The FLWR metric is: $$ds^2=-dt^2+a(t)d\vec{x}^2$$ and has a function whose dependence is on $t$. Different observers measure the time elapsed differently. Does that mean each observer measures the expansion of the universe differently?

light is said to act like both wave and particle. When we talk about an electron we say it's a particle and the wave portion is just its probability and not a physical wave like light So is there some actual wave nature of the electron itself like light or is it a wave due to its probability only...

By Newton's third law of motion there will be equal and opposite forces of gravity exerted on two objects despite their masses. Hence why does a larger objects exert a larger gravitational pull? In other words why does force of gravity increase with mass?

When the wave function of a quantum system collapses, the probability of finding it at some specific point is given depends on $||\Psi||^2$: $$ \int_{\mathbb{R}^3}{d^3 \mathbf x \; ||\Psi||^2} = 1 $$ Could this modulus square, the instant you measure, be thought as the Dirac Delta Function, beca...

The action of a field $\phi^\mu$ in flat $n$-dimensional spacetime is $$ S = \int \text{d}^n x \mathscr{L}(\phi^\mu(x),\partial_\alpha \phi^\mu(x)) $$ From an infinitesimal variation of field configuration, and discarding a boundary term, we have E-L equations $$ \frac{\partial \mathscr{L}}{\part...