### Abstract

Within the context of standard cosmology, an accelerating universe requires the presence of a third "dark" component of energy, beyond matter and radiation. The available data, however, are still deemed insufficient to distinguish between an evolving dark energy component and the simplest model of a time-independent cosmological constant. In this paper, we examine the cosmological expansion in terms of observer-dependent coordinates, in addition to the more conventional comoving coordinates. This procedure explicitly reveals the role played by the radius R_{h} of our cosmic horizon in the interrogation of the data. (In Rindler's notation, R_{h} coincides with the "event horizon" in the case of de Sitter, but changes in time for other cosmologies that also contain matter and/or radiation.) With this approach, we show that the interpretation of dark energy as a cosmological constant is clearly disfavored by the observations. Within the framework of standard FriedmannRobertsonWalker cosmology, we derive an equation describing the evolution of R_{h}, and solve it using the WMAP and Type Ia supernova data. In particular, we consider the meaning of the observed equality (or near-equality) R_{h}(t_{0}) ≅ ct_{0}, where t_{0} is the age of the universe. This empirical result is far from trivial, for a cosmological constant would drive R_{h}(t) toward ct (t is the cosmic time) only once and that would have to occur right now. Though we are not here espousing any particular alternative model of dark energy, for comparison we also consider scenarios in which dark energy is given by scaling solutions, which simultaneously eliminate several conundrums in the standard model, including the "coincidence" and "flatness" problems, and account very well for the fact that R_{h}(t_{0}) ≈ ct_{0}.

Original language | English (US) |
---|---|

Pages (from-to) | 1113-1127 |

Number of pages | 15 |

Journal | International Journal of Modern Physics D |

Volume | 18 |

Issue number | 7 |

DOIs | |

State | Published - Jul 2009 |

### Fingerprint

### Keywords

- Cosmology
- Dark energy
- Gravitation

### ASJC Scopus subject areas

- Astronomy and Astrophysics
- Space and Planetary Science
- Mathematical Physics

### Cite this

**Constraints on dark energy from the observed expansion of our cosmic horizon.** / Melia, Fulvio.

Research output: Contribution to journal › Article

*International Journal of Modern Physics D*, vol. 18, no. 7, pp. 1113-1127. https://doi.org/10.1142/S0218271809014984

}

TY - JOUR

T1 - Constraints on dark energy from the observed expansion of our cosmic horizon

AU - Melia, Fulvio

PY - 2009/7

Y1 - 2009/7

N2 - Within the context of standard cosmology, an accelerating universe requires the presence of a third "dark" component of energy, beyond matter and radiation. The available data, however, are still deemed insufficient to distinguish between an evolving dark energy component and the simplest model of a time-independent cosmological constant. In this paper, we examine the cosmological expansion in terms of observer-dependent coordinates, in addition to the more conventional comoving coordinates. This procedure explicitly reveals the role played by the radius Rh of our cosmic horizon in the interrogation of the data. (In Rindler's notation, Rh coincides with the "event horizon" in the case of de Sitter, but changes in time for other cosmologies that also contain matter and/or radiation.) With this approach, we show that the interpretation of dark energy as a cosmological constant is clearly disfavored by the observations. Within the framework of standard FriedmannRobertsonWalker cosmology, we derive an equation describing the evolution of Rh, and solve it using the WMAP and Type Ia supernova data. In particular, we consider the meaning of the observed equality (or near-equality) Rh(t0) ≅ ct0, where t0 is the age of the universe. This empirical result is far from trivial, for a cosmological constant would drive Rh(t) toward ct (t is the cosmic time) only once and that would have to occur right now. Though we are not here espousing any particular alternative model of dark energy, for comparison we also consider scenarios in which dark energy is given by scaling solutions, which simultaneously eliminate several conundrums in the standard model, including the "coincidence" and "flatness" problems, and account very well for the fact that Rh(t0) ≈ ct0.

AB - Within the context of standard cosmology, an accelerating universe requires the presence of a third "dark" component of energy, beyond matter and radiation. The available data, however, are still deemed insufficient to distinguish between an evolving dark energy component and the simplest model of a time-independent cosmological constant. In this paper, we examine the cosmological expansion in terms of observer-dependent coordinates, in addition to the more conventional comoving coordinates. This procedure explicitly reveals the role played by the radius Rh of our cosmic horizon in the interrogation of the data. (In Rindler's notation, Rh coincides with the "event horizon" in the case of de Sitter, but changes in time for other cosmologies that also contain matter and/or radiation.) With this approach, we show that the interpretation of dark energy as a cosmological constant is clearly disfavored by the observations. Within the framework of standard FriedmannRobertsonWalker cosmology, we derive an equation describing the evolution of Rh, and solve it using the WMAP and Type Ia supernova data. In particular, we consider the meaning of the observed equality (or near-equality) Rh(t0) ≅ ct0, where t0 is the age of the universe. This empirical result is far from trivial, for a cosmological constant would drive Rh(t) toward ct (t is the cosmic time) only once and that would have to occur right now. Though we are not here espousing any particular alternative model of dark energy, for comparison we also consider scenarios in which dark energy is given by scaling solutions, which simultaneously eliminate several conundrums in the standard model, including the "coincidence" and "flatness" problems, and account very well for the fact that Rh(t0) ≈ ct0.

KW - Cosmology

KW - Dark energy

KW - Gravitation

UR - http://www.scopus.com/inward/record.url?scp=68349139476&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=68349139476&partnerID=8YFLogxK

U2 - 10.1142/S0218271809014984

DO - 10.1142/S0218271809014984

M3 - Article

AN - SCOPUS:68349139476

VL - 18

SP - 1113

EP - 1127

JO - International Journal of Modern Physics D

JF - International Journal of Modern Physics D

SN - 0218-2718

IS - 7

ER -