Will The Big Bang Happen AGAIN (and Again)?

Summary

The video by PBS Space Time delves into the concept of whether the Big Bang could be part of a cyclical event, happening over and over again, and explores various models and theories that support this notion. Starting with the history and scope of the PBS Space Time show, the video examines the traditional Big Bang model, the theory of cosmic inflation, and introduces the idea of cyclic cosmology. It discusses the challenges of cyclic models, such as entropy and the universe's lifespan, and explores modern theories like the ekpyrotic universe. By touching on quantum fields, dark energy, and the potential role of extra dimensions, the video creates an engaging dialogue around the origin and continuation of the universe. While current models like inflation are well-regarded, alternate theories offer fascinating possibilities worth exploring as scientists seek to understand our universe's past and its potential future.

Highlights

• PBS Space Time celebrates its 10-year anniversary with exciting plans ahead! 🎉
• A cyclic universe might mean the Big Bang is just one of many universal resets. 🔁
• Inflation theory explains a lot but still can't dodge problems like the singularity. 🚀
• Ekpyrotic universe models come from fascinating ideas like colliding branes and higher dimensions. 🧠
• Whether the universe has no start or end, or is part of an eternal multiverse remains a captivating scientific mystery. 🕵️

Key Takeaways

• The concept of a cyclic universe, where the Big Bang may not be a unique event, is still widely considered in cosmology. 🌌
• Cosmic inflation is the mainstream theory to explain the rapid expansion of the universe, but it has its own challenges like not explaining the universe's start without singularity. 💥
• Alternate theories like the ekpyrotic universe propose ways to reconcile observed phenomena without a traditional inflationary model. 🔄
• Ekpyrotic models use concepts like branes and extra dimensions, making them as fascinating and complex as inflationary models. 🤔
• Testing between these models might not currently be possible, but future advancements may illuminate the true behavior of our universe. 🔭

Overview

In this thought-provoking episode, PBS Space Time explores whether the Big Bang was truly a singular event or part of a repeating cycle within the universe. The cosmic inflation model, which supports the notion of a singular Big Bang, faces challenges like the inevitability of a singularity at the origin of time. This leads scientists to consider alternative models such as cyclic cosmologies.

A significant alternative, the ekpyrotic universe model, suggests that our cosmos could regenerate endlessly through phases of expansion and contraction. This model incorporates theories from quantum physics, dark energy, and even posits the existence of extra spatial dimensions. It aligns with ancient cosmological ideas where the universe experiences rebirths, avoiding singularities, and potentially expanding this model's appeal among scientists.

The video leaves us pondering the ultimate question: could our universe be just one of an infinite sequence of cosmic events? Testing these theories remains difficult with current technology, but future scientific advancements may one day unveil the mysteries of the cosmos. Whether we are part of an ever-repeating cycle or a unique universe in an expansive multiverse, the journey of discovery continues.

