Dark energy two decades after: Observables, probes, consistency tests (1709.01091v2)

Abstract: The discovery of the accelerating universe in the late 1990s was a watershed moment in modern cosmology, as it indicated the presence of a fundamentally new, dominant contribution to the energy budget of the universe. Evidence for dark energy, the new component that causes the acceleration, has since become extremely strong, owing to an impressive variety of increasingly precise measurements of the expansion history and the growth of structure in the universe. Still, one of the central challenges of modern cosmology is to shed light on the physical mechanism behind the accelerating universe. In this review, we briefly summarize the developments that led to the discovery of dark energy. Next, we discuss the parametric descriptions of dark energy and the cosmological tests that allow us to better understand its nature. We then review the cosmological probes of dark energy. For each probe, we briefly discuss the physics behind it and its prospects for measuring dark energy properties. We end with a summary of the current status of dark energy research.

• The paper provides a comprehensive review of dark energy research two decades after the discovery of the accelerating universe, covering historical context, theoretical frameworks, observational probes, and future directions.
• It details key observational probes like Type Ia Supernovae, BAO, CMB, weak lensing, and galaxy clusters used to constrain dark energy parameters and test cosmological models.
• Despite consistent support for a cosmological constant, the paper emphasizes the need for future precision measurements and new probes to refine constraints and test theories beyond Lambda-CDM.

An Overview of "Dark Energy Two Decades After: Observables, Probes, Consistency Tests"

This paper by Dragan Huterer and Daniel L. Shafer provides a comprehensive review of the state of dark energy research two decades after the discovery of the accelerating universe. As the predominant component of the cosmos, dark energy is responsible for this acceleration and remains one of the central conundrums in modern cosmology. The authors systematically discuss the historical developments, theoretical frameworks, observational probes, and future directions for dark energy research.

Historical Context and Theoretical Underpinning

The discovery of the accelerated expansion of the universe in the late 1990s highlighted the inadequacy of existing cosmological models that only accommodated matter and radiation. This unexpected finding indicated the presence of dark energy, which occupies approximately 70% of the universe's total energy budget. The simplest candidate explanation for dark energy is the cosmological constant, associated with vacuum energy, represented as a constant term with an equation of state parameter $w = -1$.

However, theoretical explorations have also considered time-varying models, such as quintessence and modifications to general relativity, allowing for a dynamic $w$. Despite numerous measurements of the expansion rate and structure growth, empirical constraints remain consistent with a cosmological constant, thus elevating $\Lambda$-CDM as the reigning cosmological model while still admitting possibilities for evolving dark energy models.

Observational Probes and Their Implications

The paper provides an in-depth review of the principal cosmological probes utilized to investigate dark energy. These include:

• Type Ia Supernovae (SNe Ia): As standard candles, SNe Ia have provided pivotal evidence for dark energy. The inherent luminosity of SNe Ia, corrected for light curve stretch and color, allows precise determination of cosmic distances and expansion history, affirming the universe's acceleration.
• Baryon Acoustic Oscillations (BAO): Using the remnant acoustic peaks from the early universe's baryon-photon interactions, BAO surveys map the matter distribution and offer precise distance measurements. This feature acts as a cosmological standard ruler, providing critical data on angular diameter distances and the Hubble parameter.
• Cosmic Microwave Background (CMB): The CMB's angular power spectrum provides a high-redshift anchor point for cosmic parameters, with dark energy parameters inferred from the geometry and peak positions of the CMB anisotropies.
• Weak Lensing: Probing the large-scale structure directly, weak lensing maps distortions in background galaxy shapes induced by intervening mass, thus sensitive to both geometry and growth of cosmic structures affected by dark energy.
• Galaxy Clusters: The abundance of galaxy clusters offers insights into the growth of structures throughout cosmic history. These measurements are particularly sensitive to the amplitude of density fluctuations, providing constraints on the growth factor and dark energy's impact on structure formation.

Advances and Future Outlook

Huterer and Shafer emphasize the importance of further precision measurements and the development of new, sophisticated probes that can disentangle dark energy's effect on geometry from its impact on the growth of structures. Upcoming observational projects, both ground-based and space-based, hold promise for more detailed mapping of the universe’s expansion history and its structural development. The paper anticipates that these advancements will refine constraints on the dark energy equation of state and test theories beyond the standard cosmological constant.

While traditional observational probes remain crucial, the paper also highlights newer approaches like redshift-space distortions, peculiar velocities, and strong gravitational lensing, which offer additional pathways to constrain dark energy parameters and probe modifications of gravity.

Conclusion

This review underscores the critical role of dark energy in shaping our cosmological understanding. It bridges the past with future aspirations in cosmology, identifying robust methodologies and innovative strategies for resolving pivotal questions about the universe’s fate. The consistent support for a cosmological constant model from a multitude of observational fronts continually fuels the search for theoretical breakthroughs and deeper insights into the nature of the enigmatic dark energy.