PDA

View Full Version : horizon problem with BB theory



north
2008-Sep-03, 05:42 PM
this new to me I've never heard of this before

can anybody explain what the problem is ?

tusenfem
2008-Sep-03, 06:10 PM
1. there is no problem
2. be more specific
3. if not 2 then 1

BioSci
2008-Sep-03, 06:49 PM
this new to me I've never heard of this before

can anybody explain what the problem is ?

Here is a simple explanation: http://en.wikipedia.org/wiki/Horizon_problem


The horizon problem is a problem with the standard cosmological model of the Big Bang which was identified in the 1970s. It points out that different regions of the universe have not "contacted" each other due to the great distances between them, but nevertheless they have the same temperature and other physical properties. This should not be possible, given that the exchange of information (or energy, heat, etc.) can only take place at the speed of light. The horizon problem may have been answered by inflationary theory, and is one of the reasons for that theory's formation.

north
2008-Sep-03, 07:02 PM
well according to Joao Magueijo ( cosmologist ) there is a problem

for according to him there is no way that one corner of the Universe could communicate with the far corner of the Universe , which makes everything fundamentally the same

just saw his program on DiscoveryHD this morning

interesting

01101001
2008-Sep-03, 07:30 PM
well according to Joao Magueijo ( cosmologist ) there is a problem

Ripe to be taken to ATM?

Wikipedia: Joćo Magueijo (http://en.wikipedia.org/wiki/Jo%C3%A3o_Magueijo)


He is a pioneer of the varying speed of light (VSL) theory of cosmology, which proposes that the speed of light was much higher in the early universe, of 60 orders of magnitude faster than its present value. It is presented as an alternative to the more mainstream theory of cosmic inflation.

north
2008-Sep-03, 07:34 PM
Ripe to be taken to ATM?

Wikipedia: Joćo Magueijo (http://en.wikipedia.org/wiki/Jo%C3%A3o_Magueijo)



He is a pioneer of the varying speed of light (VSL) theory of cosmology, which proposes that the speed of light was much higher in the early universe, of 60 orders of magnitude faster than its present value. It is presented as an alternative to the more mainstream theory of cosmic inflation.

and cosmic inflation cannot , at least at this point , be proved

01101001
2008-Sep-03, 07:39 PM
and cosmic inflation cannot , at least at this point , be proved

But it is mainstream. If you wish to argue against it, ATM (http://www.bautforum.com/against-mainstream/)'s that place you know.

north
2008-Sep-03, 07:56 PM
Originally Posted by north
and cosmic inflation cannot , at least at this point , be proved


But it is mainstream. If you wish to argue against it, ATM (http://www.bautforum.com/against-mainstream/)'s that place you know.

I'm not really arguing against cosmic inflation per-say

I just wanted to know more about this horizon problem and nobody has explained it so far

so can someone explain the horizon problem , fully

01101001
2008-Sep-03, 08:01 PM
so can someone explain the horizon problem , fully

It's been explained. What part wasn't full? Can you be more specific as to what your unaddressed interest is (that doesn't involve promoting ATM)?

Maybe you want different words? Ned Wright: Cosmology Tutorial -- Part 3 (http://www.astro.ucla.edu/~wright/cosmo_03.htm)


[...] These regions have no events in common, but the two temperatures are equal to better than 1 part in 10,000. How is this possible? This is known as the "horizon" problem in cosmology.

Part 4 (http://www.astro.ucla.edu/~wright/cosmo_04.htm)


The "inflationary scenario", developed by Starobinsky and by Guth, offers a solution to the flatness-oldness problem and the horizon problem. [...]

Celestial Mechanic
2008-Sep-03, 08:15 PM
I'm not really arguing against cosmic inflation per se.
I just wanted to know more about this horizon problem and nobody has explained it so far. [Snip!]
Try the various Ned Wright links that have been posted here, and if all else fails, get thee to a library. :)

tusenfem
2008-Sep-03, 08:15 PM
Oops, sorry north I misread BH instead of BB, then the question about the horizon problem makes more sense,sort of.