Chapters

• 00:00 - 00:30: Introduction and Celebrating 10 Years of Space Time The chapter introduces new merchandise available at the store in celebration of the 10th anniversary of Space Time. It poses philosophical questions regarding the origins of the universe, such as 'How did the universe begin?' and 'How can something come from nothing?'. The chapter suggests a possible solution to these questions: perhaps the universe did not begin at all, and the Big Bang could be just one event in an infinite cycle.
• 00:30 - 01:00: History of PBS Space Time PBS Space Time premiered on YouTube on February 11, 2015, and over the past decade, it has taken viewers on incredible journeys through concepts like black holes, the boundaries of the universe, and the nature of time. The show prides itself on its mind-bending topics and the existential reflections it evokes in its audience. As they conclude their tenth year, they express gratitude for the viewers who have joined them and tease exciting plans for the upcoming year.
• 01:00 - 01:30: Audience Interaction and Merch Promotion This chapter focuses on engaging the audience for an anniversary celebration. Viewers are encouraged to interact by commenting on how long they've been watching the channel or how much of the back catalog they've watched.
• 01:30 - 02:00: Cyclic Universe in Ancient Cosmologies This chapter explores the concept of a cyclic universe, which is a common theme in various ancient cosmologies. The idea suggests a universe that undergoes endless cycles of creation and destruction, preventing the notion of a definitive end. This concept appears in the mythical cosmologies of Hinduism, Buddhism, Zoroastrianism, and in the cosmologies of the Norse, Mayans, Egyptians, and Greeks, among others. The chapter highlights the appealing nature of this idea, perhaps due to an inherent aversion to the concept of a final end to the universe.
• 02:00 - 02:30: The Limitations of Cyclic Cosmology The chapter titled 'The Limitations of Cyclic Cosmology' explores the idea of whether the universe goes through cycles similar to natural patterns like day and night or the seasons. It starts by explaining how humans often extrapolate from natural cycles and apply them to the universe, questioning whether this cyclic concept could apply universally. However, the chapter points out that current evidence does not support these cyclic cosmology theories. Present scientific understanding suggests that the universe began with the Big Bang and will continue to expand indefinitely, disputing the notion of cosmic cycles.
• 02:30 - 03:30: Introduction to Cosmic Inflation The chapter introduces cosmic inflation as an alternative to the original Big Bang model. It notes that mainstream cosmology accepts the expansion of the universe from a singular point, supported by evidence such as the recession of galaxies and the cosmic microwave background. However, some observations challenge this model, leading scientists to explore other possibilities, such as cyclic cosmology.
• 03:30 - 04:00: Challenges and Predictions of Inflation The chapter 'Challenges and Predictions of Inflation' discusses issues with the standard Big Bang model, such as the horizon problem where distant regions ended up beyond causal contact due to rapid expansion. It also addresses the unexpected flatness of the universe which implies a precise balance between matter and dark energy. The chapter further predicts relics from the early universe, like magnetic monopoles, which should persist if the universe began as a hot, dense state.
• 04:00 - 05:00: Eternal Inflation and its Issues The chapter discusses the concept of eternal inflation and addresses various issues related to it, such as the horizon, flatness, and magnetic monopole problems. It introduces cosmic inflation as a prevalent solution, which suggests a rapid exponential expansion phase in the early universe. This phase acts as the 'bang' in the big bang, enabling the universe to expand beyond causal connection and contributing to a flattened space while dispersing magnetic monopoles.
• 05:00 - 06:00: Exploring Alternative Models: Cyclic Cosmologies Cyclic cosmologies explore models where the universe undergoes endless cycles of birth, evolution, and destruction. In these models, each cycle begins with a Big Bang and ends in a Big Crunch or similar scenario, after which the universe rebounds or resets.
• 06:00 - 07:30: The Ekpyrotic Universe Model This chapter introduces the Ekpyrotic Universe Model as an alternative to the inflationary theory of the universe's expansion. It discusses the mainstream acceptance of inflation while acknowledging its limitations, particularly the prediction that inflation, if it occurred, would not completely stop, resulting in an eternally inflating universe beyond our observable patch.
• 07:30 - 10:00: Scalar Fields and Brane Cosmology This chapter discusses the concept of scalar fields in the context of brane cosmology, focusing particularly on the idea of a multiverse composed of constantly spawning bubble universes. The chapter highlights some of the criticisms and limitations of the inflationary model, such as its failure to avoid the beginning of time or an initial singularity. These shortcomings make alternative theories worth exploring.
• 10:00 - 11:30: Comparison of Ekpyrotic and Inflationary Models This chapter explores the concept of cyclic cosmologies, contrasting them with traditional inflationary models of the universe. It delves into the idea that instead of having a distinct beginning, time might be cyclical. The chapter highlights the historical context of cyclic models in ancient traditions, while also discussing their modern scientific iterations. It notes the evolution of scientific thinking from the realization of the universe's expansion towards models where the universe slows, contracts, and experiences repeated 'Big Bangs'. However, these models still grapple with the challenge of addressing the concept of a definitive cosmic beginning.
• 11:30 - 14:00: Potential Observational Differences The chapter discusses the concept of cyclic cosmologies in the universe, where the entropy and the universe's lifespan increase with every bounce. It points out that unlike inflationary theories, cyclic models can't be extrapolated indefinitely into the past because there had to be a starting point or 'a first.' Despite this limitation, the chapter highlights that cyclic cosmologies have the potential to address the horizon, flatness, and monopole problems of the universe without relying on inflation.

