Questions tagged [cosmological-inflation]

Cosmological inflation refers to an era of expansion that lasted for approximately $10^{-34}$ seconds, during which the universe expanded by a factor of approximately $10^{26}$ in every direction. This is different from ordinary space expansion and from the acceleration in expansion we experience now and questions tagged by inflation should pertain to this era and not be about the expansion of the universe in general.

121 questions with no upvoted or accepted answers
Filter by
Sorted by
Tagged with
11
votes
0answers
369 views

Equation of motion for cyclic model of the universe

I recently started to study about cyclic universe. I came across this article [1]. My question is about the action that used for describing the cyclic model: $$S = \int d^{4}x\sqrt{-g}(\frac{1}{16\pi ...
10
votes
0answers
349 views

Time diffeomorphisms breaking in inflation

I am currently working on the topic of inflation. It seems that at the stage of inflation, the universe can be described as a de Sitter space. In such a space, all spacetime diffeomorphisms are ...
9
votes
0answers
94 views

Hybrid Inflation and Supergravity

Is it necessary to add a constant term in the superpotential of hybrid inflation within supergravity in order to cancel SUSY vacuum energy at the end of inflation?
8
votes
0answers
106 views

How do inflationary models predict the generation of gravitational waves during the inflationary period?

Recent results from the BICEP2 experiment have produced a lot of talk about the primordial gravitational waves produced during the inflationary period. I would like to have some explanation about how ...
8
votes
0answers
170 views

How can dynamics be reversible if inflation-style baby universe spawning is allowed?

I just finished readying Sean Carroll's book, "From Eternity to Here", and have a question about reversibility and inflation: Assume inflation allows random quantum fluctuations to produce high ...
6
votes
0answers
115 views

Meaning of the simplest potential of quintessence models. Fields in denominator?

I am reading Sec. 1.12 of the Cosmology book by Weinberg. In this section he explains the very simple model of quintessence which attempts to provide a dynamical explanation of the smallness of the ...
6
votes
0answers
75 views

In a perturbative FRW cosmology, why do constant-density hypersurfaces define a good gauge?

It appears to be common in the discussion of perturbative FRW cosmologies to choose a gauge using hypersurfaces for special values of some quantity, like surfaces of constant density $\rho$, constant ...
6
votes
0answers
117 views

$f_{NL}$ non-Gaussianity in cosmology

In the context of cosmology, what is meant by "..arbitrary quadratic non-Gaussianity i.e non-Gaussianity that is described to leading order by a 3-point function.."? (.."quadratic non-Gaussianity" ...
5
votes
1answer
81 views

Robertson-Walker metric and cosmic homogeneity

The Robertson-Walker metric is of the form $$\tag{1} ds^2 = dt^2 - a(t)^2 \Big(\frac{dr^2}{1 - kr^2} + r^2 d\theta^2 + r^2 \sin^2\theta \, d\phi^2 \Big).$$ My question is related to the $a^2(t)$ ...
4
votes
0answers
42 views

Quantum tunneling of dark energy from black holes

There was an article posted recently suggesting there is a very deep connection between dark energy and black holes. https://phys.org/news/2019-09-black-holes-dark-energy.html I was thinking this ...
4
votes
0answers
41 views

Why does a universe with total positive energy eventually collapse in a crunch?

While reading about negative energy in the following Wikipedia page, I came across a statement that, if in a universe positive energy dominates, it will eventually collapse upon itself in a crunch. ...
4
votes
2answers
782 views

Attractive gravity has negative energy, what about repulsive gravity in the inflation phase?

Alan Guth's cosmic inflation theory posits that about 10^-38 second before the Big Bang which led to our particular universe, a tiny patch of space doubled in size more than 100 times, from sub-atomic ...
3
votes
0answers
44 views

Would other parts of the universe have a different rest frame for their own Cosmic Background Radiation?

The CMB, once the Doppler shift from the Earth's motion is subtracted, is fairly uniform, which seems to imply that all the matter that emitted it moved at more or less the same velocity when it did. ...
3
votes
1answer
76 views

Why should LTB dust be comoving?

In many research papers about inhomogeneous cosmology, one often considers spherically symmetric (LTB) spacetimes where in the co-ordinate frame $(t,r,\theta,\varphi)$ wherein the metric assumes the ...
3
votes
1answer
162 views

Can we use pressure analogy to understand inflation?

Classically, the expansion of a gas in a container requires the gas to apply outward pressure to the walls of the container. Is it also true for the Universe during inflation? How did the inflation ...
3
votes
1answer
161 views

How much uncertainty has the relic graviton background?

