8 Dark Energy and the Fate of the Universe Free Online FlashCards

What does accelerated cosmic expansion mean?

On very large scales, distances between unbound galaxies increase at an increasing rate.

What condition on density and pressure allows cosmic acceleration?

Acceleration occurs when the combined contents satisfy $\rho+3p<0$, where $\rho$ is energy density and $p$ is pressure.

What is $w$ for a cosmological constant?

For a cosmological constant, the equation-of-state parameter is $w=p/\rho=-1$, using units where the speed of light is 1.

How do dark matter and dark energy differ?

Dark matter attracts gravitationally and helps build cosmic structure; dark energy is associated with accelerated expansion.

How did Type Ia supernovae provide evidence of acceleration?

Their peak brightness estimates distance, while their redshift indicates how the universe expanded as their light traveled. Distant supernovae appeared farther away than expected if expansion were only slowing under gravity.

How do baryon acoustic oscillations trace cosmic expansion?

BAO are a preferred measurable scale in the galaxy distribution, left by sound waves in the early universe. Comparing the scale across redshifts traces expansion history.

What does the CMB help constrain in dark-energy studies?

The CMB measures conditions in the early universe; combined with later-universe measurements, it helps constrain cosmic geometry and expansion history.

What do weak lensing and galaxy clustering test?

Weak lensing and galaxy clustering track structure growth. Accelerated expansion slows the growth of large-scale structure, providing a further test.

What is the cosmological-constant model of dark energy?

It treats dark energy as a constant energy density of empty space and forms the standard $\Lambda$CDM model. It fits many observations, but quantum theory does not explain the inferred small vacuum-energy value.

What distinguishes quintessence from a cosmological constant?

Quintessence is a dynamical dark-energy model in which a field changes over time, so $w$ need not remain at $-1$.

What does modified gravity propose as an alternative?

Modified-gravity models propose that gravity itself needs adjustment on cosmic scales, rather than adding a new dark-energy substance. Tests compare both expansion and structure growth.

What is the status of recent evidence for evolving dark energy?

DESI DR2 combined with CMB data preferred a time-varying model; adding supernova data gave a preference of about 2.8–4.2 standard deviations, depending on the sample. The result is model-dependent and not a confirmed discovery.