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Observation indicates that the Accelerated Expansion of the Universe is Driven by a Form of Quintessence!

Observation indicates that the Accelerated Expansion of the Universe is Driven by a Form of Quintessence!


The dark energy is observed not to be the cosmological constant or vacuum energy!
The concordance cosmological model is now discordant!
• The 'standard' flat cosmological constant-dominated cold dark matter model is not able to explain the expansion history of the universe


The astrophysicists at Korea Institute for Advanced Study (KIAS) have announced that the accelerated expansion of the universe is observed to be driven not by the cosmological constant or vacuum energy, but rather by the quintessence, a hypothetical form of dark energy that can vary over time. The dark energy equation of state parameter (w) is accurately measured to be w=0.903±0.023 that is 4.2s away from that of the cosmological constant (w=-1). The discovery rules out the currently standard model of the universe, the 'concordance' spatially-flat cosmological constant-dominated universe (hereafter LCDM).

The LCDM model has been labelled the “concordance model” because the cosmological constraints from various observational probes have shown to be consistent with one another when their uncertainties are taken into account, and the allowed region of the cosmological parameter space included the LCDM model. Historically, the LCDM model has been accepted based on the major discoveries such as the excess density fluctuation on large scales (Park et al. 1992, 1994), extra dimming of high redshift Type Ia supernovae (Riess et al. 1998; Perlmutter et al. 1999), and the angular scale of the baryon acoustic oscillation peaks favoring a flat spatial geometry (Bennett et al. 2003). The LCDM was able to explain these observations simultaneously within observational uncertainties. A key element of this model is the accelerated expansion of space that is driven by the cosmological constant or vacuum energy. The expansion of the universe can be accelerated if the ratio of pressure to density (called 'equation of state parameter') w is less than -1/3. It has been found from various previous observations that the equation of state parameter of the dark energy component is close to w=-1, which corresponds to the cosmological constant L. The flat LCDM model has been known as the 'concordance' cosmological model for more than two decades.

For the first time after LCDM was established as the standard model, a team of astrophysicists led by those at KIAS has measured the dark energy equation of state parameter with sufficient accuracy to discover that w must differ from -1 in the flat CDM universe, namely dark energy is not the cosmological constant. A series of large-volume galaxy redshift surveys samples up to redshift ~0.8 produced by the Sloan Digital Sky Survey (SDSS) are used in the analysis, and the expansion history of the universe was measured using an extended version of the Alcock-Paczyński test (Park et al. 2019). The test exploits the fundamental fact that gravity is an isotropic force and the statistical pattern of galaxy clustering can be used as a standard shape that is conserved with time. The result is published on August 8, 2023 in the Astrophysical Journal (Dong et al. 2023).

The new analysis of the SDSS data indicates that the expansion of the universe is indeed accelerating but the acceleration is a little slower than expected in the flat LCDM universe. The dark energy equation of state parameter is measured to be w = 0.903±0.023, a 4.2 s deviation from −1! This finding of a new "w tension" inevitably leads us to discard the cosmological constant as the source for the accelerated expansion and consider alternative quintessence models (Ratra & Peebles 1988). The team is now making a more accurate measurement of w using the upcoming DESI survey data (Hahn et al. 2022) to test if w is constant or evolving. If the current work is confirmed with the larger DESI sample, a cosmology paradigm change is unavoidable.

This new measurement of w brings two dilemmas to the cosmology community according to Professor Changbom Park in the School of Physics and Quantum Universe Center (QUC) at KIAS. The first is the inability of the flat LCDM model to simultaneously agree with the two observations: the recent expansion history of the universe measured by this work and the cosmic microwave background (CMB) fluctuations, data from when the universe was only 380,000 years young. There is no concordance between observations at late and early universes. The dilemma implies that flat LCDM is not able to satisfy the observations by adjusting the amount and/or properties of the matter and dark energy in a simple way. The second is the conflict between the w and H0 tensions (Valentino et al. 2021). The H0 tension can be relieved if the expansion of the universe at late universe was more rapid than expected in LCDM. However, this would make the w tension worse. Conversely, if we choose w greater than -1, the expansion of the universe becomes slower than expected from LCDM at late epochs and then the H0 tension becomes greater.

There is a large reservoir of theoretical models that are potentially able to explain this new measurement of the dark energy equation of state parameter w as well as the H0 tension. It remains to be seen if the w tension grows in the future with larger redshift survey data and a new cosmological model can emerge to reconcile both tensions without sacrificing the great accomplishments of the current standard model of the universe.




Bennett, C. L., et al. 2003, "First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Preliminary Maps and Basic Results", The Astrophysical Journal Supplement Series, 148, 1

Hahn, C., et al. 2022, "DESI Bright Galaxy Survey: Final Target Selection, Design, and Validation", arXiv:2208.08512

Dong, F., Park, C., Hong, S. E., Hwang, H. S., Park, H., & Appleby, A. 2023, "Tomographic Alcock–Paczy´nski Test with Redshift-Space Correlation Function: Evidence for Dark Energy Equation of State Parameter w > −1", The Astrophysical Journal,

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of quintessence rather than the cosmological constant.



1. Title of Paper

- Tomographic Alcock–Paczyński Test with Redshift-Space Correlation Function
  : Evidence for Dark Energy Equation of State Parameter
w > −1


2. Authors

- Fuyu Dong, Korea Institute for Advanced Study, 1st author

- Changbom Park, Korea Institute for Advanced Study, corresponding author

- Sungwook E. Hong, Korea Astronomy and Space Science Institute        

- Juhan Kim, Korea Institute for Advanced Study

- Ho Seong Hwang, Seoul National University

- Hyunbae Park, Lawrence Berkeley National Laboratory, UC Berkeley

- Stephen Appleby, Asia Pacific Center for Theoretical Physics, POSTECH


3. Figures

Figure caption

Constraints on the cosmological parameters w and Ωm from various observations. The gold contours are the region allowed jointly by three independent observations of the late universe expansion. The magenta, yellow, and green regions are those allowed by the extended AP test (this study), baryon acoustic oscillation (BAO) scale measurement, and Type Ia supernovae (SN) brightness measurement, respectively. On the other hand, the blue region is the constraint from the cosmic microwave background (CMB) anisotropy emitted when the universe was young.

The probability distribution in the right panel shows that the dark energy equation of state parameter w jointly measured by late universe probes is 4.2s away from -1. This implies that the acceleration of the universe is driven by a kind of quintessence rather than the cosmological constant.