Tim Thompson
2008-Sep-03, 08:46 PM
can anybody explain what the problem is ?
See Cosmology: The Science of the Universe by Edward Harrison, Cambridge University Press 2000 (2nd ed), page 441, "The Horizon Riddle" and page 442, "The Horizon Problem". The basic problem is well expressed in a reflection presented by Harrison on page 455:

How can the properties of the universe be explained by causes in the past when interactions over large distances could not exist? The horizon problem became more urgent with the discovery of the smoothness and isotropy of the cosmic microwave background radiation that decoupled at the end of the early universe when the age of the universe was a few hundred thousand years. The difference in the radiation from opposite sides of the sky is less than 1 part in 100,000. Yet the emitting regions at the time of decoupling were far apart and outside each other's particle horizon. If these emitting regions do not "know" that each other exists, how can they be in identical states? The beauty of inflation is that it solves this problem.

Alan Guth introduced the idea of inflationary cosmology in 1981 to solve this problem (Inflationary universe: A possible solution to the horizon and flatness problems (http://adsabs.harvard.edu/abs/1981PhRvD..23..347G), Physical Review D 23(2): 347-356, 15 Jan 1981) and later wrote a book on the topic (The Inflationary Universe (http://www.amazon.com/Inflationary-Universe-Alan-Guth/dp/0201328402/ref=sr_1_3?ie=UTF8&s=books&qid=1220473070&sr=1-3), published in 1998).

In order to create some competition for Guth's unopposed idea of inflation, Joćo Magueijo and Andreas Albrecht came up with the idea of a variable speed of light (VSL) cosmology in 1999 (Time varying speed of light as a solution to cosmological puzzles (http://adsabs.harvard.edu/abs/1999PhRvD..59d3516A), Physical Review D 54(4): 043516, 15 Feb 1999) and Magueijo later wrote a book on his idea (Faster Than the Speed of Light: The Story of a Scientific Speculation (http://www.amazon.com/Faster-Than-Speed-Light-Speculation/dp/0142003611/ref=sr_1_1?ie=UTF8&s=books&qid=1220473285&sr=1-1), published in 2004). Magueijo was at the time considering variability of the fundamental constants as a solution to cosmological problems (i.e., Barrow & Magueijo, 1999 (http://adsabs.harvard.edu/abs/1999PhLB..447..246B), Barrow & Magueijo, 1998 (http://adsabs.harvard.edu/abs/1998PhLB..443..104B)) and evidently came to realize that the speed of light was really the only one we can play around with while maintaining sanity.

Inflationary cosmology remains the mainstream idea primarily because it presents fewer fundamental problems. It is also easier to demonstrate compatibility between inflation and other fundamental principles of physics than it is for VSL cosmologies.

Sam5
2008-Sep-03, 10:30 PM
Couldn’t it be explained a little more simply, such as by giving an analogy such as the way humans separated hundreds of thousands of years ago (or how ever long it was) and moved into Asia and then into the Western Hemisphere. While all over the world they continued to develop normally while being separated, and they had similar lifestyles and even constructed similar cities, all without having any more “contact” with one another for many millennia until the 1500s and 1600s.

For example, the Mayans didn’t have to learn how to build pyramids by first communicating with the Egyptians. Governments sprang up independently of one another. Agriculture too.

If the stuff that separated in the universe a long time ago was quite similar, why shouldn’t it continue to evolved and develop just like all the other stuff elsewhere in the universe?

Lepton
2008-Sep-04, 04:01 AM
I'm not really arguing against cosmic inflation per-say

I just wanted to know more about this horizon problem and nobody has explained it so far

so can someone explain the horizon problem , fully

I don't see how it is a problem UNLESS one wants to take a non-mainstream approach.