Will The Big Bang Happen AGAIN (and Again)? Transcription

• 00:00 - 00:30 Before we get started, just wanted to let you   know we have some new merch at the merch  store celebrating 10 years of Space Time. How did the universe begin? How can  something come from nothing? One way to   “solve” this most difficult of philosophical  conundrums is to avoid it altogether. Maybe   the universe didn’t begin at all. Maybe the  Big Bang was just one in an endless cycle.
• 00:30 - 01:00 On February 11th, 2015 a new show called PBS Space  Time appeared on YouTube. In the 10 years since,   together we've explored the insides of black holes  and ventured across the edge of the universe and   seen the beginning and end of time and the peeked  at the underlying clockwork of nature. It's been   brain-breaking and existentially humbling journey.  And we're so happy you have been on this ride with   us. Well, that ride continues. We have exciting  new plans for year 11 that will be revealed in
• 01:00 - 01:30 due course. For now, two things you can do to help  us get the anniversary celebration started. First,   post in the comments to say how long you've been  watching ... and if you joined more recently how   much of the back catalog of over 400 videos did  you actually manage to get through. And second,
• 01:30 - 02:00 merch! We’ve got a limited edition 10  year anniversary design as well as some   classic logo merch. Having this gear  doesn't just make you incredibly cool,   you also helps us keep going for another decade. No one wants the end of the universe. Maybe  that’s why the idea of a cyclic universe is so   appealing. So much so that it appears in mythical  cosmologies of Hinduism, Buddhism, Zoroastrianism,   and the cosmologies of the Norse, Mayans,  Egyptians, Greeks, and no doubt more. It makes
• 02:00 - 02:30 sense. We tend to extrapolate from the patterns  we see in nature. We see recurring cycles of day   and night, of the seasons, of life and death. Why  shouldn’t the entire universe go through cycles? Of course, the evidence has to support  our extrapolations, and we now know   that the evidence does not support cyclic  cosmologies. Right? The universe and time   itself started at the Big Bang and space  will expand forever and that’s it. Or so
• 02:30 - 03:00 says mainstream cosmology. But apparently  cyclic cosmology has not lost its appeal,   because scientists have found a way  to make it work for our universe. The original Big Bang model has all of space  originating in an infinitesimal point at the   beginning of time, and expanding from there.  This fits a lot of observations of our universe.  The recession of the galaxies  reveals the expansion of space,   and the cosmic microwave background is  pretty clearly the afterglow of an early   hot, dense state. But some observations  aren’t so easily explained. For example,
• 03:00 - 03:30 in our universe matter and energy are very evenly  spread out, but in the basic Big Bang model there   wasn’t time for this smoothing to happen before  the expansion threw distant regions beyond causal   contact. This is the horizon problem. There’s  also the fact that a basic-Big-Bang universe   isn’t expected to be so perfectly flat, which  requires an uncannily perfect balance between   matter and dark energy. And if the universe  that began in an extremely hot, dense state,   than certain relics of this phase should  persist—so-called magnetic monopoles.
• 03:30 - 04:00 Of course we’ve talked about the horizon,  flatness, and magnetic monopole problems,   as we have the most popular solution—cosmic  inflation. This proposes an extreme,   exponential expansion phase in the extremely  early universe. Inflation becomes the bang in   the big bang, and it allows an initially smooth  universe to be expanded beyond causal contact,   as well as being nicely flattened  space and scattering those pesky
• 04:00 - 04:30 monopoles far enough apart that  they're unlikely to ever be seen. To top it all off, inflation explains how the  universe got its large-scale structure. It   predicts quantum fluctuations in the inflaton  field, which became the gravitational seeds   that grew into galaxies and galaxy clusters.  Inflation goes further, predicting the those   fluctuations should lead to the same level of  lumpiness at all size scales—so-called scale   invariance. And that’s exactly what we see in  the lumps of the cosmic microwave background.
• 04:30 - 05:00 Inflation does such a good job  that it’s practically mainstream,   but there are some downsides. Modern versions  of the idea predict that if inflation happened,   then it never ended. Sure our little  patch quit with that extreme growth,   spawning our much-slower expanding universe. But  as long as inflation didn’t stop everywhere all   at once, then out there, somewhere, this eternally  inflating greater universe is blowing up forever,
• 05:00 - 05:30 constantly spawning bubble universes.  Some find this idea a little extravagant. The other issue with inflation is that  it doesn’t avoid a beginning of time,   nor a point of infinite density—a singularity at  that beginning. You can push that singulary as far   back as you like, but in inflationary models  it has to be there, just like in the regular   Big Bang. And any theory that predicts  a singularity is automatically suspect. And even if inflation didn’t have its issues, it’s  worth exploring other options. So what about the
