Inflation is doing my head in

[billyharris72]

Not the price of scopes. I’m struggling to understand cosmic inflation - not the inflation itself but the need for it.

The horizon problem, as I understand it, says it is hard to understand why the Universe is the same temperature everywhere, given the impossibility of communication between parts of it and the absence of any reason to believe that everything started at the same temperature. That last bit is obviously important - if everything starts at the same temperature then there is no need for communication for the Universe to be homogeneous.

So my question is, why wouldn't the Universe be the same temperature everywhere? The Big Bang was not an explosion (an explosion requires containment) - it's not as if you had a ball of matter with the “bang” somehow pumping energy into it (with consequent pressure and temperature differentials). More an expanding mass of [what? Space, time, matter, energy] that all originates from the same point. So why, if it is all the same thing, would it not be perfectly uniform?

Same with the release of the CMB surely - that must have been released at the same temperature everywhere as atoms started to form and photons fell out of thermal equilibrium.

As always, I suspect I've missed something important, but as it is I'm struggling to see the problem that inflation is supposed to solve.

Anyone able to help?

Billy.

[vlaiv]

I'll try, not an expert though.

It is supposed to solve following: given uniform expansion without sudden inflation, early universe would be able to equalize any temperature differences whilst still young and small in diameter. This means that matter / energy density would be equal across the universe and there would be no possibility of creating large scale structures that we observe now (galaxy clusters, local groups, voids, etc ...).

I'm not sure that even galaxies would form in such even and smooth universe. Now due to quantum mechanics and uncertainty principle - fluctuations in energy are possible. Those happened in early universe and inflation phase made sure that such fluctuations were "frozen in time" - or when universe inflated it did so at much greater speed than speed of light, thus preventing any causal influence between regions that moved apart to happen, so no thermal equilibrium any more. Some regions were left in a bit more energy state, some regions in a bit less energy state, depending on random distribution of energy uncertainty when inflation started. So when we look at CMB variations and current large scale structure of universe we see that moment, frozen in time when inflation happened (hence correlation between the two - CMB and large scale structure).

Now, this is how I understand it, but as I've said I'm no expert and all of the above could be totally wrong

 

[Paul73]

The closest that I can get is to think of the Big Bang happening everywhere at once and then space expanding from there (rather than things moving through space).

There a number of different models for the evolution of the universe..... all guaranteed to hurt our heads!

It is a facinating field of conjecture.

Paul

[billyharris72]

I was thinking about that, and suapect it has something to do with it, but can't quite make it work in my head. I can get the idea of fluctuations that lead to localised differences in density, which starts the gravitational collapse leading to stars etc. But does the even distribution of galaxies stemming from these fluctuations not imply that these would be randomly and evenly distributed and so not capable of producing large, systematic differences in temperature anyway? A bunch of randomly distributed distances around the same mean temperature would imply that relatively local scale communication would even out any differences.

If these fluctuations (the ones that caused stars and galaxies to form) were big enough then surely the Universe would not be the same in all directions.

Billy.

[vlaiv]

5 hours ago, billyharris72 said:

I was thinking about that, and suapect it has something to do with it, but can't quite make it work in my head. I can get the idea of fluctuations that lead to localised differences in density, which starts the gravitational collapse leading to stars etc. But does the even distribution of galaxies stemming from these fluctuations not imply that these would be randomly and evenly distributed and so not capable of producing large, systematic differences in temperature anyway? A bunch of randomly distributed distances around the same mean temperature would imply that relatively local scale communication would even out any differences.

If these fluctuations (the ones that caused stars and galaxies to form) were big enough then surely the Universe would not be the same in all directions.

Billy.

Ok, so let's break down timeline into chunks and see what happens in each time interval.

First is Big Bang and time right afterwards - there is no matter, only energy, very high energy density as universe is really small and all the stuff there is in universe (think all galaxies, all the matter and energy we know today) is more or less uniformly distributed around. There are small scale quantum fluctuations (small both in terms of energy and localized in space). So it is all sort of uniform, except these fluctuations. And as time goes on, any disturbance is smoothed out, simply because disturbances are small and universe is small and it is easy to reach sort of dynamic equilibrium (think boiling water - there are bubbles around, but it is more or less at the same temperature).

