Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Inflation - can someone please explain it?

  1. Dec 29, 2009 #1
    What is inflation (in big bang theory not economics)?
  2. jcsd
  3. Dec 29, 2009 #2


    User Avatar
    Science Advisor

    Inflation is a term used to describe a model of the universe immediately after the big bang where the universe underwent an exponential growth in size for a short time. The model seems to work in that many questions about the current state of the universe can be explained with the inflation theory, but could not be explained otherwise.
  4. Dec 29, 2009 #3
    Is it the case that having inflation happen makes explaining the current (or 12.9 billion years ago) state of the universe easier but we have no theory or experimental data on inflation? i.e what caused it, how fast it was, how long it took, how far it went, why is stopped, etc...
  5. Dec 29, 2009 #4
    I would love to hear a laymans explanation as well!

    I find it very suspicious, that "coincidentally", just as as the universe cooled enough for some of the intense gravity to condense into nuclear strong force, that is when the inflationary epoch ran its course. I am in a minority camp, definitely not mainstream, that considers that nuclear strong force may have spacetime bending properties: that the creation of matter should cause space to expand, and conversion of matter to energy should cause spacetime to contract. I would very much like to hear any evidence decoupling spacetime inflation from nuclear strong force creation.
  6. Dec 30, 2009 #5


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    I think that is essentially right! It's often said that inflation scenarios merely replace one kind of problem by another.

    Still, it's an interesting idea and worth learning about. You know about the currently ongoing accelerated expansion which is supposed to be driven by a constant "dark" energy density. It is a mild form of inflation, you might say.
    You can figure out how that would work. (or ask at one of the forums) The dark energy density we have today is estimated to be about 0.6 to 0.7 nanojoules per cubic meter.

    If you like math, look up "Friedman equations" in wikipedia. One of the equations has the second time-derivative of the scale factor. You have to make that positive to get acceleration. You can figure out how a constant energy density and the associated (negative) pressure could make the second derivative ä >0.

    The main thing is that if the "dark energy" that we now hypothesize is gently accelerating expansion were much much stronger---like joules per cubic meter instead of nanojoules---we would be getting something like inflation.

    So that vastly stronger "dark energy" is what they refer to as the scalar field or "inflaton" field that hypothetically drove inflation. But then we are left with the problem, as you suggested, of how do you turn the damn thing off??!!

    And exotic physics is needed just to allow the existence of the required field, as well as allowing it to decay.

    Nevertheless I'd say it's very interesting and worth thinking about. What seems unlikely and contrived to us now might someday seem quite natural, when we understand better. The fine tuning that now seems required (to get the desired amount of early expansion) may turn out to be automatic---provided by some mechanism not yet fully imagined.

    It's also worth thinking about alternatives to inflation---other scenarios that would address the early universe puzzles (horizon problem, flatness problem, microwave fluctuation spectrum....) and so would allow us to dispense with inflation as unnecessary.

    There are good people taking various different approaches and coming out with papers on different explanations for inflation, and also on ideas of how to dispense with inflation.
    Just this year there was a paper by a prominent Loop cosmologist Abhay Ashtekar. And a paper by Nobel laureate Steven Weinberg. There've been others this year---just cant think of them at the moment.

    It's an active area of research. The story's not over on inflation. Keep asking questions about it until you are satisfied.
    Last edited: Dec 30, 2009
  7. Dec 30, 2009 #6
    Inflation solves a few problems. One is that different parts of the universe ought to be lumpy. You have one part of the universe. Light doesn't have time to reach another part of the universe. How come they are the same. Another problem is that there ought to be tons of weird particles like monopoles lying around.

    So if assume that the universe expanded really, really rapidly, then these problems disappear. What happens is that the expansion smooths out the lumps, and then all of the tons of weird particles that were formed pre-inflation get spread out so that you do see them very often.

    As far as what *caused* inflation. A lot of people tried a lot of models in the early 1980's that tried to get inflation to be the result of energy released when the universe fell below a certain temperature and nuclear forces froze, but that didn't work too well. So starting in the mid-1980's people stopped thinking up detailed models to explain inflation. One big problem with this models is that it's not that hard to start inflation, but the problem is that once the universe expands very, very rapidly, how do you get it to stop?
  8. Dec 30, 2009 #7
    It's not a coincidence. Originally Alan Guth proposed inflation as the result of symmetry breaking of the strong force. However, after a few years of working on that idea, people gave up on using that as the cause of inflation because no one could get it to work right. One problem is that we know enough about the strong force to rule some things out.

