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Version: 1.0
(Sept 25, 2007)

Theoretical Cosmology Meetings

To actively encourage the field of Theoretical Cosmology in the Netherlands and to set an informal stage for the exchange of ideas, the small but growing Dutch Theoretical Cosmology community organizes Friday afternoon meetings approximately 6 times a year --- usually on the first Friday of the month. The meetings start in the afternoon with a main speaker, followed by a short break to continue with a journal club discussion on some topic of current interest. We end the afternoon with drinks. The supporting institutes, Leiden, Amsterdam and Utrecht, each take turns in hosting the event.

For more details (personalia) about the landscape of Dutch Theoretical Cosmology, click the THC landscape button on the left menu. For a list of accomplishments and the latest THC news click here.

Friday December 6, 2013 (TBA)

1400h: Seminar room

Speaker: TBA

Title: TBA

Abstract
: TBA

Friday November 1, 2013 (TBA)

1400h: Seminar room

Speaker: TBA

Title: TBA

Abstract
: TBA

Friday October 4, 2013 (TBA)

1400h: Seminar room

Speaker: TBA

Title: TBA

Abstract
: TBA

Friday September 6, 2013 (TBA)

1400h: Seminar room

Speaker: TBA

Title: TBA

Abstract
: TBA

Friday June 28, 2013 (Amsterdam/Nikhef)

1400h: Seminar room

Speaker: TBA

Title: TBA

Abstract
: TBA

Friday April 12, 2013 (University of Utrecht)

1330h: Minnaert building, room 208

Speaker: Gerasimos Rigopoulos (Technische Universitat Munchen)

Title: Gradient expansion for cosmological perturbations

Abstract
: I will discuss the use of the gradient expansion as a means of studying (sub-Hubble) non-linear cosmological perturbations, highlighting the relation to Lagrangian Perturbation Theory and the Zeldovich approximation and mapping out possible future directions.

1500h: Seminar

Speaker: Sohyun Park (Penn. State U.)

Title: Structure formation in a nonlocally modified gravity model

Abstract
: We study a nonlocally modified gravity model proposed by Deser and Woodard which gives an explanation for current cosmic acceleration. By deriving and solving the equations governing the evolution of the structure in the Universe, we show that this model predicts a pattern of growth that differs from standard general relativity (+dark energy) at the 10-30% level. These differences will be easily probed by the next generation of galaxy surveys, so the model should be tested shortly.

1600h: Journal club discussion

Speaker: Sander mooij (Nikhef)
Sander will lead a discussion for PLANCK results announced in the ESA meeting based on arXiv:1303.5076, arXiv:1303.5082 and arXiv:1303.5084.

Thursday February 28, 2013 (University of Leiden)

1400h: Institute Lorentz coffee room, Oort 269

Speaker: Alessandra Silvestri (SISSA)

Title: Testing General Relativity on Cosmological Scales

Abstract
: More than a decade after its discovery, cosmic acceleration still poses a puzzle for modern cosmology and a plethora of models of dark energy or modified gravity, able to reproduce the observed expansion history, have been proposed as alternatives to the cosmological standard model. In recent years it has become increasingly evident that probes of the expansion history are not sufficient to distinguish among the candidate models, and that it is necessary to combine those with observations that probe the dynamics of inhomogeneities. Future cosmological surveys will map the evolution of inhomogeneities to high accuracy, allowing us to test the relationships between matter overdensities, local curvature, and the Newtonian potential on cosmological scales.

I will discuss theoretical issues involved in finding an optimal framework to study deviations from General Relativity on cosmological scales, giving an overview of recent progress, with a focus on model-independent approaches. I will conclude with an outlook for potential signatures of modified gravity on astrophysical scales, where so-called screening mechanisms" may offer us a clue to new features of our underlying theory of gravity.

1530h: Seminar

Speaker: L. Senatore (Stanford/CERN)

Title: Anomalous dimensions and non-Gaussianity

Friday February 1, 2013 (Groningen university)

1400h: VIP-room (5114.0001) the Physics and Chemistry Building (Nijenborgh 4). When coming by train, one should take busline 11 or 15 to the Zernike campus, which is 20 minutes north of the Central Station. At the campus, get off at the first or second stop. The Physics building is the first big building on your right. When entering, you pass by the reception which can tell you how to get to the VIP-room.

