123 research outputs found

### Chaotic inflation with curvaton induced running

While dust contamination now appears as a likely explanation of the apparent
tension between the recent BICEP2 data and the Planck data, we will here
explore the consequences of a large running in the spectral index as suggested
by the BICEP2 collaboration as an alternative explanation of the apparent
tension, but which would be in conflict with prediction of the simplest model
of chaotic inflation. The large field chaotic model is sensitive to UV physics,
and the nontrivial running of the spectral index suggested by the BICEP2
collaboration could therefore, if true, be telling us some additional new
information about the UV completion of inflation. However, before we would be
able to draw such strong conclusions with confidence, we would first have to
also carefully exclude all the alternatives. Assuming monomial chaotic
inflation is the right theory of inflation, we therefore explore the
possibility that the running could be due to some other less UV sensitive
degree of freedom. As an example, we ask if it is possible that the curvature
perturbation spectrum has a contribution from a curvaton, which makes up for
the large running in the spectrum. We find that this effect could mask the
information we can extract about the UV physics. We also study different
different models, which might lead to a large negative intrinsic running of the
curvaton.Comment: V2: Extended version to appear in PR

### Probing correlations of early magnetic fields using mu-distortion

The damping of a non-uniform magnetic field between the redshifts of about
$10^4$ and $10^6$ injects energy into the photon-baryon plasma and causes the
CMB to deviate from a perfect blackbody spectrum, producing a so-called
$\mu$-distortion. We can calculate the correlation $\langle\mu T\rangle$ of
this distortion with the temperature anisotropy $T$ of the CMB to search for a
correlation $\langle B^2\zeta\rangle$ between the magnetic field $B$ and the
curvature perturbation $\zeta$; knowing the $\langle B^2\zeta\rangle$
correlation would help us distinguish between different models of
magnetogenesis. Since the perturbations which produce the $\mu$-distortion will
be much smaller scale than the relevant density perturbations, the observation
of this correlation is sensitive to the squeezed limit of $\langle
B^2\zeta\rangle$, which is naturally parameterized by $b_{\text{NL}}$ (a
parameter defined analogously to $f_{\text{NL}}$). We find that a PIXIE-like
CMB experiments has a signal to noise $S/N\approx 1.0 \times b_{\text{NL}}
(\tilde B_\mu/10\text{ nG})^2$, where $\tilde B_\mu$ is the magnetic field's
strength on $\mu$-distortion scales normalized to today's redshift; thus, a 10
nG field would be detectable with $b_{\text{NL}}=\mathcal{O}(1)$. However, if
the field is of inflationary origin, we generically expect it to be accompanied
by a curvature bispectrum $\langle\zeta^3\rangle$ induced by the magnetic
field. For sufficiently small magnetic fields, the signal $\langle B^2
\zeta\rangle$ will dominate, but for $\tilde B_\mu\gtrsim 1$ nG, one would have
to consider the specifics of the inflationary magnetogenesis model.
We also discuss the potential post-magnetogenesis sources of a $\langle
B^2\zeta\rangle$ correlation and explain why there will be no contribution from
the evolution of the magnetic field in response to the curvature perturbation.Comment: 23 pages, 1 figure. v2: Noted that a competing effect could
potentially be smaller than originally stated. Fixed references. Matches JCAP
versio

### On Resumming Inflationary Perturbations beyond One-loop

It is well known that the correlation functions of a scalar field in a
quasi-de Sitter space exhibit at the loop level cumulative infra-red effects
proportional to the total number of e-foldings of inflation. Using the in-in
formalism, we explore the behavior of these infra-red effects in the large N
limit of an O(N) invariant scalar field theory with quartic self-interactions.
By resumming all higher-order loop diagrams non-perturbatively, we show that
the connected four-point correlation function, which is a signal of
non-Gaussianity, is non-perturbatively enhanced with respect to its tree-level
value.Comment: 17 pages, v2: minor corrections, to appear in jca

### Constraints on Gauge Field Production during Inflation

In order to gain new insights into the gauge field couplings in the early
universe, we consider the constraints on gauge field production during
inflation imposed by requiring that their effect on the CMB anisotropies are
subdominant. In particular, we calculate systematically the bispectrum of the
primordial curvature perturbation induced by the presence of vector gauge
fields during inflation. Using a model independent parametrization in terms of
magnetic non-linearity parameters, we calculate for the first time the
contribution to the bispectrum from the cross correlation between the inflaton
and the magnetic field defined by the gauge field. We then demonstrate that in
a very general class of models, the bispectrum induced by the cross correlation
between the inflaton and the magnetic field can be dominating compared with the
non-Gaussianity induced by magnetic fields when the cross correlation between
the magnetic field and the inflaton is ignored.Comment: 34 pages, 4 figures. V2: notation improve

