# Inflation and Assuming the 'just so'

1. Mar 10, 2010

### Chimps

In essence, is it the case that there is no advancement in understanding that we still must assume that like in the classical big bang theory that conditions were 'just so' at the 'start'?

I don't have a problem with inflation but essentially the same problem remains. Conditions are assumed to be 'just so' with no predictions as to why.

The reason I mention this is that I keep hearing how inflation is such an incredible advancement yet the most fundamental aspect is not addressed. It still relies completely on the same 'just so' assumptions.

I would very much appreciate some expert opinions regarding this matter.

2. Mar 10, 2010

### bapowell

It is true that inflation does not address all the problems of standard big bang cosmology. However, it's a matter of opinion which aspects are the most fundamental. Inflation is often credited with solving the horizon and flatness problems. It solves the latter -- inflation provides a dynamics mechanism that forces the universe towards flatness. That is one fine-tuning that it removes. However, the horizon problem is not so easy, and it has to do with your 'just so' conditions at the start. Inflation leads to a Hubble-scale homogeneity. However, it seems to require homogeneity at the Hubble scale to get started in the first place (see http://arxiv.org/abs/gr-qc/9811037" [Broken].) So some fine-tuning of the initial conditions still seems to be required to get inflation going. However, this result is not universally applicable. There are ways in which such homogeneity could come about, and eternal chaotic-type inflation is one way.

Lastly, inflation does one more thing for us. It provides an elegant mechanism for generating the primordial density fluctuations, and remains one of the only explanation for the origin of super-horizon correlations seen in the CMB.

Last edited by a moderator: May 4, 2017
3. Mar 10, 2010

### Wallace

Chimps, you are correct in a way, inflation does not solve everything. But it's not true to say that there is no advancement. Inflation makes specific predictions beyond that for which the theory was formulated, and these predictions have been later observed, primarily in CMB experiments. In any case inflation is a quite general concept, rather than a specific theory. There are a number of different classes of inflationary models, and a lot of progress has been made over the last ~20 years (particularly the last ~5 years) in narrowing down the kind of inflationary model that works.

So while there are many outstanding questions, this is not because they are not being 'addressed'. Furthering our knowledge of inflation is a major driver in a lot of new and planned experiments. One major example is a number of experiments being planned to measure polarization in the CMB. If this are successful, they should tell us a lot more about inflation (or possibly find problems with the whole paradigm, if that is what the data says).

If you want to get philosophical though, even if we completely understand inflation one day, there will still be unanswered questions about what happened before that and why. If we answer them that will lead to more questions.. We will never know everything, and there will always be a point at which our knowledge falters, and we have to say it was 'just so'. The goal of science is not to remove that, but to push it as far away as possible.

4. Mar 12, 2010

### Chimps

Thanks very much for the interesting replies.

Regarding the flatness problem, is this really a problem? I thought oberservation had shown that the universe is very probably flat which means it is either flat, or in the case of inflation it doesn't have to be but it will look flat anyway. Is there any evidence suggesting that the universe is not flat?

5. Mar 12, 2010

### bapowell

Hi Chimps,
You're right about the universe being very close to flat today. The flatness problem is how did it get that way?. In the standard cosmology, a critical (flat) universe is an unstable equilibrium -- the universe tends to evolve away from flatness. In this case, given the high degree of spatial flatness observed today, the universe had to be $$10^{10}$$ times more flat in the early universe. However, in inflationary cosmology, the universe is driven towards flatness as it expands. This alleviates the fine-tuning associated with the initial density of the universe present in the standard cosmology.

6. Mar 12, 2010

### Chimps

Am I right in thinking that if the universe is 100% flat then it will stay flat anyway. If that is the case then the flatness problem is surely redundant anyway.

The value from observation seems to be so close that is it not a reasonable assumption that the universe is flat? In which case, is it even neccessary to ask how it got that way? You don't need inflation to expain it since it fits with older big bang models which assume a flat universe.

7. Mar 13, 2010

### bapowell

No. That's what I'm trying to say. In the standard cosmology, the universe always evolves away from flatness. So, if it's very flat today, then it had to be extremely flat in the early universe. This is considered a fine-tuning and gives rise to the flatness problem. Why would the universe start out in such a finely tuned initial state (with a density $$\Omega = 1.00000000001$$).

The universe is indeed very flat. But it is important to understand how it got that way because, as I mention above, it's an unstable solution. However, in an inflating universe, it doesn't matter so much what the initial value of the curvature was. The universe could have started out in a state with (essentially) arbitrary curvature and inflation would force it towards flatness. In other words, a flat universe is a dynamical attractor of the inflationary equations of motion.

8. Mar 13, 2010

### marcus

Chimps, maybe I should just chime in and say that AFAICS Powell is giving you the straight dope. The near flatness does need an explanation, we have to find a natural cause for it.

Because good old 1915 Gen Rel is basically our only theory of gravity, practically speaking. It checks out to amazing accuracy against all kinds of data (on the earth surface, in low orbit, in the solar system, on galactic scale, and beyond.)

Unless you want to throw out Gen Rel, and have something equally accurate to replace it, you have to accept that flatness is the exception, it is like Powell says unstable. A little random fluctuation and the curving crumpling starts to build up.

So for the U to be as nearly flat as we see today it had to be jillions times flatter in the past.

