ambiguity? Within our solar system it’s clear enough
which are planets and which aren’t by this definition. But what about around other stars? That brings us to the first part of the IAU
definition – a planet has to be orbiting our Sun. That’s right, other stars don’t have planets
– they have exoplanets. The word planet is reserved exclusively for
the 8 bodies of our solar system. In fact you need this combination of requirements
to know absolutely whether something is a planet by the new definition. In general, the new definitely of planet avoids
ambiguity but at the cost of arbitrariness – and you might also say of scientific usefulness. Pluto aside, the new definition allows for
two objects with extremely similar geophysical properties to be differently classed. For now the new definition makes things clearer
than they were, but as our understanding grows – particularly of exoplanets, maybe we need
to be ready to change the definition again – perhaps this time with more input from planetary
scientists. Who, after all, are THE literal experts and
more frequent users of the term.

100 thoughts on “How Many Universes Are There?

  1. Neil Degrasse Tyson would have explained this video 1,000 times simpler. I dont think this guy realizes not everyone watching this video is a astrophysicist 🤯

  2. If Alan Guth were right about the Fermi paradox, wouldn't a consequence be that we'd expect to find ourselves in a universe where intelligent life evolved shockingly early? Biologists might be puzzled by how we went from simple bacteria to humans in only, say, a million years. In the real universe, we were instead stuck on single celled organisms for billions of years, and this length of time seems substantially dependent on luck. Even if it's far less likely for multi-cellular life to evolve in millions of years, if younger universes are so dramatically more numerous than older ones, that should make up for the lower odds, and that's indeed the sort of universe we should find ourselves in.

  3. I can't say that I buy this, with regards to all of these new universes sprouting up. They may sprout up, but it's always dangerous to take people's untested hypotheses very seriously. And as mentioned, there doesn't actually seem to be evidence for this…

  4. I feel like this has gone off the deep end. About as provable at turtles all the way down. I don't see the need. I hope I live long enough to find out one way or the other.

  5. Colliding universes with opposing physics would eventually collide and destroy one another until eventually all universes destroyed. No way eternal inflation with multiverses is correct

  6. i don't buy the argument about us being the first civilization due to eternal inflation meaning newer universes and newer civilizations far outnumber older ones and this explaining the fermi paradox.
    the simple reason for this is that you can apply the same reasoning to ourselves and our own position in the history of our species.
    if the argument were correct then we'd expect to be among the first humans to ever live, but instead we are number 100 billion or so. there should be far more universes where humanity is in its infancy due to the supposed exponential growth of the multiverse.

  7. To roughly quote someone much much smarter than me a decade ago about string theory: "If a theory isn't testable, then no one should believe it."

  8. An "exoplanet" can't be an "exo" planet when viewed from within its home system, but that definition doesn't allow it to be called a "planet" even from there.

  9. Incorrect title, universe means everything so the can only be one yet presenter starts with " multi-verse" a made up word about made up fantasies, bye click.

  10. TY Matt. Best delivery of the topic I have ever heard. I still don't understand it! But I am another step closer to understanding after watching this episode. I am going to watch it 3 more times now.

  11. why would it be that we are too far away to see a colision between universes?
    I mean our univers is expanding everywhere as far as we know so there is no classic 3d center of a buble nor a edge.
    Its like asking where was the big bang, but its everywhere because all of this, including space was formed.
    So why cant the collision of universes not just happen anywhere?

  12. [DISCLAIMER: Graduated high school with a 1.8 and I am just a humble bicycle mechanic-I have little confidence in this thought experiment I'm here to start a dialogue to rest my thoughts.]


    [Thought experiment]

    Does it matter which virtual particle pair falls into a black hole? If yes, is it completely random as to which one falls in? If no, could that explain dark flow, dark energy, dark matter? If yes, are black holes themselves bubble universes? If yes, could it explain the explosive expansion of the Big Bang? If yes, at heat death of the entire observable and non-observable universe, will all "dark" matter and black holes merge to start the life cycle of the universe all over again? OR does the accumulation of "extra virtual particles" dominate gravity leaving supermassive black holes as islands of bubble universes that cannot ever interact with one another.


    [The Stackpot Theory]

    I like the idea of heat death causing a "Big Crunch" that would spark the beginning of a new universe within a universal black hole, a black hole that has fed on all of the matter/anti-matter in our universe, while it resides in the universe/space-time we are in. Think of it like the matrix. Each Big Bang would be another layer of the universe, not bubbles but stacks. But then that begs the question of what Space-time is and what does it reside in.