### Adiabatic CMB perturbations in pre-big bang string cosmology

We consider the pre-big bang scenario with a massive axion field which starts
to dominate energy density when oscillating in an instanton-induced potential
and subsequently reheats the universe as it decays into photons, thus creating
adiabatic CMB perturbations. We find that the fluctuations in the axion field
can give rise to a nearly flat spectrum of adiabatic perturbations with a
spectral tilt $\Delta n$ in the range $-0.1 \lesssim \Delta n \lesssim 1$.Comment: 15 pages, 1 figure; Version to appear in Nucl.Phys.B. Figure
improved, discussion and one reference adde

### On the non-Gaussian correlation of the primordial curvature perturbation with vector fields

We compute the three-point cross-correlation function of the primordial
curvature perturbation generated during inflation with two powers of a vector
field in a model where conformal invariance is broken by a direct coupling of
the vector field with the inflaton. If the vector field is identified with the
electromagnetic field, this correlation would be a non-Gaussian signature of
primordial magnetic fields generated during inflation. We find that the signal
is maximized for the flattened configuration where the wave number of the
curvature perturbation is twice that of the vector field and in this limit, the
magnetic non-linear parameter becomes as large as |b_{NL}| ~ 10^3. In the
squeezed limit where the wave number of the curvature perturbation vanishes,
our results agree with the magnetic consistency relation derived in
arXiv:1207.4187.Comment: 18 pages. V3: some typos fixed, matches version published in JCA

### Searching for a holographic connection between dark energy and the low-l CMB multipoles

We consider the angular power spectrum in a finite universe with different
boundary conditions and perform a fit to the CMB, LSS and supernova data. A
finite universe could be the consequence of a holographic constraint, giving
rise to an effective IR cutoff at the future event horizon. In such a model
there is a cosmic duality relating the dark energy equation of state and the
power spectrum, which shows a suppression and oscillatory behaviour that is
found to describe the low l features extremely well. However, much of the
discussion here will also apply if we actually live inside an expanding bubble
that describes our universe. The best fit to the CMB and LSS data turns out to
be better than in the standard Lambda-CDM model, but when combined with the
supernova data, the holographic model becomes disfavored. We speculate on the
possible implications.Comment: 16 pages, 5 figures, to appear in JCA

### Planckian Interacting Massive Particles as Dark Matter

The Standard Model could be self-consistent up to the Planck scale according
to the present measurements of the Higgs mass and top quark Yukawa coupling. It
is therefore possible that new physics is only coupled to the Standard Model
through Planck suppressed higher dimensional operators. In this case the WIMP
miracle is a mirage, and instead minimality as dictated by Occam's razor would
indicate that dark matter is related to the Planck scale, where quantum gravity
is anyway expected to manifest itself. Assuming within this framework that dark
matter is a Planckian Interacting Massive Particle, we show that the most
natural mass larger than $0.01\,\textrm{M}_p$ is already ruled out by the
absence of tensor modes in the CMB. This also indicates that we expect tensor
modes in the CMB to be observed soon for this type of minimal dark matter
model. Finally, we touch upon the KK graviton mode as a possible realization of
this scenario within UV complete models, as well as further potential
signatures and peculiar properties of this type of dark matter candidate. This
paradigm therefore leads to a subtle connection between quantum gravity, the
physics of primordial inflation, and the nature of dark matter.Comment: 6 pages, 1 figure, Version published in PR

### Strongly Scale-dependent Non-Gaussianity

We discuss models of primordial density perturbations where the
non-Gaussianity is strongly scale-dependent. In particular, the non-Gaussianity
may have a sharp cut-off and be very suppressed on large cosmological scales,
but sizeable on small scales. This may have an impact on probes of
non-Gaussianity in the large-scale structure and in the cosmic microwave
background radiation anisotropies.Comment: 4 page

### Patient Observers and Non-perturbative Infrared Dynamics in Inflation

We have previously derived the effect of soft graviton modes on the quantum
state of de Sitter using spontaneously broken asymptotic symmetries. In the
present paper we reinterpret this effect in terms of particle production and
relate the quantum states with and without soft modes by means of Bogoliubov
transformations. This also enables us to address the much discussed issues
regarding the observability of infrared effects in de Sitter from a new
perspective. While it is commonly agreed that infrared effects are not visible
to a single sub-horizon observer at late times, we argue that the question is
less trivial for a {\it patient observer} who has lived long enough to have a
record of the state before the soft mode was created. Though classically there
is no obstruction to measuring this effect locally, we give several indications
that quantum mechanical uncertainties may censor the effect. We then apply our
methods to find a non-perturbative description of the quantum state pertaining
to the Page time of de Sitter, and derive with these new methods the
probability distribution for the local quantum states of de Sitter and
slow-roll inflation in the presence of long modes. Finally, we use this to
formulate a precise criterion for the existence of eternal inflation in general
classes of slow-roll inflation.Comment: 37 page

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