• 05:30 - 06:00 option where, instead of asking what happened at  the beginning of time, we ask what happens if time   never had a beginning? Cyclic cosmologies exist  in many ancient traditions, but also in modern   cosmology. Soon after we noticed that the universe  was expanding, scientists came up with models in   which the universe eventually slows and starts  contracting, then bounces into a new Big Bang,   and repeats over and over. But these models  didn’t manage to do away with the beginning
• 06:00 - 06:30 of time because both entropy and the lifespan  of the universe had to increase with each   bounce. That means we couldn’t extrapolate the  bouncing back in time indefinitely. There had   to be a first. And anyway, these cyclic universes  didn’t solve the problems that inflation solves. But it turns out that cyclic cosmologies can give  us everything we want. To explain the horizon,   flatness and monopole problems without  inflation, and at the same time eliminate
• 06:30 - 07:00 that pesky beginning of the universe. In fact,  it turns out that the same type of field that   causes inflation can also be tweaked to give an  infinitely regenerating universe. This is the   idea of the ekpyrotic universe—named after the  cyclic cosmology of the ancient Greek Stoics,   in which the universe is rebirthed in  fire—ekpyrosis—between unending cycles. By   the way, this is a very different idea to the  conformal cyclic cosmology of Roger Penrose,
• 07:00 - 07:30 and I refer you to our previous episode  for that equally awesome proposal. The ekpyrotic universe was first proposed in 2001  in a paper by Justin Khoury and collaborators. Now, I’m going to come back to the ideas  of this paper, but first I want to give the   part of the story told in a followup paper  by two of the original authors. In 2002,   Paul Steinhardt and Neil Turok showed how the  same type of quantum field proposed to cause
• 07:30 - 08:00 inflation—the inflaton field—can be tweaked to  resurrect the universe rather than blowing it up. The extreme accelerating expansion of inflation  is driven by the same type of quantum field as   we think now drives the relatively  chill acceleration that we attribute   to dark energy. It’s a scalar field—the  simplest type of quantum field in that   it’s just a simple numerical property—a field  strength—in space everywhere. The field also   has a potential energy associated with that  numerical value. Sometimes the relationship
• 08:00 - 08:30 between field value and its potential energy is  simple—stronger the field the more the energy,   sometimes it's complicated. For example, in some  versions of inflation, the field value slowly   drops and energy decreases, but then the energy  reaches a minimum value and any further change   in the field would add more energy. Therefore  the field becomes stable and inflation stops. But there are lots of different ways you can  relate the field energy to the field value. And,
• 08:30 - 09:00 as shown by Steinhardt and Turok one of  those ways gives you a cyclic universe. We normally think of dark energy as being due  to a constant energy density everywhere in space   that does not change over time. But maybe dark  energy changes only very slowly. For example,   if there’s this quantum field that is slowly  changing in value—say, decreasing—with barely   noticeable changes in the associated energy. Then  we have accelerating expansion far into the future
• 09:00 - 09:30 even as the field value drops. But eventually the  energy in that field fades and becomes negative,   and as that happens acceleration slow, then  expansion slows and the universe briefly halts. And then the universe recollapses. The field  potential bottoms out in a minimum and rises   back to zero. Now, we might expect the  field to get stuck in that minimum,
• 09:30 - 10:00 however during this contraction,  gravitational potential energy is   converted into kinetic energy of the field  so that the field value blows past this   minimum. In the final phase of contraction  the field has no potential energy—no “dark   energy” equivalent—and the kinetic energy  of the field gets converted into radiation. This liberates the universe from  the constraints of this quantum   field and it quickly starts expanding again.  The radiation spawns matter and then dissipates,
• 10:00 - 10:30 the matter dominates for a while,  and finally dark energy takes over   and we find ourselves back where we  started in the roughly the modern era. So how does this version of a cyclic universe  solve all our problems? Well, the magic happens   in the contraction phase—what the authors call  the ekpyrotic period. When the universe was   at its largest, matter was so far-flung that the  only meaningful energy in the universe was in its
• 10:30 - 11:00 scalar field, and the only meaningful structure in  the universe were the quantum fluctuations in that   field. Now as the universe collapses very slowly,  those fluctuations are amplified. The shape of the   scalar field is tuned so that these fluctuations  have a scale-invariance—equal frequency for all   sizes of lumps, just as is observed in  the CMB and as is predicted by inflation.
• 11:00 - 11:30 That contraction phase also smooths out the  universe, solving the horizon problem. And   the universe does not reach arbitrarily high  temperatures, so no magnetic monopoles ever   need to be created. There’s no singularity, and  there’s no significant difference between one   cycle and the next, so this model is consistent  with cycles extending back in time forever. Steinhardt and Turok claim that the  scalar field needed to achieve all of