Jim
2008-Sep-04, 12:40 PM
See ... Alan Guth introduced the idea ... In order to create some competition ... Inflationary cosmology remains the mainstream idea ...

Thank you, Tim. I love it when you get all explanatory, especially since I can actually follow it.

Sp1ke
2008-Sep-04, 12:53 PM
For example, the Mayans didn’t have to learn how to build pyramids by first communicating with the Egyptians. Governments sprang up independently of one another. Agriculture too.

If the stuff that separated in the universe a long time ago was quite similar, why shouldn’t it continue to evolved and develop just like all the other stuff elsewhere in the universe?

It's not that everything is similar, it's that its remarkably similar. A better analogy would be if all the aliens in Star Trek evolved separately but they all ended up speaking English without ever having contact with each other.

As the Wikipedia article says:
"...the universe is in fact extremely homogeneous. For instance, the cosmic microwave background radiation (CMB), which fills the universe, is almost precisely the same temperature everywhere in the sky, about 2.725 K. The difference in temperature is so slight that it has only recently become possible to develop instruments even capable of measuring it."

Cougar
2008-Sep-04, 03:02 PM
And the problem is, in simple terms, look to the left and measure the temperature of the CMB, which has been traveling for something like 13 billion years. Now look to the right and do the same. The originating locations of those CMB photons are farther apart than even light could travel in the entire age of the universe. Therefore, those locations could never have been causally connected. Yet their temperatures are almost exactly the same, which seems to imply they must have been connected at some point. Hence, a problem.

A brief period of exponential expansion would solve this problem. Nothing in science is ever "proven," by the way. Theories have varying degrees of observational support. The big bang has quite a bit of support. Cosmic Inflation has a little, but its support is accumulating. (http://wmap.gsfc.nasa.gov/news/PressRelease_03_06.html)

Tim Thompson
2008-Sep-04, 06:08 PM
The originating locations of those CMB photons are farther apart than even light could travel in the entire age of the universe.
And the attentive reader should keep in mind that this is true for every pair of points on opposite sides of the entire sky. That is a big deal.


Couldn’t it be explained a little more simply, such as by giving an analogy such as the way humans separated hundreds of thousands of years ago ...
Not at all. As described above, look at the CMB for any pair of points antipodal to each other on the sky, and they are identical except for the very small difference in temperature. People, on the other hand, did some things common to all people as they spread around the world (i.e., build settlements and establish agriculture & etc.). But the civilizations they created were far from identical and were arguably not even "similar". The cultures & languages that developed in China, India and the fertile crescent were remarkably different from each other. So the analogy is really not a very good one anyway.

But physics is not necessarily so flexible. Suppose we pick two random points on the sky and measure the CMB spectral energy density (SED). We find that for both points the CMB SED is both (a) thermal (in the strictly defined sense) and (b) both within a millikelvin of the same black body temperature. That could be a coincidence, or it could imply that the two points had been in thermal contact long enough to reach thermal equilibrium with each other. As long as we are only talking about two points, it could easily be a coincidence. But now suppose we do the same thing for every random pair of points that exist on the sky and find the same identical result in every case. Now the coincidence argument is simply dead, the odds against such a coincidence are at least "astronomical". The unavoidable implication is that the entire sky had once been in thermal contact for a long enough time to reach thermal equilibrium, or very nearly so.

Now we are obliged to create a cosmological theory that is consistent with observation, and the old fashioned big bang cosmology fails that test. It does not allow for an opportunity to reach thermal equilibrium. Inflation and VSL cosmology were devised to solve that problem.

Sam5
2008-Sep-04, 06:53 PM
and they are identical except for the very small difference in temperature.




