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Equation 4 · Part 3 · Comparing the Main Approaches to Precision Cosmology

Symbol w_0

w(a)=w0+wa(1−a)w(a) = w_0 + w_a (1 - a)
w0w_0

What this part means

w0w_0 is one of the signed contributions combined to compute the quantity on the left.

Its job in the formula

w0w_0 is one of the signed contributions combined to compute the quantity on the left.

The passage around this formula

Evolving dark energy , in the w0-wa parametrization, keeps the sound horizon alone and instead lets dark energy’s equation of state change with cosmic time: w(a)=w0+wa(1−a)w(a) = w_0 + w_a (1 - a). where a is the cosmic scale factor (a = 1 today) and w = -1 recovers a plain cosmological constant. This is the family DESI’s DR2 BAO release put back on the table: fit jointly to DESI BAO, Planck CMB, and supernovae, a w0-wa model is preferred over flat LambdaCDM at up to several sigma, with the preferred trajectory suggesting dark energy was somewhat more negative than -1 in the past and is evolving toward less negative values now [ 3 , 4 ] .

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A subscript is a label attached below a symbol. It often selects a time step, component, category, or member of a sequence.

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