• 11:30 - 12:00 this magic is no more finely tuned than  the field needed to achieve inflation,   and so this cyclic model is just as plausible as  the inflationary model because they both result in   the same observables—at least as far as current  observational sensitivity allows. There are .. But is there any more motivation to believe that a  field of the needed variety actually exists? Well,   these guys say yes, and the mechanism  was proposed the year prior by a team
• 12:00 - 12:30 including these authors and led by Justin Khoury. This does require a little more than a  modification of the inflaton field. It   requires an entire new dimension of space. 6 new  dimensions really. One motivation for the type of   scalar field needed, with its particular potential  energy curve, lies within M-theory. This is an   encompassing framework for string theory, in which  our universe, with its 3 dimensions of space and 1   of time, exists as a single slice in a greater  object with 4 large spatial dimensions. And
• 12:30 - 13:00 with 6 coiled, compact dimensions, but we don’t  need to worry about those for this description. Our universe would be something called  a brane—short for membrane—living within   the higher dimensional space—itself  called the bulk. In this picture,   our universe is one of the boundary layers of the  bulk. We call it the visible brane. Normally this   brane is just chillin—it’s pretty empty and is  not changing in size. Things get interesting
• 13:00 - 13:30 when another free-floating brane within the  bulk—called hidden brane—smashes into us. Which,   apparently, is something that can happen.  Try not to let it keep you up at night. This   collision dumps a bunch of energy into  the visible brane, sparking a big bang. This connects to the description  of the scalar field because we can   interpret the value of that field as  the distance between the visible brane   and this incoming hidden brane. So on  the graph we saw earlier, movement to
• 13:30 - 14:00 the left—decreasing field value—corresponds  to decreasing distance between the branes. Khoury paper assumes a simpler form of the  potential than Steinhardt and Turok, with energy   decreasing exponentially as the branes approach,  which is like a purely attractive force between   the branes. In the more complex version where  potential energy decreases then increases again,   we have attraction then repulsion of the  branes. Either way, when the branes collide,
• 14:00 - 14:30 the energy is dumped into the visible brane  causing space there to start expanding. The   hidden brane recoils, propelled back  the way it came with the energy of the   bounce until eventually it’s pulled inwards again. In this interpretation, the quantum fluctuations  of matter manifest as wiggles in the incoming   hidden brane. These result in different parts  of that brane arriving at different times,
• 14:30 - 15:00 and so there are variations in the start time of  expansion in the visible brane. These ultimately   result in density and temperature fluctuations  in the resulting cosmic microwave background,   which, again, have scale invariance. And we  can interpret the solutions to the horizon,   flatness, and magnetic monopole problems in the  context of colliding branes. Both the visible   and hidden branes exist long before the  collision, and so they can reach thermal
• 15:00 - 15:30 equilibrium over a large enough region  to explain the smoothness of the CMB. The   branes can be very flat over the range that  eventually become the observable universe,   so there’s flatness achieved. And this type of  Big Bang doesn’t start as a singularity—there’s   a limit to how hot it gets—and so we don’t  need to create magnetic monopoles here either. So, did we just manage to save  the universe from ever ending,
• 15:30 - 16:00 or save it from ever starting for  that matter? And at the same time,   did we save ourselves from having to be part  of an eternally inflating multiverse? Let’s   not get ahead of ourselves. For one thing,  if this ekypyrotic behavior is due to our   universe colliding with others within a higher  dimensional space, that’s hardly less extravagant   than the eternal inflation. This M-theory stuff  may not be the cause of the peculiar potential,
• 16:00 - 16:30 but even so, we’re going to want ways to test  this against the also-untested inflationary model. Although the ekpyrotic model predicts almost  exactly the same observables as inflation,   there are potential differences. There may be  slight differences in the spectrum of density   fluctuations, but more concretely we would  expect differences in the gravitational waves   produced in the inflationary versus  ekpyrotic Big Bangs. In either case,   the extremely energetic early universe would  have generated gigantic gravitational waves,
• 16:30 - 17:00 weighted towards lower frequency in the ekpyrotic  case compared to inflation. No currently planned   detector will be able to see these waves, but it’s  conceivable that one day we’ll build a detector   that can sense they now extremely faint buzz of  ancient spacetime ripples, and read from the m   the nature of the beginning of this universe.  Those waves may also have left a signature on   the matter that formed soon after, and we may one  day be readable in the polarisation of the CMB.
• 17:00 - 17:30 Honestly, it’s crazy to even imagine that we  may one day be able to test ideas like this,   and actually have a good idea, one way or another,   whether there’s an infinite multiverse  extending through inflating space, or if   we’re just one universe in an endless temporal  chain of expanding and contracting spacetime.