Thanks for the information. :)

Jeff Root
2008-Sep-05, 01:25 AM
Tim,

Do you agree with me that even if the temperature varied greatly from
one part of the sky to another, the simple fact that cosmic background
radiation comes from every direction shows a causal connection between
the locations? Since the light is thought to have been emitted by the
formation of hydrogen atoms, protons and electrons must have been
everywhere. Do you agree that protons and electrons would not be in
many places unless those places were causally-connected? So that if
different parts of the sky were not causally-connected, we might see
protons and electrons in one part of the Universe, but not in any other
part of the Universe, but other kinds of matter, nothing like the matter
we're made of, might happen to be found in some of the other regions?
Or -- more likely -- nothing at all would be seen in other regions?

-- Jeff, in Minneapolis

Tim Thompson
2008-Sep-05, 03:37 AM
Do you agree with me that ...
Maybe. What you describe requires that the laws of physics be the same everywhere. I don't think that it necessarily requires a causal connection for that to be the case, but it certainly would make sense for the laws of physics to be the same everywhere if the universe were causally connected.

The fact that the CMB is the same temperature everywhere is significant because it requires sufficient communication between all parts of the observable universe to establish thermal equilibrium. That does necessarily require a causal connection.

north
2008-Sep-05, 03:55 AM
actually as far as CMB is concerned

I asked the question quite awhile a go , it seems that only certain galaxies out from us where compensated for while looking for CMB's

inotherwords not the whole Universe was included in the CMB " picture "

Jeff Root
2008-Sep-05, 04:51 AM
What you describe requires that the laws of physics be the same
everywhere. I don't think that it necessarily requires a causal
connection for that to be the case, but it certainly would make
sense for the laws of physics to be the same everywhere if the
universe were causally connected.

The fact that the CMB is the same temperature everywhere is
significant because it requires sufficient communication between
all parts of the observable universe to establish thermal equilibrium.
That does necessarily require a causal connection.
I see that the greater the similarity between different locations,
the greater the likelyhood that they are causally-connected, so
that a variation of .001% from one place to another is a stronger
indication of connectedness than a variation of 500%. But I don't
see that the similarity between particles in different places is any
less revealing than the similarity in temperature. If at some point
in time (early on in the Big Bang) a mess of protons and electrons
appeared where previously there were none, I would expect that
it was the result of a single cause, rather than that protons and
electrons just happened to appear in many causally-unconnected
places simultaneously. The fact (I'll take it as a given) that the
laws of physics are the same everywhere isn't sufficient to explain
why protons and electrons suddenly existed everywhere, rather
than just in a causally-connected volume.

-- Jeff, in Minneapolis

timb
2008-Sep-05, 11:59 AM
So is it true to say that the standard model of cosmogeny has two dei ex machinis: a big bang that just happened "because" and an inflation that just happened "because"? Perhaps that's a little unfair. The inflation happened because of the inflaton; perhaps the big bang happened when a random quantum fluctuation in the void created the bigbangeron.

Cougar
2008-Sep-05, 02:52 PM
actually as far as CMB is concerned... it seems that only certain galaxies out from us where compensated for while looking for CMB's... inotherwords not the whole Universe was included in the CMB " picture "

Did you have a question, or are you just trying to assert something with the least amount of foundational support possible?

Cougar
2008-Sep-05, 03:13 PM
So is it true to say that the standard model of cosmogeny has two dei ex machinis....
There is no "standard model of cosmogeny." There is a standard model of cosmology. (BTW, it's Deus ex machina)


....a big bang that just happened "because"....

Yes, the big bang theory is incomplete. As Carl Sagan said:


Ten or twenty billion years ago, something happened -- the Big Bang, the event that began our universe. Why it happened is the greatest mystery we know. That it happened is reasonably clear.


....and an inflation that just happened "because"

Here we're on shakier ground. We're not that sure inflation even occurred, but it would explain several observations that would otherwise be problems: horizon, flatness, monopoles.


...perhaps the big bang happened when a random quantum fluctuation in the void created the bigbangeron.

There is no shortage of speculations. There is a severe shortage of available data on which to base any theory.

timb
2008-Sep-06, 11:08 AM
There is no "standard model of cosmogeny." There is a standard model of cosmology. (BTW, it's Deus ex machina)


You should familiarize yourself with Latin plurals before you make a fool of yourself again.

