# Search results

1. ### Struggling with Friedmann equations

Hi! I'm struggling to derive the accelerated Friedmann equation (shame on me!)... I'll tell you what I'm doing and maybe you can find where the mistake is. First of all, we know that: H^2 = \frac{\rho}{2M^2_p}+\frac{\Lambda}{3} Now, using the Einstein Equations and doing the trace of...
2. ### Light cones in an expansive Universe

Hi everyone! I'm solving some GR problems and I have a question. The problem is that we have a metric like the FRW metric but in 1+1 dimensions, i.e.: ds^2 = -dt \otimes dt + a(t)^2 dx \otimes dt Where a(t)=t^{1/\epsilon} (for t>0) and a(t)=(-t)^{1/\epsilon} (for t<0 ). We take a vector V^\mu...
3. ### Singularities in the harmonic oscillator propagator

Hi people! Today I was doing some QFT homework and in one of them they ask me to calculate the Harmonic Oscillator propagator, which, as you may know is: W(q_2,t_2 ; q_1,t_1) = \sqrt{\frac{m\omega}{2\pi i \hbar \sin \omega (t_2-t_1)}} \times \exp \left(\frac{im\omega}{2\hbar \sin \omega...
4. ### Rindler coordinates - signals which never arrive

Hi everybody, I know that there are a lot of threads in this forum about Rindler coordinates but none of them have helped me :confused: I'll explain you my problem. First of all, my coordinates (x^0,x) (Cartesian coord., where x^0=ct) are related to the Rindler coordinates (\omega ^0,\omega)...