You might like a LAYMAN PEDAGOGICAL explanation of the math, so you can check that Powell and I are not just making a simple mistake or kidding you. One article I know is an old one 2003 by Charlie Lineweaver (world class Australian cosmologist) called "Inflation and the Cosmic Microwave Background". Here's a link in case you want to look.
http://arxiv.org/abs/astro-ph/0305179

Last edited: Mar 13, 2010
9. Mar 13, 2010

### marcus

We can see that the universe is not 100% flat, Chimps. Funhouse optics, gravitational lensing, magnified images, distorted images with circles turned to ovals, multiple images and socalled "Einstein rings". The photographs don't lie. All kinds of optical distortion done by space, by the unflat geometry of space, is visible in the pictures telescopes take.

The U geometry is obviously bumpy. Nobody claims it is actually 100% flat!

It sounds like you want to dismiss the socalled flatness problem---just wave your hands and make it go away

Maybe someone told you "the universe is flat" and you believed it. If it was an astronomer they didn't really mean exactly 100%, they were oversimplifying. Get over it

If you want to know what is really happening, cosmo is a mathematical science. We have a math model of the overall geometry, and how it evolves. The model fits the data amazingly well. Millions of datapoints. We do not understand the model completely. It leaves certain things unexplained. There are real problems, real questions. And moreover we can expect that the presentday model will be modified (as has regularly happened in many branches of mathematical science) and replaced by a betterfitting model with fewer problems. In the meantime this is the best we've got and we have to take seriously whatever it does not adequately explain, and try various add-ons and modifications and fixes. So please don't knock the socalled flatness problem. It's there. It's bugging some good people and making them think.

Here's the latest on it: a paper called "Entropic Inflation" by 3 smart guys Easson, Frampton, Smoot.
http://arxiv.org/abs/1003.1528 (Entropic Inflation)
They think they have a way that inflation doesn't need an "inflaton" field to drive it. Smoot is Nobel. Another Nobel, Weinberg, has also proposed a different mechanism for inflation that doesn't need an "inflaton".
http://arxiv.org/abs/0911.3165 (Asymptotically Safe Inflation)
These guys try to dynamite the old "just so" and dig down to a deeper layer of "just so". The "just so" of nature is an Onion and it is one of the most beautiful things humans have yet discovered. If you like it, have fun, if you don't, forget it--what you don't know can't hurt you.

Last edited: Mar 13, 2010
10. Mar 17, 2010

### Chimps

Or just completely flat in its nature? flat at the beginning and flat today.

I'm not just dismissing the flatness problem because I don't like it. Not at all.

As you and others have pointed out it would tend massively towards unflatness to such a scale that it would obviously be very unflat today - which it isn't with or without inflation. Is the fact that it is only around 10% away from flatness when it should be 'jillions' away not a sign that our methods for observation are not perfect?

11. Mar 17, 2010

### marcus

Verbal explanations don't cut it. Cosmology is a mathematical science. But if you or anyone can produce a mathematical model of the geometry of the universe, that fits the sky survey data as well as the one we have, and reproduces the observed local bumpiness, AND reproduces a largescale average curvature near zero, without inflation...then that would be interesting.

Not sure what "completely flat in its nature" would mean, Chimps. Whatever it means, it doesn't seem to correspond to reality because we know that geometry is bumpy here in the solar system and we see Einstein rings and stuff. Funhouse optics caused by clusters of galaxies. There is all kinds of weird geometric distortion that we can see with telescopes. Gravitational lensing.

Gravity works by bending geometry--there is no rational way we can pretend that real world geometry has a "nature" to be perfectly flat. Gravity wouldn't work.

The whole puzzle has to do with a large scale average. Why does the large average work out so close to zero? How does one part of space know about another part of space, say 5 billion lightyears away, so that if one part is a bit positive curved the other part can be a bit negative, and the average will work out to be near Zero? It's like a conspiracy between regions that should have no obvious means to influence each other.

If something is naturally bent and bumpy and develops funhouse optics, spontaneously, and develops black holes and all that, what has made the AVERAGE turn out to be close to zero?

You may not recognize that as a problem, and it may not stimulate your curiosity. That's fine. People have different threshholds of curiosity, and be alert to different sorts of puzzles.

I would like a rational explanation. So would many other people. If there is something about the universe like this which has no obvious explanation, I'm curious. I would like to know a MECHANISM that conduces large scale average curvature to be near zero.

It might have a clue to some deeper mechanism, at the quantum geometry level, where geometry and matter interact. We don't yet know what underlies both of them making them interact the way they do.

Our present theory of gravity is General Relativity, and it is also our model of geometry. It gives us our mathematical model of the evolving geometry of the universe. If you want to replace our model of geometry, you have to provide a replacement theory of gravity. Let me essentially repeat what I said for starters:

Verbal explanations don't cut it. Cosmology is a mathematical science. But if you or anyone can produce a mathematical model of the geometry of the universe (in other words a theory of gravity), that fits the sky survey data as well as the one we have, and reproduces the observed local bumpiness, PLUS IN ADDITION explains the largescale average curvature being near zero, without inflation...then that would be interesting.

Last edited: Mar 17, 2010
12. Mar 17, 2010

### Chimps

Thanks for the interesting reply Marcus.

I haven't got a mathematical model that can explain the universe any better than the standard model so I accept that my verbal explanations are insufficient (and probably quite tedious) but I think I can understand where I might be going wrong.

I think my idea of flatness might be slightly flawed so I will try to examine it again and hopefully reach a better understanding.

Thanks again.

13. Mar 18, 2010

### Chronos

The latest WMAP results suggest the universe is flat within a 2% margin of error. This does not suggest it is either finite or infinite.