Tim Thompson
2008-Sep-06, 05:07 PM
... in other words not the whole Universe was included in the CMB "picture"
As far as I know, this is not true. Certainly the Milky Way is a major foreground source that blocks a large chunk of the sky, but it is compensated for and removed from the COBE & WMAP all sky maps. In both cases there were also extensive catalogs of foreground sources which were then removed from the all sky data. One might dispute the effectiveness of the foreground removal, but in fact the entire sky is indeed included in corrected & compensated fashion for the CMB maps.


So is it true to say that the standard model of cosmogeny has two dei ex machinis: a big bang that just happened "because" and an inflation that just happened "because"?
Yes. We look at the universe in extraordinary detail, and then we make up a story that explains why the universe looks the way it does. We require our story to be entirely self consistent and entirely consistent with everything we know about physics, to the best of our ability to do so. We are not 100% successful, witness the failure of general relativistic cosmology to conserve energy globally. That may be revealing of a deep truth about the universe, or it may be revealing of our own ignorance. We do know that if there was inflation, then it would explain what we see. We do know that if there was a bang, then it would explain what we see. So we don't know why either happened, nor do we know that either actually happened at all. We do not know that either is existentially true, we know only that both make our cosmological models work better, so we include them in our story of the universe.

timb
2008-Sep-08, 11:29 AM
As far as I know, this is not true. Certainly the Milky Way is a major foreground source that blocks a large chunk of the sky, but it is compensated for and removed from the COBE & WMAP all sky maps. In both cases there were also extensive catalogs of foreground sources which were then removed from the all sky data. One might dispute the effectiveness of the foreground removal, but in fact the entire sky is indeed included in corrected & compensated fashion for the CMB maps.


Yes. We look at the universe in extraordinary detail, and then we make up a story that explains why the universe looks the way it does. We require our story to be entirely self consistent and entirely consistent with everything we know about physics, to the best of our ability to do so. We are not 100% successful, witness the failure of general relativistic cosmology to conserve energy globally. That may be revealing of a deep truth about the universe, or it may be revealing of our own ignorance. We do know that if there was inflation, then it would explain what we see. We do know that if there was a bang, then it would explain what we see. So we don't know why either happened, nor do we know that either actually happened at all. We do not know that either is existentially true, we know only that both make our cosmological models work better, so we include them in our story of the universe.

I appreciate that's how the story develops. In any story the smaller the number of extraordinary things that happen without explanation the better. In practice a story has to end (and start) somewhere, so, unless you're one of those people who think that with enough mathematics we can prove this is the only universe that could exist, there will always things in the story that just happened.

Kwalish Kid
2008-Sep-08, 01:13 PM
I can't really accept the horizon problem as a problem. It could simply be a product of the initial conditions of the universe. As such, it would have nor real explanation but there would be no real space to provide an explanation.

As it stands, I don't think that there is any good measure for the probability space of possible initial conditions for the universe, thus I can think of no way to determine whether or not an extremely homogeneous initial condition is rare or not.

Nereid
2008-Sep-08, 02:34 PM
I appreciate that's how the story develops. In any story the smaller the number of extraordinary things that happen without explanation the better. In practice a story has to end (and start) somewhere, so, unless you're one of those people who think that with enough mathematics we can prove this is the only universe that could exist, there will always things in the story that just happened.(bold added)

I feel there is a certain irony here, quite possibly unintended.

Imagine yourself back around the end of the 19th century, having a conversation with a well-informed, competent physicist.

Imagine introducing Quantum Electrodynamics, or Quantum Chromodynamics, two of the great (late) 20th century achievements in physics, and going into some detail on the classes of experiment relevant to QED and QCD.

How likely do you think it is that this physicist would, sooner or later, use the phrase "extraordinary things that happen without explanation"?