    I don't think it's a minority view at all. There's a family of Kaluza-Klein and string theory models that model the strong force as geometric bends in space time. As with most things, it's really easy to say "strong force is the result of bends in space-time" but when you ask "what type of bends" you end up with physicists working for thirty years, and not being able to get something that works.

    OK. The idea that people tried in the 1980's was that when ice freezes it releases energy. So the transition between the disordered disorganized pre-symmetry breaking state and the ordered state should have released a lot of energy which then got pumped into the expansion of the universe. As with the idea of strong forces being ripples in space. it's a clever idea, but the hard part is getting the details right, and people spent a few years doing that before giving up, and saying "well it will be easier if we assume that inflation has something to with a mysterious thing that we don't know about, then to get it to work with strong force interactions."

    If you are interested in this, I'm sure that there are some review papers that go through the modelling that was done in the early-1980's, as you see people come up with models, have them shot down, come up with other models, have them shot down, etc. etc. I think people went through three or four cycles before giving up.
  9. Dec 30, 2009 #8
    Dear 2, Wow! Thanks for the lowdown. Where can I search through the review papers? I havent mastered that yet...
  10. Dec 30, 2009 #9


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    So...what is an "inflaton?"

    EDIT: I don't know any QFT, just looking for a sort of qualitative description.
  11. Dec 30, 2009 #10
    Thank you for this insight. I have never come across this idea before.
  12. Dec 30, 2009 #11
    Inflation (of the cosmos) is the contemporary belief that the red shift noted while observing distant galaxies is Doppler related and that they are ballooning away from us into space at an ever accelerating pace. The more distant the galaxy, the faster it is receding.

    Unfortunately, many seem to be moving at a pace faster than the speed of light and there is no apparent reason for their acceleration, yet, with esoteric sleight of hand, some cosmologists overlook or compensate for those problems in favor of the contemporary wisdom (and, of course, the opportunity to publish).

    If you drop a white billiard ball into a container of cranberry juice, the deeper the container, the redder the billiard ball appears. There are several hypotheses to the effect that the red shift is not Doppler related, but stems from the nature of light as it traverses enormous distances.

    Light bends in the presence of gravity. When light from a star passes another celestial body, a gravitational lens effect deflects the beam toward the mass. Light also bends when shone through a prism, spreading the colors apart into a spectrum. You will note that the trajectory of the red wavelength is the least affected by the interference while violet is cast the furthest from the original direction of travel. Every particle of mass in the universe has gravitational attraction. How many gravitational lenses are there between earth and the stars of deep space? And when we observe those stars, why would we NOT expect to see some kind of color shift that increases with distance?
  13. Dec 30, 2009 #12
    I disagree what you are talking about is Hubble's observed expansion of the universe. Inflation is a extra feature added early in some big bang theories that allow a super fast, super large expansion for some period of time and then it stops and goes to the current leisurely pace (well changing pace, faster as time goes on).
  14. Dec 30, 2009 #13
    It's the mystery energy field that causes inflation. Since in quantum mechanics every field has an associated particle, the mystery energy field should have an associated particle, we now have a mystery particle.
  15. Dec 30, 2009 #14
    Nope. Inflation involves early universe.

    Yup, and after thinking about it between roughly 1950 and 1970 those theories were abandoned by most people. The trouble is that if light is getting scattered, then distant galaxies ought to look blurry. If it is getting absorbed then the energy should get re-emitted at other frequencies, and you should see a warm glow. Also it's pretty easy to think of something that blocks blue light, it's pretty hard to think of something that *shifts* light and keeps all of those spectral lines nice and sharp.

    Not very many. If there were lots of gravitational lens then looking at the universe would be like looking through a dense fog. Couldn't that cause the cosmic microwave background? Maybe. Again, people thought about that during the 1960's. The problem is that if the CMB was caused by scattered sources, you run into the opposite problem with things being too smooth.

    The trouble with gravitational lens is that its a lens and doesn't shift frequencies. There are lots of things that change light. There is absorption, scattering, lensing. The trouble is that you have to pick one.
  16. Dec 30, 2009 #15


    User Avatar
    Gold Member

    And not just a mystery particle, but some mystery mechanisms.

    1) What caused inflation to occur, and to procede at just the right rate to solve our current cosmological problems?