    [Why I started pondering this thought experiment]

    Ever since learning of virtual particles I have been fascinated by their potential implications for our universe. Soon after the theory of Hawkings Radiation was published further sparking my extreme interest in these "mathematical artifacts". Feynman says time moves forwards and backwards at the same time but you'll never know what direction you are experiencing due to our inability to observe "time" universally as it is relative. Time backwards would be the previous "stack" universe that has reached heat death. Without entropy in the previous "stack" how does Space-time behave?


  13. Neils Bohr famously shouted "Shut Up And Calculate!" And proceeded to lobby congress and the entire physics community to stop babbling total nonsense about his work, insisting his Bohrring Physics were necessary to ensure continuing progress in the physical sciences. Meanwhile, Japanese bullet train engineers invented the foundations for modern fuzzy logic, fully aware their new invention contradicted classical logic and physics. In response, the academic community ignored their success for two decades, until the use of fuzzy logic began to spread to Chinese industries. As a result, the average number of authors on any significant physics papers skyrocketed to over 120 and progress in theoretical physics slowed to crawl for 40 years, as they all knuckled down to the task at hand, and spent more money than God on the largest and most complex machines ever devised hoping to find answers, without anyone speculating on what they were doing or seriously questioning the foundations of the mathematics and physics they were using.

    These days, they prefer to just throw around enormous numbers and encourage others to speculate for them, while they solicit more funding. A simple examination has indicated that mathematics themselves appear to express particle-wave duality with, for example, Relativity containing the glaring Simultaneity Paradox and proving to also use the same mathematics as thermodynamics. Neurologists came to a similar conclusion, with their evidence indicating that our conscious thoughts emerge from our emotions and that the mind and brain routinely substitute for each other on the most fundamental level of their organization, whenever it just so happens to be more efficient. Efficiency is the new buzz word, with quantum systems being sometimes 125% efficient, which is rather difficult to ignore or summarily dismiss. However, over a trillion dollars was invested in AI research alone last year, and high energy physicists are already started to look for new jobs.

  14. Well all this is just speculation even his mathematical explanation especulacion if there is a Multiverse every Universe have its own different characteristics just like every human being have their own different health and characteristics or a type of DNA for every universe

  15. I find it hard to believe that all these universe bubbles are being created, like water from a tap, without there also being a drain somewhere that all the bubbles are collapsing into. Somewhere there is a universal/multiversal sewer system.

  16. So based on what you’re saying:
    10t/p 78 x 10t/p -32 seconds = 10t/p 10t/p 34 multiply that by:
    Number is seconds per year = 31,536,000 x 14b years gives you the total number of universes created in the lifetime of our universe, which = a lot!

    The answer to the question; “how many universes are there?” The answer would be – it’s impossible to say. Even if the numbers above were calculable and, more importantly comprehendible, we don’t know our frame of reference. We could be the first universe created or one of the last, and without knowing the answer we can’t work out the answer. So for me we aren’t asking the correct question. This question is like asking “why is blue?”.

    So my answer is… 42

  17. I don't understand why is the eternal inflation answer in any way the Fermi Paradox. If my understanding of the Fermi Paradox is correct, it is contained mainly container in our galaxy or at most in our cluster. Regarding the time and distance ever growing between Galaxies it doesn't make sense to ask why there is no trace of people in galaxies much Much younger as ours and so far away that cannot be reach anyway (even at the speed of light). Let alone in a different universe.

  18. The hypothesis of bubble universes touching each other implies that there is some kind of space or grid that they expand upon. I've heard this being referred to as the megasphere (?). We have no idea that that's the case. We only have vague ideas about what is inside our universe and the idea of an outside is beyond understanding.

  19. So if planet X aka David Bowie exists, it won't be a planet, because the end of Kuiper belt and beginning of Oort cloud should be populated. I would prefer a classification where Planet is hydrostatic (sphere) orbiting the Star, a Moon orbiting a Planet, and Planetoid being the big non-hydrostatic orbiting the star, Asteroid smaller, Comets = high excentricity, etc.

  20. If universe is created in inflatron field, that should mean that there is Some border between that universe and the field. Shouldn't then inevitably new universes be created near that border that would eventually collide with the older universe?

  21. 11:09 Even after hundreds of thousands of first hand eye witnesses, even after front page headline news on every major media outlet in December of 2017, who is it exactly that isn't seeing aliens? This is a very tired trope, the world see's aliens, you missed the bus. Stop trying to explain how mysterious it is that nobody sees aliens, whats mysterious is that the scientific community is so far behind. You guys are starting to come around to an understanding of how immensely stupid it is to constantly preach that we are alone. You're 50% there, now stop trying to convince everyone that nobody see's aliens. Seriously you're holding back the other children.