(Cue the quotes from the relevant great 20th century physicists, on the incomprensiblility of quantum mechanics, or "shut up and calculate")

At least some incredibly good stories, in modern physics, are chock-a-block full of "extraordinary things that happen without explanation" ... so perhaps that is not a good criterion to use in assessing modern cosmology either?

Sime99
2008-Sep-18, 02:54 AM
First post and I'm new to a lot of the more advanced stuff so I'm probably missing something obvious but is this not only a problem if the universe is not shaped like a 3-torus or something similar? Or is that why the torus shape was suggested?

I would have thought that assumption takes care of not only the geometry of space but the isotropic and homogeneous nature of the universe, and would allow two points on opposite sides of the sky to communicate on the other side of what we see. As an added bonus the expansion of the universe stops photons we could shine out in front of us hitting us on the back of the head.

And then does this not put bounds on the size of the universe?

Aghh! My brain hurts.

loglo
2008-Sep-20, 06:03 PM
Have a look at the Cornish et al (http://arxiv.org/abs/astro-ph/0310233) paper. I think it is what you are thinking of.

idav
2009-Jan-07, 05:58 PM
Hello folks. It's been a while since I've been on the board but I've been reading about Joćo Magueijo's ideas and the horizon problem and I'm looking for some clarification.

From the wikipedia article on the horizon problem:


When one looks out into the night sky, distances also correspond to time into the past. A galaxy measured at ten billion light years in distance appears to us as it was ten billion years ago, because the light has taken that long to travel to the viewer. If one were to look at a galaxy ten billion light years away in one direction, say "west", and another in the opposite direction, "east", the total distance between them is twenty billion light years. This means that the light from the first has not yet reached the second, the 13.7 billion years the universe has existed simply isn't long enough. In a more general sense, there are portions of the universe that are visible to us, but invisible to each other, outside each other's respective particle horizons.

In standard physical theories, no information can travel faster than the speed of light. In this context, "information" means "any sort of physical interaction". For instance, heat will naturally flow from a hotter area to a cooler one, and in physics terms this is one example of information exchange. Given the example above, the two galaxies in question cannot have shared any sort of information, they are not in "causal contact". One would expect, then, that their physical properties would be different, and more generally, that the universe as a whole would have varying properties in different areas.
This is what I don't get. Perhaps this source is just poorly worded but they make it sound as if one part of the universe has to be able to txt message another part of the universe so they can decide what temperature they're going to be today. I don't see why this is a legitimate premise to base a problem on.

To me it is sufficient to say that the current universe is the product of the big bang and the subsequent expansion. These two points in the universe that are beyond each others information horizon both came from the same theoretically infinitely small, infinitely dense point. It seems to me that once the big bang was set in motion the two points, through causality of the big bang, would end up in the same state on the average. It's like saying that two baseballs need to "talk to each other" to end up in the same position on the field. No, they just have to have similar forced applied to them.

To me premise of the horizon problem with the big bang reeks of anthropic confusion, but I'm entirely open to the idea that my contention with it is the idea that is the victim of such delusions.

Perhaps a simple clarification of terms is in order. I think I fully understand why c is constant and the "speed limit" based on Einstein's ideas but I think we must be careful to not misapply these absolutes. As an example I posit this question. If you have a solid rod of steel that is one light second long. If one per to push on point A, would it take more than a second for point b to start moving?

FYI, this isn't against the mainstream, I'm trying to understand the mainstream by asking questions.

pzkpfw
2009-Jan-07, 07:06 PM
If you have a solid rod of steel that is one light second long. If one per to push on point A, would it take more than a second for point b to start moving?

Yes, because the speed of sound in iron governs the push on B from A, not the speed of light. (It takes time for the Fe atoms to physically interact with each other).

I think I agree with your post though. Or rather, I don't understand why different bits of the Universe should end up different than others. They came from the same place (BB) and started with the same conditions - why not turn out pretty much the same?

idav
2009-Jan-07, 08:38 PM
Yes, because the speed of sound in iron governs the push on B from A, not the speed of light. (It takes time for the Fe atoms to physically interact with each other).
The speed of sound? I don't follow. It's not the speed of anything really, it's one solid object.

