Why is cosmological inflation necessary?

For the discussion of the sciences. Physics problems, chemistry equations, biology weirdness, it all goes here.

Moderators: gmalivuk, Moderators General, Prelates

D-503
Posts: 84
Joined: Sun Apr 15, 2012 11:35 pm UTC

Why is cosmological inflation necessary?

Postby D-503 » Sat Jul 05, 2014 5:54 am UTC

I'm skeptical of the theory of inflation. The universe suddenly expanding by several orders of magnitude then stopping a fraction of a second later raises more questions than it answers to my mind. I've read that inflation accounts for the homogeneity and isotropy of the observable universe, similar to the way that zooming into a picture produces pixels of only a few colors. However, this presumes that the universe was initially inhomogeneous. Is there a reason to make that assumption? It seems like the simpler theory would be to assume that the universe started out homogeneous. I've also read that inflation accounts for the observed flatness and absence of magnetic monopoles. However, I don't understand how it accounts for them. Would someone be kind enough to explain? I might find that justification more convincing.

Tchebu
Posts: 564
Joined: Thu Feb 22, 2007 12:42 am UTC
Location: Montreal

Re: Why is cosmological inflation necessary?

Postby Tchebu » Sat Jul 05, 2014 2:57 pm UTC

The main idea is to assume as little as possible about the initial state of the universe. It's pretty bad to fine-tune the laws of nature, but it's even worse to fine-tune the initial state of the universe. For laws of nature there might be some sorts of consistency conditions that we don't know about yet that might force them to be the way they are and explain the fine tuning. Historically there's a pattern of that happening. This is sort of the armchair theorist's dream.

You can't expect that sort of thing regarding the initial state of the theory, since any given theory can describe time evolution starting from any state that exists in the theory. What you CAN do is show that the dynamics of the theory are such that regardless of which state you start in, all the trajectories in state-space will quickly converge so that the initial state doesn't really matter all that much. This is what inflation does and why people take it seriously.

Assuming initial homogeneity is very stringent fine tuning. It also doesn't explain the inhomogeneities that are small but nonetheless present, unless you want to make the fine tuning even more ridiculous and hardwire the inhomogeneities into the initial state as well. Inflation, on the other hand, provides a mechanism for obtaining both the homogeneity as well as the extent to which it's broken that is pretty much insensitive to the initial state of the universe.

The flatness problem is a similar thing. Once again we get a mechanism that forces things to be the way they are regardless of the initial state of the universe.
In a universe dominated by matter and radiation, as ours has been throughout most of its history, the matter density diverges very quickly from the critical value which makes the universe flat. To have it still be flat today requires that the matter density be so ridiculously close to the critical value in the early universe that the exponential growth away from it still didn't kick in today. Turns out that a rapid expansion such as inflation (which is not driven by matter and radiation) forces the matter density to approach this critical value very quickly regardless of the initial state.

The monopole thing is basically just that we haven't found any although our current particle physics theories strongly suggest that they should be formed in the early universe in large amounts. Inflation is an excellent way to dilute the monopole density to well within experimental constraints. I don't think anyone is proposing this as the main argument for inflation, it's more of a bonus that comes with it if we want to hold on to our GUT's, which most people do.

So basically inflation provides us with a simple mechanism which takes in a very generic initial state of the universe and spits out a universe much like ours on cosmological scales.
Our universe is most certainly unique... it's the only one that string theory doesn't describe.

D-503
Posts: 84
Joined: Sun Apr 15, 2012 11:35 pm UTC

Re: Why is cosmological inflation necessary?

