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Discman
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I am very mixed-up with the terms perturbation, anisotropy and ripple.
What is the relation or difference between them?
What is the relation or difference between them?
The valleys are gravitational potential wells on all scales; they are just not causally "operative" on superhozion scales. It's not until perturbations fall back within the horizon that causal processes, like acoustic oscillations, commence.Discman said:Thank you, I suppose that is the meaning of "primordial". So, a valley will become a potential gravitational valley only in the sub horizon position?
The 10^-5 distribution is in reference to the amplitudes of the perturbations -- not their scales. For example, a scale invariant spectrum would have fluctuations of this amplitude on all scales.Still another question. Perturbations have different sizes (scales). I cannot cope that with the 1/100.000 distribution. All those different sizes and still that point-to-point difference? Strongly I have the idea there is a flaw in my thinking.
Inflation generates perturbations on superhorizon scales. The accelerated expansion causes comoving length scales to grow faster than the Hubble scale so that when inflation ends, fluctuations in the spatial curvature exist on scales surpassing what was the causal horizon during inflation.
It's all about the Fourier modes. A density perturbation in space can be resolved into its Fourier modes -- it is these modes that we track and evolve during inflation.Discman said:It becomes clearer for me all the time. Now about wavelengths. Does a big perturbation have only one long wavelength or are all the different modes present in a perturbation?
And if there are different modes in a perturbation will they become active according to the progression of the Hubble radius or have they to wait till the whole perturbation is inside the horizon?
You do indeed have fluctuations in all the SM fields, and they can in important circumstances contribute to the overall density perturbations. What sets the inflaton apart is that it is the dominant component of the energy density. Fluctuations in the inflaton field cause different parts of the universe to undergo reheating at different times, transferring the inhomogeneities in the inflaton field to the matter sector after inflation.the_pulp said:Why is it said that inflation generates perturbations on superhorizon scales? I mean, every field generates quantum perturbations just because of its existence, right? Why are the inflation quantum perturbations needed? Cant you generate the anisotropies in CMB and such with just the Standard Model + GR fields? (Ive seen the equation related to perturbations and I managed to follow them at least in a high level, but I couldn't see, conceptually, why the inflation quantum perturbations are needed)
bapowell said:You do indeed have fluctuations in all the SM fields, and they can in important circumstances contribute to the overall density perturbations. What sets the inflaton apart is that it is the dominant component of the energy density. Fluctuations in the inflaton field cause different parts of the universe to undergo reheating at different times, transferring the inhomogeneities in the inflaton field to the matter sector after inflation.
the_pulp said:Why is it said that inflation generates perturbations on superhorizon scales? I mean, every field generates quantum perturbations just because of its existence, right? Why are the inflation quantum perturbations needed? Cant you generate the anisotropies in CMB and such with just the Standard Model + GR fields? (Ive seen the equation related to perturbations and I managed to follow them at least in a high level, but I couldn't see, conceptually, why the inflation quantum perturbations are needed)
Thanks in advance!
Theories with massive bosons, like the weak interaction, are not scale invariant.phsopher said:You can't do this with vector fields (photons, gluons, gauge bosons of the weak interaction) because they are conformally invariant. What this means is basically that they are insensitive to length scales, so they don't feel the expansion of the universe and microscopic quantum fluctuations can't be stretched to superhorizon scales.
phsopher said:You can't do this with vector fields (photons, gluons, gauge bosons of the weak interaction) because they are conformally invariant. What this means is basically that they are insensitive to length scales, so they don't feel the expansion of the universe and microscopic quantum fluctuations can't be stretched to superhorizon scales.
I must say I can't think of what would be the problem with fermions but I assume there are some given that they aren't considered much in this context in the literature as far as I'm aware. Perhaps other people have more insight.
That leaves scalar fields. The only scalar field in the Standard Model is the Higgs, and like any light scalar field it will acquire a spectrum of superhorizon perturbations. However, as bapowell pointed out, if inflation is driven by another scalar field - the inlfaton - the perturbations in the Higgs field will be subdominant compared to the perturbations of the inflaton. You can't realize inflation with the Higgs itself in the vanilla Standard Model because the potential is not sufficiently flat. However, if the Higgs is coupled non-minimally to gravity then this can be done. This scenario (Higgs inflation) is however disfavored if the the recent measurement from BICEP is a signature of primordial gravitational waves.
Maybe that's a lot of information, but what to take away is that is none of the fields in the vanilla Standard Model + gravity possesses all the necessary properties for successful inflation and generation of primordial perturbation.
The vacuum energy of the inflaton field must dominate in order for the universe to accelerate!the_pulp said:1) Why is it the more dominant? (my guess is that it should have something to do with the energy at which this field should be excited)
Generally, the field value must be larger than [itex]M_{\rm Pl}[/itex] to support the required energy density.2) This explanation must surely have some counterpart in the maths. There should be some parameter in the inflaton part of the lagrangian that should be very large in order to make it the more important in the quantum fluctuations. Just to use an easy example, in fi^2 chaotic inflation, what part of the lagrangian is big enough to produce this effect in quantum fluctuations?
bapowell said:Theories with massive bosons, like the weak interaction, are not scale invariant.
the_pulp said:It was not a lot of information, you were very clear. Are not fermions invariant to scales too? (Im having in mind one lecture of susskind on renormalization -the 1st of the supersymmetry course-)
Why isn't gravity active there? Are there not curvature perturbations on superhorizon scales?Discman said:I still have a problem with the S-W effect. This effect is typical, I thought, for the super horizon areas. But gravity is not active there.
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