scholarly journals A stringy perspective on the coincidence problem

2021 ◽  
Vol 2021 (10) ◽  
Author(s):  
Francesc Cunillera ◽  
Antonio Padilla
Keyword(s):  
Phase Transition ◽  
Dark Energy ◽  
Early Universe ◽  
Low Energy ◽  
De Sitter ◽  
Coincidence Problem ◽  
String Compactifications ◽  
Slow Roll ◽  
De Sitter Vacua ◽  
Quintessence Field

Abstract We argue that, for string compactifications broadly consistent with swampland constraints, dark energy is likely to signal the beginning of the end of our universe as we know it, perhaps even through decompactification, with possible implications for the cosmological coincidence problem. Thanks to the scarcity (absence?) of stable de Sitter vacua, dark energy in string theory is assumed to take the form of a quintessence field in slow roll. As it rolls, a tower of heavy states will generically descend, triggering an apocalyptic phase transition in the low energy cosmological dynamics after at most a few hundred Hubble times. As a result, dark energy domination cannot continue indefinitely and there is at least a percentage chance that we find ourselves in the first Hubble epoch. We use a toy model of quintessence coupled to a tower of heavy states to explicitly demonstrate the breakdown in the cosmological dynamics as the tower becomes light. This occurs through a large number of corresponding particles being produced after a certain time, overwhelming quintessence. We also discuss some implications for early universe inflation.

scholarly journals de Sitter vacua in N=8 supergravity and slow-roll conditions

2013 ◽  
Vol 718 (3) ◽  
pp. 1132-1136 ◽  
Author(s):  
G. DallʼAgata ◽  
G. Inverso
Keyword(s):  
De Sitter ◽  
Slow Roll ◽  
De Sitter Vacua

scholarly journals There is no coincidence after all!

2020 ◽  
Vol 29 (14) ◽  
pp. 2043013
Author(s):  
Saurya Das
Keyword(s):  
Dark Matter ◽  
Dark Energy ◽  
Early Universe ◽  
Negative Pressure ◽  
Einstein Condensate ◽  
Coincidence Problem ◽  
Bose Einstein Condensate ◽  
Bose Einstein ◽  
Equal Density

We show that if Dark Matter is made up of light bosons, they form a Bose–Einstein condensate in the early Universe. This in turn naturally induces a Dark Energy of approximately equal density and exerting negative pressure. This explains the so-called coincidence problem.

scholarly journals INFLATIONARY DARK ENERGY FROM A CONDENSATE OF SPINORS IN A 5D VACUUM

2013 ◽  
Vol 22 (12) ◽  
pp. 1342028 ◽  
Author(s):  
PABLO ALEJANDRO SÁNCHEZ ◽  
MAURICIO BELLINI
Keyword(s):  
Dark Energy ◽  
Early Universe ◽  
Inflationary Universe ◽  
De Sitter Spacetime ◽  
De Sitter ◽  
Physical Origin ◽  
Sitter Spacetime ◽  
Interesting Candidate ◽  
Expansion Of The Universe ◽  
The Universe

What is the physical origin of dark energy? Could this energy be originated by other fields than the inflaton? In this paper, we explore the possibility that the expansion of the universe can be driven by a condensate of spinors. These spinors are free of interactions on five-dimensional (5D) relativistic vacuum in an extended de Sitter spacetime. The extra coordinate is considered as noncompact. After making a static foliation on the extra coordinate, we obtain an effective four-dimensional (4D) (inflationary) de Sitter expansion which describes an inflationary universe. In view of our results, we conclude that the condensate of spinors here studied could be an interesting candidate to explain the presence of dark energy in the early universe.

scholarly journals ON VACUUM DENSITY, THE INITIAL SINGULARITY AND DARK ENERGY

2009 ◽  
Vol 18 (14) ◽  
pp. 2343-2349 ◽  
Author(s):  
SAULO CARNEIRO ◽  
REZA TAVAKOL
Keyword(s):  
Phase Transition ◽  
Vacuum Energy ◽  
Field Potential ◽  
Scalar Fields ◽  
Vacuum Energy Density ◽  
De Sitter ◽  
Scale Invariant ◽  
Standard Cosmology ◽  
Slow Roll ◽  
Free Fields

