twin universes
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The large-scale structure of the universe.
...If the second universe experiences its expansion hindered by ours, slowed down, its density r* remains higher there, as does its temperature. We thus decide to study gravitational instability in a system composed of two populations, mutually self-attracting but repelling each other.
...According to the theory, the denser population reacts the most quickly and vigorously. It is this one that will, through gravitational instability, produce clumps of ghost matter.
...What is gravitational instability, first studied by Sir James Jeans, already mentioned?
...Consider a medium with density r, where its constituents are undergoing a certain thermal agitation velocity Vth, and let us examine whether possible density perturbations grow or dissipate. Suppose that, somewhere, a region of over-density has formed, with diameter f.
...Thermal agitation naturally tends to dissipate this perturbation. In how much time? In a time on the order of
...This is the time it takes an atom to travel distance f, and thus also the time it takes for this clump to double its diameter.
...Imagine that the thermal agitation velocity is zero. The atoms attract each other. This clump tends to collapse onto itself. We can calculate the time it takes for it to contract. In fact, a "dust cloud" imploding upon itself resembles the Big Bang in reverse:
...We then compare these two times.
...Condensation of a perturbation will occur if the accretion time is shorter than the self-dispersion time due to thermal agitation.
...Perturbations with a diameter larger than a characteristic length, called the Jeans length Lj, will amplify and form condensates, clumps of matter (clumps).
...When such a "clump" of matter forms, the matter becomes compressed and heated. The pressure forces increase and eventually stop the process.
...This is called gravitational instability or Jeans instability.
Regarding the standard model, one might think:
- Very well. After the Big Bang, the universe, expanding, cools down, and the onset of gravitational instability allows me to construct a scenario for the formation of galaxies and stars.
...If it were that simple, it would already have been done. In truth, we have no model for galaxy formation. Some believe star clusters formed first, then galaxies, then stars. Others advocate the opposite view.
...Moreover, all of this unfolds in an universe still undergoing intense expansion. The detection of galaxies with very high redshift shows they are extremely ancient (confirmed by the age of the oldest stars in our galaxy). We cannot yet handle this theoretically.
But we do know two things:
1: This gravitational instability cannot take effect as long as the matter gas remains strongly coupled to the "photon gas," as long as the universe remains ionized. Indeed, photons interact more strongly with free electrons (those escaped from atoms) than with electrons orbiting atomic nuclei. In their own way, photons form a "gas." During expansion, this gas expands, like matter, and possesses its own pressure, or radiation pressure. When matter and photons are strongly coupled, when a cloud of ionized gas tends to contract, it drags the photon gas along with it.
- But photons travel at the speed of light! How can a finite-sized gas cloud "trap photons"?
...Trapping, that is. Within this gas cloud, photons are constantly absorbed and re-emitted. Due to the frequency of these absorption-emission cycles, photons have great difficulty escaping the gaseous mass. In this sense, they are trapped (just as photons emitted at the Sun's core slowly and laboriously make their way to its surface).
...When the universe was less than 500,000 years old, not only was radiation trapped within the ionized gas masses that might have formed clumps, but radiation pressure was still too high to allow such condensations.
Conclusion: Homogeneity of the universe, or near-homogeneity, up to t = 500,000 years, according to the standard model. If anything happens, it occurs afterward.
2: There exist stars gathered into galaxies, which themselves form a large-scale structure. Some galaxies group together into clusters (Coma cluster, Virgo cluster), each containing a thousand individuals. Initially, it was thought this clustering would continue at a larger scale, leading to the idea of superclusters—clusters of clusters.
...Observations revealed something entirely different. In fact, galaxies are distributed forming what could be called "interconnected soap bubbles." Galaxy clusters are merely the "nodes" of such a distribution. Below is the result of analyzing observations (1977).
...Thus, galaxies are distributed on a very large scale (Very Large Structure) around vast voids, whose characteristic diameter is on the order of hundreds of millions of light-years.
...Following another approach, researchers attempted to reconstruct such structures, starting from a uniform distribution of matter (in a single universe, of course). The initial theory was that of planar perturbation growth, in pancakes (the "pancakes" of Zel'dovitch). However, results proved disappointing. Computer simulations did produce some cells, but they quickly dissipated due to thermal agitation. Currently, there is no convincing theory for the formation of such structures. At best, we can ensure their relative longevity by "consolidating" them with "cold dark matter."
...There is a geometric way to interpret this distribution of point masses: ordinary matter pushed away by clumps of ghost matter, a diagram already shown above.
...Consider a surface resembling a net stretched over tent pegs with blunt ends. Note in passing that the blunter the pegs, the more extended the ghost matter clump will be. The opposite occurs if the pegs are sharper. In the limit of infinitely sharp pegs, we would have "posiconic" regions: points of concentrated positive curvature.
...Here is another model, which will correspond to the following section.
Table of contents of this dossier
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