Postby D-503 » Sun Jul 20, 2014 11:08 am UTC

Making the state space converge is a compelling feature. However, I still wonder if there could be a simpler theory that achieves the same thing. What if all elementary particles shrink at a constant proportional rate (e.g. they half in size every billion years) causing space to expand exponentially, and the homogeneity of the universe is caused by a sort of diffusion that happened when the particles were all giant and overlapping, while the small scale variations were caused by quantum fluctuations?
I'm sure this idea is misguided but I would learn a lot from finding out why.
Last edited by D-503 on Sun Jul 20, 2014 2:28 pm UTC, edited 1 time in total.

andyisagod
Posts: 146
Joined: Wed Apr 23, 2008 10:19 pm UTC

Re: Why is cosmological inflation necessary?

Postby andyisagod » Sun Jul 20, 2014 2:26 pm UTC

D-503 wrote:Making the state space converge is a compelling feature. However, I still wonder if there could be a simpler theory that achieves the same thing. What if all elementary particles shrink at a constant exponential rate causing space to expand, and the homogeneity of the universe is caused by a sort of diffusion that happened when the particles were all giant and overlapping, while the small scale variations were caused by quantum fluctuations?
I'm sure this idea is misguided but I would learn a lot from finding out why.


I'm not quite sure how this could be described as simpler than inflation. The actual form of the inflationary potential might be somewhat complicated but it's still all within the realm of stuff that we know how to do, scalar fields, axion potentials etc. Introducing a whole new theory about non point like elementary particles to replace QFT seems like an incredibly complicated undertaking.

D-503
Posts: 84
Joined: Sun Apr 15, 2012 11:35 pm UTC

Re: Why is cosmological inflation necessary?

Postby D-503 » Sun Jul 20, 2014 3:02 pm UTC

I think it's simpler because it doesn't introduce a new type of physical phenomenon. We know with a pretty high level of certainty that the universe is currently expanding and it would be simpler if we could find a way to use the same phenomenon to explain the formation of the early universe. The stuff about particles shrinking might not be necessary. I think it's a more intuitive way to describe the expansion of space. I introduced the idea because it provides a way to justify an exponential rate of expansion rather than a linear one so particles in the early universe would have more time to intermingle. I've read that inflation explains how information could have traveled between disparate regions of the universe so they can become homogeneous. I think having a longer period of time when the universe was opaque (perhaps infinitely long) might accomplish the same thing. I suspect that the idea I suggested can be proven false because I've read estimates that the pre-CMB opaque universe lasted about 300,000 years, and if there is solid evidence of that, my theory doesn't stand a chance.

User avatar
LaserGuy
Posts: 4586
Joined: Thu Jan 15, 2009 5:33 pm UTC

Re: Why is cosmological inflation necessary?

Postby LaserGuy » Sun Jul 20, 2014 5:40 pm UTC

D-503 wrote:I think it's simpler because it doesn't introduce a new type of physical phenomenon. We know with a pretty high level of certainty that the universe is currently expanding and it would be simpler if we could find a way to use the same phenomenon to explain the formation of the early universe. The stuff about particles shrinking might not be necessary. I think it's a more intuitive way to describe the expansion of space. I introduced the idea because it provides a way to justify an exponential rate of expansion rather than a linear one so particles in the early universe would have more time to intermingle. I've read that inflation explains how information could have traveled between disparate regions of the universe so they can become homogeneous. I think having a longer period of time when the universe was opaque (perhaps infinitely long) might accomplish the same thing. I suspect that the idea I suggested can be proven false because I've read estimates that the pre-CMB opaque universe lasted about 300,000 years, and if there is solid evidence of that, my theory doesn't stand a chance.


Well, the age of the universe is pretty well established experimentally, so the time between the surface of last scattering and the Big Bang certainly can't be infinite. The redshift of the CMB can be measured (apparently it's z = 1091 +/- 1), so we can figure out the time difference between the age of the universe and the surface of last scattering from that, I think.

User avatar
doogly
Dr. The Juggernaut of Touching Himself
Posts: 5538
Joined: Mon Oct 23, 2006 2:31 am UTC
Location: Lexington, MA
Contact:

Re: Why is cosmological inflation necessary?

