# Tag Info

Accepted

### Age of a dark energy dominated universe

In non-dark-energy-dominated phases, $a\propto (t-t_1)^\gamma$ for some $\gamma>0$ and some reference time $t_1$. This means that $a=0$ when $t=t_1$, so that time $t_1$ is the beginning of the ...
• 6,083

### How do we account for the 'one way' drag of moving space?

In FLRW spacetime, there is a drag-like contribution to the equation of motion of a test particle, $$\frac{\mathrm{d}\vec p}{\mathrm{d}t}=-H\vec p.$$ Here $\vec p$ is the momentum with respect to the ...
• 6,083
1 vote

### How do we account for the 'one way' drag of moving space?

In the body of the paper The river model of black holes, the authors do not use the term "drag" anywhere to describe the model. They describe it as a flow, but not a flow of a viscous ...
• 102k
1 vote
Accepted

### How to find critical density?

The Friedmann eqn is: $$H^2+\frac{k}{a^2}=\frac{\rho}{3M_p^2}$$ where $M_p^2=1/8\pi G$ is the Planck mass. In this eqn we take cosmology constant as dark energy, which becomes a part of $\rho$ above. (...
Accepted

### How to understand critical density?

1: The critical density is the required density for a flat universe, so it requires only $\Omega_{\rm K}$ to be $0$, but not $\Omega_{\Lambda}$, so it also holds with dark energy if you include its ...
• 12.2k
1 vote

### Are modified theories of gravity credible?

Although the existing answers aren't wrong, I feel like this part has not been emphasized enough. NB: I'm interpreting your question as "are modified gravity theories that eliminate the need for ...
• 21.3k