In the paper [1], it is mentioned that inflation predicts that a relic graviton background is about 0.9 K (cf. cosmic neutrino background, 1.945 K, and cosmic microwave background, 2.73 K). How much ...
3
votes
0answers
325 views

Adiabatic fluctuations

In Baumann's cosmology lecture,http://www.damtp.cam.ac.uk/user/db275/Cosmology/Chapter4.pdf , chapter 4, page 89, he defines adiabatic perturbation as: Adiabatic perturbations have the property ...
3
votes
0answers
91 views

Starobinsky inflation in Supergravity

The available experimental data prefers plateau models of cosmic inflation, and among them Starobinsky inflation (aka R^2 inflation) is preferred, even if maybe not significantly. See here for summary ...
3
votes
0answers
99 views

Dynamics of the Inflaton field

The perturbations of the inflaton field $\phi$ are described by the following equation: $$\ddot\phi(x,t)+3H(t) \dot\phi(x,t)+\nabla^{2}\phi(x,t)/a(t)=-m^2\phi$$ where $a$ is the scale factor of ...
3
votes
0answers
278 views

How to plot spectral indices $n_s$ and $r$ from Planck data?

From a certain inflationary model I have obtained the theoretical values for the spectral indices $n_s$ and $r$ as functions of $N$, the e-folds number of inflation. At this point it would be ...
3
votes
0answers
339 views

How to understand that Inflation generate almost scale-invariant perturbations?

Since during the inflation, the Hubble rate is approximately constant, the perturbations generated by inflation are almost scale-invariant. How to understand that?
3
votes
1answer
342 views

Cosmological fluctuations: what is gaussian?

When we are speaking about gaussianity and non-gaussianity in a cosmological context, what is gaussian or non-gaussian in the CMB? What would a non gaussian CMB look like compared to a gaussian one? ...
3
votes
0answers
193 views

Power spectrum calculation for simple inflaton potential

I am trying to obtain an expression for the power spectrum $\Delta (k)$ of cosmological perturbations, see for example eqn (148) from the following TASI lectures. Right now, I would like to use the ...
3
votes
0answers
171 views

The speed of light during the inflationary period

Introduction: As a thought experiment, suppose I modified the value of $c$ (speed of light) in some local region and attempted to measure it with a clock placed in that same region. I will denote ...
3
votes
0answers
327 views

Never ending inflation

There are a lot of different models of inflation in cosmology, with plenty of different features and issues. I read that the problem with some models is that they introduce a never-ending inflation, ...
2
votes
0answers
45 views

Exact solution for (simple) Reheating Boltzmann equations?

Let's say we have the Universe which is exiting Inflation, with the inflaton field $\phi$ decaying into relativistic particles (radiation $R$). Without any other assumption, the equations describing ...
2
votes
0answers
65 views

Could the horizon problem be explained by a finite universe instead of inflation?

Suppose the universe is finite (either closed or open with a non-trivial topology), any point could be within the horizon of another.
2
votes
1answer
113 views

Quantum fluctuations producing CMB fluctuations also cause inflation to be eternal?

It is believed that quantum fluctuations in the inflaton field caused inflation to end at different times in different places, which led to CMB fluctuations (1 part in 100,000). Eternal inflation ...
2
votes
0answers
95 views

Can a scalar field also has positive or zero pressure?

In inflation theory, the inflaton field produce negative pressure to drive inflation. But now I’m going to ask about scalar fields in general, not the inflaton field. Can a scalar field also has ...
2
votes
0answers
60 views

Matter generation during cosmic inflation (Hawking radiation like…)

Is there any estimation of how much of the matter in the observable universe was not created during the big bang, but later during cosmic inflation? My thought is as follows: Suppose we are in the ...
2
votes
0answers
143 views

Energy density of primordial gravitational waves- two different results for the same quantity?

I want to calculate the energy density of primordial gravitational waves, $Ω_{gw}$ in a specific time. The latter is in general given by the relation : $Ω_{gw}(k,τ)= \frac{1}{\rho_{c}(τ)} \frac{d\rho_{...
2
votes
0answers
151 views

Inflation and dark energy have constant energy density - energy is created out of nothing without any source or mechanism?

