When Did Cosmic Acceleration Start ?

Pandolfi, Stefania

doi:10.1016/j.nuclphysbps.2009.07.029

“…This is because one must cross from a positive value (deceleration after inflation) to a negative one for today (acceleration). A zero acceleration q acc = 0 at about x ≈ 0.5 has been found (Pandolfi 2009) followed by an acceleration up to the present time (q 0 ≈ −0.5). Here we will prove that the condition (5) inevitably predicts a zero value for q at the central point in age x = 0.5.…”

Section: The Magic Equationmentioning

confidence: 53%

“…This picture would give us for today a decelerated expansion for the universe. But this is not what is observed: today we see an accelerated expansion, which is occurring back in time from about half the present age of the universe (Pandolfi 2009). If a(t) is the cosmological scale factor, the "deceleration" parameter q has been defined as…”

Section: Introductionmentioning

confidence: 92%

“…But nature keeps giving us surprises. About ten years ago it was found that the universe is in a state of accelerated expansion, and we know today that this state of acceleration started back in time at about one half the age of the universe (Pandolfi 2009). Initially we needed the cosmological constant to balance gravitation, to have a push to explain the expansion of the universe.…”

Section: Introductionmentioning

confidence: 99%

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Quantization of the universe as a black hole

Alfonso

¹

,

Alfonso-Faus

²

2011

Astrophys Space Sci

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Combining the kinematical definitions of the two dimensionless parameters, the deceleration q(x) and the Hubble t 0 H (x), we get a differential equation (where x = t/t 0 is the age of the universe relative to its present value t 0 ). ], determine the function H (x).A second integration gives the cosmological scale factor a(x). Differentiation of a(x) gives the speed of expansion of the universe. The evolution of the universe that results from our approach is: an initial extremely fast exponential expansion (inflation), followed by an almost linear expansion (first decelerated, and later accelerated). For the future, at approximately t ≈ 3t 0 there is a final exponential expansion, a second inflation that produces a disaggregation of the universe to infinity. We find the necessary and sufficient conditions for this disaggregation to occur. The precise value of the final age is given only with one parameter: the present value of the deceleration parameter [q(1) ≈ −0.5]. This emerging picture of the history of the universe represents an important challenge, an opportunity for the immediate research on the Universe. These conclusions have been elaborated without the use of any particular cosmological model of the universe.

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“…This picture would give us for today a decelerated expansion for the universe. But this is not what is observed: today we see an accelerated expansion, which is occurring back in time from about half the present age of the universe [10]. If a(t) is the cosmological scale factor, the "deceleration" parameter q has been defined as q(t)= -(t) a(t)/ (t) 2 (1) One would expect that, after inflation, the universe was left still expanding but slowly decelerating.…”

Section: -Introductionmentioning

confidence: 88%

Evidence for a disaggregation of the universe

Alfonso-Faus

¹

2011

Astrophys Space Sci

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Combining the kinematical definitions of the two dimensionless parameters, the deceleration q(x) and the Hubble t 0 H (x), we get a differential equation (where x = t/t 0 is the age of the universe relative to its present value t 0 ). ], determine the function H (x).A second integration gives the cosmological scale factor a(x). Differentiation of a(x) gives the speed of expansion of the universe. The evolution of the universe that results from our approach is: an initial extremely fast exponential expansion (inflation), followed by an almost linear expansion (first decelerated, and later accelerated). For the future, at approximately t ≈ 3t 0 there is a final exponential expansion, a second inflation that produces a disaggregation of the universe to infinity. We find the necessary and sufficient conditions for this disaggregation to occur. The precise value of the final age is given only with one parameter: the present value of the deceleration parameter [q(1) ≈ −0.5]. This emerging picture of the history of the universe represents an important challenge, an opportunity for the immediate research on the Universe. These conclusions have been elaborated without the use of any particular cosmological model of the universe.

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“…at z = 0. The analyses of cosmological observations in literature [106]− [111] furnish the transition redshift of the accelerating expansion as given by 0.3 < z t < 0.8.…”

Section: Non-interacting Two-fluid Modelmentioning

confidence: 99%

Two-Fluid Dark Energy Models in Bianchi Type-III Universe with Variable Deceleration Parameter

Amirhashchi

¹

,

Pradhan

²

,

Jaiswal

³

2013

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Some new exact solutions of Einstein's field equations have come forth within the scope of a spatially homogeneous and anisotropic Bianchi type-III space-time filled with barotropic fluid and dark energy by considering a variable deceleration parameter. We consider the case when the dark energy is minimally coupled to the perfect fluid as well as direct interaction with it. Under the suitable condition, the anisotropic models approach to isotropic scenario. We also find that during the evolution of the universe, the equation of state (EoS) for dark energy ω (de) , in both cases, tends to −1 (cosmological constant, ω (de) = −1), by displaying various patterns as time increases, which is consistent with recent observations. The cosmic jerk parameter in our derived models are in good agreement with the recent data of astrophysical observations under appropriate condition. It is observed that the universe starts from an asymptotic Einstein static era and reaches to the ΛCDM model. So from recently developed Statefinder parameters, the behavior of different stages of the universe has been studied. The physical and geometric properties of cosmological models are also discussed.PACS numbers: 98.80. Es, 95.36.+x

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When Did Cosmic Acceleration Start ?

Cited by 10 publications

References 36 publications

Quantization of the universe as a black hole

Quantization of the universe as a black hole

Evidence for a disaggregation of the universe

Two-Fluid Dark Energy Models in Bianchi Type-III Universe with Variable Deceleration Parameter

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