- #1
jcap
- 170
- 12
By using particle physics natural units with ##\hbar=c=1## so that Planck's length ##l_P=(8\pi G)^{1/2}## we can express Einstein's field equations as
$$G_{\mu\nu}=l_P^2\ T_{\mu\nu},$$
where ##G_{\mu\nu}## has dimension ##[\hbox{proper length}]^{-2}##, ##l_P## has dimension ##[\hbox{proper length}]##, ##T_{\mu\nu}## has dimension ##[\hbox{proper length}]^{-4}##.
In cosmology we assume the expanding FRW metric. If we assume flat space for simplicity and cartesian coordinates then we have the following line element
$$ds^2=-dt^2+a^2(t)\left(dx^2+dy^2+dz^2\right).$$
Therefore an interval of proper length in the x-direction for example is given by
$$ds=a(t)dx$$
If ##l_P## is a proper length then should it expand with the scale factor ##a(t)## or should it remain constant?
In order for it to remain constant then its corresponding comoving interval ##dx \sim 1/a## which seems unnatural to me.
Therefore I think that as ##l_P## is a proper length it should expand with the scale factor ##a(t)##.
Does this make sense?
$$G_{\mu\nu}=l_P^2\ T_{\mu\nu},$$
where ##G_{\mu\nu}## has dimension ##[\hbox{proper length}]^{-2}##, ##l_P## has dimension ##[\hbox{proper length}]##, ##T_{\mu\nu}## has dimension ##[\hbox{proper length}]^{-4}##.
In cosmology we assume the expanding FRW metric. If we assume flat space for simplicity and cartesian coordinates then we have the following line element
$$ds^2=-dt^2+a^2(t)\left(dx^2+dy^2+dz^2\right).$$
Therefore an interval of proper length in the x-direction for example is given by
$$ds=a(t)dx$$
If ##l_P## is a proper length then should it expand with the scale factor ##a(t)## or should it remain constant?
In order for it to remain constant then its corresponding comoving interval ##dx \sim 1/a## which seems unnatural to me.
Therefore I think that as ##l_P## is a proper length it should expand with the scale factor ##a(t)##.
Does this make sense?