Observational Evidence of the Accelerated Expansion of the Universe (1204.5493v1)

Abstract: The discovery of cosmic acceleration is one of the most important developments in modern cosmology. The observation, thirteen years ago, that type Ia supernovae appear dimmer that they would have been in a decelerating universe followed by a series of independent observations involving galaxies and cluster of galaxies as well as the cosmic microwave background, all point in the same direction: we seem to be living in a flat universe whose expansion is currently undergoing an acceleration phase. In this paper, we review the various observational evidences, most of them gathered in the last decade, and the improvements expected from projects currently collecting data or in preparation.

• The paper presents a synthesis of observational evidence from type Ia supernovae, galaxy clusters, CMB, and BAO confirming accelerated cosmic expansion.
• It validates the ΛCDM model and examines dark energy scenarios, including a cosmological constant and alternative scalar field hypotheses.
• The study highlights future research prospects with high-precision surveys and weak lensing methods to further constrain dark energy properties.

Observational Evidence of the Accelerated Expansion of the Universe

The paper by Pierre Astier and Reynald Pain provides a detailed examination and synthesis of observational evidence supporting the accelerated expansion of the universe. The discovery of cosmic acceleration, initially perceived through type Ia supernovae, marks a significant advancement in modern cosmology, indicating that we live in a universe currently undergoing accelerated expansion. This paper synthesizes a range of observations and analyses that corroborate this phenomenon and explores the theoretical underpinnings associated with it.

Context and Observational Evidence

The initial motivation for studying cosmic acceleration came from the standard cosmological expectation that the universe, filled with matter, would decelerate due to gravitational interactions. The parameter dubbed the "deceleration parameter" ($q_0$) long dominated cosmological discussions. However, observations starting in the late 20th century, notably by Riess et al. (1998) and Perlmutter et al. (1999), using type Ia supernovae as standard candles, suggested otherwise. These supernovae appeared dimmer than expected in a decelerating universe, indicating an accelerated expansion instead.

The expanded analysis covered in the paper includes corroboration from multiple independent observations: galaxy clusters, cosmic microwave background (CMB) measurements, and baryon acoustic oscillations (BAO). Each offers complementary data confirming the acceleration. For instance, CMB data from satellites such as WMAP and Planck provide independent evidence of a flat universe dominated by components other than baryonic matter.

Theoretical Implications

The concept of a cosmological constant, $\Lambda$, resurfaced as a leading candidate for explaining acceleration, acting as a source term in Friedmann equations with a static energy density. Models incorporating $\Lambda$ align well with observed data, leading to the widely referenced $\Lambda$CDM model. Furthermore, the introduction of "dark energy" as a broader formalism encompasses the many unexplored possibilities that could explain the negative pressure driving acceleration.

Astier and Pain emphasize the theoretical complexities and motivations posed by this observation. While a cosmological constant remains a straightforward fit, possibilities rooted in particle physics introduce scalar fields and varying equations of state parameters ($w$), with $w \sim -1$ consistent with $\Lambda$ but allowing exploration of more exotic scenarios.

Current Status and Future Directions

The synthesis in the paper suggests considerable evidence from diverse cosmological observables supporting an accelerated expansion framework. However, despite success with $\Lambda$CDM, alternatives and advancements in cosmological probes remain valuable. Future work needs to address the constancy or evolution of $w$ and consider modifications to general relativity over cosmological scales. Upcoming projects and data collection initiatives, as discussed in the paper, are essential. These include advances in galaxy surveys exploring BAO, enhanced supernovae observations, and forthcoming high-precision weak lensing studies.

Accelerometer data raises important questions in theoretical physics, linking cosmic expansion with fundamental quantum conditions or modifications to gravitational laws. The open questions concerning dark energy implications on particle physics or potential clues indicating new physics will be a focal point of research.

Conclusion

This paper by Astier and Pain represents a comprehensive review of the evidence for accelerated cosmic expansion, well-supported by multiple observational techniques and a robust theoretical framework. Although much of the evidence aligns with a universe dominated by dark energy, ongoing and future research will help elucidate the detailed properties of this mysterious energy and its implications for fundamental physics. As observational technology continually advances, the cosmology community is poised at the brink of potentially transformative discoveries.