Step two: inflation happens. Now this is how it goes. Universe is in state of dynamic equilibrium meaning somewhere it is tiny bit more energy, somewhere there is a bit less, but it kind of boils (places where there is a tiny bit more energy compared to average - just randomly move about). Now inflation starts and it expands universe at enormous speed. First thing to note is that there is still no matter as we know it, everything is in energy form, and it is really hot everywhere. As inflation happens - because energy density is so high and space expands rapidly there is a cool down everywhere, so local average density falls everywhere but because space is growing so fast that there is no way to maintain dynamic equilibrium - signal is just slower than the rate at which universe expands.

Although there is cool down everywhere it is still hot enough that only energy exists, so matter is not formed yet, things need to be cooler than this. But it is important to notice something. Remember how universe was really small and really dense? This means that now that it has expanded, there are large areas with a bit less energy and large areas with a bit more energy, and energy differences are not big, but areas (or should we call them volumes instead?) are really big. Size of this areas is not a single star system or single galaxy, or even small galaxy group. Size of these areas is comparable with largest scale structures that we know in universe, like great voids or walls or such. This means that there are regions of space large as that with a bit more energy, and a bit less energy. Ok, so this is when inflation happened.

(there are other things that happen in this process, but I'll skip those, partly because I have no clue about them, and partly because it is not important for this picture).

Next, at some point matter starts to appear, but it is still in form of hot gas all over the place, nothing solid yet, but there is something different between different areas that had just a bit different energy density. Gravity acts differently, depending on matter/energy density in that particular area. Matter starts to coalesce in regions with higher energy density, and pulls on matter in areas with lower energy density.

So we are left with regions with less matter and regions with more matter (no stars yet, just gas). Now, gravity is funny thing , it pulls with different force depending on a position, and there are local "centers" of mass that hardly move, and there are areas where pull is strong in one direction - edges between low and high density areas. So all sorts of "flows" happen and formation of local centers of mass, and these flows are responsible for all that rotation in universe that we see.

Bottom line: variations in energy density were really small, areas (or volumes) that we are talking about here have dimensions with magnitude of order hundreds of million light years, and temperature difference between those? In WMAP CMB image it shows:

Differences in in micro Kelvins. So inflation did not produce stars of galaxies, these formed by "normal" mechanisms. Inflation just created vast areas with a bit more matter and a bit less matter - but enough to stir things up when gravity started acting and expansion of universe slowed to "normal" rate.

[billyharris72]

Thanks for this explanation - it does a good job of describing these early events and how inflation could account for some of the large scale structure in the Universe.

In that sense it's good for getting a grip of why there are differences out there, that lead to the formation of galaxies etc. 

I'm still struggling with why we need it to account for the temperature of the CMB being basically the same everywhere, though. If we take the model you have set out above and remove inflation would the temperature not also be the same everywhere?

I suspect the answer is that I have fallen victim to the popular science magazine explanation of "We need x (inflation) to explain y (constant CMB)". I find in these explanations that x often does not entail y, but if they had said "We need x to explain y and reconcile it with z (in this case structure etc)" then the argument would have become clearer.

Probably still a gross simplification though. I feel forever trapped between an incomplete description and an explanation that is just too complicated. For example, yesterday I was also looking for a similarly easy answer to the question of why red giants have a lower surface temperature (i.e. why do they swell up to that extent and not a bit less, so that they have a hotter surface - hotter core = hotter surface seems intuitive)? My eventual conclusion? It's Christmas, and until Astronomy Now has a feature on integrating the hydrostatic equilibrium equations I'd better stick to the mince pies.

Billy.

[Whirlwind]

24 minutes ago, billyharris72 said:

I'm still struggling with why we need it to account for the temperature of the CMB being basically the same everywhere, though. If we take the model you have set out above and remove inflation would the temperature not also be the same everywhere?

It's a statistical argument. If there was a continuous universe with no inflation then random variations will eventually mean elements isolated from each other will become more statistically varied than we see. If you took isolated elements (pots) of the universe and left them to evolve on their own with no influence then there is an almost infinite number of ways that those 'pots' can evolve.  To have all of those 'pots' evolve in exactly the same way where there is an almost infinite number of ways they can vary is infinitesimally small (but not quite zero).  This is effectively entropy where an ordered system will always evolve to a disordered system.  So either we live in an extremely unlikely universe or, much more likely, the pots were linked in some way in the past which is why everything looks a lot more ordered than we would expect.   

