“Standard candles (left) and standard rulers (right) are two different techniques astronomers used to measure the expansion of space at various times/distances in the past. Based on how quantities like luminosity or angular size/diameter change with distance, we can infer the expansion history of the Universe. Standard candles involve looking at objects whose intrinsic brightness is known at all cosmic distances, while standard rulers involve looking at features such as the physical size of a known object or the average separation distance between any two galaxies (imprinted from baryon acoustic oscillations during the early stages of the Big Bang) that evolve as the Universe expands.” (Big Think, Ask Ethan: How can ultra-distant galaxies move so fast?)
Here, we must realize one thing. We don’t know the luminosity of the most distant objects. There can be dark nebulae between Earth and those objects. Another thing is that. There are two directions in which those objects move. The horizontal and vertical. The vertical movement is the movement away from our galaxy. And the horizontal movement is the movement to the side from the original direction of our galaxy.
This means that if we were to find a galaxy. That is the opposite of our galaxy, and both galaxies orbit the center in the same direction. That means we would not see horizontal movement at all. But the vertical distancing, the redshift of that galaxy, will be incredible. The redshift measures the vertical movement. Spectral lines in that distant galaxy turn red. The horizontal movement is measured by using different methods.
If this galaxy is found. That could bring us closer to proving the existence of dark flow. If dark flow exists. And galaxies orbit the same point. That tells us. The universe has a mass center. Or. There is a point that puts the entire universe in orbit around it.
The answer is in the position of the observer. The phenomenon is opposite to the case where two cars collide at a speed of 50 km/h. This means that. When car 1 has a speed of 50 km/h. And a car 2 also has a speed of 50 km/h, the impact speed. It is 50 km/h + 50 km/h. And. That is 100 km/h. So, the effect is similar to that of a car impacting a standing wall at a speed of 100 km/h. Same way. If two cars are distancing themselves.
To the opposite direction. And both of them have a speed of 50 km/h, the distancing speed is also 100 km/h. This is one of the things that we can just say. That everything is relative. When two electrons collide in the particle accelerators at a speed of 80% of the speed of light. That means their impact speed is 160% of the speed of light. Those particles will not cross the speed of light. But their mutual speed is higher.
The mutual speed of two objects can be different from the speed of each of the objects. In the same way, when two photons travel in opposite directions. Their mutual speed. It is. Two times faster. Than the speed of light.
Then to galaxies. Measurements of the distancing speed of galaxies. It is measured by using the Doppler effect. This means that the wavelength of the radiation becomes longer. Then two objects are distancing. This means that spectral lines travel to the red. And when another object gets closer to us, that turns the wave movement shorter.
This is the effect, called blueshift. But. We must realize that gravitation pulls that radiation longer. And this means that. Near black holes, all objects seem slower. Than they really are. So galaxies cause an effect on the object that comes closer, seeming to be slower than they really are. And objects that travel away seem to be distancing faster than they really do. We can call it an effect. There gravity stretches light. As a virtual redshift.
But then. If. We are looking. At the most distant galaxies that are on the opposite side of the universe. We must realize that the speed always behaves the same way. When two galaxies are moving away from each other. Their distancing speed behaves like the distancing speed between two cars. The speed at which the systems measure. It is the speed of galaxy 1 + the speed of galaxy 2.
But then another thing is this. Gravitation stretches light. This means that every gravitational center. Seems to be in longer distances than they really are. Gravitation stretches light on both sides of the measurement line. The galaxy that sends light pulls that light back. That causes a virtual redshift that is stronger than the real redshift.
When that light travels to the Milky Way, the gravity of our galaxy pulls the wave movement from the front. And that means that. Also, our galaxy has the effect of that redshift. In the cases of galaxies. The gravity stretches light so strongly that it has an effect on redshift. If we think that the effect of the gravitational redshift is very small in the case of light-years.
But in the long distances. Like distances of megaparsecs, even small errors. Turn bigger. One parsec is 3,26 ly and a megaparsec is a million parsecs.
In the same way as in the cases. That measurement tool makes a 1 mm error. In the 100m distances. That error might not seem big. But. When we try to measure distances. Like Earth's distance to Jupiter. Those errors turn into an enormous scale.
https://bigthink.com/starts-with-a-bang/how-galaxies-move-fast/
https://en.wikipedia.org/wiki/Dark_flow
https://en.wikipedia.org/wiki/Doppler_effect
https://en.wikipedia.org/wiki/Parsec
