# How necessary is inflation given the laws of physics?

How deeply is cosmic inflation required for the laws of physics? Is inflation required for a universe remotely like ours, or is it simply a contingent on the starting conditions of the universe?

For example, is time as we experience it a byproduct of inflation, something that could not exist without it?

The (limited) research I've done seems to suggest that inflation is deeply embedded in the laws of physics, but I don't really understand that. If it is so, how can our part of the universe not expand (within a galaxy), yet the laws of physics still apply here?

• "is time as we experience it a byproduct of inflation" - Are you referring to a particular way of "experiencing time"? Particular how? What would be an alternative? – safesphere Jul 14 at 16:57
• Possible duplicates of last subquestion (v2): physics.stackexchange.com/q/2110/2451 and links therein. – Qmechanic Jul 14 at 17:28

Time has nothing to do with inflation. Clocks would tick even if the universe had not inflated.

Inflation is not “deeply embedded” in the laws of physics. You can have the Standard Model and a Big Bang based on General Relativity without having inflation.

Inflation is an ad hoc add-on to cosmological models to explain certain features of our universe — such as its homogeneity, isotropy, flatness, lack of magnetic monopoles, etc. — that would be hard to understand without it. Most inflationary models uses a so-far-unobserved scalar “inflaton” field to cause a brief period of inflationary expansion. We know that scalar fields exist (the Higgs field is scalar) and they can have the negative pressure that is required for inflation. Thus many physicists see inflation as fitting comfortably and plausibly into existing ideas about particle physics and cosmology, but inflation is not required by them.

Addendum for @safesphere: A scalar Higgs field is a critical part of the Standard Model of particle physics. With this field, the model is in impressive agreement with all observations of electromagnetic, weak, and strong interactions between various particles at, say, the Large Hadron Collider. Without it, the model utterly fails. For mainstream physicists, this evidence more than suffices to consider the Higgs field to “exist”. Whenever a physicist says “X exists”, she means “A model with X in it works really well to explain what we observe”.

• "we know that scalar fields exist (the Higgs field is scalar)" - How do we know this? Has the Higgs mechanism been observed? I recall CERN has (almost, 4.9 out of 5 sigma) found a new particle that could be the Higgs boson, but could be something else. Is there any direct experimental evidence of the existence of the scalar field permeating all space? Or is it simply made up to save the current limited theory? Like we make up the unobserved "dark energy" to conceal the fact that the FLRW cosmology mismatches the observation of the matter amount in the universe by 95%. (Good answer +1) – safesphere Jul 14 at 16:48
• @safesphere If you don’t accept the mainstream view that the Standard Model with its Higgs field is in good agreement with observation (including the W and Z masses generated by the Higgs mechanism), then perhaps a site like PSE that discusses only mainstream physics is not a good fit for your skepticism. If you don’t accept the standard model of particle physics and you don’t accept the standard model of cosmology, then there isn’t much I can discuss with you. Sorry! – G. Smith Jul 14 at 16:54
• Calling scientific consensus “dogma” is the mark of a True Skeptic! – G. Smith Jul 14 at 17:40
• I have added an addendum just for you! – G. Smith Jul 14 at 17:58
• The measure problem in eternal inflation should be a new question. Unfortunately, it is one I won’t be able to answer. As for probability, physicists fortunately just ignore what philosophers think about it and go about their calculations and experiments. I have no idea how to do quantum mechanics without dealing with probabilities. Do you? – G. Smith Jul 15 at 5:15

In the beginning there was the original Big Bang model, that explained the expansion of the universe using general relativity ( which described successfully gravity at the scale of the universe), and the known particle physics interactions.

But then, when the cosmic wave background radiation was measured, it showed a uniformity in any direction at the level of $$10^{-5}$$ .

This is also the horizon problem and it is inextricably tied with the fact that General Relativity is the theory of the Big Bang , with special relativity for flat spaces.

The horizon problem (also known as the homogeneity problem) is a cosmological fine-tuning problem within the Big Bang model of the universe. It arises due to the difficulty in explaining the observed homogeneity of causally disconnected regions of space in the absence of a mechanism that sets the same initial conditions everywhere

For a general uniformity in temperature to exist in the universe at the time of the photon decoupling at 380.000 years after the Big Bang the particles in the various regions of space should be able to interact and come to a thermodynamic equilibrium. This cannot happen at the time before the photon decoupling because of relativity, there are regions of the universe which do not interact with each other, due to the light cone geometry, so the uniformity is not explainable with thermodynamics.

The theory of inflation by introducing an effective quantum mechanical theory, allows for the uniformity observed, as it is explained in the link.

This is a model that explains observations, though there is not as yet a definitive quantization of gravity.

• There is no "horizon problem", because there is no reason to assume that the Big Bang was not symmetrical. Separated areas did not need to "interact and come to a thermodynamic equilibrium", because they were never out of the equilibrium due to symmetry. "The mechanism that sets the same initial conditions everywhere" is symmetry. – safesphere Jul 14 at 16:27
• @safesphere you can have your theory on this, but the mainstream model , I have given links, is as I have described. – anna v Jul 14 at 19:05
• I don't think inflation is "mainstream". It has been challenged time and again in print. This site sticks to the known and confirmed laws of physics and their legitimate challenges. For example, there is no prohibition that I am aware of to ask about Modified Gravity as opposed to Newtonian Gravity or General Relativity. Thus you are free to refer to inflation as a working theory and I am not prohibited by any "mainstream inquisition" from challenging this theory by logic. The question about symmetry has been asked before and remains unanswered. – safesphere Jul 14 at 19:57