I can understand that a photon in an expanding universe can be redshifted and lose its energy by its wavelength being stretched from space expansion. But in the case of inflation and dark energy, it ...
2
votes
0answers
66 views

Inflationary model that predicts both gravitational waves and isocurvature perturbations

I 'm searching for inflationary models that predict both gravitational waves and isocurvature (isothermal) perturbations. If I understood well, de Sitter inflation predicts only curvature (adiabatic)...
2
votes
1answer
190 views

Is Poplawski's “Cosmology with torsion” a “variable speed of light” cosmology?

In his 2010 paper "Cosmology with torsion", Poplawski mentions that, in the hypothesized formation of a new temporally-local universe within a black hole, its "expansion is not visible for observers ...
2
votes
0answers
191 views

Is there any primordial gravitational wave(form) that could explain dark energy?

First, I asked myself the question: Is there any gravitational waveform, however fantastical, that could explain dark energy. I suppose for a ubiquitous gravitational wave to be an explanation for ...
2
votes
0answers
64 views

Initial value dependence of slow-roll inflation

Observables in single field slow-roll inflation (lets assume a monomial potential for simplicity) depend only on the number of e-folds at the pivot-scale, $n_s=1-2/N_*$, $r=8/N_*$ etc. On the other ...
2
votes
0answers
51 views

Is the CMB Cold spot growing or shrinking?

What do we know about the nature of this region of the sky? is that region entirely devoid of galaxies? Could it be a bubble void, and we are simply unable to see the galaxies that are on the ...
2
votes
0answers
184 views

Drawbacks of single-field inflationary models with $\phi^2$ or $\phi^4$ potential?

I am aware that existing models of inflation, especially the single field chaotic inflation with quadratic or quartic potential, e.g. $$ V = \frac12 m^2 \phi^2 \qquad \text{or} \qquad \lambda \phi^4 $$...
2
votes
0answers
37 views

Correlation length during phase transitions in early Universe

During phase transitions of the second kind topological defects may form on the bounds of two areas separated by correlation length. In early Universe during phase transitions correlation length $l_{...
2
votes
0answers
108 views

Effective field theory of inflation in the slow roll case

I'm reading this set of notes by Daniel Baumann on the effective field theory of inflation but I can't quite understand one step in section 3.1.1 concerning slow roll inflation. Basically, the author ...
2
votes
1answer
99 views

Inflation as simply expansion of “previous space time”?

Suppose space-time is expanding at an ever accelerating rate now, and supposing it continues doing so in the future, won't our universe at some point in the far far distant future look exactly like it ...
2
votes
0answers
79 views

Units and missing constants in quintessence expressions?

In cosmology, quintessence is an alternative to the cosmological constant. In this approach (described here), we consider a scalar field $\phi$ and its self-interacting potential $V\left(\phi\right)$ ...
2
votes
0answers
91 views

Question on Equipartition at Reheating?

Apologies in advance if this is a silly question. I am fimiliar with the idea of Reheating in cosmology, as discussed briefly here. I have been reading several papers which make the assumption of '...
2
votes
0answers
423 views

BICEP2 and string theory

Can anybody elaborate on the implications of the BICEP2 result for string theory? The discussion here What experiment would disprove string theory? suggests that refuting string theory is rather ...
2
votes
0answers
1k views

superhorizon mode and cmb

During the period of inflation, perturbations went out of the Hubble horizon (Hubble radius, R = 1/aH) because the Hubble radius was decreasing during that time. After inflation, the Hubble radius ...
2
votes
0answers
173 views

Cosmic Inflation: Lower Expansion Rate with than without?

When I read the Cosmic Inflation diagram at https://en.wikipedia.org/wiki/File:Horizonte_inflacionario.svg which I rotated to make it readable: Plot 1 http://yukterez.enabled.io/inflation.png where ...
2
votes
0answers
68 views

Is Inflation modelled by a field?

If Inflation is modelled by a field - is this a classical field or a quantum field? If classical are there good reasons not to quantise it? What are the implications of such a quantisation?
2
votes
0answers
178 views

Can decompactification explain the inflation of the early universe?

I've just reread chapter 11 of this book where it is explained among other things, that our four dimensional universe could be unstable concerning a decompactification transition, since potential ...
2
votes
0answers
195 views

Is it really true that slow-roll inflation is incompatible with string theory?

In this blog post, Motl claimed that slow-roll inflation is incompatible with string theory. Is that really true, and why do people think so?
2
votes
0answers
175 views

Is eternal inflation Lorentz invariant?

Start without general relativity. Consider a metastable vacuum over good ol'-fashioned Minkowski space. It decays. A bubble forms and the domain wall expands. The domain wall is timelike, and ...