The Accelerating Universe

Abstract

In this article we review the discovery of the accelerating universe using type Ia supernovae. We then outline ways in which dark energy -component that causes the acceleration -is phenomenologically described. We finally describe principal cosmological techniques to measure large-scale properties of dark energy. This chapter therefore complements articles by Caldwell and Linder (2010) in this book who describe theoretical understanding (or lack thereof) of the cause for the accelerating universe.Evidence for the missing component. Inflationary theory [Guth (1981)] explains how tiny quantum-mechanical fluctuations in the early universe could grow to become structures we see on the sky today. One of the factors that motivated inflation is that it predicts that the total energy density relative to the critical value is unity, Ω ≡ ρ/ρ crit = 1. This inflationary prediction convinced many theorists that the universe is precisely flat.Around the same time that inflation was proposed, a variety of dynamical probes of the large-scale structure in the universe were starting to indicate that the matter energy density is much lower than the value needed to make the flat. Perhaps the most specific case was made by the measurements of the clustering of galaxies, which are sensitive to the parameter combination Γ ≡ Ω M h, where Ω M is the energy density in matter relative to critical, and h is the Hubble constant in units of 100 km/s/Mpc. The measured value at the time was Γ 0.25 (with rather large errors). One way to preserve a flat universe was to postulate that the Hubble constant itself was much lower than the measurements indicated (h ∼ 0.7), so that Ω M = 1 but h ∼ 0.3 [Bartlett et al. (1995)]. Another possibility was the presence of Einstein's cosmological constant (see the Caldwell article in this book), which was World Scientific Book -9.75in x 6.5in bookchapter 2 Book Title suggested as far back as 1984 as the possible missing ingredient that could alleviate tension between data and matter-only theoretical predictions [Peebles (1984); Turner et al. (1984)] by making the universe older, and allowing flatness with a low value of the matter density. Type Ia supernovae and cosmologyThe revolutionary discovery of the accelerating universe took place in the late 1990s, but to understand it and its implications, we have to step back a few decades.Type Ia supernovae. Type Ia supernovae (SN Ia) are explosions seen to distant corners of the universe, and are thought to be cases where a rotating carbonoxygen white dwarf accretes matter from a companion star, approaches the Chandrasekhar limit, starts thermonuclear burning, and then explodes. The Ia nomenclature refers to spectra of SN Ia, which have no hydrogen, but show a prominent Silicon (Si II) line at 6150Å. SN Ia had been studied extensively by Fritz Zwicky who also gave them their name [Baade and Zwicky (1934)], and by Walter Baade, who noted that SN Ia have very uniform luminosities [Baade (1938)]. Light from type Ia supernovae brightens and fades over a period of ab...

Cosmological exact solutions of Petrov type D. A mixture of two fluids: dark energy and radiation

“…For the Hubble parameter (H), it was estimated that if t −→ 0 for both fluids or only one of them, H −→ ∞ and if t −→ ∞, then, H −→ H 0 , where H 0 = √ 3Λ/3. The deceleration parameter q → −1 if t → ∞ in combination of both fluids or if only dark energy is present (R = 0), this indicates an accelerated expansion of the universe in accordance with observations of Type la Supernovas [15,14,9]. If there is only radiation, (Λ = 0) the parameter q → 1 indicates a deceleration process.…”

Dependence on supernovae light-curve processing in void models

“…We now have a concordance model in which the dimming of distant SNe Ia [1][2][3][4][5][6][7][8][9][10], anisotropies in the cosmic microwave background (CMB) [11,12], and the signature of baryon acoustic oscillation (BAO) [13,14] cannot be explained by considering only baryonic and dark matter. The most popular solution is to introduce an additional component with negative pressure, the so-called dark energy (e.g., [15][16][17][18][19]). Some other proposals have been presented since then.…”

Supernova light-curve fitters and dark energy

“…It has become clear that different cosmological observations, such as the dimming of distant SNe Ia [1][2][3], anisotropies in the cosmic microwave background (CMB) [4], and the signature of baryon acoustic oscillation (BAO) [5,6] cannot be explained with a cosmological model that contains only baryonic and dark matter according to a FRW standard model. The most popular solution is to introduce an extra component with negative pressure, the so-called dark energy (e.g., [7][8][9][10][11]).…”