Speaker: Thorsten Battefeld (Univ. of Gottingen)

Title:Particle production during and after Inflation

Abstract
: Additional light degrees of freedom at certain locations in field space (ESPs) are ubiquitous in moduli spaces and can have interesting effects during and after inflation: particle production at such locations and the associated back-reaction can yield additional contributions to correlation functions of curvature fluctuations, affect the speed of the inflaton(s) (trapped inflation) and drastically alter the nature of preheating. In this talk, I first examine a single grazing ESP encounter as a warm-up, before discussing multi-field preheating at ESPs in the vicinity (but not at) the VEV of the inflatons (as in trapped inflation). The latter can be efficient if ESPs are dense, and/or if ESPs happen to be close to special locations due to prolonged narrow resonances that are absent if preheat matter fields become light at the VEV of the inflatons.

1530h: Journal club discussion

Speaker: Ivonne Zavala
Ivonne will talk about A Sinister Universe'' by Adshear, Martinec and Wyman based on: arXiv:1301.2598.

Friday December 7, 2012 (University of Utrecht)

1400h: Minnaert building, room 401

Speaker: Joao Magueijo (Imperial)

Title: Matters of gravity: testing modified gravity in the Solar system

Abstract
: Contrary to popular belief, on very large distances visible matter stubbornly refuses to "fall" according to the laws of gravity of both Newton and Einstein. The paradox has led to the introduction of dark matter, purporting to explain the observed surplus of gravitational pull. The logical possibility remains that there is no dark matter, what you see is all there is, and that the paradox simply signals the break down of the Einstein-Newton theory of gravity. I will review alternative theories of gravity that do away with the need for dark matter. Surprisingly, Solar system gravitational experiments (such as those associated with the LISA Pathfinder mission, Lunar Laser Ranging and Very Large Baseline Interferometry) might settle the score between the two approaches.

1530h: Journal club discussion

Speaker: Tomislav Prokopec (UU)
Tomislav will talk about solid inflation'' based on the following papers:
arXiv:1210.0569, arXiv:gr-qc/9811024 and arXiv:gr-qc/9708005.

Friday November 2, 2012 (NIKHEF/UVA)

1400h: Seminar room H320

Speaker: Christian Byrnes (University of Sussex)

Title: Primordial black holes as a tool for constraining non-Gaussianity

Abstract
: Primordial Black Holes (PBH's) can form in the early Universe from the collapse of large density fluctuations. Tight observational limits on their abundance constrain the amplitude of the primordial fluctuations on very small scales which can not otherwise be constrained, with PBH's only forming from the extremely rare large fluctuations. The number of PBH's formed is therefore highly sensitive to small changes in the shape of the tail of the fluctuation distribution, which depends on the amount of non-Gaussianity present. We study how local non-Gaussianity of arbitrary size (parameterised by fNL and gNL) affects the PBH abundance and the resulting constraints on the amplitude of the fluctuations on very small scales. Intriguingly, even non-linearity parameters of order unity have a significant impact on the PBH abundance. The sign of the non-Gaussianity is particularly important, with the constraint on the allowed fluctuation amplitude tightening by an order of magnitude as fNL changes from just -0.5 to 0.5. We show that if PBH's are observed in the future, then regardless of the amplitude of the fluctuations, non-negligible negative fNL would be ruled out. Finally we show that gNL may have an even larger effect on the number of PBH's formed than fNL.

1530h: Journal club discussion

Speaker: I-Sheng Yang (ITFA)
I-Sheng will speak on "Non-Accidental Inflation: Model Building in the Planck Era."

Abstract
: We argue that if Planck and other future observations discover certain global structures related to the initial conditions of inflation, we need models explaining such initial conditions and inflation together by design, instead of by accident. We point out several different initial conditions which are possibly observable and require new models to combine them with slow-roll inflation.

Friday October 5, 2012 (University of Leiden)

1400h: Lorentz seminar room

Speaker: Wessel Valkenburg

Title: Promoting the Copernican Principle to an observational fact

Abstract
: The one fundamental ingredient in today's cosmological models is the Cosmological Principle, which is a cosmological extension of the Copernican Principle. However, what happens if you construct a cosmology based on observations only? The possibility that we occupy a position in space time which is not representative for the entire universe, has an impact on our knowledge of the late time universe. I will address the consequences of such a pragmatic approach and I will show how we can and we far we are from observationally establishing the Copernican Principle, which is possibly the last missing observation necessary to put today's standard cosmological model on a firm basis.

