There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable.We started off our series on The Greatest Story Ever Told by talking about inflation, or a period when the Universe was expanding exponentially. We see plenty of evidence that inflation happened from looking at the geometry of the Universe (it looks like it was stretched flat),
There is another theory which states that this has already happened. -Douglas Adams
at the fact that the physical properties of the Universe start off with the same initial conditions (implying they once were in contact with one another despite being billions of light years apart), and at the spectrum of primordial fluctuations, which are exactly the kind that we expect to arise from inflation.
Well, what comes next?
If you guessed "the Big Bang," you're on the right track. Inflation took care of three important things for us, and made the Universe flat, nearly-perfectly uniform, and gave us slight imperfections from that uniformity. But beyond that, we need to wind up with a Universe full of matter, anti-matter, and energy. We need that Universe to be very hot, extremely dense, and expanding very rapidly -- but not exponentially -- with the knowledge that it's going to cool.
So how do you do it? How do you go from an inflating Universe:
to one that's set to give us the seeds of the one we live in today? The problem with the inflationary Universe is that you take a teeny-tiny region of space that has a whole bunch of random stuff in it, and you stretch it out, exponentially, over many billions (and possibly many factors of 10 more that mere billions) of light years. No matter how much matter and energy was in that little region, you've taken that amount and stretched it across a region that's bigger than the observable Universe, dropping the density down to something like one proton per Universe.
(Image credit: Ned Wright.) Whatever the temperature of your Universe was at the onset of inflation, it becomes awfully cold once inflation gets going. Like, absolute zero-ish cold. And yet, we want to create a big bang, and make all the matter and energy in the Universe today. How do we do this?
The same way this hydroelectric dam works. The water at the top of the falls has a whole bunch of potential energy that is stored in it. As the water falls lower and lower in elevation, that potential energy gets converted into kinetic energy, or the energy associated with motion. But it can also be used to turn turbines and generate electricity. This remarkably simple and useful phenomenon -- that energy can be turned from one form into another -- is the foundation of much of our modern technology.
Well, during inflation, something is driving the Universe to expand exponentially. Our best theoretical work tells us that this is some sort of incredibly large vacuum energy, which is a fancy way of saying,
during this time (inflation), space itself -- this exponentially expanding space -- has an incredibly large amount of potential energy stored within itself.So this magic word, reheating, is the process where all of this potential energy is turned into another form of energy, known as mass.
How does this happen, physically? In our hydroelectric dam, gravity provides the potential energy, and when you slide down to a lower gravitational energy (at a lower elevation), the water gains speed and momentum, which can be used for turning turbines and generating energy.
In the inflating Universe, the thing causing inflation itself holds the potential energy. Do we know exactly what that thing causing inflation is? No. We can extrapolate the physics we know now and invent a new class of particle that could cause inflation -- the inflaton -- but we don't know for certain that is a reasonable extrapolation. If any other particle, even just one of them, such as a quark, an electron, or a photon, has any interaction with this inflaton, then it will pull the potential energy out of the field the same way the gravitational potential energy of a ball gets pulled out and converted into other forms as it rolls down a hill.
For one take, see Mark Trodden's post involving reheating. (But do bear in mind that I'm a little more conservative than Mark is when stating whether the existence of the inflaton is a definite or not.)
This question, the one of how we went from an inflating Universe to the one dominated by matter and energy that we live in today, is one of the great open questions of physics and astronomy. It's great ground for theorists, because we have many good ideas and some observational constraints, but there is plenty of room for imagination. On this front, personally? I'm content to say "I don't know" for the time being. We know inflation happened, but we don't know what caused it. We know that it ended at some point and gave rise to the big bang, but we don't know what the physical mechanism was that caused that.
I'll say that again. So far -- to recap -- we don't know what the physical mechanism was that either caused inflation to happen in the first place or caused it to end and give rise to our matter-and-energy-filled Universe. This will be a recurring theme in the very early and very late Universe: having evidence that a phenomenon happened, but not knowing what the correct mechanism for that phenomenon is. I'll see you again for part 3 (spoiler alert), where we finally will get to talking about the big bang!