Postby doogly » Sun Jul 20, 2014 9:29 pm UTC

Also, there is no fundamental notion of particles, it's quantum fields, so their "size" is something that would be very difficult to dynamically modify, and even more so across Hubble horizons.
LE4dGOLEM: What's a Doug?
Noc: A larval Doogly. They grow the tail and stinger upon reaching adulthood.

Keep waggling your butt brows Brothers.
Or; Is that your eye butthairs?

D-503
Posts: 84
Joined: Sun Apr 15, 2012 11:35 pm UTC

Re: Why is cosmological inflation necessary?

Postby D-503 » Mon Jul 21, 2014 4:31 am UTC

LaserGuy wrote:Well, the age of the universe is pretty well established experimentally, so the time between the surface of last scattering and the Big Bang certainly can't be infinite. The redshift of the CMB can be measured (apparently it's z = 1091 +/- 1), so we can figure out the time difference between the age of the universe and the surface of last scattering from that, I think.


But the red-shift from the CMB is presumably the red-shift of the surface of last scattering? Since the universe was opaque before then there would be no light to measure the red-shift of. AFAICT the 300,000 year value is an extrapolation from the model rather than something we have observational evidence of.

doogly wrote:Also, there is no fundamental notion of particles, it's quantum fields, so their "size" is something that would be very difficult to dynamically modify, and even more so across Hubble horizons.


Why would the "size" of quantum fields be difficult to dynamically modify? To me QFs shrinking and space expanding are equivalent theories, and don't we assume space expands at the same rate across Hubble horizons?

Tchebu
Posts: 564
Joined: Thu Feb 22, 2007 12:42 am UTC
Location: Montreal

Re: Why is cosmological inflation necessary?

Postby Tchebu » Mon Jul 21, 2014 4:53 am UTC

To me QFs shrinking and space expanding are equivalent theories

If they are equivalent, then the horizon problem should still exist, just translated into the weird "shrinking particles" language.
Our universe is most certainly unique... it's the only one that string theory doesn't describe.

User avatar
doogly
Dr. The Juggernaut of Touching Himself
Posts: 5538
Joined: Mon Oct 23, 2006 2:31 am UTC
Location: Lexington, MA
Contact:

Re: Why is cosmological inflation necessary?

Postby doogly » Mon Jul 21, 2014 5:08 am UTC

What in a QFT would you want to dynamically change in order to have the effect of changing the cross section? Which interactions would you change the cross section for? All of them, by the same rate? How?
LE4dGOLEM: What's a Doug?
Noc: A larval Doogly. They grow the tail and stinger upon reaching adulthood.

Keep waggling your butt brows Brothers.
Or; Is that your eye butthairs?

D-503
Posts: 84
Joined: Sun Apr 15, 2012 11:35 pm UTC

Re: Why is cosmological inflation necessary?

Postby D-503 » Mon Jul 21, 2014 5:33 am UTC

doogly wrote:What in a QFT would you want to dynamically change in order to have the effect of changing the cross section? Which interactions would you change the cross section for? All of them, by the same rate? How?

I don't know enough about QFT to say. To redress the questions I'm trying to get at, I've read that there is evidence that the universe is expanding at an accelerating rate. In this case, I think exponential growth is the simplest assumption to make because it has only a single parameter. In an exponential growth curve there is no zero point, it just keeps doubling in size at a fixed interval, so if the universe grows along such a curve, the pre-CMB phase could have lasted infinitely long. Given that, all regions of the universe could have interacted with each-other at some point in the past allowing it to become homogeneous, removing one of the reasons to use the theory of inflation. However, this directly contradicts the model that says the pre-CMB phase lasted around 300,000 years. I want to know if there is observational evidence to support that timeline, or if it is an extrapolation from a model that makes different assumptions. I also want to know if exponential growth is consistent with our observations of the post-CMB universe, or if it would have consequences in the pre-CMB universe that would make it impossible.