  22. Yo , from all the stuff i have watched for around a year , i understood that there is 68perc dark energy , 18 % dark matter rest regular mat. Witch all came from the big bang , witch is misconception , the dark matter and energy were there the big bang was like turning the oven on , just heating the things on and afther cooling they became matter ,
    The idea that all there is came from nothing means we are in a black hole or you are talking crap
    It is unthinkably stupid to belive everything came from nothing it is not possible , thank you. If you think you can fit everything just nowhere what reason does this have at all besides it is not possible in any dimensia or eternety ..
    Even a black hole is a thing where everything is in one singularity but it has horizon and oculys volume of space ect..

  23. If you are going to ask why is the univerce expanding it might not , it may just float , on a 5d field , just going backwards and around in time in an infinity , witch might be as big as plank lenght or actually infinite , it is not our job to know

  24. Dark energy (100% objects in the universe)might have transformed into dark matter e=mc2 causing huge preassure diferences , spawning the big bang creating regular matter via spacetime curvature = exeleration and some other thermo quantum forces

  25. I don't get the Fermi Paradox explanation.
    -The Fermi Paradox is formulated so, that according to the known properties of our own universe and what we assume about life/intelligence/technological civilizations, there should be a multitude of other observable technological civilizations all around us.
    -The multi-universe theory is formulated in a way, that does not contradict any of the known properties of our own universe or about technological civilizations
    The conclusion to me seems to be, that the multi-universe theory can not change our understanding of the Fermi Paradox.

    OK, I admit, that my intuitions about probabilities have been wrong before. Can anyone explain to me, how the existence or non-existence of technological civilizations in other unrelated universes either a) changes the known (or assumed) properties of our own universe or in any other way disproves the conclusion above?

  26. Woah, where's the rest of the video? I was anticipating how "space time" was going to be worked into the ending.

  27. My second take on my problem with the Fermi Paradox explanation:
    One of the ideas, that is often formulated together with the anthropic principle is the idea, that we are a totally normal, average species, and everything about us is totally normal, typical and average.
    So, our universe was about 13.8 billion years old, when humanity first appeared, and we therefor assume, that it takes an average universe about 13.8 billion years to produce its first technological civilization.
    The Fermi Paradox problem is, that according to all we know so far, and assuming, we are the first technological civilization ever, we don't really understand, WHY it took the universe so long to produce us. The circumstances, that we deem necessary for our existence should have occurred multiple times before, even in the time cone of our observable universe.
    So, the multiverse theory by far can't solve the Fermi Paradox, at best it shifts the question from: "Why did it take the universe 14 billion years to produce us?" to "Why does a given random universe on average need 14 billion years to produce its first technological civilization?"

  28. dumb question : if our universe is 14 billion years old from the big bang, when we were the size of an orange, our universe is 93 billion light years across, how did it get that big if nothing is faster than the speed of light? wouldn't the universe be a maximum of 14 billion light years across?

  29. How "discrete" are all these bubble universes? If they are as discrete as regular bubbles, I'd like to know how physicists justify this. Thanks.

  30. What if Bubbles are not expanding at the same rate (speed of light). Maybe even faster!?! How knows what crazy things he an happen in other bubbles!

  31. The answer to that question is "one". If you ask a similar, but differently worded question you might get a different answer.

  32. So can a universe form inside a universe? What would THAT look like from the perspective of someone just outside the new universe?

  33. Did he brush over what happens when A universe expands into another universe and could that reverse the process of heat death?

  34. I believe Universes are way too distant to ever collide with each other and we may never become a Type II Civilization, let alone a Type V Trans-Universal as this video suggests. If a Multiverse exists, it means that we may or may not have been the first, but certainly not the last Universe in the Cosmos. And the physics of different Universes would be the same overall, just in different stages of their life cycles. While young Earth creationists have been largely debunked, I am fairly certain that the young Multiverse theory also does not have credence. As old and as vast as the Multiverse is, those numbers probably pale in comparison to the actual reality of it.

  35. Can new universes be created inside another one? And what is between the universes? The length required between two to merge was indeed a real thing. Ten to the time of minus fifty something.

  36. Re intelligent life and the Fermi Paradox. I can see that for intelligent life universes, younger vastly outnumber older, but why does that change anything at all in this one? Seems irrelevant to me. What did I miss?

  37. Eight…the answer is eight. Dick Van Patten taught us all that eight is enough.

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