1530h: Journal club discussion

Speaker: Alexey Boyarsky
Alexey will report on the dark matter session in COSMO 13, in particular the discussions about the 130 GeV line found in arXiv:1204.2797, arXiv:1205.1045, and the caveats in interpreting it as coming from dark matter annihilation, see arXiv:1205.4700. Another two papers on this subject one might consult are: arXiv:1209.4562, arXiv:1206.1616.

Friday June 1, 2012 (Nikhef Institute)

1400h: Seminar room H331

Speaker: Gianmassimo Tasinato (University of Portsmouth, UK)

Title: Non-linear interactions and exact solutions in massive gravity

Abstract
: I will start discussing the basic features of a covariant massive gravity model, which is a candidate for a ghost-free non-linear extension of Fierz-Pauli. I will then analyze how strong coupling effects make the phenomenology of this model particularly interesting. First, I will show how non-linear interactions are able to screen the effects of additional degrees of freedom in this theory, leading to predictions compatible with general relativity. Then, I will discuss a general class of exact solutions of this theory, containing cosmological configurations able to describe present day acceleration of our universe with no need of a positive cosmological constant. I will conclude discussing the surprising properties of the dynamics of fluctuations around some of these configurations.

1530h: Journal club discussion

Speaker: Damien George (Nikhef)
Gorge will discuss Accelerated Expansion from Negative Lambda'' based on arXiv:1205.3807.

Friday May 11, 2012 (Nikhef Institute, Amsterdam)

1400h: Nikhef Building (oppesite to the Science Park 904 building), H331

Speaker: Sera Cremonini (DAMTP)

Title: A Critical Look at Effective Field Theory Violations During Inflation

Abstract
: I will discuss the validity of effective field theory methods and the decoupling of heavy fields during inflation. In particular, I will consider inflationary models in which the inflaton is coupled to a heavy (super-Hubble) degree of freedom which is initially in its vacuum state. A temporary departure from inflation can lead to a period of non-adiabaticity, during which effective field theory methods are known to fail. However, the locality of the event and energy conservation lead to a tight bound on the size of the effects of the heavy field, and therefore on the resulting corrections to the power spectrum. These results further motivate the use of effective field theory methods to characterize inflationary dynamics, and focus the question of observability of additional degrees of freedom during inflation to near the Hubble scale or below.

1530h: Journal club discussion

Speaker: Ana Achucarro (U. of Leiden)
Ana will discuss her most recent paper on Heavy fields, reduced speeds of sound and decoupling during inflation'' : arXiv:1205.0710.

Friday April 13, 2012 (University of Utrecht)

1400h: Minnaert building, MG 211

Speaker: Thomas Konstandin (CERN & DESY)

Title: Electroweak baryogenesis - a status report

Abstract
: After a review of the mechanism of electroweak baryogenesis I discuss its viability in different models in the light of recent experimental results. In particular, I discuss the status in the MSSM and in a class of singlet extensions that serve as effective theories to composite Higgs models.

1530h: Journal club discussion

Speaker: Jan Weenink (U. of Utrecht)
Jan will give an overview of recent developments in inhomogeneous cosmologies. The discussion will be based on:
arXiv:1203.4567, "Cosmology when living near the Great Attractor" by Valkenburg & Bjaelde;
arXiv:1110.2587, "Topological quintessence" by Bueno Sanchez & Perivolaropoulos;
arXiv:1203.6776 , "Inhomogeneous vacuum energy" by Wands et al;
arXiv:1203.6776 , "Inhomogeneous non-Gaussianity" by Byrnes et al.

Friday March 2, 2012 (University of Leiden)

1400h: Seminar in Lorentz Seminar Room

Speaker: Mikhail Shaposhnikov

Title: Scale-invariant alternatives to general relativity

Abstract
: We study the general class of gravitational field theories constructed on the basis of scale invariance (and therefore absence of any mass parameters) and invariance under transverse diffeomorphisms (TDiff), which are the 4-volume conserving coordinate transformations. We show that these theories are equivalent to a specific type of scalar-tensor theories of gravity (invariant under all diffeomorphisms) with a number of properties, making them phenomenologically interesting. In particular, they lead to the evolution of the universe supported by present observations: inflation in the past, followed by the radiation and matter dominated stages and accelerated expansion at present. All mass scales in this type of theories come from one and the same source. The massless particle spectrum of these theories contains the graviton and a new particle -- dilaton, which has only derivative couplings and thus escapes the fifth force constraints.