Standard cosmology poses a number of important questions. Apart from its singular origin, it possesses early and late accelerating phases required to account for observations. The vacuum energy has been considered as a possible way to resolve some of these questions. The vacuum energy density induced by free fields in an early de Sitter phase has earlier been estimated to be proportional to H4, while more recently it has been suggested that the QCD condensate induces a term proportional to H at late times. These results have been employed in models which are nonsingular and inflationary at early times and accelerating at late times. Here we cast these models in terms of scalar fields and study the corresponding spectrum of primordial perturbations. At early times the spectrum is found to be not scale-invariant, thus implying that slow roll inflation is still required after the phase transition induced by the vacuum. At late times the corresponding scalar field potential is harmonic, with a mass of the order of the Hubble scale — a result that may be understood in the light of the holographic conjecture.

scholarly journals Thermodynamic analysis of universes with the initial and final de Sitter eras

2015 ◽  
Vol 30 (31) ◽  
pp. 1550158 ◽  
Author(s):  
H. Moradpour ◽  
M. T. Mohammadi Sabet ◽  
A. Ghasemi
Keyword(s):  
Dark Energy ◽  
Energy Density ◽  
Production Process ◽  
Particle Production ◽  
Apparent Horizon ◽  
Thermal Fluctuations ◽  
Mutual Interaction ◽  
De Sitter ◽  
Coincidence Problem ◽  
The Universe

Our aim is studying the thermodynamics of cosmological models including initial and final de Sitter eras. For this propose, bearing Cai–Kim temperature in mind, we investigate the thermodynamic properties of a dark energy (DE) candidate with variable energy density, and show that the state parameter of this dark energy candidate [Formula: see text] should obey the [Formula: see text] constraint, whiles there is no interaction between the fluids filled the universe, and the universe is not in the de Sitter eras. Additionally, based on the thermal fluctuation theory, we study the possibility of inducing fluctuations to the entropy of the DE candidate due to a mutual interaction between the cosmos sectors. Therefore, we find a relation between the thermal fluctuations and the mutual interaction between the cosmos sectors, whiles the DE candidate has a varying energy density. Finally, bearing the coincidence problem in mind, we derive a constraint on the vacuum energy, and investigate its relation with the entropy evolution of the DE candidate. We also point to a model with initial and final de Sitter eras in which a gravitationally induced particle production process leads to change the expansion eras, whiles the corresponding pressure is considered as the cause of current accelerated phase. We study its thermodynamics, and show that such processes may also leave thermal fluctuations into the system. We also find an expression between the thermal fluctuations and the particle production rate. Finally, we use Hayward–Kodama temperature to get a relation for the horizon entropy in models including the gravitationally induced particle production process. Our study shows that the first law of thermodynamics is available on the apparent horizon whiles, the gravitationally induced particle production process, as the DE candidate, may add an additional term to the Bekenstein limit of the horizon. The relation between the validity of the second law of thermodynamics and the gravitationally particle production process is also studied.

scholarly journals HYBRID CHAPLYGIN GAS

2009 ◽  
Vol 24 (07) ◽  
pp. 541-555 ◽  
Author(s):  
HONGSHENG ZHANG ◽  
ZONG-HONG ZHU ◽  
LIHUA YANG
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Transient Behavior ◽  
Chaplygin Gas ◽  
De Sitter ◽  
Coincidence Problem ◽  
Effective Equation ◽  
Phantom Divide ◽  
Perturbation Growth

Hybrid Chaplygin gas model is put forward, in which the gases play the role of dark energy. For this model the coincidence problem is greatly alleviated. The effective equation of state of the dark energy may cross the phantom divide w = -1. Furthermore, the crossing behavior is decoupled from any gravity theories. In the present model, w < -1 is only a transient behavior. There is a de Sitter attractor in the future infinity. Hence, the big rip singularity, which often afflicts the models with matter whose effective equation of state less than -1, naturally disappears. There exist stable scaling solutions, both at the early universe and the late universe. We discuss the perturbation growth of this model. We find that the index is consistent with observations.

scholarly journals Sharpening the boundaries between flux landscape and swampland by tadpole charge