Physical Science
Comments
This is awesome. The first time I read about this was in 'The Elegant Universe', I don't think I understood anything about it then. This explains the process really well.
Posted by: Nick | January 19, 2010 7:05 AM
Ethan, what do you make of Ashtekar's recent arxiv.org/abs/0912.4093 claim that loop quantum cosmology explains inflation, without an "inflaton" ?
and, thanks again for the series. you're really great at this.
Posted by: David | January 19, 2010 10:48 AM
Ethan, this is really cool stuff I'd never heard much about before, but more than a little confusing for a layman like me. What's getting me is the difference between inflation and the bang. The impression I've always been under is that the bang is what scattered matter all over the universe, but the way you describe inflation makes it seem like inflation is not just the flattening of the universe but also scattering everything. So is the bang just sort of an event that changes the conditions from cold to hot and creates the elements we're familiar with due to a release of all this potential energy? During inflation, is matter sort of uniformly distributed, and after the big bang all that energy and new matter is what makes everything clump together? I get the feeling that my difficulty is partly one of perspective, like when thinking about inflation the distances involved are probably much smaller than after the big bang.
Sorry for all the low-level questions, but I am awfully clueless about this sort of thing. Looking forward to the rest of the story!
Posted by: Sol | January 19, 2010 11:36 AM
I am near the end of Sean Carroll's "From Eternity to Here: The Quest for the Ultimate Theory of Time" and he is talking about some of this also. He seems far less conservative than you, Ethan.
Posted by: NewEnglandBob
| January 19, 2010 11:42 AM
Ethan, if I understood you correctly, you seem to say that inflation happened before the big bang. This claim was stated more explicitly in your previous post. And you specifically linked to Wikipedia's page about inflation ( http://en.wikipedia.org/wiki/Cosmic_inflation ) where it state the opposite of your claim, and I quote: "inflation is the theorized exponential expansion of the universe at the end of the grand unification epoch, 10−36 seconds after the Big Bang"
So, you either talking about a different big bang, or I'm really dumb and shouldn't be reading this stuff.
Posted by: Ali | January 19, 2010 1:33 PM
what the heck are u talking about??????
Posted by: jelly | January 19, 2010 1:47 PM
Ali -
In a sense there are 2 Big Bangs. When we realized the universe was expanding we surmised that if you "play the tape backwards" the point we would reach would be the Big Bang. This was the original Big Bang.
Then we realized, due to the uniformity of the cosmic microwave background, that there must have been a period of rapid inflation BEFORE the original Big Bang.
Today, most people cite the start of the period of inflation as the Big Bang (as you note the timing difference is almost zero).
Posted by: healthphysicist | January 19, 2010 3:20 PM
David,
It's a possibility that what he's setting forth describes our Universe at some level. If I were a betting man, I'd bet against LQC as being the correct description of our Early Universe, but there is much more work that needs to be done before we have any way of determining its validity.
NewEnglandBob,
Yes, he is less conservative (about theoretical physics) than I am. Most other theoretical physicists are compared to me, but they're still aware that declaring "this is what happened, and this is how it happened" is premature. Talking about the mechanics of inflation still involves a considerable amount of speculation.
Ali,
Wikipedia is making an assumption that is not necessarily valid. We can say the Universe is 13.7 billion years old. When it was younger, it was hotter. When it was 380,000 years old, it was so hot that neutral atoms couldn't form. When it was 3 minutes old, it was so hot that nuclei couldn't form. And, if you extrapolate back farther, when it was 10^-30 seconds old (give or take), it was energetic enough that you should have inflation.
But it's incredibly irresponsible to extrapolate back before inflation based on what we know. Because of this, I prefer to start the clock on the big bang at the end of inflation; which I'll do in part 03.
