Robert Brandenberger
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View article: Back-Reaction of Super-Hubble Fluctuations, Late Time Tracking, and Recent Observational Results
Back-Reaction of Super-Hubble Fluctuations, Late Time Tracking, and Recent Observational Results Open
Previous studies have suggested that the back-reaction of super-Hubble cosmological fluctuations on a symmetric background space-time, with respect to being homogeneous and isotropic, could behave like a dynamical relaxation of the cosmolo…
View article: Visions in quantum gravity
Visions in quantum gravity Open
To deepen our understanding of Quantum Gravity and its connections with black holes and cosmology, building a common language and exchanging ideas across different approaches is crucial. The Nordita Program “Quantum Gravity: From gravitati…
View article: A Non-Inflationary Axion and ALP Misalignment Mechanism
A Non-Inflationary Axion and ALP Misalignment Mechanism Open
Based on considerations of quantum gravity, global symmetries which lead to axions as pseudo Nambu-Goldstone bosons after low scale symmetry breaking cannot be exact at the Planck scale. Here, we show that Planck-suppressed terms which yie…
View article: Collective field theory of gauged multi-matrix models: integrating out off-diagonal strings
Collective field theory of gauged multi-matrix models: integrating out off-diagonal strings Open
A bstract We study a two-matrix toy model with a BFSS-like interaction term using the collective field formalism. The main technical simplification is obtained by gauge-fixing first, and integrating out the off-diagonal elements, before ch…
View article: Cosmological Magnetic Fields from Ultralight Dark Matter
Cosmological Magnetic Fields from Ultralight Dark Matter Open
We propose a mechanism for the generation of magnetic fields on cosmological scales that is operative after recombination. An essential ingredient is an instability (of parametric resonance type) of the electromagnetic field driven by an o…
View article: Back-Reaction of Super-Hubble Fluctuations, Late Time Tracking and Recent Observational Results
Back-Reaction of Super-Hubble Fluctuations, Late Time Tracking and Recent Observational Results Open
Previous studies suggested that the back-reaction of super-Hubble cosmological fluctuations could lead to a dynamical relaxation of the cosmological constant. Moreover, this mechanism appears to be self-regulatory, potentially leading to a…
View article: Visions in Quantum Gravity
Visions in Quantum Gravity Open
To deepen our understanding of Quantum Gravity and its connections with black holes and cosmology, building a common language and exchanging ideas across different approaches is crucial. The Nordita Program "Quantum Gravity: from gravitati…
View article: Do Observations Prefer Thawing Quintessence?
Do Observations Prefer Thawing Quintessence? Open
In light of recent observations by the Dark Energy Spectroscopic Instrument (DESI), we study evidence for thawing quintessence over a cosmological constant as dark energy, with emphasis on the effect of the choice of priors. Working with a…
View article: Swampland conjectures constraints on dark energy from a highly curved field space
Swampland conjectures constraints on dark energy from a highly curved field space Open
We study the interplay of the trans-Planckian censorship conjecture (TCC) and the swampland distance conjecture (SDC) in the context of multifield dark energy in a curved field space. In this scenario, the phase of accelerated expansion is…
View article: Collective field theory of gauged multi-matrix models: Integrating out off-diagonal strings
Collective field theory of gauged multi-matrix models: Integrating out off-diagonal strings Open
We study a two-matrix toy model with a BFSS-like interaction term using the collective field formalism. The main technical simplification is obtained by gauge-fixing first, and integrating out the off-diagonal elements, before changing to …
View article: Clock Fields and Logarithmic Decay of Dark Energy
Clock Fields and Logarithmic Decay of Dark Energy Open
We investigate the physical measurability of the infrared instability of a de Sitter phase in the formalism recently proposed. We find that the logarithmic decay of the effective cosmological constant is only measurable if an additional cl…
View article: On the Origin of the $SO(9) \rightarrow SO(3) \times SO(6)$ Symmetry Breaking in the IKKT Matrix Model
On the Origin of the $SO(9) \rightarrow SO(3) \times SO(6)$ Symmetry Breaking in the IKKT Matrix Model Open
We suggest a dynamical mechanism which explains why in the supersymmetric IKKT matrix model the $SO(9)$ symmetry of the Lagrangian is spontaneously broken to $SO(3) \times SO(6)$, allowing only three large classical spatial dimensions to e…
View article: Embedded domain walls and electroweak baryogenesis
Embedded domain walls and electroweak baryogenesis Open
Embedded walls are domain wall solutions which are unstable in the vacuum but stabilized in a plasma of the early Universe. We show how embedded walls in which the electroweak symmetry is restored can lead to an efficient scenario of elect…
View article: Infrared Sensitivity of Cosmological Probes In The Presence of Axion Field Fluctuations
Infrared Sensitivity of Cosmological Probes In The Presence of Axion Field Fluctuations Open
We study the effects of long wavelength entropy fluctuations on cosmological probes such as galaxy clustering, luminosity distance, and CMB temperature anisotropies. Specifically, we consider fluctuations of a massless spectator scalar fie…