1530h: Journal club discussion

Speaker: Dmytro akubovskyi will lead the discussionon on Cores in warm dark matter haloes: a Catch 22 problem'' by Andrea V. Maccio', Sinziana Paduroiu, Donnino Anderhalden, Aurel Schneider, Ben Moore, arXiv:1202.1282.

Friday February 3, 2012 (University of Leiden)

1400h: Seminar in Lorentz Seminar Room

Speaker: Julien Lesgourges (CERN)

Title: The intriguing radiation content of the universe

Abstract
: In the minimal cosmological model, during the radiation dominated stage, the energy of the universe is attributed to photons and to the three known families of active neutrinos, with standard properties. This assumption is still poorly constrained by the data, but in the last few years a number of cosmological and particle physics experiment raised some marginal preference for a radiation density excess. Within the next twelve months, cosmological experiments like Planck or BOSS will trigger some spectacular progress in understanding the abundance and properties of cosmological radiation. We will try to shed some light on these issues, first by clarifying the observable effects of extra light relics in cosmology, and second by summarizing the theoretical challenges for neutrino physics and cosmology that may result from a possible excess detection.

1530h: Journal club discussion

Speaker: Ted van der Aalst will lead the discussionon on two papers on conformal invariance in inflation: arXiv:1108.0874 (Creminelli), arXiv:1103.4164 (Antoniadis, Mazur, Mottola).

There will be another talk at Utrecht (MG 401) given by Chiara Caprini (CEA, Saclay) at 2pm on February 20, Monday. Everyone is welcome.

Title: Gravitational waves from first order phase transitions

Abstract
: Gravitational waves can constitute a unique probe of the very early universe. This seminar concentrates on a particular source of a stochastic background of gravitational waves, namely a primordial first order phase transition. The gravitational wave signal is generated by the collision of the broken phase bubbles and by the magnetohydrodynamic turbulence in the primordial plasma stirred by the bubble collision. It will be shown how the main features of the gravitational wave spectrum can be predicted by simple, general arguments based on the source properties, such as its time evolution and space structure. Detection prospects will be discussed in particular for the electroweak phase transition, in connection with the new NGO/eLISA mission of ESA.

Friday December 2, 2011 (University of Leiden)

Special Note: Avi Loeb will be delivering the Sackler Lectures in Leiden Nov 21-25.  These will include an introduction to 21 cm cosmology, which will be the topic of Pritchard's talk on December 2.  Everyone is welcome to attend both events.

1400h: Seminar in Lorentz Seminar Room

Speaker: Jonathan Pritchard (Imperial)

Title: Exploring the dawn of cosmic structure with the 21 cm line and CO mapping

Abstract: The first billion years after the Big Bang is the period where the first stars and galaxies form yet this period remains one at the frontier of observations.  In this talk, I will discuss new techniques that promise the possibility of transforming our understanding of this period.  First, I will discuss applications of radio observations of the redshifted 21 cm line. Observations of the sky averaged global signal with single dipole experiments and measurements of fluctuations with interferometers could offer new insights into reionization and early X-ray and UV emission of the first galaxies.  I will also discuss the possibility for "intensity mapping" in atomic and molecular lines as an alternative to traditional galaxy surveys.  Focussing on CO intensity mapping during the epoch of reionization, I will explore how such measurements could provide a large scale context for our understanding of how galaxies form and produce metals.

1530h: Journal club discussion

Speaker: Sander Mooij will lead the discussion about the article "The principle of relative locality," by Giovanni Amelino-Camelia, Laurent Freidel, Jerzy Kowalski-Glikman and Lee Smolin, arXiv:1101.0931.

November, 2011

There will be three Amsterdam-area cosmology events in November:

1) APP at the University of Leiden, November 3

2) Special THC Seminar at the University of Amsterdam, Friday November 4

1400h in room C4.174 (right across the elevator on the 4th floor)

Speaker: Thomas Hartman (Princeton)

Title: Holography in de Sitter Space with a Free CFT

Because de Sitter space shares some features with anti-de Sitter, it is tempting to apply holography and the AdS/CFT correspondence to cosmology, but specific theories are difficult to construct. I will describe a proposal for an explicit holographic duality in four-dimensional de Sitter space, based on higher spin gravity. The boundary CFT is free and therefore provides an exactly solveable toy model for quantum gravity in de Sitter space.