Perhaps my typo confused you? I meant to say, "If one were to push on point A..."

I would think the Fe atoms are already physically reacting to each other by being bonded by the electromagnetic force. Perhaps this question I've posed is just a manifestation of me failing by using a real world example in such a macroscopic context. Would the force applied at point A actually cause a wave of compression that must reach the other end before motion takes place?


I think I agree with your post though. Or rather, I don't understand why different bits of the Universe should end up different than others. They came from the same place (BB) and started with the same conditions - why not turn out pretty much the same?
Exactly, if anything I would think that we'd be struggling to find out why there are any matter density/temperature variations in the CMB at all. It suggests that the big bang or subsequent expansion was not uniform and we don't yet know understand what other forces are at work shaping the composition of matter.

speedfreek
2009-Jan-07, 08:51 PM
Would the force applied at point A actually cause a wave of compression that must reach the other end before motion takes place?

Yes, and that wave moves at the speed of sound for that material - it is the speed at which a vibration moves along the rod.

http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#4

idav
2009-Jan-07, 08:58 PM
Oh that's what he was getting at...So I was even more wrong than I thought. Surprises are great! Awesome, thanks guys. I love finding the exact answer to a question of mine!

Now can anyone address the rest of my post? The part that pzkpfw seemed to agree with? On what data do we base the conclusion that the CMB is too "smooth"? Why is that line of "too smooth" drawn where it is? You can just tell me that I might as well enroll in college right now if I want to understand any time this decade...but please try!

speedfreek
2009-Jan-07, 11:37 PM
Here is my simplistic (and lumpy!) take on it:

The age of the universe and the size of the observable universe today tells us how fast things must have been moving apart to begin with. For the opposite edges of our observable universe to have become so distant from each other in only 13.7 billion years means that, on the face of it, they can never have been causally connected except at the very beginning. It seems that pretty much as soon as the universe started to expand, any differences in the initial conditions would have had no way to even themselves out across the universe.

Why can't the universe have been smooth to begin with? The answer lies in the uncertainty principle. When the observable universe was really small, quantum fluctuations down at the microscopic scale should have introduced much more unevenness than we see in the CMBR, which was emitted 300,000 years later. The WMAP images do show small differences, and indeed those differences were enough to form the seeds of the large scale structures that formed later. But when our observable universe was only a little larger than the size of fluctuations themselves they should have introduced large differences across our observable universe.

(Remember, our observable universe wasn't necessarily the whole thing, even right at the beginning. We measure our observable universe by the time that light takes to travel, so in the merest fractions of a second after the beginning it was absolutely tiny, but that doesn't mean it is all there was - the whole caboodle might already have been meters, kilometers, light-years across, or even infinite!)

We have a two-fold problem here. If the initial quantum lumpiness of our observable universe had time to smooth out before it all got too large to make any difference, the universe would have been too smooth for galaxies to form later. But if that initial lumpiness didn't have time to smooth out before it all got too large, it should have been a lot lumpier than it was!

What inflation does is provide a mechanism that allows something between the two. If the observable universe had time to almost smooth out when it was very small (it can't be completely smooth due to uncertainty) and then it inflated exponentially in a very short space of time, an inflation such that the distance between points down at the Planck scale was increasing faster than light, then that smoothness would have been spread across a very large area, but an area containing hugely magnified (and thus smoothed out) quantum fluctuations.

After inflation we are left with an almost smooth observable universe a few centimetres across, rather than a very smooth or very lumpy one! Those few centimetres that represent the observable universe would have been many magnitudes smaller than the inflationary volume and the whole universe might have already been many magnitudes larger than our observable universe before it inflated!

pzkpfw
2009-Jan-07, 11:50 PM
Thanks; nicely (and understandably!) written.