Posted by: Ethan Siegel | January 19, 2010 3:23 PM
Here's my thinking...
The universe is a single unitary object, the smallest object we know at about 40 billion light years in diameter. It is solid space-time and only space time. Space-time at the moment comes in two states, compressed or uncompressed. Folded or flat in other words.
The rest state for space-time is flat. Folded is a higher energy state. Folded space-time will unfold when it can, but space-time resists change, the faster and greater the change the greater the resistance.
At present most of space-time is flat, with real teeny folds moving through. But not objects per se, for folds are a state space-time finds itself in when it is under compression. It isn't fold objects that move through space-time, but the fold state. In effect, you have a string of space-time marking the path of a fold as these tiny little beads of space-time fold and unfold. And, as noted above, space-time resists change.
This resistance is expressed as the curvature of space-time. That is, space-time bends around a fold as it moves, changing its path of motion. The faster a fold moves the greater the curvature of space-time, and thus the greater the mass.
Back at the big bang the universe was entirely compressed at first, which means it was entirely folded. That is, it was entirely a fold. Not until the universe had expanded to the point some of it could unfold was it possible for anything to move within the universe. In this case volumes of flat space-time, which acted much as folded space-time does today. Thus at the start there was no mass, because there could be no mass. With no mass, which puts a speed limit on how fast things can move, there was no limit to how fast the universe could expand. When it became possible for objects to exists --- whether flat or fold based --- when it became possible for mass to arise the brakes on expansion were applied.
There's more to this, but this should give you the basic idea.
Posted by: mythusmage
| January 21, 2010 5:12 AM
Mythusmage - Mass certainly can affect acceleration, but it does not affect speed. And even massless objects cannot exceed c.
Posted by: DataJack | January 21, 2010 4:52 PM
If I understand correctly the initial inflation period is the template for the BB and the coelesence of matter after and during the BB is an indicator of the mass/energy density dictated by inflation and expanded by the BB.
My question is; Was there a specific geometric shape to the dominant energy densities during inflation?
Woundn't the Void-supercluster distibution be more indicative of an inflation remnant rather than a BB remnant?
The mass density of today indicates the energy density of the past?
If the seperation of gravity from the strong and weak is indicative of the BB,then there is a relation that portrays inflation as a limited threshold event which would state the equivalence of energy density and mass creation,Whereas the Anisotropies of the CMB is indicative of the BB, the final energy density state of inflation would seem exeedingly small indeed.
Posted by: Sphere Coupler | January 21, 2010 10:43 PM
#10
Datajack,
We're not talking about objects as we know them, we are talking about the phenomena from which objects arise. Let's go over the basics again.
The universe is a single, unitary object. This object is made of a thing we call space-time. This space-time is, at present, under compression. Not total compression, partial compression, which allows space-time to exist as both compressed and uncompressed. Or folded and flat.
Folded or flat, that is a property of space-time, not an object in and of itself. Because the universe is a single, indivisible object, strictly speaking nothing inside the universe can move. Space-time within the universe cannot move, only switch from one state to another. In short it is the fold state or flat state that moves, in a sort of chain where a string of space-time becomes folded, then flat again.
Space-time resists change. The greater the extent of the change and the faster it occurs, the greater the resistance and thus the greater the curvature produced. The result is a reduction in velocity.
It is this curvature, this resistance to fold movement, that we call mass. Mass is a phenomenon arising from the motion of the very foundation of the universe as we know it. If for some reason folds did not "move" there would be no mass, and so the universe would be utterly empty and we wouldn't be here.
Our universe depends on motion and without motion our universe wouldn't exist.
Posted by: mythusmage
| January 22, 2010 1:58 AM
A singularity occurring in billions of locations can act as a single point.
I thought this was known...maybe not.
Posted by: Sphere Coupler | January 29, 2010 3:22 AM
how does falling water from the damm turn its potential enery to kenetic energy? the water is only falling a short distance though.
Posted by: Enrique | March 15, 2010 3:28 PM