3) Strong Coupling and Holography in Cosmology Workshop at the University of Amsterdam, November 7-8

Friday Sept 30, 2011 (University of Amsterdam)

1400h: Seminar in A1.10

Speaker: PierStefano Corasaniti (LUTH, Observatory of Paris)

Title: A new approach to halo counting statistics: path-integral, non-spherical halo collapse and primordial non-Gaussianity

Abstract: The mass distribution of dark matter halos in the universe carries unique information on the non-linear gravitational processes that have shaped the late time cosmic structure formation. Due to the complexity of the non-linear collapse of dark matter density fluctuations, the halo mass function has been mainly investigate using numerical N-body simulations.  In this talk I will review some recent theoretical advancements based on a path-integral formulation of the Excursion Set theory and present an analytical computation of the halo mass function for Gaussian and non-Gaussian initial conditions in the case of an effective stochastic barrier model which captures the main features of the non-spherical collapse of dark matter halos. This provides a mathematical description of halo mass function that explicitly depends on physical model parameters and which we find to be in remarkable agreement with Gaussian and non-Gaussian numerical N-body simulation results.

1530h: Journal club discussion

Speaker: Jan Pieter van der Schaar (University of Amsterdam)
Jan Pieter will lead a discussion about the article "The Multiverse Interpretation of Quantum Mechanics" by Susskind and Bousso, arXiv:1105.3796.

Schedule 2010-2011

Friday June 10, 2011
(University of Utrecht)

1400h: Seminar in Minnaert Gebouw 211 (lecture hall on second floor)

Speaker: David Polarski (Montpellier, France)

Title: Dark Energy

Abstract: The Universe has presumably entered a stage of accelerated expansion rate at low redshifts. This is a radical departure from conventional cosmology. Various ways to address this fundamental problem have been proposed and we will review some of them.

1530h: Seminar

Speaker: Richard Woodard

Title: The Graviton Propagator in de Donder Gauge on de Sitter Background

Abstract: We construct the graviton propagator on de Sitter background in exact de Donder gauge. We prove that it must break de Sitter invariance, just like the propagator of the massless, minimally coupled scalar. Our explicit solutions for its two scalar structure functions preserve spatial homogeneity and isotropy so that the propagator can be used within the larger context of inflationary cosmology, however, it is simple to alter the residual symmetry. Because our gauge condition is de Sitter invariant (although no solution for the propagator can be) renormalization should be simpler using this propagator than one based on a noncovariant gauge. It remains to be seen how other computational steps compare.  Preprint: 1106.0925

Note: Richard is also giving an ITP Colloquium next Wed at 4pm (Title: "A Non-Technical (Would I Lie?) Discussion of the Problem of Quantum Gravity'')

Friday May 6, 2011 (University of Leiden)

1400h: Seminar in de Sitter Lecture Hall (NOTE ROOM CHANGE)

Speaker: Martin Sloth (CERN)

Title: Cosmological Perturbations and IR Issues in quasi de Sitter Universes

Abstract: Using simple semiclassical relations it is possible to show that the conventional cosmological correlation functions  are affected by significant IR corrections in quasi de Sitter space-times when averaged over very large volumes (in the "large box"). The IR effects apparently imply a breakdown of perturbation theory in the large box on sufficiently long time scales, for example the time between self-reproduction and reheating in chaotic inflation.  An interpretation of the apparent breakdown of the perturbative expansion of gravity  and the relation to the black hole information paradox will also be briefly discussed. Then we will show how one can define "IR-safe" observables seen by a post-inflationary observer today (in the "small box"), leading to a  cosmological RG equation connecting "large box" and "small box" observers. Finally, we demonstrate how an observer today might be able to observe the beginning of the end of perturbative de Sitter imprinted in small statistical inhomogeneities/anisotropies at short scales.

1530h: Journal club discussion in Lorentz Seminar Room

Speaker: Pablo Ortiz (Leiden University)

Pablo will lead a discussion about the article "Gauge-flation: Inflation from non-abelan gauge fields" (arXiv:1102.1513).