2021 ◽  
Vol 2021 (12) ◽  
Author(s):  
Keiya Ishiguro ◽  
Hajime Otsuka
Keyword(s):  
Flux Compactifications ◽  
Scalar Potential ◽  
Low Energy ◽  
The Other ◽  
Effective Theory ◽  
Taylor Model ◽  
De Sitter ◽  
Cancellation Condition ◽  
Vacuum Structure ◽  
De Sitter Vacua

Abstract We investigate the vacuum structure of four-dimensional effective theory arising from Type IIB flux compactifications on a mirror of the rigid Calabi-Yau threefold, corresponding to a T-dual of the DeWolfe-Giryavets-Kachru-Taylor model in Type IIA flux compactifications. By analyzing the vacuum structure of this interesting corner of string landscape, it turns out that there exist perturbatively unstable de Sitter (dS) vacua in addition to supersymmetric and non-supersymmetric anti-de Sitter vacua. On the other hand, the stable dS vacua appearing in the low-energy effective action violate the tadpole cancellation condition, indicating a strong correlation between the existence of dS vacua and the flux-induced D3-brane charge (tadpole charge). We also find analytically that the tadpole charge constrained by the tadpole cancellation condition emerges in the scalar potential in a nontrivial way. Thus, the tadpole charge would restrict the existence of stable dS vacua, and this fact underlies the statement of the dS conjecture. Furthermore, our analytical and numerical results exhibit that distributions of $$ \mathcal{O}(1) $$ O 1 parameters in expressions of several swampland conjectures peak at specific values.

scholarly journals Unimodular vs nilpotent superfield approach to pure dS supergravity

2021 ◽  
Vol 2021 (1) ◽  
Author(s):  
Sukruti Bansal ◽  
Silvia Nagy ◽  
Antonio Padilla ◽  
Ivonne Zavala
Keyword(s):  
String Theory ◽  
General Framework ◽  
Recent Progress ◽  
High Energy ◽  
De Sitter Spacetime ◽  
De Sitter ◽  
New Approach ◽  
Sitter Spacetime ◽  
Novel Approach ◽  
De Sitter Vacua

Abstract Recent progress in understanding de Sitter spacetime in supergravity and string theory has led to the development of a four dimensional supergravity with spontaneously broken supersymmetry allowing for de Sitter vacua, also called de Sitter supergravity. One approach makes use of constrained (nilpotent) superfields, while an alternative one couples supergravity to a locally supersymmetric generalization of the Volkov-Akulov goldstino action. These two approaches have been shown to give rise to the same 4D action. A novel approach to de Sitter vacua in supergravity involves the generalisation of unimodular gravity to supergravity using a super-Stückelberg mechanism. In this paper, we make a connection between this new approach and the previous two which are in the context of nilpotent superfields and the goldstino brane. We show that upon appropriate field redefinitions, the 4D actions match up to the cubic order in the fields. This points at the possible existence of a more general framework to obtain de Sitter spacetimes from high-energy theories.

scholarly journals Weak gravity bounds in asymptotic string compactifications

2021 ◽  
Vol 2021 (6) ◽  
Author(s):  
Brice Bastian ◽  
Thomas W. Grimm ◽  
Damian van de Heisteeg
Keyword(s):  
Black Holes ◽  
Moduli Space ◽  
Mass Formula ◽  
Complex Structure ◽  
Mass Ratio ◽  
De Sitter ◽  
String Compactifications ◽  
Type Iib String Theory ◽  
Bps States ◽  
Weak Gravity

Abstract We study the charge-to-mass ratios of BPS states in four-dimensional $$ \mathcal{N} $$ N = 2 supergravities arising from Calabi-Yau threefold compactifications of Type IIB string theory. We present a formula for the asymptotic charge-to-mass ratio valid for all limits in complex structure moduli space. This is achieved by using the sl(2)-structure that emerges in any such limit as described by asymptotic Hodge theory. The asymptotic charge-to-mass formula applies for sl(2)-elementary states that couple to the graviphoton asymptotically. Using this formula, we determine the radii of the ellipsoid that forms the extremality region of electric BPS black holes, which provides us with a general asymptotic bound on the charge-to-mass ratio for these theories. Finally, we comment on how these bounds for the Weak Gravity Conjecture relate to their counterparts in the asymptotic de Sitter Conjecture and Swampland Distance Conjecture.

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