Wikipedia:Reference desk/Archives/Science/2019 August 17

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August 17

Anisotropic vs isotropic nature of spacetime in the universe

This question is regarding the expansion or contraction of the universe.

Is there any WP article that provides information on how to verify and validate the assumption of isotropic spacetime? Any other resource might also be helpful.

The FLRW General_metric relies on the assumption that the universe is homogeneous and isotropic. The FLRW metric is used in developing the Lambda-CDM_model of cosmology and is the basis of the narrative in Expansion_of_the_universe. For now lets ignore the topic of homogeneous vs inhomogeneous cosmology. But what if the universe is anisotropic implying that density of mass-energy and properties of spacetime are different along various directions of n-dimensional spacetime?

The WP article on anisotropy only has a small paragraph about cosmological anisotropy: "Physicists from University of California, Berkeley reported about their detection of the cosine anisotropy in cosmic microwave background radiation in 1977. Their experiment demonstrated the Doppler shift caused by the movement of the earth with respect to the early Universe matter, the source of the radiation. Cosmic anisotropy has also been seen in the alignment of galaxies' rotation axes and polarisation angles of quasars."

The articles on Gravitational_collapse, Chronology_of_the_universe and Big_Crunch weren't sufficient to indicate if different parts of the universe can be expanding or contracting in different ways. Why wouldn't a massive cosmic entity cause parts of the universe to contract as well as other connected parts in an orthogonal direction to expand while bending spacetime?

I am interested in this topic because I can analytically reason that if galaxies are contracting faster than the rest of the universe, regardless of whether the universe as a whole is contracting or static or expanding, many of the experimental observations of modern cosmology as well as predictions of general relativity would be true and would explain the current universe.

An observer from any galaxy would note that all galaxies were "moving away from each other" if galaxies were collapsing faster than the rest of spacetime.

And in particular, the observers in each galaxy would note increasing mass-energy density if galaxies were collapsing faster than an overall collapsing universe without the need for introducing concepts like dark energy and dark matter. Also in this case of more rapidly collapsing galaxies in a collapsing universe, ergodic regions could envelope a galaxy to cause internal refraction and reflection of electromagnetic radiation within the envelope that might be observed as "cosmic background radiation" from within the envelope. Such an envelope around massive entities that scatters light along with destructive interference of light on a cosmic scale could be an answer for why the night sky is sufficiently dark (an answer to Olbers'_paradox)

It doesn't seem to me that we would obtain the current observations of red-shifted radiation from cosmic entities or other indicators of "increasing separation" among galaxies if and only if the universe were expanding.

How might I go about discussing this topic and collecting verifiable resources so that I can make better progress in researching the topic of anisotropic universe?

--Zailagu (talk) 06:17, 17 August 2019 (UTC)

The universe is likely isotropic, see this. --2600:1700:90E0:E040:74E7:7757:2992:8BE7 (talk) 06:49, 17 August 2019 (UTC)

Well, cosmologists generally agree that an isotropic, expanding universe best explains what we observe from multiple lines of evidence. You seem to be coming at this from the angle of trying to find other ways (such as the collapsing universe you propose) to explain certain observations, like cosmic redshift. The thing is, you can always propose ad hoc hypotheses to explain observations (maybe the Solar System is a cosmic zoo, with a giant hologram projected around it by our alien wardens to simulate a universe), which is why we have principles like Occam's razor. If the universe is collapsing, what was the initial state before the collapse, and why is it collapsing? There's nowhere near enough mass in the observable universe (as far as we can tell) to cause it to collapse gravitationally. On the other hand, the Lambda-CDM model tidily explains many observations, including cosmic redshift, the cosmic microwave background, and the proportion of elements in the universe (including the metallicity of star populations). You also mention dark matter, but dark matter doesn't really have much to do with cosmological isotropy, as far as I understand it (someone please correct me if I'm wrong). Rather, we have a lot of observations that indicate there's a bunch of mass sitting around in the universe that doesn't interact electromagnetically, so we call it "dark" matter. All conclusions in science are provisional, so it's possible tomorrow we could find something mind-blowing that totally overturns our theories (which is what basically happened with relativity and quantum mechanics), but until then there's a reason why the "standard" models are standard. --47.146.63.87 (talk) 07:15, 17 August 2019 (UTC)

References

An interesting paper on an apparent though still hotly debated anisotropy that has been observed, specifically regarding the alignment of distant quasars: [1]. Hypotheses include: A) Selection bias has not been completely handled yet (on both radio and visual observations); B) Many quasars in a region may be descendants of a single more massive object, and are expected to have similar axes of rotation; C) ¯\_(ツ)_/¯ Someguy1221 (talk) 09:04, 17 August 2019 (UTC)