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Table 1.
Comparison between a barotropic fluid with constant ωs > 0 and EDE paradigms.
| Matter with pressure | Early dark energy |
|---|---|
| Thermodynamics | |
| 1. Constant ωs > 0, satisfying the Zel’dovic limit. Direct thermodynamical interpretation. | 1. Time-varying EoS. Nearly constant density. |
| 2. ρs(z)∝(1 + z)3(1 + ωs), not designed to decay. | 2. Both ωEDE and ρEDE decay after recombination. |
| Physical origin and properties | |
| 1. Hypothetical matter or radiation component with pressure. Elastic properties. | 1. Typically scalar fields (e.g., axion-like); non-barotropic dynamics. |
| 2. Always present but generally subdominant, affecting early expansion. | 2. Fine-tuned to dominate briefly in the early universe and vanish later. |
| Observable effects | |
| 1. Modifies early background expansion. Directly detectable. | 1. Alters recombination history. Directly undetectable. |
| 2. Affects primordial evolution. Diluted at late times. | 2. Alleviates tensions by dynamically turning off. |
Notes. The barotropic fluid with constant ωs > 0 is referred to as matter with pressure.
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