[michaelmorris]

22 hours ago, billyharris72 said:

Thanks for this explanation - it does a good job of describing these early events and how inflation could account for some of the large scale structure in the Universe.

In that sense it's good for getting a grip of why there are differences out there, that lead to the formation of galaxies etc. 

I'm still struggling with why we need it to account for the temperature of the CMB being basically the same everywhere, though. If we take the model you have set out above and remove inflation would the temperature not also be the same everywhere?

 

I think you might be confusing 'random' with 'uniform'.  Random distributions are always going to look clumped.  Over a large enough spacial and temporal scale random will look like uniform. 

 

[vlaiv]

23 hours ago, billyharris72 said:

Thanks for this explanation - it does a good job of describing these early events and how inflation could account for some of the large scale structure in the Universe.

In that sense it's good for getting a grip of why there are differences out there, that lead to the formation of galaxies etc. 

I'm still struggling with why we need it to account for the temperature of the CMB being basically the same everywhere, though. If we take the model you have set out above and remove inflation would the temperature not also be the same everywhere?

I suspect the answer is that I have fallen victim to the popular science magazine explanation of "We need x (inflation) to explain y (constant CMB)". I find in these explanations that x often does not entail y, but if they had said "We need x to explain y and reconcile it with z (in this case structure etc)" then the argument would have become clearer.

Probably still a gross simplification though. I feel forever trapped between an incomplete description and an explanation that is just too complicated. For example, yesterday I was also looking for a similarly easy answer to the question of why red giants have a lower surface temperature (i.e. why do they swell up to that extent and not a bit less, so that they have a hotter surface - hotter core = hotter surface seems intuitive)? My eventual conclusion? It's Christmas, and until Astronomy Now has a feature on integrating the hydrostatic equilibrium equations I'd better stick to the mince pies.

Billy.

It looks like there is something to it. Here is a quote from Wikipedia about inflation:

"It explains the origin of the large-scale structure of the cosmos. Quantum fluctuations in the microscopic inflationary region, magnified to cosmic size, become the seeds for the growth of structure in the Universe (see galaxy formation and evolution and structure formation).[2] Many physicists also believe that inflation explains why the Universe appears to be the same in all directions (isotropic), why the cosmic microwave background radiation is distributed evenly, why the Universe is flat, and why no magnetic monopoles have been observed."

It does state that many physicists believe that inflation explains isotropy of Universe. It does not list source for this statement, nor does it provide any further insight as to why they believe so or how does it might explain isotropy of Universe.

[billyharris72]

1 hour ago, michaelmorris said:

I think you might be confusing 'random' with 'uniform'.  Random distributions are always going to look clumped.  Over a large enough spacial and temporal scale random will look like uniform. 

You may well have a point there, but does this help explain why we need inflation? Given that a random distribution is unlikely to produce any large scale structure, does this not imply that, even without Inflation, we would expect the CMB to have approximately the same temperature everywhere?

The picture I have in my head (and which is probably wrong) is of a large matrix of normally distributed values. Each cell is analogous to the "pots" described in Whirlwind's post above, and there is clear variation in values across the matrix. 

If we now replace each value with the mean of its value and its local surrounding cells we will rapidly end up with a (roughly) uniform distribution of values, even though cells that are far apart have never influenced each other. No need for anything like inflation in that scenario.

I fully accept I'm wrong on this, but can anyone point out where?

Billy.

Edit: of course the above depends on the size of the matrix, the distribution etc. If it's simply a matter of the CMB being more even than any sensible parameters would suggest then I'm happy with that.

[vlaiv]

19 minutes ago, billyharris72 said:

You may well have a point there, but does this help explain why we need inflation? Given that a random distribution is unlikely to produce any large scale structure, does this not imply that, even without Inflation, we would expect the CMB to have approximately the same temperature everywhere?

The picture I have in my head (and which is probably wrong) is of a large matrix of normally distributed values. Each cell is analogous to the "pots" described in Whirlwind's post above, and there is clear variation in values across the matrix. 