User avatar
PM 2Ring
Posts: 3715
Joined: Mon Jan 26, 2009 3:19 pm UTC
Location: Sydney, Australia

Re: Why is cosmological inflation necessary?

Postby PM 2Ring » Mon Jul 21, 2014 8:36 am UTC

Take a look at the Graphical timeline of the Big Bang. (All blue texts in that diagram are links). Notice that inflation (in the standard inflation theory) happens quite early, triggered by the phase change at the end of the grand unification epoch. If we delay inflation (or some equivalent process) we not only need to explain an alternative trigger mechanism, we have to allow it to operate for a longer time period in order to get the observed homogeneity.

Inflation reduced the density of matter and of energy in the universe, so if we delay it, the pre-inflation processes last longer, but the post-inflation processes should still take roughly the same amount of time as they do in the standard model, since those processes depend on the types of matter and energy present and on the energy density. Eg, the era of Big Bang nucleosynthesis couldn't begin until protons existed and the universe was cool enough for helium nuclei to be stable, and it had to stop when the density and temperature dropped below the values necessary for fusion to occur.

andyisagod
Posts: 146
Joined: Wed Apr 23, 2008 10:19 pm UTC

Re: Why is cosmological inflation necessary?

Postby andyisagod » Mon Jul 21, 2014 9:30 pm UTC

D-503 wrote:I think it's simpler because it doesn't introduce a new type of physical phenomenon. We know with a pretty high level of certainty that the universe is currently expanding and it would be simpler if we could find a way to use the same phenomenon to explain the formation of the early universe.


Just to be clear you are the one introducing a whole new physical phenomenon. Inflation fits nicely into the theories we already have. As you say we know the universe is currently expanding all inflation is saying is that it expanded (much) faster for a period in the past.

I've read that there is evidence that the universe is expanding at an accelerating rate. In this case, I think exponential growth is the simplest assumption to make because it has only a single parameter.


This definitely doesn't match our observations. Compare what the size of the universe as a function of time would look like in an always exponentially expanding universe vs one in which you have inflation which then ends followed by slower expansion and you can clearly see these aren't equivalent.

User avatar
drachefly
Posts: 197
Joined: Thu Apr 23, 2009 3:25 pm UTC

Re: Why is cosmological inflation necessary?

Postby drachefly » Mon Jul 21, 2014 9:50 pm UTC

Tchebu wrote:The monopole thing is basically just that we haven't found any although our current particle physics theories strongly suggest that they should be formed in the early universe in large amounts.


What?

User avatar
thoughtfully
Posts: 2253
Joined: Thu Nov 01, 2007 12:25 am UTC
Location: Minneapolis, MN
Contact:

Re: Why is cosmological inflation necessary?

Postby thoughtfully » Mon Jul 21, 2014 10:23 pm UTC

drachefly wrote:
Tchebu wrote:The monopole thing is basically just that we haven't found any although our current particle physics theories strongly suggest that they should be formed in the early universe in large amounts.


What?

Not in any verified theories, but stuff like monopoles (and other topological defects), baryon nonconservation, etc. feature fairly consistently in most GUT and beyond models.
Image
Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away.
-- Antoine de Saint-Exupery

User avatar
doogly
Dr. The Juggernaut of Touching Himself
Posts: 5538
Joined: Mon Oct 23, 2006 2:31 am UTC
Location: Lexington, MA
Contact:

Re: Why is cosmological inflation necessary?

Postby doogly » Mon Jul 21, 2014 10:29 pm UTC

Right, the unverified predict an unobserved thing, and this worries some people.

I would love for these people's problems in my life.
LE4dGOLEM: What's a Doug?
Noc: A larval Doogly. They grow the tail and stinger upon reaching adulthood.

Keep waggling your butt brows Brothers.
Or; Is that your eye butthairs?


Return to “Science”

Who is online

Users browsing this forum: No registered users and 8 guests