Friday April 1, 2011 (University of Amsterdam)

1400h: Seminar in C4.174 (right across the elevator on the 4th floor)

Speaker: James Fergusson (DAMTP, Univ of Cambridge)

Title: Constraining nonGaussianity with the CMB

Abstract: Detection of nonGaussianity would be very significant as it would not only rule out single field slow roll inflation but it's form would provide a clear indication as to which other class of models would replace it. However due to the complexity of calculating higher order correlators, which are best suited to constraining it, work so far has focused exclusively on a couple of simple cases. In this talk I will propose a new approach which allows us not only to constrain nonGaussianity of any type, but to also generate simulated nonGaussian maps for any given model, detect and eliminate all known contaminants, and directly reconstruct the form of the nonGaussianity from the data both at late times and primordialy.

1600h: Colloquium (note special event, time and place) in room C1.12

Speaker: Amina Helmi (University of Groningen)

Title: Lessons from Near-field Cosmology

Abstract: Near-field cosmology is an active area of research in current astrophysics. It focuses on testing the cosmological model on the scales of the Milky Way galaxy and its nearest neighbours.  I will review what we have learned from observations and modelling of these systems in recent years.

Friday March 4, 2011 (University of Amsterdam)

1400h: Seminar in C4.174 (right across the elevator on the 4th floor)

Speaker: Leonardo Senatore (Stanford University)

Title: The Effective Field Theory of Inflation and of Multifield Inflation

Abstract: I will describe the effective field theory of inflation and of multifield inflation, i.e. the most general theory describing the fluctuations around a quasi de Sitter background, in the case of single and multi-field models.  This approach allows us to describe in a unique Lagrangian all possible inflationary models.  It also allows us to fully explore the spectrum of the possible signatures that can be generated by inflation.  Finally, it represents the most direct way of mapping cosmological observations into parameters of a fundamental Lagrangian.  This is particularly important given the on-going experimental effort from the WMAP and Planck satellites, as well as from the Large Scale Structure surveys.  I will describe how this effective Lagrangian is constructed, the most important signatures that we can infer from it, and finally how current observations are already beginning to constrain its parameters.

1530h: Journal club discussion in ITFA common room (C4.278)

Speaker: Ted van der Aalst (University of Leiden)

Ted will be leading the discussion about A Minimal Inflation Scenario, arXiv:1101.4948

Friday February 4, 2011 (University of Leiden)

1400h: Seminar in Lorentz Seminar Room

Speaker: Alexander Westphal (DESY)

Title: Simple exercises to flatten your potential

Abstract:
We show how backreaction of the inflaton potential energy on heavy scalar fields can flatten the inflationary potential, as the heavy fields adjust to their most energetically favorable configuration. This mechanism operates in previous UV-complete examples of axion monodromy inflation - flattening a would-be quadratic potential to one linear in the inflaton field - but occurs more generally, and we illustrate the effect with several examples.  Special choices of compactification minimizing backreaction may realize chaotic inflation with a quadratic potential, but we argue that a flatter potential such as power-law inflation $V(\phi) \propto \phi^p$ with $p<2$ is a more generic option at sufficiently large values of $\phi$.

1530h: Special Seminar

Speaker: Bret Underwood (McGill)

Title: A Breathing mode for Compactifications

Abstract: How do we study the physics of theories with extra dimensions?  The easiest thing to do is to reduce the higher dimensional description to a 4-dimensional effective description.  This results in a number of scalar fields describing, for instance, fluctuations of higher dimensional scalar fields (dilaton) or the volume of the compact space (volume modulus).  But the fields in the effective theory must be constructed with care: artifacts from the higher dimensions, such as higher dimensional diffeomorphisms and constraint equations, can restrict or remove putative degrees of freedom entirely.  The effective theory including these effects resembles in many ways cosmological perturbation theory.  The constraints and diffeomorphisms generically lead the dilaton and volume modulus to combine into a single degree of freedom in the effective theory, the "breathing mode".  This implies that models of inflation or moduli stabilization involving extra dimensions which treat these moduli as separate have the wrong degrees of freedom.

The constraints and diffeomorphisms generically lead the dilaton and volume modulus to combine into a single degree of freedom in the effective theory, the "breathing mode".  This implies that models of inflation or moduli stabilization involving extra dimensions which treat these moduli as separate have the wrong degrees of freedom.