    2) What caused inflation to "switch off" everywhere in the Universe at the same time after it had expanded 1026 times or more? If it hadn't done so, there would be anisotropies in matter distribution as we survey the high-redshift domains in various directions.
  17. Dec 30, 2009 #16
    And also there is the problem with reheating. If the universe expanded really, really fast it should have cooled, right? So how did the universe get hot again?
  18. Dec 30, 2009 #17


    User Avatar
    Gold Member

    Obviously, the degeneration of inflatons into ordinary matter is a highly exothermic process. :devil: See? All fixed! :tongue:
  19. Dec 30, 2009 #18


    User Avatar
    Science Advisor

    Reheating occurred when the energy in the inflaton field which drove the expansion was released into thermal energy.

    In the standard inflationary picture, this happens naturally when the inflaton field reaches the bottom of its potential: it starts to oscillate around the minimum, which causes it to decay.
  20. Dec 30, 2009 #19
    My misunderstanding. I was equating inflation w/expansion. I've been away from astronomy/cosmology for some 35 years and have not kept up with the current vernacular.
    So if you look at a light source while standing in the red 'shadow' of its spectrum it appears fuzzy? What about someone observing that source from its violet 'shadow'? Do they assume the galaxy is approaching? I would rest easier if cosmologists didn't use the expansion of the universe to explain away velocities greater than C. What is the current data on distant galaxies when observed in frequencies above/below the visible spectrum. Is the Doppler effect apparent there, also?
    Presumably the ratio of matter to aether in local space is 1 atom/m3 - and there are a lot of 'm's btween here and the more remote galaxies. We don't know what elemental gasses occupy deep space and even with that minute density, the more distant light sources may appear red for much the same reason that the sky is blue.
    If there was a BigBang, then we should be able to determine an epicenter somewhere in the known universe. If galaxies appear to be receding at a relatively even pace at 360° originating at our point of view, the matter of BB is HIGHLY suspect.
  21. Dec 30, 2009 #20
    If it's due to scattering, yes. If you look at the moon through a haze that forms halos, the moon looks blurry.

    Yes. Lyman alpha is a very strong line in the ultraviolet, and you start detecting tons of quasars and hydrogen gas clouds. There are things like the Lyman alpha forest and the Gunn-Peterson trough. You have a sharp emission line by a quasar. And then you have a bunch of absorption lines on one side of that line. The standard interpretation is that you have a highly redshifted emission line, and then in between the galaxy and the earth you have a bunch of gas clouds at smaller redshift whose shadow you can see.

    And you can calculate cross sections and absorption coefficients. If you shine a beam of light through hydrogen/helium gas, there are only about a dozen or so known things that can happen to them.

    1) Yes we do. It's a hydrogen/helium mix with trace amounts of other stuff
    2) Distant light sources don't appear red. They are shifted. If you have a UV object and you shift that, it looks blue. Also you get back to the scattering problem

    Big bang happened everywhere.
  22. Jan 2, 2010 #21
    It is my understanding that signature absorption lines can be shifted by differentials in the temperatures of the media traversed by a beam before it is observed. Discounting the possibility that the effect of space is neutral (actually its properties may be so subtle as to not be detectable by current methodology) then over vast distances wouldn't a material density of even 1 atom per CuM be enough to result in the same shift as a 'cloud' would render in a local venue? Wouldn't each photon suffer multiple resonant scatterings.
    A simple axiom, implication and conclusion:
    Axiom: Something must exist before it can change or be changed.
    Implication: Cause and effect (change) is derived from existence
    Conclusion: No phenomenon can be derived from its own derivative, so the phenomenon of existence was not the result of any process - cause and effect, change, creation, whatever you wish to call it.

    If existence is not a function of time, then the 'age' of the cosmos is not 'temporary', and that which is not temporary is 'eternal'. The 'age' of BB seems to be guaged at only 14B years. If the cosmos were going to suffer an entrophy death, it seems it should have happened by now.
  23. Jan 2, 2010 #22


    User Avatar
    Science Advisor

    The frequencies of absorption lines are almost completely unaffected by the intervening medium. What you've written here only holds true for continuous spectra, where, for instance, a cool intervening medium will absorb some higher-frequency light and emit more low-frequency light, lowering the average frequency of the spectrum as a whole. But since the entire process is a process of absorption and reemission of light, it just doesn't affect the frequencies of emission lines.
  24. Jan 3, 2010 #23
    If galaxies are in motion, they have trajectories. Although complicated by the possibility of the expansion/inflation (I hope you are not inferring creation) of space between celestial bodies, from any point of reference each trajectory ought to be tracable back in time to produce at least some approximation of its position 14B years ago. And even if the point of observation is not fixed, compensating calculations can be derived to accommodate those dynamics.

