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.

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 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   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,   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.

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   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.

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,   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   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   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   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,   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   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  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,   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   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,   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,   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   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.

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   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   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   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   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   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, 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,   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 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,   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,   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,   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.

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.