The Universe was born from the Big Bang, and the commonly held idea is that it was an explosion starting from a particular point. Such a situation would therefore mean that there is a center from which everything began. But while tempting, this portrayal of the Big Bang is incorrect. So, if an explosion is not at the origin of the Big Bang, does the Universe still have a center?
An explosion begins at a specific point and quickly spreads outward. The materials that spread the fastest are the ones that move outward the fastest. The farther an observer stands from the center of the explosion, the less material likely to reach it. The energy density decreases with time, and decreases more rapidly with distance from the center. Thus, it will always be possible for an observer to reconstruct the center of the explosion.
However, this situation does not correspond to the Universe that we observe. The Universe looks the same over long distances:same densities, same energies, same distributions of galaxies, etc. Distant objects, moving away more quickly from us, do not seem to have the same age as closer objects, which move at lower speeds; they are younger.
Instead of an explosion, general relativity predicts an expansion. A universe that begins from a hot and dense state, whose very spatial fabric extends. The common idea is that this event began at a particular point. But this idea is of course wrong. General relativity simply predicts the existence of a region filled with matter and energy from which the Universe evolved according to the laws of gravity.
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According to the cosmological principle, the Universe is homogeneous and isotropic, that is to say that its appearance is independent of the direction in which it is observed; in other words, its general appearance does not depend on the position of the observer. Thus, regardless of the direction of observation, the Universe displays the same physical properties (density, temperature, etc.). This principle finds several experimental confirmations, in particular in the isotropy of the cosmic microwave background, the aspect of the background being the same at all points in the Universe.
This uniform light witnessing the Big Bang confirms that it occurred simultaneously in all directions, thus rendering obsolete the very idea of searching for a center in the Universe. Especially since general relativity does not predict the finiteness of the Universe; it could very well have been infinite from the start, the notion of center would then no longer have any meaning.
In addition, considerations on the metric, that is to say the dynamics and the geometry of space-time, go in the direction of an absence of a center. Indeed, to locate the center of a space, it is necessary to have a system of coordinates. In general relativity, this coordinate system is spacetime and its four dimensions (x, y, z and t). However, at the time of the Big Bang, this space-time is not yet built, so there is no way to be able to locate an event in a space that does not yet exist.
And if the Universe does have a center, it can be anywhere; the part of the Universe that is accessible to us for observation is insufficient to reveal this information to us. To consider this hypothesis, we would have to detect a consequent anisotropy in the temperatures, the densities of matter or the distributions of galaxies. But so far, on large scales, the Universe seems to be the same in all directions. Thus, any observer can theoretically be the center of the Universe.
There is no precise zone from where the Universe entered into expansion under the impetus of the Big Bang, only a time from which this expansion began, approximately 13.8 billion years.
The Big Bang is therefore an event affecting the entire observable Universe at a given time. This is why observing over great distances in all directions means observing the past. And so it is for this reason that, regardless of the direction, the Universe sports uniform properties.