    If space, itself, is expanding, why is it not measurable locally but only at enormous distances?
    Last edited: Jan 3, 2010
  25. Jan 3, 2010 #24


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    What can be viewed as the cumulative effect of many small local expansions is large enough to detect and is, in fact, measured.

    I will try to answer your question carefully, please tell me anything that doesn't make sense to you.

    The current rate of expansion of distances is 1/140 of one percent every million years. Given limited time and the limited accuracy of distance measurements, the local detection of expansion is not practical. You can do some numbers and see for yourself.

    And there is a deeper problem. Our context is General Relativity, and since there is nothing in General Relativity called "space, itself" it must not be "space, itself" that is expanding. What expands, in this context, are distances (defined a certain way) between observers who are at rest with respect to the ancient matter that gave off the cosmic microwave background.

    You are at rest relative to the CMB if your microwave sky is approximately all the same temperature, with no doppler hotspot. At rest can only be determined with finite accuracy. Another reason local measurements are a limited option.

    It has been determined that the solar system is moving about 370 km/s in the direction of Leo. This motion relative CMB can be taken out of observational data, so that our corrected observations are then from the standpoint of an observer at rest relative to the CMB.

    We see the universe, in this case, from the standpoint of an observer at rest with respect to the hot ionized gas of the early universe, and relative to the ancient light which that gas emitted.

    The Hubble law v = Hd is valid for distances between stationary observers. By definition the distances are as if measured by freezing the expansion process and timing light signals.

    When you see v = Hd, remember that d is such a distance, and v is the current rate that distance d is expanding.

    In a universe where geometry is governed by General Relativity none of us have the right to assume that the distance between stationary observers will not change, will not increase or decrease. Gen Rel says that Euclidean geometry can't be taken for granted. Euclid may be approximated in some situations, but is not to be relied on. Same with Special Rel geometry. When and where it applies, there will be some reason that causes a standard flat non-expanding geometry to be (approximately) applicable.


    You asked about local detection. Assume that a galaxy sends us some light and that while the light is in transit, distances expand by a factor of 2. When the light arrives, the galaxy will be twice as far as when it emitted the light.

    The wavelength of the light will also be twice what it was when emitted.

    You can think of that as a detection of the cumulative effect of many infinitesimal (local) lengthenings that occurred along the way. So although any one (local) lengthening would have been too small to detect, one is in effect detecting the whole lot of them by measuring their cumulative result.

    The wavelength of the light is increased by exactly the same factor as distances at large are increase. Hopefully since the wavelength is a small distance, this will go some ways towards as satisfactory response to your question.

    As a footnote I should mention that there are several different definitions of distance that find uses in cosmology. The one I'm using here is the one appropriate to the very basic Hubble law, and to the commonly used Friedmann model. You imagine freezing the expansion process at the present, or at some given moment, and timing light signals, essentially radar-ranging with expansion stopped. The idea of being at rest with respect to CMB (or equivalently, before we had the CMB, with respect to the expansion process) helps make simultaneity precise here.
    Last edited: Jan 3, 2010
  26. Jan 3, 2010 #25
    That's different from my understanding. It's very hard to shift an absorption line, because any sort of change involves an interaction with the medium which causes something much more complex than a simple frequency shift from happening.

    If it's made up of ionized hydrogen then no, since cross sections for interactions are really low. Now if the photons are moving through neutral hydrogen, then yes the light will interact with the medium, and we see this happening with the most distant quasars (the Lyman alpha forest and the Gunn-Peterson trough). But when it goes through a cloud, what happens is that you get new absorption lines, you don't get previous lines shifting.

    Also the cross sections for light moving through hydrogen is pretty well studied, since you can pass light through hydrogen on earth and see how it behaves. There aren't any known effects that would case frequency shifts. Scattering, yes. Absorption, yes. Shifting, no.

    I'm very dubious about using "word logic" to say things about physics because it turns out that words tend not to be precise enough to allow for clear thinking. When you say something "exists" what are you saying? It turns out that when you use words, you are saying something quite complex and fuzzy. Does a vacuum exist?

    I've seen stranger things happen so I don't trust "word logic" when it comes to these sorts of things. In any case, we are talking about physics and not math, and axioms don't work in physics.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook