What causes the existence of the Cosmic Microwave Background (CMB) rest frame, and how does it relate to Lorentz symmetry?

Context

This question explores the origin of the CMB rest frame, which is the frame of reference where the CMB appears most uniform. It questions why, given Lorentz symmetry (the principle that the laws of physics are the same for all observers in uniform motion), there is a preferred frame associated with the average velocity of the matter that emitted the CMB. The user also asks about the possibility of large-scale fluid structures, like eddies, existing within the matter that generated the CMB and how confident we are that such structures don't violate the cosmological principle.

Simple Answer

• The CMB rest frame exists because the early universe was very hot and dense.
• In this hot, dense state, matter and radiation interacted strongly, creating a nearly uniform plasma.
• This plasma had an average velocity, defining the CMB rest frame.
• Lorentz symmetry isn't broken; it's just that we're observing the universe from a specific point within it.
• We're reasonably confident large-scale eddies aren't present because they'd leave distinct signatures in the CMB, which haven't been observed.

Detailed Answer

The Cosmic Microwave Background (CMB) rest frame is a consequence of the universe's early conditions. In the primordial universe, shortly after the Big Bang, the cosmos was a hot, dense plasma composed of photons, electrons, and baryons (protons and neutrons). These particles were in constant interaction, frequently scattering off each other. This intense interaction established a state of thermal equilibrium. As the universe expanded and cooled, this plasma eventually reached a point where the photons decoupled from the matter. This decoupling occurred when the temperature dropped enough for electrons and protons to combine into neutral hydrogen, a process known as recombination. The photons released during this decoupling are what we observe today as the CMB. The CMB rest frame corresponds to the frame in which the CMB radiation appears most uniform, which reflects the average velocity of the plasma at the time of decoupling.

The existence of a CMB rest frame doesn't necessarily violate Lorentz symmetry, which states that the laws of physics are the same for all observers in uniform motion. Lorentz symmetry still holds true; however, we, as observers, are situated within a specific frame of reference in the universe. The CMB rest frame is simply a convenient frame for describing the large-scale structure and evolution of the universe. From our vantage point, we observe the CMB with a specific dipole anisotropy, which is a slight variation in temperature across the sky. This dipole is interpreted as our motion relative to the CMB rest frame. By subtracting this dipole, we can analyze the smaller temperature fluctuations in the CMB, which provide crucial information about the early universe and the formation of structures like galaxies and galaxy clusters.

The question mentions the possibility of spontaneous momentum symmetry breaking, drawing an analogy to viscous interactions causing matter to adopt a common velocity. While there are similarities, it's crucial to remember that the early universe's plasma was driven by the strong electromagnetic interactions between charged particles, rather than solely viscous forces. These interactions played a dominant role in establishing a shared velocity among the particles. The process is more accurately described as the result of frequent scattering and thermalization, leading to a highly uniform energy distribution. The uniformity of the CMB, after accounting for our motion, provides strong evidence for this thermal equilibrium at the time of decoupling. Furthermore, if there had been significant momentum symmetry breaking due to, say, exotic particles or previously unknown physical processes, these would likely leave noticeable traces in the CMB, which have yet to be observed.

The idea of large-scale fluid structures, such as eddies, within the matter that created the CMB, is an interesting consideration. However, the level of uniformity observed in the CMB places tight constraints on the presence and size of such structures. If large-scale eddies existed in the early universe, they would have imprinted distinctive patterns onto the CMB. These patterns would appear as temperature anisotropies on specific angular scales. The fact that the observed CMB is so remarkably uniform, with temperature fluctuations of only about one part in 100,000, suggests that any such eddies must have been either very small or very weak. Sophisticated statistical analyses of the CMB data, performed by missions such as COBE, WMAP, and Planck, have searched for such non-Gaussian features, but so far, no evidence of significant large-scale fluid structures has been found. These analyses help to strengthen our confidence that the cosmological principle holds to a good approximation.

The cosmological principle, which states that the universe is homogeneous and isotropic on large scales, is a fundamental assumption in cosmology. The remarkable uniformity of the CMB strongly supports this principle. Although the question posits the possibility of large scale fluid structures undermining the cosmological principle, the current CMB observations combined with statistical analyses do not provide enough information to confirm the hypothesis. While the cosmological principle does not rule out local variations or smaller scale inhomogeneities, it asserts that these variations average out over sufficiently large volumes. Our understanding of the early universe and the physics of the CMB allows us to make precise predictions about the expected patterns of temperature fluctuations. By comparing these predictions with the actual observations, we can test the validity of the cosmological principle and place limits on the presence of any deviations from homogeneity or isotropy.