What are the Main Theories for the Current State and Structure of the Universe?

[Darkmatrix]

Hi, I'm wondering what the main theories for the state/structure of the universe are and what they consist of? Not the creation but the current state/structure of it.

 

[bapowell]

The so-called Lambda-CDM (or LCDM or $\Lambda$CDM) is the standard concordance model that best describes the evolution and current state of the universe. It is the standard hot big bang model that describes a universe expanding and cooling from an early hot and dense period, through the cold dark matter (CDM)-assisted formation of large scale structure, to the current epoch of dark energy-dominated expansion (the Lambda in LCDM). LCDM assumes a scale invariant spectrum of Gaussian, adiabatic density perturbations as the initial seeds of structure formation, which so far provides the best fit to data.

 

[BillSaltLake]

I think that perfect scale invariance of the spectrum is not what is observed and that the degree of deviation from scale invariance is very important. Am I correct in the assertion that if it were perfect scale invariance, then the density deviations over length scales typical of the whole observed last scattering surface would be much less than observed?

 

[bapowell]

I think that perfect scale invariance of the spectrum is not what is observed and that the degree of deviation from scale invariance is very important. Am I correct in the assertion that if it were perfect scale invariance, then the density deviations over length scales typical of the whole observed last scattering surface would be much less than observed?

It is true that the Harrison-Zel'dovich (scale invariant) spectrum is claimed by WMAP7 to be ruled out at 3 sigma, favoring instead a power law spectrum with a spectral index, n, centered at around n=0.97. However, when model selection is taken into consideration, this conclusion no longer holds up to scrutiny. Model selection concerns itself with the question: suppose we have a base model, where n = 1 is set and not free to vary (the Harrison-Zel'dovich (HZ) model), and suppose we have a competing model where n is allowed to vary freely within some range. The second model has one more additional free parameter than the first, and will consequently provide a better fit to the data simply on account of this additional freedom. Model selection works to ensure that the addition of this free parameter is warranted by the data in the first place. So, it's not simply a matter of counting sigmas; one must also determine whether the data favors a more complex model. When this analysis is carried out, it is found that the evidence against the HZ spectrum is not substantial. See http://arxiv.org/abs/arXiv:0912.1614.

The HZ spectrum can also be made to fit by considering more general reionization scenarios:
http://arxiv.org/abs/arXiv:1003.4763

 

[sardar]

There are several main theories for the current state and structure of the universe, including the Big Bang Theory, the Steady State Theory, and the Inflationary Theory.

The Big Bang Theory suggests that the universe began as a singularity, a point of infinite density and temperature, and has been expanding and cooling ever since. This theory is supported by the observation of cosmic microwave background radiation, which is the residual heat left over from the Big Bang.

The Steady State Theory, on the other hand, proposes that the universe has always existed in a steady state, with matter being continuously created to fill in the gaps left by the expansion of the universe. This theory has largely been disproven by observations of the cosmic microwave background radiation and the discovery of the expansion of the universe.

The Inflationary Theory combines elements of both the Big Bang and Steady State theories, suggesting that the universe underwent a period of rapid expansion in its early stages, followed by a more gradual expansion. This theory is supported by observations of the large-scale structure of the universe and the distribution of matter and energy.

Other theories, such as the Multiverse Theory and the String Theory, also attempt to explain the current state and structure of the universe, but these are still being studied and debated among scientists.

Overall, our current understanding of the universe's state and structure is constantly evolving as new evidence and theories emerge. It is a fascinating and complex topic that continues to captivate scientists and researchers.