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spaghetti3451
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What is the main motivation behind the theory of cosmological perturbations? Is it the observational data, observational hints, or perhaps the theory of inflation?
Cosmological perturbations are an attempt to mathematically model the fact that our universe isn't perfectly uniform in density. The idea is to assume you can define an isotropic, homogeneous space-time, and get from that idealized model to our real universe by adding perturbations to that basic space-time.failexam said:What is the main motivation behind the theory of cosmological perturbations? Is it the observational data, observational hints, or perhaps the theory of inflation?
Yes, they're independent of inflation.failexam said:Is the theory of cosmological perturbations independent of the theory of inflation, or the are the theories interconnected and one cannot survive without the other?
If the theory of inflation is proven to be wrong, will there still be a need for a theory of cosmological perturbations?
The metric doesn't exist in Newtonian gravity.failexam said:Thanks for the detailed answer.
On a different note, why does the Newtonian theory of cosmological perturbations neglect perturbations of the metric?
All scales. Perturbations larger than the Hubble radius get "frozen" and tend not to evolve much (they do evolve some, just not much).failexam said:Well, I guess then, in general relativity, the metric does perturb.
Does the metric perturb on all scales? Or does the metric perturb only on scales larger than the Hubble radius?
The "modes" I mentioned before are waves. In a gas, they become pressure (sound) waves.failexam said:Would it be possible to explain in a bit more detail?
I don't understand what the wave has to do with the perturbation of the metric. Also, how does the slowing down of the rate of expansion help?
It is a 2-tensor covariant object. But the covariance of the metric is not the same as invariance: the metric does change under a change in coordinates. The covariance means that it changes in a very specific way under a change in coordinates. This statement doesn't say that the metric itself has any particular symmetries (though it is always possible to find coordinates for which the metric is diagonal). In particular, if the metric is to describe our universe it can't be symmetric under rotations because the universe we observe is different in different directions.failexam said:Hey, it's alright with your explanation.
I was wondering if you can explain the following sentence.
The first step in the analysis of metric fluctuations is to classify them according to their transformation properties under spatial rotations.
I always thought that the metric is a two-tensor covariant object. Where does this business of classifying them according to their transformations under spatial rotations come from?
Chalnoth said:Slowing down the rate of expansion increases the Hubble radius.
Chalnoth said:In particular, if the metric is to describe our universe it can't be symmetric under rotations because the universe we observe is different in different directions.
The Hubble radius is simply ##c/H##.failexam said:You mention that slowing down the rate of expansion increases the Hubble radius. But isn't the Hubble radius increasing all the time anyway, regardless of the expansion (or not) of the universe?
The assumption of isotropy is approximate, not exact. The existence of the Earth and stars and galaxies prove that it's not exactly the same in every direction. If it were exactly the same in every direction, our universe would be a perfectly-uniform gas with no structure.failexam said:You say that the universe we observe is different in different directions, but isn't is a basic assumption (i.e. the cosmological principle) that the universe is isotropic?
Chalnoth said:it is always possible to find coordinates for which the metric is diagonal
True. But it's also usually not possible to find a single coordinate chart which is valid for the entire spacetime.PeterDonis said:In a sufficiently small neighborhood of a particular event, yes. But it's not always possible to find a single coordinate chart in which the metric is diagonal throughout the entire spacetime.
Chalnoth said:it's also usually not possible to find a single coordinate chart which is valid for the entire spacetime.
The theory of cosmological perturbations is a mathematical framework used to study the evolution of small fluctuations in the density and temperature of the early universe. These fluctuations are believed to have played a crucial role in the formation of large-scale structures, such as galaxies and galaxy clusters, in the universe.
The theory of cosmological perturbations was motivated by the need to explain the observed large-scale structures in the universe. Early observations showed that the universe is not completely homogeneous and that there are variations in the density and temperature of matter on large scales. This theory was developed to understand the origins and evolution of these fluctuations.
The theory of cosmological perturbations is an essential part of the standard model of cosmology, which describes the evolution of our universe from the Big Bang to the present day. It helps to explain how small initial fluctuations in the early universe grew and evolved to form the large-scale structures that we observe today.
There is strong evidence to support the theory of cosmological perturbations, including observations of the cosmic microwave background radiation, which is believed to be the oldest light in the universe. These observations show slight temperature variations that are consistent with the predictions of the theory. Additionally, observations of the large-scale structure of the universe, such as the distribution of galaxies, also support this theory.
While the theory of cosmological perturbations has been successful in explaining many observations, there are still some open questions and challenges. For example, the theory does not fully explain the observed distribution of galaxies on small scales. There are also ongoing efforts to better understand the initial conditions and mechanisms that gave rise to the fluctuations in the early universe. Further research and observations are needed to address these challenges and improve our understanding of this theory.