2 ms·
We consider it matter because of the scaling of its energy density. If say the universe expands by a factor, the energy density of matter ρ is ρ ∝ 1/a^3, for ra
by PartiallyTyped 2y ago
We consider it matter because of the scaling of its energy density. If say the universe expands by a factor, the energy density of matter ρ is ρ ∝ 1/a^3, for radiation it is ρ ∝ 1/a^4. The cosmological constant is actually a constant.
Radiation redshifts as the universe expands, but "matter" "just" "spreads".
- MaxikCZ 2y agoIsn't redshifting decrease in wavelength? The way even a mesh can reflect radio waves I always accounted to "the wave is so big it can't fit trough the holes of the mesh", so in that sense the radiation redshift equals to it spreading (even tho on interaction it manifests in a single point.
- PartiallyTyped 2y agoRedshift is an increase in wavelengths and a decrease in frequency. As an object moves further away from a reference point, all waves emitted from that object have an increased wavelength. Imagine a basketball player bouncing a ball up and down at a fix frequency. The wavelength is inversely proportional to the frequency. The wave here is the ball bouncing up and down. Now imagine the moment the ball hits the ground they start moving away from you at a fixed speed. Because they have accelerated, light now needs to travel some additional distance since the ball at the top is further away from you than the ball at the bottom was. But their new frequency remains the same because the displacement is now accounted for in the wavelength. If they stop moving, opposite happens and it blueshifts. If they keep accelerating, the distance increases and so does the redshifting.