The universe's expansion and redshift.

The universe's expansion happens at a lower speed than the speed of light. But why some measurements can give other values? The reason for that is in redshift. When two particles or objects travel away from each other. 

That forms a redshift that seems their distance speed appears to be faster than it is. When the object goes away from the other, the observer of the other object seems that the wavelength, that comes from another particle turns longer. 

That causes the effect that spectral lines go to the red side of the spectrum. When two photons are traveling on opposite sides that causes a situation, where the other photon seems to travel twice at the speed of light if the observer stands on the other photon. 

When we think about two cars that impact with a speed, both of them drive 50 km/h, that means the impact speed is 50+50=100 km/h. Same way when those cars drive in opposite directions, their distance speed is 50+50 km/h. And if there is radar on those cars, those radars measure the distance speed as 100 km/h, if that radar is targeted to another particle. 

That means that our side of the universe will recede from the opposite side of the universe with a speed that should be twice as they recede from the point, where the Big Bang happens. But things like black holes, and other gravity fields make those measurements difficult. Gravity stretches light, and all gravity fields make redshift stronger than it should be. 

The black holes are things that can pull lightwaves into straight form. They can cause very strong virtual redshift, and that means almost all black holes might seem to be farther than they are. Because black holes stretch light, it means that black holes turn the wavelength of all wavelengths of radiation. 

And that thing means that the redshift can turn gravity waves into electromagnetic waves if that massive gravity can pull those gravity waves enough. That thing can mean that the gravity field can stretch gamma rays into the IR radiation simply pulling the waves longer. 

Energy travels out from the universe. There can be wave movement outside the universe, but the problem is that the energy level of that material energy is lower than the energy minimum in the universe. Radiation cannot travel inside the universe until the energy level outside the universe rises so high, that energy can fall into the universe. That thing makes it impossible to see things outside the universe. 

When we think about the most exotic and interesting things in the universe, called dark matter and dark energy. Gravity can turn the dark energy into some other wavelength. Dark energy is mysterious energy or wave movement, that should interact with gravity fields. The gravity field can turn dark energy visible if it can stretch that thing enough. Gravity should interact with dark energy. 

https://bigthink.com/starts-with-a-bang/expanding-universe-break-speed-light/