View article: Accretion onto oscillating cosmic string loops
Accretion onto oscillating cosmic string loops Open
Cosmic string loops are non-linear density fluctuations which form in the early universe and could play an important role in explaining many phenomena which are in tension with the standard ΛCDM model. Hence, the accretion process onto cos…
View article: A Superfluid Dark Matter Cosmic String Wake
A Superfluid Dark Matter Cosmic String Wake Open
We study the effects of superfluid dark matter on the structure of a cosmic string wake, considering both the effects of regular and quantum pressure terms. We consider the total fluid to consist of a combination of baryons and dark matter…
View article: Cosmic Strings from Thermal Inflation
Cosmic Strings from Thermal Inflation Open
Thermal inflation was proposed as a mechanism to dilute the density of cosmological moduli. Thermal inflation is driven by a complex scalar field possessing a large vacuum expectation value and a very flat potential, called a “flaton”. Suc…
View article: Swampland Conjectures Constraints on Dark Energy from a Highly Curved Field Space
Swampland Conjectures Constraints on Dark Energy from a Highly Curved Field Space Open
We study the interplay of the trans-Planckian censorship conjecture (TCC) and the swampland distance conjecture (SDC) in the context of multifield dark energy in a curved field space. In this scenario, the phase of accelerated expansion is…
View article: Stringy spacetime uncertainty principle and a modified trans-Planckian censorship criterion
Stringy spacetime uncertainty principle and a modified trans-Planckian censorship criterion Open
We study the implications of the stringy spacetime uncertainty relation (STUR) for inflationary cosmology. By demanding that no fluctuation modes that exit the Hubble radius are affected by the nonlocality resulting from the STUR, we find …
View article: Back-reaction of long-wavelength cosmological fluctuations as measured by a clock field
Back-reaction of long-wavelength cosmological fluctuations as measured by a clock field Open
We consider the back-reaction of cosmological fluctuations on the local expansion rate averaged over a space-like hypersurface of constant value of a clock field. We show that in the infrared limit, the fluctuations lead to a decrease in t…
View article: Back-reaction of long-wavelength cosmological fluctuations as measured by a clock field
Back-reaction of long-wavelength cosmological fluctuations as measured by a clock field Open
We consider the back-reaction of cosmological fluctuations on the local expansion rate averaged over a space-like hypersurface of constant value of a clock field. We show that in the infrared limit, the fluctuations lead to a decrease in t…
View article: N-Body Simulation of Early Structure Formation from Cosmic String Loops
N-Body Simulation of Early Structure Formation from Cosmic String Loops Open
By means of N-body simulations, we study early structure formation in the presence of a scaling distribution of cosmic string loops. Cosmic string loops dominate the high redshift halo mass function while the fluctuations seeded by the sta…
View article: Stringy Spacetime Uncertainty Principle and a Modified Trans-Planckian Censorship Criterion
Stringy Spacetime Uncertainty Principle and a Modified Trans-Planckian Censorship Criterion Open
We study the implications of the stringy space-time uncertainty relation (STUR) for inflationary cosmology. By demanding that no fluctuation modes that exit the Hubble radius are affected by the nonlocality resulting from the STUR, we find…
View article: Decay of ALP condensates via gravitation-induced resonance
Decay of ALP condensates via gravitation-induced resonance Open
Oscillating scalar field condensates induce small amplitude oscillations of the Hubble parameter which can induce a decay of the condensate due to a parametric resonance instability [1]. We show that this instability can lead to the decay …
View article: Superstring cosmology — a complementary review
Superstring cosmology — a complementary review Open
In this review, a number of approaches to superstring cosmology which make use of key features which distinguish string theory from point particle theories are discussed, with particular emphasis on emergent scenarios. One motivation for t…
View article: Spontaneous symmetry breaking in the BFSS model: analytical results using the Gaussian expansion method
Spontaneous symmetry breaking in the BFSS model: analytical results using the Gaussian expansion method Open
We apply the Gaussian expansion method to the BFSS matrix model in the high temperature limit. When the (Euclidean) BFSS action is expanded about a Gaussian ansatz, it is shown that the SO(9) symmetry is spontaneously broken, analogous to …
View article: Accretion onto Oscillating Cosmic String Loops
Accretion onto Oscillating Cosmic String Loops Open
Cosmic string loops are non-linear density fluctuations which form in the early universe and could play an important role in explaining many phenomena which are in tension with the standard $Λ$CDM model. Hence, the details of the accretion…
View article: Superfluid dark matter flow around cosmic strings
Superfluid dark matter flow around cosmic strings Open
We consider a cosmic string moving through a gas of superfluid dark matter (SFDM) particles and analyze how it affects the dark matter distribution. We look at two different cases: first, a cosmic string passing through an already condense…