Observational Probes of Cosmic Acceleration (1201.2434v2)

Abstract: The accelerating expansion of the universe is the most surprising cosmological discovery in many decades, implying that the universe is dominated by some form of "dark energy" with exotic physical properties, or that Einstein's theory of gravity breaks down on cosmological scales. The profound implications of cosmic acceleration have inspired ambitious experimental efforts to measure the history of expansion and growth of structure with percent-level precision or higher. We review in detail the four most well established methods for making such measurements: Type Ia supernovae, baryon acoustic oscillations (BAO), weak gravitational lensing, and galaxy clusters. We pay particular attention to the systematic uncertainties in these techniques and to strategies for controlling them at the level needed to exploit "Stage IV" dark energy facilities such as BigBOSS, LSST, Euclid, and WFIRST. We briefly review a number of other approaches including redshift-space distortions, the Alcock-Paczynski test, and direct measurements of H_0. We present extensive forecasts for constraints on the dark energy equation of state and parameterized deviations from GR, achievable with Stage III and Stage IV experimental programs that incorporate supernovae, BAO, weak lensing, and CMB data. We also show the level of precision required for other methods to provide constraints competitive with those of these fiducial programs. We emphasize the value of a balanced program that employs several of the most powerful methods in combination, both to cross-check systematic uncertainties and to take advantage of complementary information. Surveys to probe cosmic acceleration produce data sets with broad applications, and they continue the longstanding astronomical tradition of mapping the universe in ever greater detail over ever larger scales.

• The paper systematically reviews four primary observational methods—Type Ia supernovae, BAO, weak lensing, and galaxy clusters—to quantify cosmic acceleration and test dark energy models.
• It evaluates the strengths and challenges of each technique by examining calibration issues, systematic uncertainties, and the benefits of cross-correlation methods.
• The findings emphasize that future multi-band surveys and space-based missions will significantly enhance the precision of cosmic acceleration measurements.

Insightful Overview of "Observational Probes of Cosmic Acceleration"

The paper "Observational Probes of Cosmic Acceleration" provides a comprehensive review of various methodologies for measuring the accelerating expansion of the universe, a pivotal discovery in cosmology that suggests the dominance of dark energy or potential deviations from General Relativity (GR) on cosmological scales. This essay provides a detailed summary of the paper's principal components, highlighting the strengths of the methodologies discussed, potential systematic uncertainties, and the implications for future developments in cosmology.

Core Methodologies

The paper extensively reviews four primary observational techniques for probing cosmic acceleration:

1. Type Ia Supernovae (SNe Ia): SNe Ia serve as standardizable candles for measuring cosmic distances. The authors elaborate on the methodology of using the luminosity-distance relation derived from the empirical correlation between supernova peak luminosities and light curve shapes. The statistical power of SNe Ia measurements is robust, yet systematic errors like dust extinction and photometric calibration pose significant challenges. Observations suggest future programs should focus considerably on rest-frame near-infrared photometry to mitigate dust-related uncertainties.
2. Baryon Acoustic Oscillations (BAO): The BAO method relies on the imprint of sound waves in the early universe on the large-scale structure as a standard ruler, providing a means to measure the distance-redshift relation in absolute units. The BAO signal's resilience to non-linear structure formation enhances its stability as a cosmological probe. Nevertheless, achieving the method's statistical potential necessitates vast surveys, highlighting the significance of upcoming and future projects like the Dark Energy Spectroscopic Instrument (DESI) and potential space missions.
3. Weak Gravitational Lensing (WL): WL measures the subtle distortion of background galaxy shapes due to foreground mass distributions, directly probing the matter distribution in the universe. The paper underscores the technical complexities associated with WL, such as shape measurement accuracy, redshift estimation, and intrinsic alignments. The authors emphasize that space-based observations could substantially improve WL measurements due to enhanced PSF stability and precise instrument calibration.
4. Galaxy Clusters: The abundance and distribution of galaxy clusters as a function of mass and redshift provide insights into both the expansion history and the growth of structure. The dominant concern here is the calibration of mass-observable relations, where WL holds promise as a cross-correlation tool to calibrate cluster masses.

Systematic Uncertainties and Mitigation

The review details systematic uncertainties intrinsic to each method, reflecting the complexity of extracting precision cosmological parameters. For SNe Ia, the focus is on calibration and extinction corrections, whereas for WL, the challenges lie in shear measurement and intrinsic alignments. The paper argues for multi-faceted observation strategies and cross-correlation methods to mitigate these systematics effectively.

Implications for Cosmology

The discussions in the paper highlight the central role of these observational techniques in constraining dark energy models and testing GR. Moreover, the authors stress the value of a balanced program employing multiple methods to leverage complementary strengths and cross-check potential systematic biases. They speculate that future space missions and large-scale ground-based surveys could significantly enrich our understanding of dark energy, particularly if systematic uncertainties are successfully minimized.

Future Prospects

The paper anticipates that the combination of forthcoming optical, near-infrared, and radio surveys will gradually map out much of the available cosmic volume, significantly improving precision in measuring $D(z)$ and $H(z)$. Additionally, the incorporation of cross-correlation techniques across disciplines may help refine redshift estimation and calibration processes, enhancing the overall reliability of cosmological inferences.

In summary, "Observational Probes of Cosmic Acceleration" provides a detailed foundation for understanding and furthering research in measuring cosmic acceleration, underscoring the importance of methodology refinement and systematic error mitigation in future surveys. The dialogue set forth in the paper stimulates a concerted effort towards a coherent understanding of dark energy through diverse observational lenses, paving the way for transformative advancements in cosmology.