For me, then, the initial issue seems like the audio problem:

(small signal + static) x Amplification = big signal + big static

[That analogy breaks, because inflation seems kind of like the Amplification being ramped up so fast that the static doesn't get time to affect the small signal too much - and that makes no sense (in my audio example).]

speedfreek
2009-Jan-08, 12:26 AM
I should say that this is not exactly my field of expertise (I tend to concentrate of what has happened since the CMBR was emitted) so take my explanation on approval!

idav
2009-Jan-08, 02:27 PM
Thanks for you post speedfreek, it did clear up some things.
It seems that pretty much as soon as the universe started to expand, any differences in the initial conditions would have had no way to even themselves out across the universe.That's one hell of a problem isn't it? So essentially without the cosmic inflation theory our universe doesn't fit into thermodynamic models? The second law doesn't apply because the system isn't closed, it's kind of just, "broken"?

Another thing that bothers me about this whole issue and the acceptance of inflation is that it seems that inflation was literally conjured up to fix the problems in the big bang theory. That doesn't sound like science. Everything I read says that inflation is completely unsubstantiated by any observations. So why is it accepted as mainstream? Granted, it's not like we're making important decisions based on this information. This entire area of interest is but a mental adventure, but perhaps not for long!:)

(Removed comment that could insult some people. pzkpfw)

StupendousMan
2009-Jan-08, 07:20 PM
Another thing that bothers me about this whole issue and the acceptance of inflation is that it seems that inflation was literally conjured up to fix the problems in the big bang theory.


Yes, that was the motivation for inflation theory: the observation that the CMB is more uniform than it "ought to be" (for causally uncorrected regions). The theory was devised to explain a puzzling observation.



That doesn't sound like science. Everything I read says that inflation is completely unsubstantiated by any observations.

??

The observation of the uniform CMB was the basis for inflation. The theory has been modified to be consistent with these observations as our detectors have improved.

How is this "completely unsubstantiated by any observations?" It's BASED on observations.

idav
2009-Jan-08, 07:54 PM
Right, but it's based on that which it is trying to resolve. Again I'm not trying to be ATM here. I'm sure those of you that understand these issues with far better articulation than I find it perfectly suitable. I guess I'm just not there yet.

Hahaha, I see someone removed a comment of mine. You slick admins you, at least you get what I was saying.:shhh::p

speedfreek
2009-Jan-08, 08:41 PM
The thing is, there isn't just the horizon problem to deal with. The Big-Bang also suffers from a flatness problem and a monopole problem too. Inflationary theory solves all three of these separate and seemingly complicated problems using one relatively simple process, and that is why inflation is such a compelling theory.

http://en.wikipedia.org/wiki/Cosmic_inflation

Cougar
2009-Jan-08, 09:06 PM
Another thing that bothers me about this whole issue and the acceptance of inflation is that it seems that inflation was literally conjured up to fix the problems in the big bang theory.

As StupendousMan said, certain big bang puzzles became motivating factors behind the continued development of inflation.... but I believe if you read Alan Guth's 1997 book, The Inflationary Universe, you will find that Guth did not originally set out to "patch up" the big bang. Besides, Guth was a particle physicist, not a cosmologist.


Everything I read says that inflation is completely unsubstantiated by any observations.

WMAP has carefully measured the "geometry" of space, and the data indicate it is flat, or Euclidean, throughout the visible universe. This is a supporting observation for inflation. I believe there are numerous other supporting observations, including the slight but measured polarization of the CMB, but....


So why is it accepted as mainstream?

The above-referenced supporting observations are suggestive. They are preliminary nails in the coffin, but the lid is not closed particularly tight. I think most scientists understand inflation theory is still pretty tentative and far from being strongly supported by observation. On the other hand, the theory is looked upon very favorably because in one simple event, it explains three major questions that arise in a big bang scenario. Some say, sort of jokingly, that the theory explains these problems so easily and naturally, it must be right.