If we now replace each value with the mean of its value and its local surrounding cells we will rapidly end up with a (roughly) uniform distribution of values, even though cells that are far apart have never influenced each other. No need for anything like inflation in that scenario.

I fully accept I'm wrong on this, but can anyone point out where?

Billy.

Edit: of course the above depends on the size of the matrix, the distribution etc. If it's simply a matter of the CMB being more even than any sensible parameters would suggest then I'm happy with that.

Ok, I think I know what the problem is (and I did not even know that it existed as a problem )

It is known as Horizon problem, and inflation theory solves it.

I've found a 3 minute video that kind of explains it, but it does leave few questions unanswered.

 

 

[laowhoo]

I'm gonna read this thread but first thought I'd start w/ what I just re-visited yesterday, wherein your (OP) question is directly addressed/dismissed/what have you. The plasma cosmologists say it's all for naught, but of particular interest to me is the work of Halton Arp (Dobson's in here too), and the primitive art of those who may have been present to witness night sky anomalies when the solar system was still settling.

Cosmology Quest in 2 parts

https://www.youtube.com/watch?v=KmotCQCxQEI

https://www.youtube.com/watch?v=m-2uvQ_MJz8

(But I also love Guth, and his original model predicts that the size of the actual universe is to the observable universe [last estimated at 85 billion light years across] what the size of the observable universe is to an atom. But as Susskind reminds us, we can never know.) Here's a link to the plasma cosmologists' Thunderbolts Project Electric Universe site

https://www.thunderbolts.info/wp/

and YouTube page

https://www.youtube.com/user/ThunderboltsProject/videos

BTW, maybe fully half the astrophysics community no longer subscribes to the standard cosmological model. I'm probably more of a Big Bounce adherent along "standard" model lines, but I'm also reminded by Susskind that the Holographic Model is THE standard model in cosmology. If that's true, then what to do with any of it?

For archaeoastronomy buffs, you might like this, Symbols of an Alien Sky in 3 parts

https://www.youtube.com/watch?v=t7EAlTcZFwY&list=PLgA1Fo2P4HzCyZlhWGbziXrj91QZYZGyK&index=2

https://www.youtube.com/watch?v=tRV1e5_tB6Y&list=PLgA1Fo2P4HzCyZlhWGbziXrj91QZYZGyK&index=3

https://www.youtube.com/watch?v=34wtt2EUToo&list=PLgA1Fo2P4HzCyZlhWGbziXrj91QZYZGyK&index=4

There's also Penrose's lecture Aeons Before the Big Bang

https://www.youtube.com/watch?v=4YYWUIxGdl4

and Faith, Fantasy, and the Big Questions

https://www.youtube.com/watch?v=aaIdJMxP6bA

Jaoa's big bang

https://www.amazon.com/Faster-Than-Speed-Light-Speculation/dp/1422358836

etc., etc.

(Feel I'd be remiss to leave out Hamlet's Mill, if only as a reminder that we often don't credit our ancestors enough.  And here's to Zwicky who taught us how to properly respect mere academecians as spherical [removed word].)

[Tiki]

On ‎23‎/‎12‎/‎2017 at 15:03, billyharris72 said:

So my question is, why wouldn't the Universe be the same temperature everywhere?

 

According to quantum mechanics, there is a natural limit to how accurately one is able to specify the position and energy of an object. This means that it is very unlikely that any two objects will have the same temperature let alone a 'gazillion' objects such as in the early universe. Whilst the temperature of the early universe might very nearly be the same everywhere it won't be exactly the same. These 'quantum fluctuations' are expected to have evolved into relatively large differences (about 10% according to mid-1970's calculations) by the present day.

There is a natural limit to how well we can predict the interactions of snooker balls on a snooker table. If we specify the position of the balls and the initial velocity of the cue-ball as accurately as possible in accord with quantum mechanics then it is found that we are unable to predict what happens after a dozen or so cue-ball interactions. The equations governing snooker-ball paths are well known and are presumably more simple than the equations governing the evolution of the universe. By analogy, doesn't it make sense that the Universe should appear less isotropic than it actually does?

Although the horizon problem originates from the 1950's, it became much more of a problem once the CMB was detected and the scientific importance of Big Bang cosmologies became to be more widely realized.

 

I hope this helps.